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Chapter 2. Frontend

Table of Contents

The frontend is an HTTP endpoint that provides a REST API. In this chapter, we discuss how you can interact with dCache using this interface.

In this chapter, we will assume that dCache is running a frontend service on dcache.example.org on port 3880 with TLS encryption enabled. Therefore, all the example URLs will start https://dcache.example.org:3880/

REST overview

REST is a design principal, rather than a specific protocol. One of the key ideas of REST is that clients use only a very few operations when interacting with dCache. In fact, only four standard HTTP operations are needed: GET, POST, DELETE and PATCH.

Despite the small number of operations, clients can still discover and control a wide range of aspects of dCache by targeting different URLs. These URLs are referred to as resources, with the different requests having similar semantics: a GET request discovers the current status of a resource, a POST request creates a new resource, a DELETE request removes or destroys a resource and a PATCH request modifies an existing resource.

The REST API that frontend provides is non-standard. This allows us to expose some of dCache’s advance features that are not available through other protocols.

All REST API resources start /api/v1/. The next path element in the resource groups together related concepts; for example resources that start namespace (/api/v1/namespace) represent dCache’s namespace: files and directories. Resources that start events (/api/v1/events) represent the Server-Sent Events (SSE) support and its management interface. Resources that start /user (/api/v1/user) describe the identity of dCache users.

A number of resources are intended for administrative operations and require special privileges. These resources are not documented here, but in a separate admin-focused book.

Swagger

Swagger is a standard way of describing a REST API as a JSON object. All the REST API calls are described, along with the arguments and the possible return values.

A Swagger description of a REST API may be used to build documentation for that API. It may also be used to generate REST clients in almost any language.

Each dCache frontend provides a swagger description of its API. The swagger JSON description is available with the path /api/v1/swagger.json; for example, from the URL https://dcache.example.org:3880/api/v1/swagger.json. A GET request to this location returns a JSON object describing all dCache REST operations.

curl -s https://prometheus.desy.de:3880/api/v1/swagger.json | jq .
|{
|  "swagger": "2.0",
|  "info": {
|    "description": "User and administration interaction with dCache",
|    "version": "Swagger Server",
|    "title": "",
|    "contact": {
|      "name": "dCache support team",
|      "email": "support@dCache.org"
|    },
|    "license": {
|      "name": "Apache 2.0",
|      "url": "http://www.apache.org/licenses/LICENSE-2.0"
|    }
|  },
|  "basePath": "/api/v1",
|[...]
|  "externalDocs": {
|    "description": "Wiki",
|    "url": "https://github.com/dCache/dcache/wiki/Restful-API"
|  }

Swagger UI

The Swagger UI is a JavaScript application that reads the Swagger JSON description of a REST API and builds an interactive web-page. This page allows you to explore the API, read the documentation and even try out the operations.

dCache frontend comes bundled with the Swagger UI application. You can try the Swagger UI by pointing your browser at /api/v1/ (e.g,. https://dcache.example.org:3880/api/v1/).

Top of Swagger UI page

Note the Swagger JSON description of dCache REST API and corresponding Swagger UI include all resources, whether they are user-focused or admin-focused. You will see a number of resources that are not described here and that regular dCache users are not authorised to use.

As an example, by scrolling down the page, you will find a section on dCache’s namespace operations.

Overview of namespace operations

Clicking on the GET /namespace/ box expands this operation to reveal detailed information about this operation. This includes which parameters this operation accepts and what values might be returned.

Documentation on GET /namespace operation

Clicking on the “Try it out” button modifies the display so that the page now accepts values. A large Execute button also appears. Entering the desired values and clicking on the Execute button will make a REST request. The result of the REST operation is also shown.

Trying out a GET on /namespace

You can also enter username and password information by clicking on the Authorize button near the top of the page. Swagger UI will use this credential when making subsequent requests.

You can find out more about Swagger UI at Swagger UI home page.

Authentication

The frontend service supports several different forms of authentication. Each REST request may be authenticated using username and password (HTTP BASIC), Kerberos, OpenID-Connect, Macaroons, SciToken. In addition, the TLS connection supports authentication with X.509 client certificates. However, it is possible that your dCache instance is configured to support only a subset of these options.

Bearer tokens (such as macaroons, SciToken and OpenID-Connect) may be specified by including them in the Authorization HTTP request header; e.g., Authorization: Bearer <token>. They may also be specified in the URL, with the authz query parameters; e.g. https://dcache.example.org:3880/api/v1/user?authz=M[...]z

If the client supplies an X.509 client certificate when establishing the secure TLS connection then that identity is used only if no other authentication information is available. If the HTTP request contains the Authorization request header or the authz query parameter then that information is used to authenticate and authorise the request.

REST with curl

In most of this document, we will give the resource path (from which the request URL is derived), indicate a request type (e.g., GET or POST), necessary input (if any) and output (if any).

Whether it is to gain experience or to build a simple script, the curl command is a simple way to interact with the dCache REST API. In the following section, we describe how to use curl with frontend.

Formatting the response JSON

Although a simple curl invocation will make a GET request and show the response, the format may not be laid out for easy reading. Therefore, it is often useful to reformat the JSON output. The following example shows the JSON response reformatted using the jq command:

curl -s https://dcache.example.org:3880/api/v1/user | jq .
|{
|  "status" : "ANONYMOUS"
|}

The | jq . uses the jq command to format the JSON response. Since curl is no longer providing data on standard output (stdout), it provides progress information instead. This would upset the output, so the -s option is used to tell curl not to emit such progress information.

Including JSON in POST request

When making a POST request with curl that sends JSON as input, it is important to specify the Content-Type request header. This is because curl will, by default, use the wrong type, which will prevent dCache from accepting the request.

For example, to send a POST request with the following JSON input:

{
  "action": "mkdir",
  "name": "new-dir"
}

The curl command might look like:

curl -u paul -X POST -H 'Content-Type: application/json' \
|        -d '{"action":"mkdir", "name":"new-dir"}' \
|        https://dcache.example.org:3880/api/v1/namespace/Users/paul
|Enter host password for user 'paul':
|{"status":"success"}

Discovering Location response header

In several cases, a POST request will create a new resource with a name chosen by dCache. A common pattern is for dCache to return the URL of this new resource in the Location HTTP response header.

The -D- curl option returns all response headers, allowing you to see the Location response header:

curl -D- -u paul -X POST \
|        https://dcache.example.org:3880/api/v1/events/channels
|Enter host password for user 'paul':
|HTTP/1.1 201 Created
|Date: Tue, 09 Apr 2019 20:50:07 GMT
|Server: dCache/5.1.0-SNAPSHOT
|Location: https://dcache.example.org:3880/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w
|Access-Control-Allow-Origin: *
|Access-Control-Allow-Methods: GET, POST, DELETE, PUT
|Access-Control-Allow-Headers: Content-Type, Authorization, Suppress-WWW-Authenticate
|Content-Length: 0
|

In this example, the new resource has the URL https://dcache.example.org:3880/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w.

Active transfers

The transfers resource (/api/v1/transfers) represents all ongoing transfers. This information is collected periodically and made available as snapshot views of all concurrent transfers.

By default, a GET request returns information about transfers from the most recent snapshot.

The following example shows a typical response when there are no active transfers:

{
  "items": [],
  "currentOffset": 0,
  "nextOffset": -1,
  "currentToken": "e4c5759b-f9b3-4165-9bd5-074b8e022596",
  "timeOfCreation": 1554815277797
}

In this example, there are no active transfers so the items property is empty. The currentOffset and nextOffset properties indicate that all available data is shown.

The currentToken is a unique reference for this snapshot. It may be supplied as a query parameter when making subsequent GET requests to refer to this snapshot (and not any other snapshot).

Finally, the timeOfCreation gives the Unix time (in milliseconds) when this snapshot was created.

The following example shows the response to a GET request when there are on-going transfers:

{
  "items": [
    {
      "cellName": "webdav-secure-grid",
      "domainName": "dCacheDomain",
      "serialId": 1554815749796000,
      "protocol": "HTTP-1.1",
      "process": "",
      "pnfsId": "0000AC8078894C74493B8F1BE11DC2672D9C",
      "path": "/Users/paul/test-1",
      "pool": "pool2",
      "replyHost": "2001:638:700:20d6:0:0:1:3a",
      "sessionStatus": "Mover PoolName=pool2 PoolAddress=pool2@pools/641: Waiting for completion",
      "waitingSince": 1554815749796,
      "moverStatus": "RUNNING",
      "transferTime": 8021,
      "bytesTransferred": 16384,
      "moverId": 641,
      "moverSubmit": 1554815749839,
      "moverStart": 1554815749839,
      "subject": {
        "principals": [
          {
            "primaryGroup": false,
            "gid": 0,
            "name": "0"
          },
          {
            "clientChain": [
              "2001:638:700:20d6:0:0:1:3a"
            ],
            "address": "2001:638:700:20d6:0:0:1:3a",
            "name": "2001:638:700:20d6::1:3a"
          },
          {
            "name": "paul.millar@desy.de"
          },
          {
            "primaryGroup": true,
            "gid": 1001,
            "name": "1001"
          },
          {
            "name": "/C=DE/O=GermanGrid/OU=DESY/CN=Alexander Paul Millar"
          },
          {
            "uid": 2002,
            "name": "2002"
          },
          {
            "primaryGroup": false,
            "gid": 2002,
            "name": "2002"
          },
          {
            "primaryGroup": false,
            "name": "dteam"
          },
          {
            "fqan": {
              "group": "/dteam",
              "capability": "",
              "role": ""
            },
            "primaryGroup": true,
            "name": "/dteam"
          },
          {
            "name": "paul"
          },
          {
            "loA": "IGTF_AP_CLASSIC",
            "name": "IGTF-AP:Classic"
          }
        ],
        "readOnly": false,
        "publicCredentials": [],
        "privateCredentials": []
      },
      "userInfo": {
        "username": "paul",
        "uid": "2002",
        "gid": "1001",
        "primaryFqan": {
          "group": "/dteam",
          "capability": "",
          "role": ""
        },
        "primaryVOMSGroup": "/dteam"
      },
      "valid": true,
      "uid": "2002",
      "gid": "1001",
      "transferRate": 2,
      "vomsGroup": "/dteam",
      "timeWaiting": "0+00:00:09"
    }
  ],
  "currentOffset": 0,
  "nextOffset": -1,
  "currentToken": "fbfd7d39-9959-4135-bf9a-5f65355346f5",
  "timeOfCreation": 1554815757860
}

The format describing a transfer is not yet fixed and may be subject to change.

Filtering

The list of active transfers may be limited by specifying different query parameters.

The following filters are supported:

Name Select only transfers…
state in state
door initiated with this door
domain inititated in this specific domain
prot using the named protocol
uid initiated by this user
gid initiated by members of this group
vomsgroup initiated by a member of this voms group
path transfers involving this path
pnfsid transfers involving this PNFS-ID
client transfers involving this client

An an example, the query /api/v1/transfers?uid=1000&prot=HTTP-1.1 would list all current HTTP transfers involving the user with uid 1000.

Pagination

On an active dCache, there may be many too many concurrent transfers to return them all in a single JSON response. To support this, a query can target a specific snapshot (rather than the latest snapshot) and limiting the number of responses.

The token query parameter may be used to select a specific snapshot. The value is the currentToken value. In the above example, the currentToken value is fbfd7d39-9959-4135-bf9a-5f65355346f5, so repeated queries that target this snapshot have token=fbfd7d39-9959-4135-bf9a-5f65355346f5 as a query parameter.

The offset and limit query parameters select a subset of values. If not specified then they default to 0 and 2147483647 respectively.

Therefore, a GET request targeting the path /api/v1/transfers?token=fbfd7d39-9959-4135-bf9a-5f65355346f5&offset=0&limit=10 returns the first ten transfer from a specific snapshot.

A subsequent GET request targeting the path /api/v1/transfers?token=fbfd7d39-9959-4135-bf9a-5f65355346f5&offset=10&limit=10 returns the next ten transfers, and so on.

Sorting

The output is sorted. The priority of different fields is controlled by the sort query parameter, which takes a comma-separate list of field names. The default value is door,waiting.

Doors

Doors are protocol-specific network services that allow clients to interact with files stored in dCache; for example, a WebDAV door allows clients to read and write files using the WebDAV protocol, in addition to creating directories and renaming files.

Uploading and downloading files is not supported in the REST API; therefore, a client must find a door that supports an appropriate protocol if the client needs to transfer files.

The doors resource (/api/v1/doors) represents all doors within dCache. This allows a client to discover which protocols are supported and what are their endpoints.

A GET request on this resource yields a complete set of all doors. The response is a JSON array of JSON objects, with each JSON object describing a door.

Here is an example response:

[
  {
    "protocol": "ftps",
    "version": "1.0.0",
    "root": "/",
    "addresses": [
      "dcache.example.org"
    ],
    "port": 21,
    "load": 0,
    "tags": [
      "glue",
      "srm",
      "storage-descriptor"
    ],
    "readPaths": [
      "/"
    ],
    "writePaths": [
      "/"
    ]
  },
  {
    "protocol": "https",
    "version": "1.1",
    "root": "/",
    "addresses": [
      "dcache.example.org"
    ],
    "port": 2443,
    "load": 0,
    "tags": [
      "glue",
      "srm",
      "storage-descriptor"
    ],
    "readPaths": [
      "/"
    ],
    "writePaths": [
      "/"
    ]
  },
  {
    "protocol": "gsiftp",
    "version": "1.0.0",
    "root": "/",
    "addresses": [
      "dcache.example.org"
    ],
    "port": 2811,
    "load": 0,
    "tags": [
      "glue",
      "srm",
      "storage-descriptor"
    ],
    "readPaths": [
      "/"
    ],
    "writePaths": [
      "/"
    ]
  },
  {
    "protocol": "https",
    "version": "1.1",
    "root": "/",
    "addresses": [
      "dcache.example.org"
    ],
    "port": 443,
    "load": 0,
    "tags": [
      "cdmi",
      "dcache-view"
    ],
    "readPaths": [
      "/"
    ],
    "writePaths": [
      "/"
    ]
  }
]

In this example, three doors are listed: an FTPS door listening on port 21, an gsiftp (GridFTP) door listening on port 2811, and a WebDAV door listening on port 443.

The root property gives the door’s root path. The value is the absolute path in dCache equivalent to the door’s root path. For example, if the root property value is /data/atlas and a client downloads the file /2019/data-1.root through this door then the client will receive the file /data/atlas/2019/data-1.root.

The addresses indicates on which address(es) the door is listening.

The port indicates the TCP port number on which the door is listening.

The tags are arbitrary metadata describing aspects of the door; for example, describing for which kind of use the door is intended. This information is advisory, but allows dCache to advertise doors that are for specialist users.

The load is a number between 0 and 1, indicating how busy the door is currently.

The readPaths and writePaths describe generic limitations the door will impose; for example, only allowing write activity on a subset of the namespace.

Identity

The identity resources are about someone’s identity within dCache. There is currently one resource that represents the user making the request: /user (/api/v1/user). A GET request to this resource returns information about the user making the request.

If no credentials are presented then the user is the ANONYMOUS user:

{
  "status" : "ANONYMOUS"
}

If the user authenticates, then information about that user is returned:

{
  "status": "AUTHENTICATED",
  "uid": 2002,
  "gids": [
    2002,
    0
  ],
  "username": "paul",
  "homeDirectory": "/Users/paul",
  "rootDirectory": "/"
}

Namespace

With the namespace part of the API, you can discover information about a specific file or directory, list the contents of a directory, delete and rename files, and modify a file’s or directory’s QoS.

Discovering metadata

Resources that describe files and directories in dCache are found beneath the namespace resource (/api/v1/namespace) by simply appending the dCache path to this path. For example, the resource namespace (/api/v1/namespace) represents the root directory and namespace/upload (/api/v1/namespace/upload) represents the upload directory.

To discover information about a path in dCache, make a GET request to the file’s or directory’s resource.

The following example shows the root directory’s resource:

{
  "fileMimeType" : "application/vnd.dcache.folder",
  "fileType" : "DIR",
  "pnfsId" : "000000000000000000000000000000000000",
  "nlink" : 11,
  "mtime" : 1554697387559,
  "creationTime" : 1554696069369,
  "size" : 512
}

Additional information may be requested by specifying different query parameters in the GET request. These additional fields are not included by default as fetching them would slow down the query.

There are three additional fields: locality, locations and qos; for example, to enable fetching locality and qos information, the GET request would include ?locality=true&qos=true in the URL.

The locality flag adds information about whether data is currently available. With this flag, the output includes the extra field fileLocality in the output for files. This flag does not provide any extra information for directories. The possible values for fileLocality are summarised in the following table:

Name Semantics
ONLINE data is available now.
NEARLINE data is not available now; an automated process can make the data available on demand.
ONLINE_AND_NEARLINE data is available now, but might require an automated activity to make it available in the future.
UNAVAILABLE data is not available now; sysadmin intervention may be needed to make it available.
LOST data is not available and there is no process to obtain it.

The locations flag adds information about where data is currently located. With this flag, the output includes the locations field. This field’s value is a JSON array of pool names.

The qos flag adds information about the current QoS of a file or directory. With this flag, the output includes the currentQoS field and optionally the targetQoS field. The former describes the current QoS for this file or directory. If dCache is currently transitioning a file to a different QoS then the targetQoS field is present, describing which QoS the file should have. See QoS Management to understand more about these QoS values. How to trigger QoS changes is described in Modifying QoS.

If the path does not exist, then the HTTP response has a status code 404, with the following JSON response:

{
  "errors": [
    {
      "message": "Not Found",
      "status": "404"
    }
  ]
}

Listing directories

A GET request that targets a directory resource URL may include the query argument children=true. For example, a request to list the root directory would target the resource /namespace (/api/v1/namespace) and the complete path would be /api/v1/namespace/?children=true.

The response is as before, but now also includes the children property. The value is a JSON array of JSON objects, where each JSON object describes a child of this directory. The same information is provided for each child as is provided for the parent directory.

The following example shows a typical root directory listing.

{
  "fileMimeType": "application/vnd.dcache.folder",
  "children": [
    {
      "fileName": "lost+found",
      "fileMimeType": "application/vnd.dcache.folder",
      "fileType": "DIR",
      "pnfsId": "000000000000000000000000000000000001",
      "nlink": 2,
      "mtime": 1554696070327,
      "creationTime": 1554696070327,
      "size": 512
    },
    {
      "fileName": "Users",
      "fileMimeType": "application/vnd.dcache.folder",
      "fileType": "DIR",
      "pnfsId": "00007EF0F064738E420099E7BDA672500DC2",
      "nlink": 30,
      "mtime": 1554696093632,
      "creationTime": 1554696089487,
      "size": 512
    },
    {
      "fileName": "VOs",
      "fileMimeType": "application/vnd.dcache.folder",
      "fileType": "DIR",
      "pnfsId": "0000F0C3D9A2EA9F4681970BF3D414A311ED",
      "nlink": 15,
      "mtime": 1554696092837,
      "creationTime": 1554696089424,
      "size": 512
    },
    {
      "fileName": "upload",
      "fileMimeType": "application/vnd.dcache.folder",
      "fileType": "DIR",
      "pnfsId": "00003405A2416C8D4317AA3833352F967A9A",
      "nlink": 14,
      "mtime": 1554726185595,
      "creationTime": 1554697387559,
      "size": 512
    }
  ],
  "fileType": "DIR",
  "pnfsId": "000000000000000000000000000000000000",
  "nlink": 11,
  "mtime": 1554697387559,
  "creationTime": 1554696069369,
  "size": 512
}

If the target is not a directory then adding children=true has no effect on the output.

Deleting files and directories

A file or directory may be deleted by making a DELETE request on the target’s corresponding resource. For example, a DELETE request targeting /api/v1/namespace/Users/paul/old-data would attempt to delete the file /Users/paul/old-data.

If the target is a directory then it must be empty before the operation will be successful.

The following example shows the response when successfully deleting a file:

{
  "status": "success"
}

Creating directories

A new directory may be created using a POST request targeting the resource corresponding to the parent directory. This request must supply a JSON object as the HTTP entity, with the action property of mkdir and the `name property containing the new directory’s name.

The following example creates a new directory called my new dir.

{
  "action": "mkdir",
  "name": "my new dir"
}

The response, if successful shows the status success:

{
  "status": "success"
}

Moving and renaming

To rename or move a file or directory, make a POST request to the resource representing the source file or directory; for example, to rename the file /Users/paul/my-data, the POST request would target a URL with the path /api/v1/namespace/Users/paul/my-data.

The POST request must contain a JSON object with the action property value mv, and the destination property set to the new name or path. If the destination path is relative (i.e., does not start with /) then it is resolved against the request’s path parameter.

The following example renames the source to new-name.

{
  "action": "mv",
  "destination": "new-name"
}

If the operation is successful then HTTP response has a 200 status code, and the response entity is the JSON object:

{
  "status": "success"
}

Modifying QoS

Each file or directory has a corresponding QoS value. To modify this assigned QoS, make a POST request to the resource representing the file or directory; for example, to modify the QoS of the file /Users/paul/my-data, the POST request would target /api/v1/namespace/Users/paul/my-data.

The POST request must contain a JSON object with the action property set to qos, and the target property set to the desired QoS. The following example requests the target have QoS tape:

{
  "action": "qos",
  "target": "tape"
}

QoS transitions that target a file may take some time to complete. While the transition is taking place, a GET request on the resource representing this file will continue to show the file’s QoS unchanged but will include an additional field targetQoS that indicates to which QoS dCache is transitioning this file. See Discovering metadata for more details.

See QoS Management to understand more about the different QoS target values, including performance characteristics.

QoS Management

The qos-management resources are a set of resources for working with different QoS classes. These resources all start (/api/v1/qos-management).

The current QoS class of a file or directory is represented by that file or directory’s resource. See Discovering metadata for more details on how to query the current QoS and modify QoS values.

The resources that start qos-management/qos (/api/v1/qos-management/qos) represent the different QoS classes.

Files and directories in dCache have an associated QoS class. The QoS class for a file describes how that file is handled by dCache; for example, what performance a user may reasonably expect when reading from that file. The QoS class for a directory describes what QoS class a file will receive when it is written into that directory. Currently, all directory QoS classes have an equivalent file QoS class with the same name.

A GET request on qos-management/qos/file (/api/v1/qos-management/qos/file) provides a list of file QoS class names. Similarly, a GET request on qos-management/qos/directory (/api/v1/qos-management/qos/directory) provides a list of directory QoS class names.

The following example shows the response to a GET request targeting /api/v1/qos-management/qos/file:

{
  "name": [
    "disk",
    "tape",
    "disk+tape",
    "volatile"
  ],
  "message": "successful",
  "status": "200"
}

The same response is returned when querying the qos-management/qos/directory resource.

In both cases, the name property is a JSON array of QoS class names.

Each individual QoS class is represented by the resource qos-management/qos/<type>/<name> (/api/v1/qos-management/qos/<type>/<name>). For example, the resource qos-management/qos/file/tape represents the QoS tape for files.

A GET request on a QoS class resource returns information about that resource. The following example shows the response to a GET request targeting the resource qos-management/qos/file/disk (/api/v1/qos-management/qos/file/disk):

{
  "status": "200",
  "message": "successful",
  "backendCapability": {
    "name": "disk",
    "transition": [
      "tape",
      "disk+tape"
    ],
    "metadata": {
      "cdmi_data_redundancy_provided": "1",
      "cdmi_geographic_placement_provided": [
        "DE"
      ],
      "cdmi_latency_provided": "100"
    }
  }
}

The returned JSON provides two groups of information.

The transition list shows all allowed transitions if a file currently has the QoS class disk. In this example, the transition from QoS class disk to QoS class volatile is not allowed. See Modifying QoS for more information about changing a file’s or directory’s QoS.

The metadata object contains information about the disk QoS class. The three values in this example (cdmi_data_redundancy_provided, cdmi_geographic_placement_provided and cdmi_latency_provided) come from the CDMI specification, see section 16.5.

Space reservations

Space reservations are a promise to store a given amount of data. They may be used when uploading a reasonable sized dataset, to avoid running out of storage space midway through the upload.

Resources beneath space (/api/v1/space) represent space reservations.

A GET request on the space/tokens (/api/v1/space/tokens) resource provides information about space reservations. The response is a JSON array containing a JSON object. Each of these JSON objects provides information about a space reservation.

The following example shows the returned JSON for two space reservations:

[
  {
    "id": 2,
    "voGroup": "/atlas",
    "retentionPolicy": "REPLICA",
    "accessLatency": "ONLINE",
    "linkGroupId": 2,
    "sizeInBytes": 268435456000,
    "creationTime": 1554782633504,
    "description": "ATLASSCRATCHDISK",
    "state": "RESERVED"
  },
  {
    "id": 3,
    "voGroup": "/atlas",
    "retentionPolicy": "REPLICA",
    "accessLatency": "ONLINE",
    "linkGroupId": 2,
    "sizeInBytes": 107374182400,
    "creationTime": 1554782633548,
    "description": "ATLASDATADISK",
    "state": "RESERVED"
  }
]

Query parameters in the URL may be used to limit the reservations listed; for example, accessLatency=ONLINE limits the response to those reservations with online access-latency and voGroup=/atlas limits the response to those reservations with /atlas ownership.

The following filters are supported:

Name Select only reservations…
id with this id.
voGroup with this VO group.
voRole with this VO role.
accessLatency with this Access Latency.
retentionPolicy with this Retention Policy.
groupId created from a linkgroup with this id.
state with this current state.
minSize with a capacity (in bytes) larger than this capacity
minFreeSpace with a free capacity (in bytes) larger than this capacuty

Multiple filters may be combined in a single query, with the effects being accumulative: each filter (potentially) reducing the number of reservations listed.

For example, the query parameter voGroup=/atlas&minSize=200000000000 (/api/v1/space/tokens?voGroup=/atlas&minSize=200000000000) restricts the returned list of space reservations to those that are both owned by VO group /atlas and have a reserved capacity is at least 200 GB.

Storage Events

Storage events is a mechanism where dCache can let you know when something of interest has happened. To do this, dCache uses a W3C standard protocol: Server-Sent Events (SSE).

SSE is a protocol that allows an HTTP client to receive events with a minimal delay. It is widely supported, with libraries existing in all major languages in addition to built-in support in all major web browsers.

All REST activity to do with storage events targets the events resource (/api/v1/events) with different resources below this having more specific roles.

Management overview

Although the SSE protocol is a standard, there is no standard management interface to allow you to control which events you are interested in receiving. Therefore, dCache has a proprietary interface that allows you to discover what possibilities exist, discover the current configuration and modify that configuration.

The SSE protocol targets a specific endpoint. In dCache, this endpoint for receiving events is called a channel. It is expected that each client will have its own channel: channels are not shared between clients. A client will create its own channel and then configures this channel to receive all events the client is interested in.

Although it is not forbidden, a client could create multiple channels. However, this is unnecessary, as a channel can receive any number of events of any type. Clients creating multiple channels is also discouraged, as each user is allowed only a limited number of channels.

Storage events are grouped together into broadly similar types, called event types; for example, all events that simulate the Linux filesystem notification system “inotify” have the inotify event type. Events that are to do with SSE support itself (or other low-level aspects) have the SYSTEM event type.

When a client creates a channel, it initially receives only SYSTEM events. To start receiving interesting events, the client must create subscriptions. A subscription is a description of which events (of a specific event type) are of interest. There is a JSON object, called a selector, that describes which events (of all possible events emitted by an event type) are of interest. The exact format of the selector depends on the event type.

A channel can have multiple subscriptions. Each subscription is independent: they could come from the same event type, or from different event types. A client can add and remove subscriptions as its interest in events changes. For example, a client that is showing the contents of a specific directory might subscribe to learn of changes to that directory; when the user changes directory, so the subscriptions would change accordingly.

Top-level information

A GET request on the events (/api/v1/events) resource returns a JSON object providing information that is independent of any channel and any event type.

The channels property value describes information about channels in general, rather than about a specific channel. Channels are automatically deleted if they are not used for long enough. The time before this happens may be adjusted by the client. The lifetimeWhenDisconnected property describes this policy, with the values in seconds.

Additionally, each user is allowed to have only a limited number of channels. Once this limit is reached, attempts to create more channels will fail. This limit is recorded in the maximumPerUser property.

The following shows a typical response to a GET request on the events resource:

{
  "channels": {
    "lifetimeWhenDisconnected": {
      "maximum": 86400,
      "minimum": 1,
      "default": 300
    },
    "maximumPerUser": 128
  }
}

In the above example, new channels are garbage collected automatically after five minutes. An individual channel may be configured to be garbage collected on a different schedule: as quickly as after one second, or as long as after a day. Users are limited to having at most 128 channels concurrently.

Understanding event types

The events/eventTypes resource (/api/v1/events/eventTypes) describes information about different events types, independent of any subscriptions.

A GET request against this resource provides a list of available event types:

[
  "inotify",
  "metronome"
]

In the above example, two event types are shown: inotify and metronome. The SYSTEM event type is not shown since a channel is always subscribed to this event type and cannot control the delivery of those events.

Individual event types are represented by the resource events/eventTypes/<name>. For example, the inotify event type is represented by the resource events/eventTypes/inotify (/api/v1/events/eventTypes/inotify).

A GET request targeting the event type resource returns a JSON object providing basic information about this event type.

For example, the following JSON is the response to a GET request targeting the events/eventTypes/metronome (/api/v1/events/eventTypes/metronome) resource

{
  "description": "a configurable stream of messages"
}

Similar information is available from the events/eventTypes/metronome resource:

{
  "description" : "notification of namespace activity, modelled after inotify(7)"
}

There are two further resources per resource type: one that represents the selector and one that represents the data supplied with events of this type.

Event Type: Selectors

The events/eventTypes/<name>/selector resource (/api/v1/events/eventTypes/<name>/selector) represents the selector for this event type. A selector is supplied when subscribing to events of a specific type, to describe which events are of interest.

A GET request on this resource returns the JSON Schema of the selector. When subscribing to this event type, the selector must satisfy this JSON Schema. In addition to describing the structure, the schema also describes the semantics of each of the arguments, including any default values that are used if not specified.

metronome

The following is the response to a GET request targeting the events/eventTypes/<name>/selector resource:

{
  "$id": "http://dcache.org/frontend/events/metronomeSelectors#",
  "$schema": "http://json-schema.org/draft-06/schema#",
  "type": "object",
  "properties": {
    "frequency": {
      "title": "The trigger frequency",
      "description": "How often events are fired, in Hz.",
      "type": "number",
      "minimum": 0.0033333333333333335,
      "maximum": 1000000
    },
    "delay": {
      "title": "The delay between successive triggers",
      "description": "The time between two triggers, in seconds.",
      "type": "number",
      "minimum": 1e-06,
      "maximum": 300
    },
    "message": {
      "title": "The event payload",
      "description": "The data sent with each event.  A ${username} is replaced by the user's username and ${count} is replaced by the message number.",
      "minLength": 1,
      "type": "string",
      "default": "tick"
    },
    "count": {
      "title": "The number of events",
      "description": "The number of events to generate before cancelling the subscription.  If not specified then the events are supplied until the subscription is explicitly cancelled by the client.",
      "type": "number"
    }
  },
  "oneOf": [
    {
      "required": [
        "frequency"
      ]
    },
    {
      "required": [
        "delay"
      ]
    }
  ],
  "additionalProperties": false
}

This describes how, when subscribing for metronome events, either the frequency or delay argument must be provided. The count and message argument may be provided, but are not required.

The following is a valid selector that requests events be sent every two seconds with the data "Message 1", "Message 2", "Message 3" and so on.

{
  "message": "Message ${count}",
  "delay": 2
}

The following JSON object is a valid selector that requests events at 1 kHZ for two seconds. Each event will have the same data "tick".

{
  "freqency": 1000,
  "count": 2000
}
inotify

Selectors for inotify are more complicated, but a JSON Schema that describes them is represented by the events/eventTypes/inotify/selector resource.

The following JSON Schema is returned by a GET request targeting this resource.

{
  "$id": "http://dcache.org/frontend/events/namespaceSelectors#",
  "$schema": "http://json-schema.org/draft-06/schema#",
  "description": "path of a directory to watch",
  "type": "object",
  "required": [
    "path"
  ],
  "properties": {
    "path": {
      "title": "The path of the file or directory to watch",
      "description": "The target must exist when the request is made.  The watch will follow the target, if it is moved.",
      "pattern": "^/(.*[^/])?$",
      "type": "string"
    },
    "flags": {
      "title": "Control which events are selected",
      "description": "See inotify(7) for the meaning of these flags.",
      "type": "array",
      "items": {
        "type": "string",
        "enum": [
          "IN_ACCESS",
          "IN_ATTRIB",
          "IN_CLOSE_WRITE",
          "IN_CLOSE_NOWRITE",
          "IN_CREATE",
          "IN_DELETE",
          "IN_DELETE_SELF",
          "IN_MODIFY",
          "IN_MOVE_SELF",
          "IN_MOVED_FROM",
          "IN_MOVED_TO",
          "IN_OPEN",
          "IN_ALL_EVENTS",
          "IN_CLOSE",
          "IN_MOVE",
          "IN_DONT_FOLLOW",
          "IN_EXCL_UNLINK",
          "IN_MASK_ADD",
          "IN_ONESHOT",
          "IN_ONLYDIR"
        ]
      }
    }
  },
  "additionalProperties": false
}

The path property is required, while the flags property is optional.

The following selector selects all inotify events for the directory /data/uploads.

{
  "path": "/data/uploads"
}

The following selector watches the /Users/paul/docs directory for any files or directories being created, renamed or deleted. For files, an IN_CREATE event is sent at the start of an upload and an IN_CLOSE_WRITE event is sent when the upload is complete.

{
  "path": "/Users/paul/docs",
  "flags": [
    "IN_CREATE",
    "IN_DELETE",
    "IN_MOVE_FROM",
    "IN_MOVE_TO",
    "IN_CLOSE_WRITE"
  ]
}

Event Type: Events

The events/eventTypes/<eventType>/event resource (/api/v1/events/eventTypes/<eventType>/event) represents the events generated by this event type.

A GET request returns a JSON Schema that describes the information included with events of this type.

metronome

The events/eventTypes/metronome/event resource (/api/v1/events/eventTypes/metronome/event) represents the events from the metronome event type.

A GET request will provide JSON object like:

{
  "$id": "http://dcache.org/frontend/events/metronomeEvents#",
  "$schema": "http://json-schema.org/draft-06/schema#",
  "type": "string"
}

This JSON Schema describes a simple JSON String which (for metronome) corresponds to the message field in the metronome selector. Therefore, the default event data is simply "tick".

inotify

The events generated by the inotify event type are described in the events/eventTypes/inotify/event (/api/v1/events/eventTypes/inotify/event`) resource.

A GET request targeting this resource returns the following JSON Schema:

{
  "$id": "http://dcache.org/frontend/events/namespaceEvents#",
  "$schema": "http://json-schema.org/draft-06/schema#",
  "description": "the description of a change in the namespace",
  "type": "object",
  "oneOf": [
    {
      "$ref": "#/definitions/nonMoveChildEvent"
    },
    {
      "$ref": "#/definitions/moveChildEvent"
    },
    {
      "$ref": "#/definitions/selfEvent"
    },
    {
      "$ref": "#/definitions/managementEvent"
    }
  ],
  "definitions": {
    "childEvent": {
      "required": [
        "name"
      ],
      "properties": {
        "name": {
          "title": "The name of the target",
          "description": "The filename of the filesystem object that triggered this event.",
          "type": "string",
          "minLength": 1
        }
      }
    },
    "nonMoveChildEvent": {
      "allOf": [
        {
          "$ref": "#/definitions/childEvent"
        },
        {
          "properties": {
            "mask": {
              "title": "One or more flags that describe this event",
              "description": "The semantics are based on inotify(7)",
              "type": "array",
              "minitems": 1,
              "maxitems": 2,
              "items": {
                "type": "string",
                "enum": [
                  "IN_ACCESS",
                  "IN_ATTRIB",
                  "IN_CLOSE_WRITE",
                  "IN_CLOSE_NOWRITE",
                  "IN_CREATE",
                  "IN_DELETE",
                  "IN_MODIFY",
                  "IN_OPEN",
                  "IN_ISDIR"
                ]
              }
            }
          }
        }
      ]
    },
    "moveChildEvent": {
      "allOf": [
        {
          "$ref": "#/definitions/childEvent"
        },
        {
          "required": [
            "mask",
            "cookie"
          ],
          "properties": {
            "mask": {
              "title": "One or more flags that describe this event",
              "description": "The semantics are based on inotify(7)",
              "type": "array",
              "minitems": 1,
              "maxitems": 2,
              "items": {
                "type": "string",
                "enum": [
                  "IN_MOVED_FROM",
                  "IN_MOVED_TO",
                  "IN_ISDIR"
                ]
              }
            },
            "cookie": {
              "title": "move association",
              "description": "An id that is the same for the MOVED_FROM and MOVED_TO events from a single namespace operation.",
              "type": "string",
              "minLength": 1
            }
          }
        }
      ]
    },
    "selfEvent": {
      "required": [
        "mask"
      ],
      "properties": {
        "mask": {
          "title": "One or more flags that describe this event",
          "description": "The semantics are based on inotify(7)",
          "type": "array",
          "minitems": 1,
          "maxitems": 2,
          "items": {
            "type": "string",
            "enum": [
              "IN_DELETE_SELF",
              "IN_MOVE_SELF",
              "IN_ISDIR"
            ]
          }
        }
      }
    },
    "managementEvent": {
      "required": [
        "mask"
      ],
      "properties": {
        "mask": {
          "title": "One or more flags that describe this event",
          "description": "The semantics are based on inotify(7)",
          "type": "array",
          "minitems": 1,
          "maxitems": 3,
          "items": {
            "type": "string",
            "enum": [
              "IN_IGNORED",
              "IN_Q_OVERFLOW",
              "IN_UNMOUNT"
            ]
          }
        }
      }
    }
  }
}

This JSON Schema describes how inotify events are one of four types: a move-child event, a non-move-child event, a self event and a management event.

Move-child events

A move child event is where the subscription’s path is a directory and either a directory item (a file or directory) is moved into the subscription’s path, or a directory item is moved out of that path. Renaming a file within a directory is treated as a move operation and generates two move-child events.

Here are some examples to clarify this.

A move-child event is generated if the watched directory is /Users/paul/data and a file is moved from /Users/paul/old-data/example.dat to /Users/paul/data/example.dat. The event might look like:

{
  "name": "example.dat",
  "mask": [
    "IN_MOVED_TO"
  ],
  "cookie": "123456789abcdef"
}

Similarly, a move-child event is generated if the watched directory is /Users/paul/data and a directory is moved from /Users/paul/data/2018 to /Users/paul/old-data/2018. Here’s how that event would look:

{
  "name": "2018",
  "mask": [
    "IN_MOVED_TO",
    "IN_ISDIR"
  ],
  "cookie": "1133557799bbddf"
}

Finally, two move-child events are generated if the file old-name.txt in /Users/paul/data is renamed to new-name.txt and kept in the same directory.

Here are those two events:

{
  "name": "old-name.txt",
  "mask": [
    "IN_MOVED_FROM"
  ],
  "cookie": "1111555511115555"
}

and

{
  "name": "new-name.txt",
  "mask": [
    "IN_MOVED_TO"
  ],
  "cookie": "1111555511115555"
}

The order of these two events is not guaranteed; however the two events will have the same cookie property value.

If a client is watching multiple directories then the event’s subscription property describes which event corresponds to which watched directory (or file). The subscription property is part of every event’s envelope, so isn’t shown here.

If a file is moved from one watch directory to another then the client will receive two move-child events with the same cookie property value, much like a rename within the same directory.

Non-move child events

Non-move events are generated when a subscription’s path is a directory and something happens with a directory item (a file or directory) within that watched directory.

The following event indicates that a new file within the watched directory has been created. This new file has the name my-new-file.dat.

{
  "name": "my-new-file.dat",
  "mask": [
    "IN_CREATE"
  ]
}

The following event indicates that a new directory within the watched directory has been created. This new directory has the name my-new-dir.

{
  "name": "my-new-dir",
  "mask": [
    "IN_CREATE",
    "IN_ISDIR"
  ]
}

This event indicates that some metadata associated with the file or directory called important.dat has changed. A client must fetch fresh metadata to learn what has changed.

{
  "name": "important.dat",
  "mask": [
    "IN_ATTRIB"
  ]
}
Self events

Self events are events that describe changes to the watched path itself, rather than the watched directory’s child. If the subscription is targeting a file (rather than a directory) then the client will receive only self events and management events.

The following event indicates that the subscription’s path, which is a directory, has moved.

{
  "mask": [
    "IN_MOVE_SELF",
    "IN_ISDIR"
  ]
}

The following event indicates that the subscription’s path, which is a file, has been deleted.

{
  "mask": [
    "IN_DELETE_SELF"
  ]
}

Any IN_DELETE_SELF event will trigger the automatic removal of the subscription.

Management events

Management events are those that are to do with the delivery of inotify events, rather than the direct result of activity in the watched portion of the namespace.

The IN_IGNORED event indicates that no further events will be sent on this subscription. Here is an example:

{
  "mask": [
    "IN_IGNORED"
  ]
}

The IN_Q_OVERFLOW event indicates that dCache-internal resources were exhausted when attempting to deliver events. It is possible that events were lost as a result of this. Therefore, the client should take steps to resynchronise its state with dCache’s namespace.

Here is an example of such an event.

{
  "mask": [
    "IN_Q_OVERFLOW"
  ]
}

Channel lifecycle

A channel provides the endpoint that an SSE client will use to receive events. A channel is also the entity that receives subscription requests.

The events/channels (/api/v1/events/channels) resource represents all channels for the current user.

A GET request against this resource returns a JSON List of JSON Strings, one for each of the currently available channels. This list is initially empty:

[]

In order to receive any events, a client must connect to the channel’s SSE endpoint. This requires that a client first create a channel.

A POST request to the events/channels (/api/v1/events/channels) resource creates a new channel. The new channel is represented by the events/channels/<id> (/api/v1/events/channels/<id>) resource.

The complete URL of this resource is provided in the Location HTTP response header:

curl -D- -u paul -X POST \
|        https://dcache.example.org:3880/api/v1/events/channels
|Enter host password for user 'paul':
|HTTP/1.1 201 Created
|Date: Tue, 09 Apr 2019 20:50:07 GMT
|Server: dCache/5.1.0-SNAPSHOT
|Location: https://dcache.example.org:3880/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w
|Access-Control-Allow-Origin: *
|Access-Control-Allow-Methods: GET, POST, DELETE, PUT
|Access-Control-Allow-Headers: Content-Type, Authorization, Suppress-WWW-Authenticate
|Content-Length: 0
|

In the above example, the new channel is represented by the resource events/channels/pf_B1dEed98IVKqc9BNa-w.

A subsequent GET request targeting the events/channels resource will show this channel:

[
  "https://dcache.example.org:3880/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w"
]

A channel may be given an identifier by the client. Having a client identifier is optional and has no impact on how the channel operates; instead, it allows a client to discover a channel it created previously.

A channel is assigned an identifier by the client when creating a channel by including a JSON object in the POST request with the client-id property:

{
  "client-id": "test-1"
}

In this example, the newly created channel will be given the client identifier test-1.

The client identifier may then be used to select specific channels when querying the existing channels. This is done by specifying the identifier as a query parameter in a GET request targeting the events/channels resource.

As above, a GET request without any query parameter will show all the channels that are currently available to this user:

[
  "https://dcache.example.org:3880/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w",
  "https://dcache.example.org:3880/api/v1/events/channels/IQ7U7sA0gpnpu9QGz-Hbxg"
]

In this example, the user currently has two channels: the channel created earlier without a Client ID and the channel created with Client ID test-1.

By including the client identifier in the client-id query parameter, the response from the GET request selects only those channels created with that client identifier.

The following is the response to a GET request targeting the events/channels resource while selecting test-1 client ID (/api/v1/events/channels?client-id=test-1)

[
  "https://dcache.example.org:3880/api/v1/events/channels/IQ7U7sA0gpnpu9QGz-Hbxg"
]

In this example, only the channel created with the test-1 client identifier is returned.

If the query parameter is included in the GET request without any value (/api/v1/events/channels?client-id=) then the response includes only channels created without a client identifier:

[
  "https://dcache.example.org:3880/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w"
]

Information about a specific channel may be obtained by a GET request against that channel’s resource: events/channels/<id> (/api/v1/events/channels/<id>).

Note: clients also receive events from a channel by making a GET request to a channel’s resource. A request to receive SSE events must include the HTTP request header Accept: text/event-stream. Therefore, to avoid ambiguity, a query to discover a channel’s metadata should include the Accept: application/json HTTP request header.

The response to a GET request targeting a channel endpoint is a JSON object that describes this channel. The timeout property describes the amount of time, in seconds, a client may be disconnected from the channel before the channel is automatically deleted.

The following shows the response to a GET request targeting a specific channel:

{
  "timeout": 300
}

In this example, the channel will be automatically deleted if the client remains disconnected for more than five minutes.

The timeout value may be modified by making a PATCH request that targets the channel’s resource. This PATCH request must contain a JSON object with the timeout property providing the updated duration, in seconds, after which the channel is automatically removed.

The following JSON Object is an example of the HTTP entity sent with a PATCH request.

{
  "timeout": 3600
}

In the above example, the PATCH request attempts to modify this channel’s timeout to one hour.

After this request is successfully processed, a GET request that targets this channel’s resource will show the updated timeout value:

{
  "timeout": 3600
}

Further down, there are several complete examples showing how to receive events.

Once a client is finished receiving events, it can remove a channel by issuing a DELETE request that targets the channel’s resource.

curl -u paul -X DELETE \
|        https://dcache.example.org:3880/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w
|Enter host password for user 'paul':

Subsequent attempts to use this channel will return a 404 status code, as will attempts to query the channel’s status or modify the channel.

curl -D- -u paul -H 'Accept: application/json' \
|        https://dcache.example.org:3880/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w
|Enter host password for user 'paul':
|HTTP/1.1 404 Not Found
|Date: Tue, 09 Apr 2019 21:11:57 GMT
|Server: dCache/5.1.0-SNAPSHOT
|Content-Type: application/json
|Access-Control-Allow-Origin: *
|Access-Control-Allow-Methods: GET, POST, DELETE, PUT
|Access-Control-Allow-Headers: Content-Type, Authorization, Suppress-WWW-Authenticate
|Content-Length: 51
|
|{"errors":[{"message":"Not Found","status":"404"}]}

The channel will also no longer appear in the response to GET requests targeting the events/channels resource.

A client is not required to remove the channel it created: dCache will automatically delete any leftover channel once the client remains disconnected for long enough. However, it is recommended clients explicitly delete a channel if the channel is no longer needed. This is because each dCache user is only allowed a limit number of concurrent channels and it may take some time before an abandoned channel is automatically deleted.

Subscriptions

A channel must subscribe to receive events. A channel’s subscriptions are represented by the events/channels/<id>/subscriptions (/api/v1/events/channels/<id>/subscriptions) resource.

A GET request targeting the events/channels/<id>/subscriptions resource will return the current list of subscriptions.

As a channel initially has no subscriptions, this list is initially empty:

[]

The resource events/channels/<id>/subscriptions/<eventType> (/api/v1/events/channels/<id>/subscriptions/<eventType>) represents the subscriptions of this channel of event type <eventType>.

For example, the events/channels/pf_B1dEed98IVKqc9BNa-w/subscriptions/inotify resource represents the inotify subscriptions of the channel events/channels/pf_B1dEed98IVKqc9BNa-w.

To subscribe to events from <eventType>, a client issues a POST request targeting this resource. The POST request entity is the selector for this subscription, and must be valid according to this eventType selector JSON Schema.

For example, to subscribe to metronome events with the default message every two seconds to the events/channels/pf_B1dEed98IVKqc9BNa-w channel, the client issues a POST request to events/channels/pf_B1dEed98IVKqc9BNa-w/subscriptions/metronome with the HTTP entity:

{
  "delay": 2
}

This selector is valid for the metronome selector schema (see the events/eventTypes/metronome/selector resource).

If the subscription is accepted then the HTTP response contains the subscription resource as the Location response header.

The following example shows a subscription to the metronome event source.

curl -D- -u paul -X POST -H 'Content-Type: application/json' \
|        -d '{"delay":2}' \
|        https://dcache.example.org:3880/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w/subscriptions/metronome
|Enter host password for user 'paul':
|HTTP/1.1 201 Created
|Date: Tue, 09 Apr 2019 21:29:30 GMT
|Server: dCache/5.1.0-SNAPSHOT
|Location: https://dcache.example.org:3880/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w/subscriptions/metronome/53db4a4a-d04b-47ec-acee-b475772586ed
|Access-Control-Allow-Origin: *
|Access-Control-Allow-Methods: GET, POST, DELETE, PUT
|Access-Control-Allow-Headers: Content-Type, Authorization, Suppress-WWW-Authenticate
|Content-Length: 0
|

The Location response header contains a resource that represents this subscription.

This new subscription is now included in the response to a GET request targeting this channel’s subscription resource events/channels/pf_B1dEed98IVKqc9BNa-w/subscriptions (/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w/subscriptions):

[
  "https://dcache.example.org:3880/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w/subscriptions/metronome/53db4a4a-d04b-47ec-acee-b475772586ed"
]

A GET request on the resource representing the subscription returns the selector used to generate the subscription.

For example, the resource representing the above subscription is events/channels/pf_B1dEed98IVKqc9BNa-w/subscriptions/metronome/53db4a4a-d04b-47ec-acee-b475772586ed (/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w/subscriptions/metronome/53db4a4a-d04b-47ec-acee-b475772586ed). A GET request on this resource returns the selector:

{
  "delay": 2
}

Once events from this subscription are no longer needed, the subscription may be removed by issuing a DELETE request against the subscription resource.

The following example shows the above subscription being removed.

curl -u paul -X DELETE \
|        https://dcache.example.org:3880/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w/subscriptions/metronome/53db4a4a-d04b-47ec-acee-b475772586ed
|Enter host password for user 'paul':

Once the subscription is removed, the channel will stop receiving events from this subscription.

In addition, this subscription will no longer be included in the response from a GET request that targets the channel’s subscription resource events/channels/pf_B1dEed98IVKqc9BNa-w/subscriptions (/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w/subscriptions):

[]

Receiving events: SSE

The SSE protocol is sufficiently simple that it is possible to see events using curl. In this section, we explore enough to demonstrate receiving events.

Note Using curl as an SSE client is meant only to provide an easy way to see the effect of subscribing to events. This is not a recommended procedure.

First, to receive SSE events, the client makes a GET request to the resource representing the channel, specifying it will accept the MIME type text/event-stream

curl -u paul -H 'Accept: text/event-stream' \
|        https://dcache.example.org:3880/api/v1/events/channels/pf_B1dEed98IVKqc9BNa-w
|Enter host password for user 'paul':
|^C

The curl command does not return straight away, but blocks. Any event will be delivered as a series of lines describing the event type, the subscription and the event itself. Therefore, you must interrupt curl (typically, by typing Control+C) in order to stop it from waiting for further events.

The following examples show the effect of subscribing to events, along with their delivery.

Let’s show a complete example, where a channel is created and curl is set to receive any events. While that is happening, a metronome subscription is used to deliver a single event.

First, we create a channel.

curl -u paul -X POST -D- \
|        https://dcache.example.org:3880/api/v1/events/channels
|Enter host password for user 'paul':
|HTTP/1.1 201 Created
|Date: Wed, 10 Apr 2019 10:18:30 GMT
|Server: dCache/5.2.0-SNAPSHOT
|Location: https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw
|Access-Control-Allow-Origin: *
|Access-Control-Allow-Methods: GET, POST, DELETE, PUT
|Access-Control-Allow-Headers: Content-Type, Authorization, Suppress-WWW-Authenticate
|Content-Length: 0
|

The channel resource is events/channels/MwcXzif2nF3NkHWcjk8sGw (/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw).

We now start receiving events by making a GET request to this channel resource, accepting a text/event-stream response:

curl -u paul -H 'Accept: text/event-stream' \
|        https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw
|Enter host password for user 'paul':

This event-watching curl command will wait indefinitely to receive events.

While this is happening, we create a new metronome subscription in a separate terminal. The metronome selector indicates that metronome should deliver a single event and then unsubscribe itself.

curl -u paul -X POST -H 'Content-Type: application/json' \
|        -d '{"delay":1,"count":1}' \
|	https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/metronome
|Enter host password for user 'paul':

When this has been accepted, the output from the (already running) event-watching curl command changes:

curl -u paul -H 'Accept: text/event-stream' \
|        https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw
|Enter host password for user 'paul':
|event: SYSTEM
|data: {"type":"NEW_SUBSCRIPTION","subscription":"https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/metronome/2748b2a8-10f4-4c4b-ac73-8351f5107822"}
|
|event: metronome
|id: 0
|data: {"event":"tick","subscription":"https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/metronome/2748b2a8-10f4-4c4b-ac73-8351f5107822"}
|
|event: SYSTEM
|data: {"type":"SUBSCRIPTION_CLOSED","subscription":"https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/metronome/2748b2a8-10f4-4c4b-ac73-8351f5107822"}

This output shows three events. The full description of this output is provided by the SSE specification. In summary, an empty line separates events and each event has one or more :-separated key-value pairs, with certain keywords (event, data, id) having a specific meaning.

event
The event type that generated this event. In this example, two events are SYSTEM events and one metronome event.
id
An optional, unique identifier for this event. This is used by the client when reconnecting to indicate the last event it processed. dCache will deliver any events that occurred while the client was disconnected.
data
Information about an event. For dCache events, this field is always a JSON object.

The two SYSTEM events describe the creation of a new subscription and the (automatic) removal of that subscription by metronome after the final event was sent.

The metronome event is the single event generated by the subscription.

Here is the metronome event’s data, reformatted to make it a little easier to read:

{
  "subscription": "https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/metronome/2748b2a8-10f4-4c4b-ac73-8351f5107822",
  "event": "tick"
}

This overall JSON format is the same for all events: there is always a subscription property and an event property.

The subscription property identifies the subscription that selected this event. This property’s value is the resource that represents this subscription. This allows a client to have multiple subscriptions and identify which events are from which subscriptions.

The event property is the specific information about the specific event. The general format for this information is described by the event JSON Schema for this event type, as represented by the events/eventTypes/<eventType>/event resource. For example, metronome event JSON is described by the events/eventTypes/metronome/event resource.

We can also add an inotify subscription and trigger some events. While keeping the event-watching curl process running, we add an inotify subscription by issuing a POST request to events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/inotify:

curl -u paul -X POST -H 'Content-Type: application/json' \
|        -d '{"path": "/Users/paul"}' \
|	https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/inotify
|Enter host password for user 'paul':

The inotify selector is

{
  "path": "/Users/paul"
}

This selector subscribes the channel to all inotify events for the directory /Users/paul.

The event-watching curl process will see this new subscription as a SYSTEM event:

|event: SYSTEM
|data: {"type":"NEW_SUBSCRIPTION","subscription":"https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/inotify/AACFqb-WHkhFAYm6_xQY1Jf3"}

Now, when we create a new directory in the /Users/paul directory, we will see a corresponding event.

In a separate terminal, we create this new directory using a REST API call.

curl -u paul -X POST -H 'Content-Type: application/json' \
|        -d '{"action": "mkdir", "name": "new-directory"}' \
|	https://dcache.example.org:3880/api/v1/namespace/Users/paul
|Enter host password for user 'paul':
|{"status":"success"}

In the event-watching curl process, we are notified of this new directory

|event: inotify
|id: 1
|data: {"event":{"name":"new-directory","mask":["IN_CREATE","IN_ISDIR"]},"subscription":"https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/inotify/AACFqb-WHkhFAYm6_xQY1Jf3"}

Reformatting this event’s data, the information is:

{
  "event": {
    "name": "new-directory",
    "mask": [
      "IN_CREATE",
      "IN_ISDIR"
    ]
  },
  "subscription": "https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/inotify/AACFqb-WHkhFAYm6_xQY1Jf3"
}

The inotify event data for this event:

{
  "name": "new-directory",
  "mask": [
    "IN_CREATE",
    "IN_ISDIR"
  ]
}

We can then rename this new directory from new-directory to my-data.

curl -u paul -X POST -H 'Content-Type: application/json' \
|        -d '{"action": "mv", "destination": "/Users/paul/my-data"}' \
|	https://dcache.example.org:3880/api/v1/namespace/Users/paul/new-directory
|Enter host password for user 'paul':
|{"status":"success"}

The following two events are delivered to the event-watching curl process:

|event: inotify
|id: 2
|data: {"event":{"name":"new-directory","cookie":"0r6/JbKH+oZ0D2ETnzGMQA","mask":["IN_MOVED_FROM","IN_ISDIR"]},"subscription":"https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/inotify/AACFqb-WHkhFAYm6_xQY1Jf3"}
|
|event: inotify
|id: 3
|data: {"event":{"name":"my-data","cookie":"0r6/JbKH+oZ0D2ETnzGMQA","mask":["IN_MOVED_TO","IN_ISDIR"]},"subscription":"https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/inotify/AACFqb-WHkhFAYm6_xQY1Jf3"}

The inotify event data for these two events is:

{
  "name": "new-directory",
  "cookie": "0r6/JbKH+oZ0D2ETnzGMQA",
  "mask": [
    "IN_MOVED_FROM",
    "IN_ISDIR"
  ]
}

and

{
  "name": "my-data",
  "cookie": "0r6/JbKH+oZ0D2ETnzGMQA",
  "mask": [
    "IN_MOVED_TO",
    "IN_ISDIR"
  ]
}

Notice that the cookie value is the same for the IN_MOVE_FROM and IN_MOVE_TO events: 0r6/JbKH+oZ0D2ETnzGMQA. This allows the client to realise that these two events are from the same rename operation.

As a final example, we will upload the file interesting-results.dat using the WebDAV door:

curl -u paul -LT interesting-results.dat \
|       https://dcache.example.org/Users/paul/
|Enter host password for user 'paul':

This generates the following update in the event-watching curl process output:

|event: inotify
|id: 4
|data: {"event":{"name":"interesting-results.dat","mask":["IN_CREATE"]},"subscription":"https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/inotify/AACFqb-WHkhFAYm6_xQY1Jf3"}
|
|event: inotify
|id: 5
|data: {"event":{"name":"interesting-results.dat","mask":["IN_OPEN"]},"subscription":"https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/inotify/AACFqb-WHkhFAYm6_xQY1Jf3"}
|
|event: inotify
|id: 6
|data: {"event":{"name":"interesting-results.dat","mask":["IN_MODIFY"]},"subscription":"https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/inotify/AACFqb-WHkhFAYm6_xQY1Jf3"}
|
|event: inotify
|id: 7
|data: {"event":{"name":"interesting-results.dat","mask":["IN_CLOSE_WRITE"]},"subscription":"https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/inotify/AACFqb-WHkhFAYm6_xQY1Jf3"}
|
|event: inotify
|id: 8
|data: {"event":{"name":"interesting-results.dat","mask":["IN_ATTRIB"]},"subscription":"https://dcache.example.org:3880/api/v1/events/channels/MwcXzif2nF3NkHWcjk8sGw/subscriptions/inotify/AACFqb-WHkhFAYm6_xQY1Jf3"}

This is five events describing changes within dCache from this upload of data.

The initial IN_CREATE event describes the creation of this new file’s namespace entry. At this point, the file has no size and cannot be read.

The IN_OPEN indicates that dCache has started the process of accepting data for this file.

The IN_MODIFY event indicates that the client is sending data for this file. In Linux, the IN_MODIFY event is sent whenever data has been written. In dCache, this event is rate limited, so there is a minimum time between successive IN_MODIFY events. This is to avoid overloading when the system is busy. Nevertheless there can be many IN_MODIFY events delivered if the upload takes some time.

The IN_CLOSE_WRITE event indicates that the client has finished sending the file’s data and the file is ready to be read.

The final IN_ATTRIB event describes how the namespace entry is updated with new information about the file. For example, by setting the file’s size.

The order of the IN_CLOSE_WRITE and IN_ATTRIB events is not guaranteed.