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Nearline Storage Plugins

Pools may copy files to nearline storage such as tape (a process also refered to as “flushing”) and recall those files at a later point (aka “staging”).

To interface with such nearline storage systems, a driver is needed. The default driver shipped with dCache merely calls out to a configurable script. Although such scripts are easy to develop, the requirement to call out to an external script for every file being flushed to nearline storage or staged from nearline storage significantly limits the scalability of this approach. In particular for tape systems it is crucial to batch together recalls from tapes to minimize tape mounts, however a script invocation per file limits how many files can be recalled at a time.

dCache has a third party accessible plugin system. One of the pluggable components is the nearline storage interface.

Nearline Requests

A nearline storage drive must support three types of requests: flush to the nearline storage, stage from the nearline storage, and removal of a file from a nearline storage. dCache calls out to the driver providing a corresponding request object, either org.dcache.pool.nearline.spi.FlushRequest, org.dcache.pool.nearline.spi.StageRequest, or org.dcache.pool.nearline.spi.RemoveRequest. The request object describes the file that is to be flushed, staged or removed, and acts as a callback through which the driver reports progress, completion or failure of the request.

Each request object has a unique ID and dCache may cancel a nearline request at any time using this ID.

Identifying Replicas

In dCache, a physical copy of a logical file is called a replica. Thus pools contain replicas. It is the responsibility of the nearline storage driver to copy replicas from the pool to the nearline storage and to copy replicas back from the nearline storage into the pool.

Traditionally, dCache pools have stored replicas in the local file system, and a flush or stage request would refer to the replica in the local file system by path. Starting with dCache 3.0, replica store plugins may provide alternative backends such as CEPH and thus replicas in a pool may not be accessible through the local file system. For this reason, dCache 3.0 and newer identify the replica in the pool by URI. Unless an alternative replica store is used, such a URI will always use the file: scheme. If compatibility with versions earlier than 2.17 is desired, a driver should refrain from using the getReplicaUri method.

Once flushed to a nearline storage, the replica is identified by a URI generated by the driver. This URI should use the nearline storage type (typically enstore or osm) as the scheme and the nearline storage instance name as the authority. The rest of the URI can be used to encode other information needed by the driver to identify the file. dCache stores the generated URI in its name space. Upon recalling a replica from nearline storage or removing it from nearline storage, the URI is used to identify it.

Request Lifecycle

From the point of view of dCache, the request goes through three steps: QUEUED, ACTIVE and then either COMPLETED or FAILED. It is the responsibility of the driver to queue and schedule requests as appropriate. This is because only the driver knows e.g. the tape layout and can schedule requests to minimize tape mounts. The driver must however issue the appropriate callbacks to dCache to signal the transition from QUEUED to ACTIVE and from ACTIVE to either COMPLETED or FAILED.

A nearline request has a completion deadline. Once the deadline is reached, the pool will likely cancel the request. This is not a guarantee that it will be cancelled, nor is this a guarantee that it will not be cancelled earlier. A driver may use the deadline in it scheduling decision.

Upon activating a request, the driver must be prepared that dCache aborts the operation. E.g. upon flushing a file, dCache will verify that the file still exists in the name space and if not will abort the flush. Similarly, a driver must be prepared that the activation is not instantaneous. E.g. upon staging a file from tape, the pool does not reserves space for the file until the stage is activated and thus activation may block until space becomes available (files are garbage collected or previously staged files are migrated away from the pool). For this reason the activation callback is asynchronous, providing a ListenableFuture as a result. The driver can obtain the result of the activation from this future once it is available.

The Nearline Storage SPI

A third party plugin must implement two interfaces: An implementation of org.dcache.pool.nearline.spi.NearlineStorageProvider acts as a loader or factory for the nearline storage, while org.dcache.pool.nearline.spi.NearlineStorage is that actual driver instance.

dCache uses the Java service loader facility to discover plugins. To make the plugin discoverable by dCache, the compiled classes and a file called META-INF/services/org.dcache.pool.nearline.spi.NearlineStorageProvider have to be packaged as a jar file. The org.dcache.pool.nearline.spi.NearlineStorageProvider file must contain the fully qualified class name of the NearlineStorageProvider implementation. Place this jar in a sub-directory underneath /usr/local/share/dcache/plugins/. After restarting the dCache pool, the driver should be visible in the admin shell.

To simplify setting up the basic structure of a nearline storage plugin, we provide a Maven archetype. See below for further details.


Although it appears simple, the NearlineStorageProvider interface can be difficult to implement as it is an asynchronous interface. Only an asynchronous interface allows the nearline storage SPI to scale to tens or hundreds of thousands simulatenous requests. Compared to the classic callout to a script, even a blocking implementation will provide significant advantages.

To simplify development, dCache provides the AbstractBlockingNearlineStorage base class. Subclasses implement the abstract flush, stage and remove methods in a blocking fashion and the base class deal with request activation, cancellation and completion or failure callbacks. Subclasses provide three thread pools to use, allowing full control over concurrency and request queueing.

In fact, the script driver shipped with dCache is a subclass of AbstractBlockingNearlineStorage.

Maven Archetype

A Maven Archetype is a template for new Maven projects. We provide an archetype as a starting point for writing nearline storage plugins for dCache. To instantiate the archetype, run

$ mvn archetype:generate

Maven will prompt you to select an archetype from a list. Select org.dcache:dcache-nearline-plugin-archetype. Maven will then prompt you to enter a group ID (typically starts with a domain name under your control in reverse order, e.g. org.example), an artifact ID (which identifies your Maven project), a project version number, a Java package name (often starts with the group ID), a descriptive string, and a driver name that identifies it in the admin interface.

The dCache version defaults to the archetype version. It can be altered by refusing to accept the entered values. This will cause Maven to prompt for the values a second time and this time is also prompts for the dCache version. This should rarely be necessary for a nearline storage plugin, as the SPI does not change often. There is a good chance that your plugin will work with many different versions of dCache (we provide no guarantees though!).


$ mvn archetype:generate
[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[INFO] Building Maven Stub Project (No POM) 1
[INFO] ------------------------------------------------------------------------
[INFO] >>> maven-archetype-plugin:2.2:generate (default-cli) > generate-sources @ standalone-pom >>>
[INFO] <<< maven-archetype-plugin:2.2:generate (default-cli) < generate-sources @ standalone-pom <<<
[INFO] --- maven-archetype-plugin:2.2:generate (default-cli) @ standalone-pom ---
[INFO] Generating project in Interactive mode
[INFO] No archetype defined. Using maven-archetype-quickstart (org.apache.maven.archetypes:maven-archetype-quickstart:1.0)
Choose archetype:
1: https://download.dcache.org/nexus/content/repositories/releases/ -> org.dcache:xrootd4j-channelhandler-plugin-archetype (-)
2: https://download.dcache.org/nexus/content/repositories/releases/ -> org.dcache:xrootd4j-authz-plugin-archetype (-)
3: https://download.dcache.org/nexus/content/repositories/releases/ -> org.dcache:dcache-nearline-plugin-archetype (-)
Choose a number or apply filter (format: [groupId:]artifactId, case sensitive contains): : 3
Define value for property 'groupId': : org.example
Define value for property 'artifactId': : test-nearlinestorage
Define value for property 'version':  1.0-SNAPSHOT: :
Define value for property 'package':  org.example: : org.example.dcache
[INFO] Using property: dcache = 2.16.5
Define value for property 'description': : Test NearlineStorage to demonstrate the archetype
Define value for property 'name':  org.example.dcache.test-nearlinestorage: : org.example.test
Confirm properties configuration:
groupId: org.example
artifactId: test-nearlinestorage
version: 1.0-SNAPSHOT
package: org.example.dcache
dcache: 2.16.5
description: Test NearlineStorage to demonstrate the archetype
name: org.example.test
 Y: : y
[INFO] ----------------------------------------------------------------------------
[INFO] Using following parameters for creating project from Archetype: dcache-nearline-plugin-archetype:2.16.5
[INFO] ----------------------------------------------------------------------------
[INFO] Parameter: groupId, Value: org.example
[INFO] Parameter: artifactId, Value: test-nearlinestorage
[INFO] Parameter: version, Value: 1.0-SNAPSHOT
[INFO] Parameter: package, Value: org.example.dcache
[INFO] Parameter: packageInPathFormat, Value: org/example/dcache
[INFO] Parameter: package, Value: org.example.dcache
[INFO] Parameter: version, Value: 1.0-SNAPSHOT
[INFO] Parameter: name, Value: org.example.test
[INFO] Parameter: dcache, Value: 2.16.5
[INFO] Parameter: groupId, Value: org.example
[INFO] Parameter: description, Value: Test NearlineStorage to demonstrate the archetype
[INFO] Parameter: artifactId, Value: test-nearlinestorage
[INFO] project created from Archetype in dir: /private/tmp/test-nearlinestorage
[INFO] ------------------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 08:50 min
[INFO] Finished at: 2016-07-14T10:28:50+02:00
[INFO] Final Memory: 16M/304M
[INFO] ------------------------------------------------------------------------

The result is a ready to compile Maven project:

$ cd test-nearlinestorage/
$ tree
├── LICENSE.txt
├── README.md
├── pom.xml
└── src
    └── main
        ├── assembly
        │   └── plugin.xml
        ├── defaults
        │   └── org.example.test.properties
        ├── docs
        │   └── README.md
        ├── java
        │   └── org
        │       └── example
        │           └── dcache
        │               ├── PluginNearlineStorage.java
        │               └── PluginNearlineStorageProvider.java
        └── resources
            └── META-INF
                └── services
                    └── org.dcache.pool.nearline.spi.NearlineStorageProvider

10 directories, 8 files
$ mvn package
[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[INFO] Building Test NearlineStorage to demonstrate the archetype 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ test-nearlinestorage ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] Copying 0 resource
[INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ test-nearlinestorage ---
[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding UTF-8, i.e. build is platform dependent!
[INFO] Compiling 2 source files to /private/tmp/test-nearlinestorage/target/classes
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ test-nearlinestorage ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /private/tmp/test-nearlinestorage/src/test/resources
[INFO] --- maven-compiler-plugin:3.1:testCompile (default-testCompile) @ test-nearlinestorage ---
[INFO] No sources to compile
[INFO] --- maven-surefire-plugin:2.12.4:test (default-test) @ test-nearlinestorage ---
[INFO] No tests to run.
[INFO] --- maven-jar-plugin:2.4:jar (default-jar) @ test-nearlinestorage ---
[INFO] Building jar: /private/tmp/test-nearlinestorage/target/test-nearlinestorage-1.0-SNAPSHOT.jar
[INFO] --- maven-assembly-plugin:2.6:single (make-assembly) @ test-nearlinestorage ---
[INFO] Reading assembly descriptor: src/main/assembly/plugin.xml
[INFO] Copying files to /private/tmp/test-nearlinestorage/target/test-nearlinestorage-1.0-SNAPSHOT
[WARNING] Assembly file: /private/tmp/test-nearlinestorage/target/test-nearlinestorage-1.0-SNAPSHOT is not a regular file (it may be a directory). It cannot be attached to the project build for installation or deployment.
[INFO] Building tar: /private/tmp/test-nearlinestorage/target/test-nearlinestorage-1.0-SNAPSHOT.tar.gz
[INFO] ------------------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.510 s
[INFO] Finished at: 2016-07-14T10:33:29+02:00
[INFO] Final Memory: 23M/306M
[INFO] ------------------------------------------------------------------------

The resulting tar file can be shipped to customers and unpackaged in /usr/local/share/dcache/plugins. Obviously you need to modify the generated files to do something useful first.


The simplest functional nearline storage driver is probably the copy driver that ships with dCache. It sequentially copies the replicas to and from another directory in the local file system. The main class is CopyNearlineStorage, although the bulk of the code is in the base class FileSystemNearlineStorage, itself a subclass of AbstractBlockingNearlineStorage.

For a more complicated example, check out the script driver. It demonstrates configurable concurrency. The main class is ScriptNearlineStorage.

For a full blown asynchronous driver, check out the [endit driver] (https://github.com/gbehrmann/dcache-endit-provider) developed at NDGF to interface with the Endit tape integration system.