All of dCache makes use of the cell package. It is a framework for a distributed and scalable server system in Java. The dCache system is divided into cells which communicate with each other via messages. Several cells run simultaneously in one domain.
Each domain runs in a separate Java virtual machine and each
cell is run as a separate thread therein. Domain names have to be unique.
The domains communicate with each other via TCP using connections that are
established at start-up. The topology is controlled by the location manager
service. In the standard configuration, all domains connect with the
dCacheDomain, which routes all messages to the appropriate domains. This
forms a star topology.
Only for message communication
The TCP communication controlled by the location manager service is for
the short control messages sent between cells. Any transfer of the data
stored within dCache does not use these connections; instead, dedicated
TCP connections are established as needed.
A single node provides the location-manager service. For a single-host dCache
instance, this is localhost;
for multi-host dCache instances, the hostname of the node providing this service
must be configured using the serviceLocatorHost property.
The domain that hosts the location manager service is also configurable. By default,
the service runs within the dCacheDomain domain; however, this may be changed by
setting the dcache.broker.domain property. The port that
the location manager listens on is also configurable, using the
dcache.broker.port property; however, most sites may leave this
property unaltered and use the default value.
Within this framework, cells send messages to other cells
addressing them in the form
<cellName>@<domainName>.
This way, cells can communicate without knowledge about the host
they run on. Some cells are well known, i.e. they
can be addressed just by their name without
@<domainName>. Evidently,
this can only work properly if the name of the cell is unique
throughout the whole system. If two well known cells with the same name are present, the
system will behave in an undefined way. Therefore it is wise to
take care when starting, naming, or renaming the well known
cells. In particular this is true for pools, which are well known cells.
A domain is started with a shell script
bin/dcache start <domainName>.
The routing manager and location manager cells are started in
each domain and are part of the underlying cell package
structure. Each domain will contain at least one cell in
addition to them.