As mentioned earlier, dCache relies on Apache ZooKeeper, a distributed directory and coordination service.
Embedded vs standalone
Apache ZooKeeper may be deployed either embedded inside dCache or as a standalone installation separate from dCache. Embedded means the ZooKeeper servers runs as a dCache service with a dCache domain and can be monitored as a dCache cell. No separate software packages needs to be installed. Standalone means that ZooKeeper packages are installed independently of dCache packages and that the ZooKeeper daemon runs independently of any dCache domains.
There is no doubt that the embedded deployment is easier, but we strongly urge sites to use a standalone ZooKeeper installation for production setups. A standalone installation may be deployed on the same hardware as dCache (typically the head nodes). A separate installation has the advantage of allowing the two products to be upgraded independently. It also improves robustness that dCache restarts do not also cause ZooKeeper restarts. Since dCache relies on ZooKeeper at a rather low level, running an embedded ZooKeeper as a regular dCache service cause several chicken and egg problems.
Single or clustered
ZooKeeper may be deployed in a single server setup or a clustered multiserver setup. For clustered deployments, 2n + 1 severs are required to allow n servers to fail. Typically one would have 3 or 5 ZooKeeper servers to allow 1 or 2 to be lost. An embedded deployment is always limited to a single server.
Reads from ZooKeeper scale with the number of servers. Writes however tend to become slower the more servers are added. This is because each ZooKeeper server replicates the full state and any update has to be written to a quorum of servers.
Data stored in ZooKeeper is stored persistently on disk on each ZooKeeper server. Currently dCache does not write a lot of data to ZooKeeper and IO performance is not critical. Any write will however be followed by an fsync, so keep latency in mind, that is, avoid distributed file systems, NFS, and use a battery backed write back cache for RAID.
Authorization and firewalls
ZooKeeper may be configured to require authentication, however dCache currently does not support this. It is of utmost importance that access to your ZooKeeper instance is protected by a firewall. This is no different that however cell communication had to be protected by firewalls in previous releases of dCache.
Future versions of dCache may support authenticating with a ZooKeeper server and may support encrypted communication.
As dCache currently does not support authentication, all information written into ZooKeeper will have no ACLs. This means that site may switch off the ACL support (the
skipACL option), which ZooKeeper documentation claims will result in some performance benefits. The overall benefit of this will likely be limited, unless the ZooKeeper nodes are heavily loaded.
To start a single server ZooKeeper as an embedded dCache service, place the
zookeeper service inside a dCache domain:
FHS packages of dCache default to
/var/lib/dcache/zookeeper for the ZooKeeper persistent state.
We recommend deploying this domain on the same server as your core domain and to place the zookeeperDomain earlier than the core domain in the layout file. It is valid to place the
zookeeper service in a domain with other services, but for stability when restarting services we recommend decoupling ZooKeeper as much as possible.
We recommend reading the ZooKeeper admin guide for details on deploying ZooKeeper. Many Linux distributions have prepackaged ZooKeeper.
Configuring dCache to connect to ZooKeeper
dcache.zookeeper.connection property needs to be defined on every dCache server to identify the ZooKeeper server or servers. In a clustered ZooKeeper deployment this property is set to a comma separated list of ZooKeeper servers endpoints. The ZooKeeper admin guide should contain details on further configuration options that can be specified in the connection string; however, this seemed to be missing at the time of writing. Instead, the ZooKeeper programmers guide ZooKeeper Sessions section contains some details (search for “connection string”).
Each domain in dCache will connect to a single ZooKeeper endpoint. If dCache is configured with multiple ZooKeeper endpoints, one is chosen at random. Should that fail, another endpoint is used automatically. Note that ZooKeeper places a limit on the number of concurrent connections from the same IP address (see
maxClientCnxns configuration parameter). Be sure that this number is large enough for the maximum number of domains running on a single dCache node.
The property defaults to
localhost:2181. Except when deploying a single server installation, the property has to be defined on all dCache nodes to identify the ZooKeeper servers.
Inspecting ZooKeeper through dCache
System cell offers two new commands,
zk ls and
zk get to list and read the data stored in ZooKeeper. Since all domains use the same ZooKeeper instance, the behaviors of these commands is the same in all domains. ZooKeeper data is organized hierarchically. Each element is called a znode and may have children as well as a value. The name space is organized like a POSIX file system, with slash as a name separator. E.g.
zk ls / lists the children under the root znode.
ZooKeeper ships with its own client that allows many more operations. This can be used to connect to ZooKeeper independently of dCache. If you modify the data managed by dCache, all warranty is voided.
If the ZooKeeper service is embedded inside dCache, the
zookeeper cell can be accessed through the admin interface and various statistics can be queried. The cell also has a pinboard with the most recent log messages and allows the log level to be adjusted on the fly.
Expected error messages
dCache will connect to one of the configured ZooKeeper servers. If it is unavailable it will try the next and keep retrying until successful. Error messages like the following are to be expected:
28 Apr 2016 13:02:25 (System)  ZooKeeper connection to localhost/0:0:0:0:0:0:0:1:2181 failed (Connection refused), attempting reconnect. 28 Apr 2016 13:02:27 (System)  ZooKeeper connection to localhost/127.0.0.1:2181 failed (Connection refused), attempting reconnect.
This may even happen with a single server deployment if that server is unavailable. In particular if embedding ZooKeeper inside a dCache service, such messages will occur in the log whenever the domain hosting ZooKeeper is starting or stopping (see aforementioned issue about chickens and eggs).