Managing addresses, and especially ports is a drag. ZooKeeper can be used as a service registry. Servers can register themselves and clients can find services there. The zc.zkzeo package provides support for registering ZEO servers and a ZEO client storage that gets addresses from ZooKeeper.
To run a ZEO server, and register it with ZooKeeper, first create a ZEO configuration file:
<zeo> address 127.0.0.1 </zeo> <zookeeper> connection zookeeper.example.com:2181 path /databases/demo </zookeeper> <filestorage> path demo.fs </filestorage>
The ZEO configuration file has the same options as usual, plus a zookeeper section with two options:
- A ZooKeeper connection string. This is typically a list of HOST:PORT pairs separated by commas.
- The path at which to register the server. The path must already exist. When the server starts, it will register itself by creating a subnode of the path with a name consisting of it’s address.
(You can also specify a ZooKeeper session timeout, in milliseconds, with a session-timeout option.)
When specifying the ZEO address, you can leave off the port and the operating system will assign it for you.
To start the server, use the zkrunzeo script:
$ bin/zkrunzeo -C FILENAME
where FILENAME is the name of the configuration file you created.
The zc.monitor package provides a simple extensible command server for gathering monitoring data or providing run-time control of servers. If zc.monitor is in the Python path, zc.zkzeo can start a monitor server and make it’s address available as the monitor property of of a server’s ephemeral port. To request this, we use a monitor-server option in the zookeeper section:
<zeo> address 127.0.0.1 </zeo> <zookeeper> connection zookeeper.example.com:2181 path /databases/demo monitor-server 127.0.0.1 </zookeeper> <filestorage> path demo.fs </filestorage>
The value is the address to listen on.
With the configuration above, if we started the server and looked at the ZooKeeper tree for ‘/databases/demo’ using the zc.zk package, we’d see something like the following:
>>> import zc.zk >>> zk = zc.zk.ZooKeeper('zookeeper.example.com:2181') >>> zk.print_tree('/databases/demo') /demo /127.0.0.1:64211 monitor = u'127.0.0.1:11976' pid = 5082
Some notes on the monitor server:
- A monitor server won’t be useful unless you’ve registered some command plugins.
- zc.monitor isn’t a dependency of zc.zkzeoc and won’t be in the Python path unless you install it.
You can define a client in two ways, from Python and using a configuration file.
From Python, use zc.zkzeo.client:
>>> import zc.zkzeo >>> client = zc.zkzeo.client( ... 'zookeeper.example.com:2181', '/databases/demo', ... max_disconnect_poll=1)
You pass a ZooKeeper connection string and a path. The Client constructor will create a client storage with addresses found as sub-nodes of the given path and it will adjust the client-storage addresses as nodes are added and removed as children of the path.
You can pass all other ZEO.ClientStorage.ClientStorage arguments, except the address, as additional positional and keyword arguments.
You’re usually not really interested in getting a storage object. What you really want is a database object:
>>> db = zc.zkzeo.DB( ... 'zookeeper.example.com:2181', '/databases/demo', ... max_disconnect_poll=1)
or often, just a database connection:
>>> conn = zc.zkzeo.connection( ... 'zookeeper.example.com:2181', '/databases/demo', ... max_disconnect_poll=1)
In configuration files, use a zkzeoclient storage section:
%import zc.zkzeo <zodb> <zkzeoclient> zookeeper zookeeper.example.com:2181 server /databases/demo max-disconnect-poll 1 </zkzeoclient> </zodb>
The options for zkzeoclient are the same as for the standard ZODB zeoclient section, except:
- There’s an extra required zookeeper option used to provide a ZooKeeper connection string.
- There can be only one server option and it is used to supply the path in ZooKeeper where addresses may be found.
- Fixed bug: The path key on the zookeeper server-configuration section was required, and shouldn’t have been.
- Register the host name from the ZEO address setting with ZooKeeper. (This is often an empty string, which zc.zk turns into the fully-quelified domain name.)
- Fixed bug in handling the monitor-server. The actuall address setting was ignored.
- Fixed a packaging bug.
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