Replication

Introduction

Vesper supports asynchronous replication of data between multiple datastore instances using a message queue to broadcast data changes.

This approach allows for more flexible set of topologies than normally found in database servers (e.g. enabling disconnected nodes and hierarchical topologies) while off-loading reliability and scalability requirements to the message queue broker, of which there are several proven, enterprise-class options available to choose from.

Requirements

stomp.py (easy_install stomp.py if you didn’t include the “replication” extra when you installed vesper [e.g. easy_install "vesper[replication]"])

But note that the current version only works on Python 2.6 or later – easy_install http://stomppy.googlecode.com/files/stomp.py-2.0.2.tar.gz for compatibility with older versions of Python.

A STOMP-compatible message queue broker. The following message queue brokers have been tested:

• ActiveMQ 5.3.0 or greater. Recommended for production use.
• coilmq (easy_install coilmq). (Doesn’t support Python 2.4)
• MorbidQ 0.8 or greater (easy_install Twisted morbid)

Installing the “tests” extra (e.g. easy_install "vesper[tests]") will install coilmq or MorbidQ and Twisted if you are running Python 2.4.

Configure Vesper instances

Each instance requires the following configuration variables to be set::

#each instance needs a different branch id
branchId="0A"
#revision history must be activated for replication to work
saveHistory = True
#list of (hostname, port) pairs identifying the message broker
#(specifying more than one will enable failover)
replication_hosts=[('localhost', 61613)]
#channel can be any name
replication_channel="VESPER_DATA"

If you are plan to use MorbidQ as your message broker also add send_stomp_ack=False to your configuration because MorbidQ doesn’t support replying with STOMP ack messages.

Run a message broker

coilmq

Coilmq can be started by running coilmq from your shell.

ActiveMQ

To enable the STOMP protocol on ActiveMQ, add a connector to its configuration file, e.g.::

<transportConnectors>
  <transportConnector name="stomp" uri="stomp://localhost:61613"/>
</transportConnectors>

See http://activemq.apache.org/stomp.html and http://twiki.cern.ch/twiki/bin/view/EGEE/MsgTutorial for more info.

Testing

You can verify you have the basic components installed correctly by running the replication unit tests found in tests/replicationTest.py, for example::

cd ./tests
python replicationTest.py

By default, the tests will launch (and stop) its own instance of a message broker (either coilmq or MorbidQ if you are running Python 2.4). To have the tests use an different message broker, use –mq host:port as a command line argument, for example::

python replicationTest.py --mq test-queue:61613