As shown above, the most common way to load balance for Zope setups is to create multiple identical Zope instances sharing the same database via the ZEO.

The less common way, shown above, is to create redundant ZEO servers which shares the same database. So, we must make sure that failure in one ZEO server won’t stop our Zope application from running correctly. The solution is to keep multiple copies of the Data.fs synchronized and available under separate ZEO servers. But syncing can be expensive in the case of bulky database. In any case, Zope allows multiple server addresses where the Zope instance can look for a particular ZEO server to get a particular database. The configuration is to be done in zope.conf:

    <zodb 1>
        <zeoclient>
            server 192.168.0.1:8800
            server 192.168.0.2:8800
            server 192.168.0.3:8800
            storage 1
            # ZEO client cache, in bytes
            cache-size 20MB
            # Uncomment to have a persistent disk cache
            #client zeo1
        </zeoclient>
    </zodb>

In this case, the Zope instance will sequentially iterate among the server addresses to find a ZEO server to access database ‘1′.

This setup will introduce the ZEO server using mkzeoinstance and we will have to modify the configuration files, specifically zeo.conf and zope.conf. That’s it! No need to change the code. Connecting and dealing with objects is all the same.

ZEO server is the native solution for database load balancing. it allows us to access the Data.fs over the network. Also, it takes care of object locking!

a. Configuring zeo.conf

In zeo.conf, what is important is to specify the database files it should manage.

    <zeo>
        address 8800
        read-only false
        invalidation-queue-size 100
        # pid-filename $INSTANCE/var/ZEO.pid
        # monitor-address PORT
        # transaction-timeout SECONDS
    </zeo>
    <filestorage 1>
        path $INSTANCE/var/Data1.fs
    </filestorage>
    <filestorage 2>
        path $INSTANCE/var/Data2.fs
    </filestorage>

So now we have our database stored at the ZEO server.

b. Configuring zope.conf

Now we must configure zope.conf to get access to the database via the ZEO server.

    <zodb 1>
        <zeoclient>
        server localhost:8800
            storage 1
            # ZEO client cache, in bytes
            cache-size 20MB
            # Uncomment to have a persistent disk cache
            #client zeo1
        </zeoclient>
    </zodb>
    <zodb 2>
        <zeoclient>
            server localhost:8800
            storage 2
            # ZEO client cache, in bytes
            cache-size 20MB
            # Uncomment to have a persistent disk cache
            #client zeo2
        </zeoclient>
    </zodb>

When the ZEO server and Zope instance are started, the Zope instance will look for the ZEO server and it will connect to the database. The Zope instance will keep on searching at a regular interval if it fails to find the ZEO.

We don’t have to make any changes in the code to migrate from the previous setup to this setup.