[Pascal Peregrina]

I would like to know what is the difference between these 2 ZODB config params : <key name="pool-size" datatype="integer" default="7"/> <key name="version-pool-size" datatype="integer" default="3"/>

Before ZODB 3.3.1, there's a hard limit on the number of simultaneously open DB Connections. Well, there are two hard limits, one for DB.open() calls that pass a non-empty string as the value of the optional version= argument (that limit is version-pool-size), and the other is for all other DB.open() calls (connections not "in a version", & that limit is pool-size). If you don't use versions (which are, as Chris said, deprecated), version-pool-size has no effect. If you try to open more Connections than the appropriate limit, the DB.open() call will block until a Connection of the requested flavor (version versus non-version) is closed. Because the _usual_ outcome in this case is that the call blocks forever, in ZODB 3.3.1 there are no hard limits here (although these config options still exist, and exceeding them logs advisory messages).