Re: KiokuDB::Backend::BDB and replication

Hi,


2009/5/24 atomicstack <[email protected]>:
> I notice BerkeleyDB::Manager has attributes for replication, but no mention
> in the documentation. Has anybody had luck using it in a replicated
> environment? I'm tinkering with MemcacheDB but the tests seem to fail when
> BDB::Manager calls BerkeleyDB::Hash->new. My XS knowledge isn't great so I'm
> not sure where/how to start digging into BerkeleyDB to see if it's honouring
> the replication flags properly...

That flag is probably a leftover from my experiments, it's not supported yet.

Adding replication to the BerkeleyDB module is possible, but requires
quite a bit of XS work, unfortunately.

There are several steps required to support replication in BerkeleyDB
(the module):

1. Berkeley DB (the library) support 4 modes of memory management.
BerkeleyDB.xs uses only one of these, where the library allocates
everything (instead of using a callback for instance). Unfortunately
this mode is incompatible with the threaded environment, which is
necessary for replication (bdb spawns a background thread for this).

2. The XS api for replication is not written yet. This is
comparatively easy though.

3. the replication API doesn't support automatic forwarding to the
current master, only finding out who the master is. This means that
the RPC layer also has to be supported. To allow transparent writes on
the slaves all writes must be forwarded to the current master and then
replicated to the slave. The RPC api is also not too hard to expose.

4. After RPC is available, writing a layer to automatically forward
all write operations to the master is still a big undertaking. For
instance if there is an open transaction with write operations that
probably means all reads need to be done off the master too, so to
benefit from replication this might not be a good approach, and
something a little more explicit is necessary.