An interesting article from Lucid Imagination employee Mark Miller about Scaling Solr. It gives a lot of good practices and tips on how to get the best out of one indexing machine, and explains how to use Solr built-in replication system.
Consider the following pseudo-code:
$nid = node_save($data);
$node = node_load($nid);
Because node_save() executes a mutator query (an INSERT or UPDATE statement) is has to be executed on the master, so the master can propagate the changes to the slaves. Because node_load() uses a read-only query, it can go to the master or any of the available slaves. Because of the lack of synchronization between master and slaves, there is one obvious caveat: when we execute node_load() the slaves might not have been updated.
