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Slony operates by attaching triggers to both the source and target tables. Due to this, creating a Slony set on a very active database can cause locking contention. Why does it need triggers? The triggers on the source system capture insert, update, and delete activities and forward them to the remote system. On subscriber nodes, the triggers block any insert, update, or delete activity that does not originate from Slony itself.
The triggers also make it possible to switch between which node is the subscriber and which is the origin without any further table locks. Keep this in mind when copying data via Slony, or the locks could cause query timeouts and customer complaints. Try to schedule new sets and set modifications during ...
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