Is it safe to compact a CouchDB database that has continuous replication?
couchdb, database-replication
Solution
Yes, this is perfectly safe.
Compaction works by constructing the new compacted state in memory, then writing that new state to a new database file and updating pointers. This is because CouchDB has a very firm rule that the internals of the database file never gets updated, only appended to with an fsync. This is why you can rudely kill CouchDB's processes and it doesn't have to recover or rebuild the database like you would in other solutions.
This means that you need extra disk space available to re-write the file. So, trying to compact a CouchDB database to prevent full disk warnings is usually a non-starter.
Also, replication uses the internal representation of sequence trees (b+trees). The replicator is not streaming the entire database file from disk onto the network pipe.
Lastly, there will of course be an increase in system resource utilization. However, your tests should have shown you roughly how much this costs on your system vs an idle CouchDB, which you can use to determine how closely you're pushing your system to the breaking point.
Problem
We have a couple of production couchdb databases that have blown out to 30GB and need to be compacted. These are used by a 24/7 operations website and are replicated with another server using continuous replication. From tests I've done it'll take about 3 mins to compact these databases. Is it safe to compact one side of the replication while the production site and replication are still running?