The long road to Mongo's durability

Anyone running in database circles will probably have at some point heard someone joke about how MongoDB loses data. Unsurprisingly, data persistence is something that database people tend to take seriously; so seriously that it’s got a name: durability, perhaps best known as the “D” at the end of “ACID”. The property states that after a transaction has committed, it will remain committed even in the event of power loss, crashes, or other errors.

For a long time MongoDB wasn’t durable and could easily lose data, but has since mostly redeemed itself. Here’s the story of how MongoDB won its “D”.

MongoDB clients have a setting called WriteConcern that dictates the level of certainty that they should have before considering data they add or change to be persisted. For the first four years of the data store’s life its default setting was 0, which meant that the clients didn’t even wait for server acknowledgement to consider a write successful. Confirming that the requests had made it to the outgoing socket buffer of the local host was “good enough”.

It doesn’t take much to see the problem here. Any number of common real life occurrences could cause that data to be lost: the client machine crashing, the failure of the connected mongod instance, or an interruption in the network connection that leads to a communication error.

Until version 1.8 (released March 2011), MongoDB didn’t have journaling. Changes were committed in memory and for performance reasons only flushed to disk about once a minute. Again, the problems here are obvious in that a crash would lose you a minute’s worth of data that you’d thought was committed.

In 1.8 journaling was added, but still came with a caveat: it was only synced to disk on a regular “commit interval” of ~100 ms. Once again, a crash would cause the loss of any data that hadn’t yet been written during the current interval. A second WriteConcern option called j (for “journaling”; the first is named w) was added that lets clients specify that they want to wait for the journal to sync to disk before returning.

The DBAs who joked about MongoDB losing data were right. For a long time it had multiple options at its disposal to burn the data it managed. On the plus side, they were all great help for the benchmarks that help fuel its initial craze; persistence operations are very fast when you don’t wait to see whether they worked.

To their credit, the company did eventually close most of these holes. MongoDB 1.8 brought journaling, and as of November 2012 its client libraries set WriteConcern to 1 by default; meaning that a write is acknowledged only after it’s been confirmed to have propagated to a replica set’s primary. Real durability is possible by setting the j option, although the data store’s design around sync intervals continues to make its use not performant.

MongoDB may still be missing three letters of “ACID”, but these days it’s got one on the board 1.

1 MongoDB is durable assuming that the right configuration tweaks are in place (WriteConcern w = 1 and j = true).

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