April 2011
Intermediate to advanced
68 pages
1h 27m
English
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Chapter 4. Data Safety and Consistency
Tip #29: Write to the journal for single server, replicas for multiserver
In an ideal world, all writes would be instantly, permanently saved to disks and be instantly retrievable from anywhere. Unfortunately, this is impossible in the real world, you can either take more time to make sure your data is safe or save it faster with less safety. MongoDB gives you more knobs to twiddle in this area than it has knobs for everything else combined and it’s important to understand your options.
Replication and journaling are the two approaches to data safety that you can take with MongoDB.
Generally, you should run with replication and have at least one of your servers journaled. The MongoDB blog has a good post on why you shouldn’t run MongoDB (or any database) on a single server.
MongoDB’s replication automatically copies all of your writes to other servers. If your current master goes down, you can bring up another server as the new master (this happens automatically with replica sets).
If a member of a replica set shuts down uncleanly and it was not
running with --journal, MongoDB makes no guarantees about
its data (it may be corrupt). You have to get a clean copy of the data:
either wipe the data and resync it or load a backup and fastsync.
Journaling gives you data safety on a single server. All operations are written to a log (the journal) that is flushed to disk regularly. If your machine crashes but the hardware is OK, you can restart the server ...
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