MongoDB London 2013

Some very rough notes from MongoDB London 2013.

Session 1 - Performance

Keep indexes in memory
Data in memory if you can
Slow queries can be configured to appear in logs
Use SSDs
Growing documents is bad
Do an ‘explain’ on queries
Padding factor
DB locks when writing
Sharding to scale writes
Optionally read from slaves, but they may not have the written data yet
Write concern level configurable
Can set importance level of writes based on the node that has acknowledged the write
You can define your own _id structure to help querying
Use short field names - use an abstraction layer
Covered indexes
Dropping collections is faster than removing
mongostat
Run your own benchmark - benchrun
serverdensity.com/mdb (@davidmytton)
Document per day, pre-allocated, then use the inc operator

An uneven number of nodes is advised
There’s a mesh of heartbeats between the nodes
An arbiter node only exists for voting - it stores no data
You can have hidden nodes, for “backup” purposes only
You can give a node a slaveDelay so the replication is delayed
Servers can be tagged, e.g. { datacenter: "new york" }
There are 5 read preference modes
You can test all this on a single machine
Failure points:
- Power
- Network
- Data Center (5 nodes safest: 2 (primary) + 2 (primary) + 1 (backup DC))
- Multi-node failure can occur, e.g. 2 out of 3 fail
When there’s only one node, the whole cluster becomes read-only
You can disable indexing if you want to, e.g. on a backup node that isn’t ever queried