MongoDB Sharding Internals
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Eliot Horowitz's presentation at MongoSV on December 3, 2010
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MongoDB Sharding Internals
- 1. Sharding Internals Eliot Horowitz @eliothorowitz MongoSV December 3, 2010
- 2. MongoDB Sharding • Scale horizontally for data size, index size, write and consistent read scaling • Distribute databases, collections or a objects in a collection • Auto-balancing, migrations, management happen with no down time
- 3. • Choose how you partition data • Can convert from single master to sharded system with no downtime • Same features as non-sharding single master • Fully consistent
- 4. Range Based MIN MAX LOCATION A F shard1 F M shard1 M R shard2 R Z shard3 • collection is broken into chunks by range • chunks default to 200mb or 100,000 objects
- 5. Architecture Shards mongod mongod mongod ... Config mongod mongod mongod Servers mongod mongod mongod mongos mongos ... client
- 6. Shards • Can be master, master/slave or replica sets • Replica sets gives sharding + full auto- failover • Regular mongod processes
- 7. Config Servers • 3 of them • changes are made with 2 phase commit • if any are down, meta data goes read only • system is online as long as 1/3 is up
- 8. mongos • Sharding Router • Acts just like a mongod to clients • Can have 1 or as many as you want • Can run on appserver so no extra network traffic • Cache meta data from config servers
- 9. Writes • Inserts : require shard key, routed • Removes: routed and/or scattered • Updates: routed or scattered
- 10. Queries • By shard key: routed • sorted by shard key: routed in order • by non shard key: scatter gather • sorted by non shard key: distributed merge sort
- 11. Splitting • Take a chunk and split it in 2 • Splits on the median value • Splits only change meta data, no data change
- 12. Splitting T1 MIN MAX LOCATION A Z shard1 T2 MIN MAX LOCATION A G shard1 G Z shard1 T3 MIN MAX LOCATION A D shard1 D G shard1 G S shard1 S Z shard1
- 13. Balancing • Moves chunks from one shard to another • Done online while system is running • Balancing runs in the background
- 14. Migrating T3 MIN MAX LOCATION A D shard1 D G shard1 G S shard1 S Z shard1 T4 MIN MAX LOCATION A D shard1 D G shard1 G S shard1 S Z shard2 T5 MIN MAX LOCATION A D shard1 D G shard1 G S shard2 S Z shard2
- 15. Setting it Up • Start servers • add shards: db.runCommand( { addshard : "10.1.1.5" } ) • turn on partitioning: db.runCommand( { enablesharding : "test" } • shard a collection: db.runCommand( { shardcollection : "test.data" , key : { num : 1 } } )
- 16. User profiles • Partition by user_id • Secondary indexes on location, dates, etc... • Reads/writes know which shard to hit
- 17. User Activity Stream • Shard by user_id • Loading a user’s stream hits a single shard • Writes are distributed across all shards • Can index on activity for deleting
- 18. Photos • Can shard by photo_id for best read/write distribution • Secondary index on tags, date
- 19. Logging Possible Shard Keys • date • machine, date • logger name
- 20. Download MongoDB http://www.mongodb.org and let us know what you think @eliothorowitz @mongodb 10gen is hiring! http://www.10gen.com/jobs
Editor's Notes
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- for inconsistent read scaling\n\n
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- don’t shard by date\n\n
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