2010 mongo sv-shardinginternals

Sharding Internals Eliot Horowitz @eliothorowitz MongoSV December 3, 2010

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

• 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

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 64mb or 100,000 objects

Architecture Shards mongod mongod mongod ... Conﬁg mongod mongod mongod Serversmongodmongodmongod mongos mongos ... client

Shards• Can be master, master/slave or replica sets• Replica sets gives sharding + full auto- failover• Regular mongod processes

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

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

Writes• Inserts : require shard key, routed• Removes: routed and/or scattered• Updates: routed or scattered

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

Splitting• Take a chunk and split it in 2• Splits on the median value• Splits only change meta data, no data change

SplittingT1 MIN MAX LOCATION A Z shard1T2 MIN MAX LOCATION A G shard1 G Z shard1T3 MIN MAX LOCATION A D shard1 D G shard1 G S shard1 S Z shard1

Balancing• Moves chunks from one shard to another• Done online while system is running• Balancing runs in the background• Any mongos can initiate• All data transfer happens directly between shards• Once initiated, mongos can go without issue

MigratingT3 MIN MAX LOCATION A D shard1 D G shard1 G S shard1 S Z shard1T4 MIN MAX LOCATION A D shard1 D G shard1 G S shard1 S Z shard2T5 MIN MAX LOCATION A D shard1 D G shard1 G S shard2 S Z shard2

Live Migrations• Migrate initiated• Initial data copied• Delta copied until steady state + secondary ack• Start commit• Copy ﬁnal delta, wait for secondary ack• Finish commit

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User proﬁles• Partition by user_id• Secondary indexes on location, dates, etc...• Reads/writes know which shard to hit

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

Photos• Can shard by photo_id for best read/write distribution• Secondary index on tags, date

LoggingPossible Shard Keys• date• machine, date• logger name

2010 mongo sv-shardinginternals

by Dwight Merriman on Jun 01, 2011

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