1. The Secret Sauce of
Sharding
Eliot Horowitz
@eliothorowitz
MongoSF
May 24 2011

2. Architecture
Shards
mongod mongod mongod
...
mongod mongod mongod
mongod mongod mongod
Config
Servers
mongod
mongos mongos ...
mongod
mongod
client client client client

3. Common Setup
• A common setup is 3 shards with 3 servers
per shard: 3 masters, 6 slaves
• Can add sharding later to an existing replica
set with no down time
• Can have sharded and non-sharded
collections

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 64mb or 100,000 objects

5. 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

6. 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

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

8. 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

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

10. 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

11. Balancing
• Moves chunks from one shard to another
• Done online while system is running
• Balancing runs in the background

12. 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

13. 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 } } )

14. Live Migration
• Copy initial data set
• Copy anything modified since start
• Commit start
• Copy delta
• Finish commit

15. Download MongoDB
http://www.mongodb.org
and let us know what you think
@eliothorowitz @mongodb
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