Доклад Хенрика Инго на HighLoad++ 2014.

1. Solutions Architect, MongoDB
Henrik Ingo
@h_ingo
Introduction to Sharding

2. Agenda
•Scaling Data
•MongoDB'sApproach
•Architecture
•Configuration
•Mechanics

3. MongoDB's Approach to Sharding

4. Partitioning
• User defines shard key
• Shard key defines range of data
• Key space is like points on a line
• Range is a segment of that line

5. Initially 1 chunk
Default max chunk size: 64mb
MongoDB automatically splits & migrates chunks
when max reached
Data Distribution

6. Queries routed to specific shards
MongoDB balances cluster
MongoDB migrates data to new nodes
Routing and Balancing

7. MongoDB Auto-Sharding
• Minimal effort required
– Same interface as single mongod
• Two steps
– Enable Sharding for a database
– Shard collection within database

8. Architecture

9. What is a Shard?
• Shard is a node of the cluster
• Shard can be a single mongod or a replica set

10. Sharding infrastructure

11. Configuration

12. Example Cluster

13. mongod --configsvr
Starts a configuration server on the default port (27019)
Starting the Configuration Server

14. mongos --configdb <hostname>:27019
For 3 configuration servers:
mongos --configdb <host1>:<port1>,<host2>:<port2>,<host3>:<port3>
This is always how to start a new mongos, even if the cluster is already running
Start the mongos Router

15. mongod --shardsvr
Starts a mongodwith the default shard port (27018)
Shard is not yet connected to the rest of the cluster
Shard may have already been running in production
Start the shard database

16. On mongos:
– sh.addShard(‘<host>:27018’)
Adding a replica set:
– sh.addShard(‘<rsname>/<seedlist>’)
Add the Shard

17. db.runCommand({ listshards:1 })
{ "shards" :
[{"_id”: "shard0000”,"host”: ”<hostname>:27018” } ],
"ok" : 1
}
Verify that the shard was added

18. Enabling Sharding
• Enable sharding on a database
sh.enableSharding(“<dbname>”)
• Shard a collection with the given key
sh.shardCollection(“<dbname>.people”,{“country”:1})
• Use a compound shard key to prevent duplicates
sh.shardCollection(“<dbname>.cars”,{“year”:1,
”uniqueid”:1})

19. Tag Aware Sharding
• Tag aware sharding allows you to control the
distribution of your data
• Tag a range of shard keys
– sh.addTagRange(<collection>,<min>,<max>,<tag>)
• Tag a shard
– sh.addShardTag(<shard>,<tag>)

20. Routing Requests

21. Cluster Request Routing
• Targeted Queries
• Scatter Gather Queries
• Scatter Gather Queries with Sort

22. Cluster Request Routing: Targeted Query

23. Routable request received

24. Request routed to appropriate shard

25. Shard returns results

26. Mongos returns results to client

27. Cluster Request Routing: Non-Targeted Query

28. Non-Targeted Request Received

29. Request sent to all shards

30. Shards return results to mongos

31. Mongos returns results to client

32. Cluster Request Routing: Non-Targeted Query
with Sort

33. Non-Targeted request with sort received

34. Request sent to all shards

35. Query and sort performed locally

36. Shards return results to mongos

37. Mongos merges sorted results

38. Mongos returns results to client

39. Shard Key

40. Shard Key
• Shard key is immutable
• Shard key values are immutable
• Shard key must be indexed
• Shard key limited to 512 bytes in size
• Shard key used to route queries
– Choose a field commonly used in queries
• Only shard key can be unique across shards
– `_id` field is only unique within individual shard

41. Shard Key Considerations
• Cardinality
• Write Distribution
• Query Isolation
• Reliability
• Index Locality

42. {
_id: ObjectId(),
user: 123,
time: Date(),
subject: “...”,
recipients: [],
body: “...”,
attachments: []
}
Example: Email Storage
Most common scenario, can be applied to 90% cases
Each document can be up to 16MB
Each user may have GBs of storage
Most common query: get user emails sorted by time
Indexes on {_id}, {user, time}, {recipients}

43. Example: Email Storage
Cardinality
Write
Scaling
Query
Isolation
Reliability
Index
Locality

44. Example: Email Storage
Cardinality
Write
Scaling
Query
Isolation
Reliability
Index
Locality
_id Doc level One shard
Scatter/gat
her
All users
affected
Good

45. Example: Email Storage
Cardinality
Write
Scaling
Query
Isolation
Reliability
Index
Locality
_id Doc level One shard
Scatter/gat
her
All users
affected
Good
hash(_id) Hash level All Shards
Scatter/gat
her
All users
affected
Poor

46. Example: Email Storage
Cardinality
Write
Scaling
Query
Isolation
Reliability
Index
Locality
_id Doc level One shard
Scatter/gat
her
All users
affected
Good
hash(_id) Hash level All Shards
Scatter/gat
her
All users
affected
Poor
user Many docs All Shards Targeted
Some users
affected
Good

47. Example: Email Storage
Cardinality
Write
Scaling
Query
Isolation
Reliability
Index
Locality
_id Doc level One shard
Scatter/gat
her
All users
affected
Good
hash(_id) Hash level All Shards
Scatter/gat
her
All users
affected
Poor
user Many docs All Shards Targeted
Some users
affected
Good
user, time Doc level All Shards Targeted
Some users
affected
Good

48. @h_ingo
Questions?

49. Solutions Architect, MongoDB
Henrik Ingo
@h_ingo
Thank you!