This document discusses tag-based sharding in MongoDB. It provides an overview of MongoDB clusters and components like replica sets, shards, config servers, and mongos. It also explains key concepts like chunks and migrations. Additionally, it outlines the steps to split chunks, migrate data between shards, and perform normal MongoDB operations. Finally, it covers how to implement tag-based sharding by tagging shards and defining tag ranges for chunks.

1. Tag-based sharding
Distribute your data as you need
MongoDB User Group
Madrid, October 13th 2015
Juan Antonio Roy Couto

2. Who am I?
Juan Antonio Roy Couto
Financial Software Developer
Twitter: @juanroycouto
Linkedin: https://www.linkedin.com/in/juanroycouto
Personal blog: http://www.juanroy.es
Contributor at: http://www.mongodbspain.com
Charrosfera member: http://www.charrosfera.com
Email: juanroycouto@gmail.com
MongoDB User Group
Tag-based sharding

3. ❏ Cluster overview
❏ Definitions
❏ Steps for balancing
❏ Steps to split a chunk
❏ Migration steps
❏ Normal MongoDB operation
❏ Pre-splitting
❏ Commands to split a chunk
❏ Tag-based sharding overview
❏ Tag your shards
❏ Tag your chunk ranges
Table of Contents
MongoDB User Group
Tag-based sharding

4. ❏ Replica set
❏ Shards
❏ config servers
❏ config database
❏ mongos
Cluster overview
MongoDB User Group
Tag-based sharding

5. Cluster overview
Replica Set
● High availability
● Data safety
● Disaster recovery
MongoDB User Group
Tag-based sharding
Replica Set
Secondary
Secondary
Primary

6. Scale out
Even data distribution across all of the
shards based on a shardkey
A shardkey range belongs to only one
shard
More efficient queries
Cluster overview
Shards
MongoDB User Group
Tag-based sharding
Cluster
Shard 0 Shard 2Shard 1
A-I J-Q R-Z

7. Cluster overview
MongoDB User Group
Tag-based sharding

8. Cluster overview
Config servers
MongoDB User Group
Tag-based sharding
● config database
● Identical information (consistency check).
● Metadata:
○ Cluster shards list
○ Data per shard (chunk ranges)
○ ...
● Don’t sync from each other.
● Default Config server (All mongos read it)

9. Cluster overview
config database
Collections:
● changelog: splits and migration information
● chunks *
● collections * (only sharded)
● databases *
● lockpings
● locks
● mongos
● settings
● shards *
● system.indexes
● tags
● version *
MongoDB User Group
Tag-based sharding

10. ● Receives client requests and returns results.
● Reads the metadata and sends the query to the necessary
shard/shards
● Does not store data
● Keeps a cache version of the metadata. We can refresh it by:
○ mongos>db.runCommand( { flushRouterConfig : 1 } )
○ or restarting the server
Cluster overview
mongos
MongoDB User Group
Tag-based sharding

11. MongoDB User Group
Tag-based shardingDefinitions
● Range: Data division based on the values of the shardkey.
● Chunk: They are not physical data. Chunks are just a logical
grouping of data into ranges (64MB by default).
● Split: Chunk division. No data is moved.
● Migration: Chunk movements between shards in order to get an
even distribution. Only one chunk is moved at a time.
● Balanced system: The same number of chunks per shard.
● Balancer: Checks if a migration is needed and starts it.

12. MongoDB User Group
Tag-based sharding
Split
Migration
Steps for balancing

13. Steps to split a chunk
MongoDB User Group
Tag-based sharding
Shard 0
Final
Shard 0
Chunk 1 Chunk 1
Chunk 2
Chunk 2
Chunk 3
Config
server
mongos
1. Needs chunk 2 to be
splitted?
2. Split points
list
3. Update
metadata
4. Refresh
cache

14. Migration steps
MongoDB User Group
Tag-based sharding
Shard 1 (To)
Shard 0 (From)
Chunk 4
Chunk 1
Chunk 2
Chunk 3
Config
server
mongos
1. Is balancer running?
2. Is the system imbalance?
3. Pick chunk 3
4. Split chunk 3?
5. Begin (1)
6. Deletes finished?(2)
7. Read this chunk(3)
8. Transfer(4)
9. Update metadata(5)
10. Remove chunk 3 from
shard 0(6)
11. Refresh mongos cache Chunk 3
1
4
7
8
9
1
1

15. Normal MongoDB operation
MongoDB User Group
Tag-based sharding
Shard 0 Shard 1 Shard 2 Shard 3
mongos
Client
Migrations

16. Useful for storing data directly
in the shards (massive
data loads).
Avoid bottlenecks.
MongoDB does not need to
split or migrate chunks.
After the split, the migration
must be finished before
data loading.
Pre-splitting
MongoDB User Group
Tag-based sharding
Cluster
Shard 0 Shard 2Shard 1
Chunk 1
Chunk 5
Chunk 3
Chunk 4
Chunk 2

17. Splitting a chunk:
mongos>for (var i=0; i<20, i++) {
sh.splitAt(“testdb.presplit”, { x : 1000*i } );
}
Querying existing chunks:
mongos>use config
mongos>db.chunks.find( { ns : “testdb.presplit” } )
Commands to split a chunk
MongoDB User Group
Tag-based sharding

18. Tags are used when you want to pin ranges to a specific shard.
Tag-based sharding overview
MongoDB User Group
Tag-based sharding
shard0
EMEA
shard1
APAC
shard2
LATAM
shard3
NORAM

19. mongos>sh.addShardTag(“shard0”, “EMEA”)
mongos>sh.addShardTag(“shard1”, “APAC”)
mongos>sh.addShardTag(“shard2”, “LATAM”)
mongos>sh.addShardTag(“shard3”, “NORAM”)
Tag your shards
MongoDB User Group
Tag-based sharding

20. mongos>sh.addTagRange( namespace, minimum, maximum, tag )
mongos>sh.addTagRange( “testdb.tagrange”,
{ “x” : 0 },
{ “x” : 1000 },
“EMEA” )
minimum: the minimum value (inclusive) of the shard key range to include in the tag.
maximum: the maximum value (exclusive) of the shard key range to include in the tag.
Tag your chunk ranges
MongoDB User Group
Tag-based sharding

21. Questions?
Questions?
MongoDB User Group
Tag-based sharding

22. We are looking for writers
mongodbspain.com
MongoDB User Group
Tag-based sharding

23. Thank you for your attention!
MongoDB User Group
Tag-based sharding
Madrid, October 13th 2015
Juan Antonio Roy Couto