Introduction to Shardmonster¶
Mongo has great support for sharding. However, the system must be entirely the same version. This means that you cannot selectively move data from Mongo 2.4 to Mongo 3.0 (for example). Big bang upgrades of databases are scary (and we’ve had downtime in the past when we tried to do an upgrade to 2.6). Writing this ourselves allows us to move data between different shards of differing versions at will.
Terminology¶
This is made more difficult by the fact that a Mongo server can contain multiple databases.
Cluster - The complete set of mongo servers that store all data.
Location - A server (or replica set) combined with a database name that contains data. A single cluster will contain multiple locations.
Realm - A collection that spans the cluster. May have data stored in multiple versions of Mongo. A realm may span multiple locations (depending on how much the data has been sharded). A cluster may contain multiple realms of data.
Shard - A set of data denoted by a field (the shard field) and a value (the shard key). The shard, during a migration, may be stored in multiple locations. A realm typically contains multiple shards. A location also contains multiple shards.