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Sharded Cluster Query Routing

MongoDB mongos instances route queries and write operations to shards in a sharded cluster. mongos provide the only interface to a sharded cluster from the perspective of applications. Applications never connect or communicate directly with the shards.

The mongos tracks what data is on which shard by caching the metadata from the config servers. The mongos uses the metadata to route operations from applications and clients to the mongod instances. A mongos has no persistent state and consumes minimal system resources.

The most common practice is to run mongos instances on the same systems as your application servers, but you can maintain mongos instances on the shards or on other dedicated resources.

Changed in version 3.2: For aggregation operations that run on multiple shards, if the operations do not require running on the database’s primary shard, these operations can route the results to any shard to merge the results and avoid overloading the primary shard for that database.

Routing Process

A mongos instance uses the following processes to route queries and return results.

How mongos Determines which Shards Receive a Query

A mongos instance routes a query to a cluster by:

1. Determining the list of shards that must receive the query.
2. Establishing a cursor on all targeted shards.

In some cases, when the shard key or a prefix of the shard key is a part of the query, the mongos can route the query to a subset of the shards. Otherwise, the mongos must direct the query to all shards that hold documents for that collection.

Example

Given the following shard key:

{ zipcode: 1, u_id: 1, c_date: 1 }

Depending on the distribution of chunks in the cluster, the mongos may be able to target the query at a subset of shards, if the query contains the following fields:

{ zipcode: 1 }
{ zipcode: 1, u_id: 1 }
{ zipcode: 1, u_id: 1, c_date: 1 }

How mongos Handles Query Modifiers

If the result of the query is not sorted, the mongos instance opens a result cursor that “round robins” results from all cursors on the shards.

If the query specifies sorted results using the sort() cursor method, the mongos instance passes the $orderby option to the shards. The primary shard for the database receives and performs a merge sort for all results before returning the data to the client via the mongos.

If the query limits the size of the result set using the limit() cursor method, the mongos instance passes that limit to the shards and then re-applies the limit to the result before returning the result to the client.

If the query specifies a number of records to skip using the skip() cursor method, the mongos cannot pass the skip to the shards, but rather retrieves unskipped results from the shards and skips the appropriate number of documents when assembling the complete result. However, when used in conjunction with a limit(), the mongos will pass the limit plus the value of the skip() to the shards to improve the efficiency of these operations.

Detect Connections to mongos Instances

To detect if the MongoDB instance that your client is connected to is mongos, use the isMaster command. When a client connects to a mongos, isMaster returns a document with a msg field that holds the string isdbgrid. For example:

{
   "ismaster" : true,
   "msg" : "isdbgrid",
   "maxBsonObjectSize" : 16777216,
   "ok" : 1
}

If the application is instead connected to a mongod, the returned document does not include the isdbgrid string.

Broadcast Operations and Targeted Operations

In general, operations in a sharded environment are either:

• Broadcast to all shards in the cluster that hold documents in a collection
• Targeted at a single shard or a limited group of shards, based on the shard key

For best performance, use targeted operations whenever possible. While some operations must broadcast to all shards, you can ensure MongoDB uses targeted operations whenever possible by always including the shard key.

Broadcast Operations

mongos instances broadcast queries to all shards for the collection unless the mongos can determine which shard or subset of shards stores this data.

Multi-update operations are always broadcast operations.

The remove() operation is always a broadcast operation, unless the operation specifies the shard key in full.

Targeted Operations

All insert() operations target to one shard.

All single update() (including upsert operations) and remove() operations must target to one shard.

Important

All update() and remove() operations for a sharded collection that specify the justOne or multi: false option must include the shard key or the _id field in the query specification. update() and remove() operations specifying justOne or multi: false in a sharded collection without the shard key or the _id field return an error.

For queries that include the shard key or portion of the shard key, mongos can target the query at a specific shard or set of shards. This is the case only if the portion of the shard key included in the query is a prefix of the shard key. For example, if the shard key is:

{ a: 1, b: 1, c: 1 }

The mongos program can route queries that include the full shard key or either of the following shard key prefixes at a specific shard or set of shards:

{ a: 1 }
{ a: 1, b: 1 }

Depending on the distribution of data in the cluster and the selectivity of the query, mongos may still have to contact multiple shards [1] to fulfill these queries.

[1]	mongos will route some queries, even some that include the shard key, to all shards, if needed.

Sharded and Non-Sharded Data

Sharding operates on the collection level. You can shard multiple collections within a database or have multiple databases with sharding enabled. [2] However, in production deployments, some databases and collections will use sharding, while other databases and collections will only reside on a single shard.

Regardless of the data architecture of your sharded cluster, ensure that all queries and operations use the mongos router to access the data cluster. Use the mongos even for operations that do not impact the sharded data.

[2]	As you configure sharding, you will use the enableSharding command to enable sharding for a database. This simply makes it possible to use the shardCollection command on a collection within that database.

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