FAQ: Sharding with MongoDB¶

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Is sharding appropriate for a new deployment?
Can I select a different shard key after sharding a collection?
Why are my documents not distributed across the shards?
How does mongos detect changes in the sharded cluster configuration?
What does writebacklisten in the log mean?
How does mongos use connections?

This document answers common questions about Sharding. See also the Sharding section in the manual, which provides an overview of sharding, including details on:

Is sharding appropriate for a new deployment?¶

Sometimes. However, if your data set fits on a single server, you should begin with an unsharded deployment as sharding while your data set is small provides little advantage .

Can I select a different shard key after sharding a collection?¶

No.

There is no automatic support in MongoDB for choosing a different shard key after sharding a collection. This reality underscores the importance of choosing a good shard key. If you must change a shard key after sharding a collection, the best option is to:

dump all data from MongoDB into an external format.
drop the original sharded collection.
configure sharding using a more ideal shard key.
pre-split the shard key range to ensure initial even distribution.
restore the dumped data into MongoDB.

Although you cannot select a different shard key for a sharded collection, starting in MongoDB 4.2, you can update a document’s shard key value unless the shard key field is the immutable _id field. For details on updating the shard key values, see Change a Document’s Shard Key Value.

Before MongoDB 4.2, a document’s shard key field value is immutable.

Why are my documents not distributed across the shards?¶

The balancer starts distributing data across the shards once the distribution of chunks has reached certain thresholds. See Migration Thresholds.

In addition, MongoDB cannot move a chunk if the number of documents in the chunk exceeds a certain number. See Maximum Number of Documents Per Chunk to Migrate and Indivisible/Jumbo Chunks.

How does `mongos` detect changes in the sharded cluster configuration?¶

mongos instances maintain a cache of the config database that holds the metadata for the sharded cluster.

mongos updates its cache lazily by issuing a request to a shard and discovering that its metadata is out of date. To force the mongos to reload its cache, you can run the flushRouterConfig command against each mongos directly.

What does `writebacklisten` in the log mean?¶

The writeback listener is a process that opens a long poll to relay writes back from a mongod or mongos after migrations to make sure they have not gone to the wrong server. The writeback listener sends writes back to the correct server if necessary.

These messages are a key part of the sharding infrastructure and should not cause concern.

How does `mongos` use connections?¶

Each mongos instance maintains a pool of connections to the members of the sharded cluster. Client requests use these connections one at a time; i.e. requests are not multiplexed or pipelined.

When client requests complete, the mongos returns the connection to the pool. These pools do not shrink when the number of clients decreases. This can lead to an unused mongos with a large number of open connections. If the mongos is no longer in use, it is safe to restart the process to close existing connections.

To return aggregated statistics related to all of the outgoing connection pools used by the mongos, connect a mongo shell to the mongos with , and run the connPoolStats command:

copy

db.adminCommand("connPoolStats");

See the System Resource Utilization section of the UNIX ulimit Settings document.

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