FAQ: Sharding with MongoDB¶
- Shard Keys and Considerations for Shard Key Selection
- Query Routing
- High Availability
- Data Partitioning with Chunks and Chunk Migration Process
- Troubleshoot Sharded Clusters
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?¶
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
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.
mongos detect changes in the sharded cluster configuration?¶
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shard and discovering that its metadata is out of date.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
writebacklisten in the log mean?¶
The writeback listener is a process that opens a long poll to relay
writes back from a
migrations to make sure they have not gone to the wrong server. The
writeback listener sends writes back to the correct server if
These messages are a key part of the sharding infrastructure and should not cause concern.
mongos use connections?¶
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.