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- FAQ: Sharding with MongoDB
FAQ: Sharding with MongoDB¶
On this page
- Is sharding appropriate for a new deployment?
- How does sharding work with replication?
- Can I change the shard key after sharding a collection?
- What happens to unsharded collections in sharded databases?
- How does MongoDB distribute data across shards?
- What happens if a client updates a document in a chunk during a migration?
- What happens to queries if a shard is inaccessible or slow?
- How does MongoDB distribute queries among shards?
- How does MongoDB sort queries in sharded environments?
- How does MongoDB ensure unique
_idfield values when using a shard key other than
- I’ve enabled sharding and added a second shard, but all the data is still on one server. Why?
- Is it safe to remove old files in the
- How does
- Why does
mongoshold connections open?
- Where does MongoDB report on connections used by
- What does
writebacklistenin the log mean?
- How should administrators deal with failed migrations?
- What is the process for moving, renaming, or changing the number of config servers?
- When do the
mongosservers detect config server changes?
- Is it possible to quickly update
mongosservers after updating a replica set configuration?
- What does the
- How do indexes impact queries in sharded systems?
- Can shard keys be randomly generated?
- Can shard keys have a non-uniform distribution of values?
- Can you shard on the
- What do
moveChunk commit failederrors mean?
- How does draining a shard affect the balancing of uneven chunk distribution?
This document answers common questions about horizontal scaling using MongoDB’s sharding.
Is sharding appropriate for a new deployment?¶
If your data set fits on a single server, you should begin with an unsharded deployment.
Converting an unsharded database to a sharded cluster is easy and seamless, so there is little advantage in configuring sharding while your data set is small.
Still, all production deployments should use replica sets to provide high availability and disaster recovery.
How does sharding work with replication?¶
Can I change the shard key after sharding a collection?¶
There is no automatic support in MongoDB for changing a 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.
What happens to unsharded collections in sharded databases?¶
How does MongoDB distribute data across shards?¶
Sharding must be specifically enabled on a collection. After enabling sharding on the collection, MongoDB will assign various ranges of collection data to the different shards in the cluster. The cluster automatically corrects imbalances between shards by migrating ranges of data from one shard to another.
What happens if a client updates a document in a chunk during a migration?¶
mongos routes the operation to the “old” shard, where
it will succeed immediately. Then the shard
instances will replicate the modification to the “new” shard before
the sharded cluster updates that chunk’s “ownership,” which
effectively finalizes the migration process.
What happens to queries if a shard is inaccessible or slow?¶
If a shard is inaccessible or unavailable, queries will return with an error.
However, a client may set the
partial query bit, which will then
return results from all available shards, regardless of whether a
given shard is unavailable.
If a shard is responding slowly,
mongos will merely wait
for the shard to return results.
How does MongoDB distribute queries among shards?¶
The exact method for distributing queries to shards in a
cluster depends on the nature of the query and the configuration of
the sharded cluster. Consider a sharded collection, using the
user_id, that has
For a query that selects one or more values for the
mongosdetermines which shard or shards contains the relevant data, based on the cluster metadata, and directs a query to the required shard or shards, and returns those results to the client.
For a query that selects
user_idand also performs a sort:
mongoscan make a straightforward translation of this operation into a number of queries against the relevant shards, ordered by
user_id. When the sorted queries return from all shards, the
mongosmerges the sorted results and returns the complete result to the client.
For queries that select on
These queries must run on all shards:
mongosmust parallelize the query over the shards and perform a merge-sort on the
How does MongoDB sort queries in sharded environments?¶
If you call the
cursor.sort() method on a query in a sharded
mongod for each shard will sort its
results, and the
mongos merges each shard’s results before returning
them to the client.
How does MongoDB ensure unique
_id field values when using a shard key other than
If you do not use
_id as the shard key, then your
application/client layer must be responsible for keeping the
field unique. It is problematic for collections to have duplicate
If you’re not sharding your collection by the
_id field, then you
should be sure to store a globally unique identifier in that
field. The default BSON ObjectId works well in
I’ve enabled sharding and added a second shard, but all the data is still on one server. Why?¶
Next, keep in mind that the default chunk size is 64 MB. As a result, in most situations, the collection needs to have at least 64 MB of data before a migration will occur.
Additionally, the system which balances chunks among the servers attempts to avoid superfluous migrations. Depending on the number of shards, your shard key, and the amount of data, systems often require at least 10 chunks of data to trigger migrations.
You can run
db.printShardingStatus() to see all the chunks
present in your cluster.
Is it safe to remove old files in the
mongod creates these files as backups during normal
shard balancing operations. If some error occurs during a
migration, these files may be
helpful in recovering documents affected during the migration.
Once the migration has completed successfully and there is no need to recover documents from these files, you may safely delete these files. Or, if you have an existing backup of the database that you can use for recovery, you may also delete these files after migration.
mongos use connections?¶
Each client maintains a connection to a
mongos instance maintains a pool of connections to the
members of a replica set supporting the sharded cluster. Clients use
one at a time. Requests are not multiplexed or pipelined. When client
requests complete, the
mongos returns the connection to the
mongos hold connections open?¶
Where does MongoDB report on connections used by
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.
How should administrators deal with failed migrations?¶
Failed migrations require no administrative intervention. Chunk migrations always preserve a consistent state. If a migration fails to complete for some reason, the cluster retries the operation. When the migration completes successfully, the data resides only on the new shard.
What is the process for moving, renaming, or changing the number of config servers?¶
See Sharded Cluster Tutorials for information on migrating and replacing config servers.
When do the
mongos servers detect config server changes?¶
mongos updates its cache lazily by issuing a request to a
shard and discovering that its metadata is out of date. There is no
way to control this behavior from the client, but you can run the
flushRouterConfig command against any
to force it to refresh its cache.
Is it possible to quickly update
mongos servers after updating a replica set configuration?¶
mongos instances will detect these changes without
intervention over time. However, if you want to force the
mongos to reload its configuration, run the
flushRouterConfig command against to each
What does the
maxConns setting on
This is particularly useful for a
mongos if you have a client
that creates multiple connections and allows them to timeout rather
than closing them.
In this case, set
maxIncomingConnections to a value slightly
higher than the maximum number of connections that the client creates, or the
maximum size of the connection pool.
How do indexes impact queries in sharded systems?¶
If the query does not include the shard key, the
mongos must send the query to all shards as a
“scatter/gather” operation. Each shard will, in turn, use either the
shard key index or another more efficient index to fulfill the query.
If the query includes multiple sub-expressions that reference the
fields indexed by the shard key and the secondary index, the
mongos can route the queries to a specific shard and the
shard will use the index that will allow it to fulfill most
efficiently. See this presentation
for more information.
Can shard keys be randomly generated?¶
Shard keys can be random. Random keys ensure optimal distribution of data across the cluster.
Sharded clusters, attempt to route queries to specific shards when queries include the shard key as a parameter, because these directed queries are more efficient. In many cases, random keys can make it difficult to direct queries to specific shards.
Can shard keys have a non-uniform distribution of values?¶
Yes. There is no requirement that documents be evenly distributed by the shard key.
However, documents that have the same shard key must reside in the same chunk and therefore on the same server. If your sharded data set has too many documents with the exact same shard key you will not be able to distribute those documents across your sharded cluster.
Can you shard on the
You can use any field for the shard key. The
_id field is a common
Be aware that
ObjectId() values, which are the default value of
_id field, increment as a timestamp. As a result, when used as
a shard key, all new documents inserted into the collection will
initially belong to the same chunk on a single shard. Although the
system will eventually divide this chunk and migrate its contents to
distribute data more evenly, at any moment the cluster can only direct
insert operations at a single shard. This can limit the throughput of
inserts. If most of your write operations are updates, this limitation
should not impact your performance. However, if you have a high insert
volume, this may be a limitation.
To address this issue, MongoDB 2.4 provides hashed shard keys.
moveChunk commit failed errors mean?¶
At the end of a chunk migration, the shard must connect to the config database to update the chunk’s record in the cluster metadata. If the shard fails to connect to the config database, MongoDB reports the following error:
ERROR: moveChunk commit failed: version is at <n>|<nn> instead of <N>|<NN>" and "ERROR: TERMINATING"
When this happens, the primary member of the shard’s replica set then terminates to protect data consistency. If a secondary member can access the config database, data on the shard becomes accessible again after an election.
How does draining a shard affect the balancing of uneven chunk distribution?¶
The sharded cluster balancing process controls both migrating chunks from decommissioned shards (i.e. draining) and normal cluster balancing activities. Consider the following behaviors for different versions of MongoDB in situations where you remove a shard in a cluster with an uneven chunk distribution:
- After MongoDB 2.2, the balancer first removes the chunks from the draining shard and then balances the remaining uneven chunk distribution.
- Before MongoDB 2.2, the balancer handles the uneven chunk distribution and then removes the chunks from the draining shard.