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Production Considerations (Sharded Clusters)¶
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Starting in version 4.2, MongoDB provides the ability to perform multi-document transactions for sharded clusters.
The following page lists concerns specific to running transactions on a sharded cluster. These concerns are in addition to those listed in Production Considerations.
Sharded Transactions and MongoDB Drivers¶
For transactions on MongoDB 4.2 deployments (replica sets and sharded clusters), clients must use MongoDB drivers updated for MongoDB 4.2.
On sharded clusters with multiple mongos instances,
performing transactions with drivers updated for MongoDB 4.0 (instead
of MongoDB 4.2) will fail and can result in errors, including:
Note
Your driver may return a different error. Refer to your driver’s documentation for details.
| Error Code | Error Message |
|---|---|
| 251 | cannot continue txnId -1 for session ... with txnId 1 |
| 50940 | cannot commit with no participants |
Performance¶
Single Shard¶
Transactions that target a single shard should have the same performance as replica-set transactions.
Multiple Shards¶
Transactions that affect multiple shards incur a greater performance cost.
Note
On a sharded cluster, transactions that span multiple shards will error and abort if any involved shard contains an arbiter.
Time Limit¶
To specify a time limit, specify a maxTimeMS limit on
commitTransaction.
If maxTimeMS is unspecified, MongoDB will use the
transactionLifetimeLimitSeconds.
If maxTimeMS is specified but would result in transaction that
exceeds transactionLifetimeLimitSeconds, MongoDB will use
the transactionLifetimeLimitSeconds.
To modify transactionLifetimeLimitSeconds for a sharded
cluster, the parameter must be modified for all shard replica set
members.
Read Concerns¶
Multi-document transactions support "local",
"majority", and "snapshot" read concern
levels.
For transactions on a sharded cluster, only the
"snapshot" read concern provides a consistent snapshot
across multiple shards.
For more information on read concern and transactions, see Transactions and Read Concern.
Write Concerns¶
You cannot run transactions on a sharded cluster that has a shard
with writeConcernMajorityJournalDefault set to false
(such as a shard with a voting member that uses the in-memory
storage engine).
Note
Regardless of the write concern specified for the
transaction, the commit operation for a
sharded cluster transaction includes some parts that use {w:
"majority", j: true} write concern.
Arbiters¶
Transactions whose write operations span multiple shards will error and abort if any transaction operation reads from or writes to a shard that contains an arbiter.
See also Three Member Primary-Secondary-Arbiter Shards for transaction restrictions on shards that have disabled read concern majority.
Three Member Primary-Secondary-Arbiter Shards¶
For a sharded cluster with three-member PSA shards, you may have
disabled read concern “majority”
(i.e. --enableMajorityReadConcern false or
replication.enableMajorityReadConcern: false) to avoid cache pressure.
- On sharded clusters,
If a transaction involves a shard that has disabled read concern “majority”, you cannot use read concern
"snapshot"for the transaction. You can only use read concern"local"or"majority"for the transaction. If you use read concern"snapshot", the transaction errors and aborts.Transactions whose write operations span multiple shards will error and abort if any of the transaction’s read or write operations involves a shard that has disabled read concern
"majority".
- To check if read concern “majority” is disabled,
- You can run
db.serverStatus()and check thestorageEngine.supportsCommittedReadsfield. Iffalse, read concern “majority” is disabled.
Backups and Restores¶
Warning
mongodump and mongorestore
cannot be part of a backup strategy for 4.2+ sharded clusters
that have sharded transactions in progress, as backups created with
mongodump do not maintain the atomicity guarantees
of transactions across shards.
For 4.2+ sharded clusters with in-progress sharded transactions, use one of the following coordinated backup and restore processes which do maintain the atomicity guarantees of transactions across shards:
Chunk Migrations¶
Chunk migration acquires exclusive collection locks during certain stages.
If an ongoing transaction has a lock on a collection and a chunk migration that involves that collection starts, these migration stages must wait for the transaction to release the locks on the collection, thereby impacting the performance of chunk migrations.
If a chunk migration interleaves with a transaction (for instance, if a transaction starts while a chunk migration is already in progress and the migration completes before the transaction takes a lock on the collection), the transaction errors during the commit and aborts.
Depending on how the two operations interleave, some sample errors include (the error messages have been abbreviated):
an error from cluster data placement change ... migration commit in progress for <namespace>Cannot find shardId the chunk belonged to at cluster time ...
Outside Reads During Commit¶
During the commit for a transaction, outside read operations may try to read the same documents that will be modified by the transaction. If the transaction writes to multiple shards, then during the commit attempt across the shards
- Outside reads that use read concern
snapshotor"linearizable", or are part of causally consistent sessions (i.e. include afterClusterTime) wait for all writes of a transaction to be visible. - Outside reads using other read concerns do not wait for all writes of a transaction to be visible but instead read the before-transaction version of the documents available.
See also
Additional Information¶
See also Production Considerations.