Transactions

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Transactions API

Transactions and Atomicity
Transactions and Operations
Transactions and Sessions
Read Concern/Write Concern/Read Preference
General Information
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In MongoDB, an operation on a single document is atomic. Because you can use embedded documents and arrays to capture relationships between data in a single document structure instead of normalizing across multiple documents and collections, this single-document atomicity obviates the need for distributed transactions for many practical use cases.

For situations that require atomicity of reads and writes to multiple documents (in a single or multiple collections), MongoDB supports distributed transactions. With distributed transactions, transactions can be used across multiple operations, collections, databases, documents, and shards.

Transactions API

➤ Use the Select your language drop-down menu in the upper-right to set the language of the following example.

This example highlights the key components of the transactions API. In particular, it uses the callback API. The callback API:

starts a transaction
executes the specified operations
commits the result (or aborts on error)

The callback API incorporates retry logic for certain errors. The server tries to rerun the transaction after a TransientTransactionError or UnknownTransactionCommitResult commit error.

Starting in MongoDB 6.2, the server does not retry the transaction if it receives a TransactionTooLargeForCache error.

Important

Use the MongoDB driver for your MongoDB version.
When using drivers, each operation in the transaction must pass the session to each operation.
Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
You can create collections in transactions implicitly or explicitly. See Create Collections and Indexes in a Transaction.

static bool
with_transaction_example (bson_error_t *error)
{
   mongoc_client_t *client = NULL;
   mongoc_write_concern_t *wc = NULL;
   mongoc_collection_t *coll = NULL;
   bool success = false;
   bool ret = false;
   bson_t *doc = NULL;
   bson_t *insert_opts = NULL;
   mongoc_client_session_t *session = NULL;
   mongoc_transaction_opt_t *txn_opts = NULL;
   /* For a replica set, include the replica set name and a seedlist of the
    * members in the URI string; e.g.
    * uri_repl = "mongodb://mongodb0.example.com:27017,mongodb1.example.com:" \
    *    "27017/?replicaSet=myRepl";
    * client = mongoc_client_new (uri_repl);
    * For a sharded cluster, connect to the mongos instances; e.g.
    * uri_sharded =
    * "mongodb://mongos0.example.com:27017,mongos1.example.com:27017/";
    * client = mongoc_client_new (uri_sharded);
    */
   client = get_client ();
   /* Prereq: Create collections. Note Atlas connection strings include a majority write
    * concern by default.
    */
   wc = mongoc_write_concern_new ();
   mongoc_write_concern_set_wmajority (wc, 0);
   insert_opts = bson_new ();
   mongoc_write_concern_append (wc, insert_opts);
   coll = mongoc_client_get_collection (client, "mydb1", "foo");
   doc = BCON_NEW ("abc", BCON_INT32 (0));
   ret = mongoc_collection_insert_one (coll, doc, insert_opts, NULL /* reply */, error);
   if (!ret) {
      goto fail;
   }
   bson_destroy (doc);
   mongoc_collection_destroy (coll);
   coll = mongoc_client_get_collection (client, "mydb2", "bar");
   doc = BCON_NEW ("xyz", BCON_INT32 (0));
   ret = mongoc_collection_insert_one (coll, doc, insert_opts, NULL /* reply */, error);
   if (!ret) {
      goto fail;
   }
   /* Step 1: Start a client session. */
   session = mongoc_client_start_session (client, NULL /* opts */, error);
   if (!session) {
      goto fail;
   }
   /* Step 2: Optional. Define options to use for the transaction. */
   txn_opts = mongoc_transaction_opts_new ();
   mongoc_transaction_opts_set_write_concern (txn_opts, wc);
   /* Step 3: Use mongoc_client_session_with_transaction to start a transaction,
    * execute the callback, and commit (or abort on error). */
   ret = mongoc_client_session_with_transaction (session, callback, txn_opts, NULL /* ctx */, NULL /* reply */, error);
   if (!ret) {
      goto fail;
   }
   success = true;
fail:
   bson_destroy (doc);
   mongoc_collection_destroy (coll);
   bson_destroy (insert_opts);
   mongoc_write_concern_destroy (wc);
   mongoc_transaction_opts_destroy (txn_opts);
   mongoc_client_session_destroy (session);
   mongoc_client_destroy (client);
   return success;
}
/* Define the callback that specifies the sequence of operations to perform
 * inside the transactions. */
static bool
callback (mongoc_client_session_t *session, void *ctx, bson_t **reply, bson_error_t *error)
{
   mongoc_client_t *client = NULL;
   mongoc_collection_t *coll = NULL;
   bson_t *doc = NULL;
   bool success = false;
   bool ret = false;
   BSON_UNUSED (ctx);
   client = mongoc_client_session_get_client (session);
   coll = mongoc_client_get_collection (client, "mydb1", "foo");
   doc = BCON_NEW ("abc", BCON_INT32 (1));
   ret = mongoc_collection_insert_one (coll, doc, NULL /* opts */, *reply, error);
   if (!ret) {
      goto fail;
   }
   bson_destroy (doc);
   mongoc_collection_destroy (coll);
   coll = mongoc_client_get_collection (client, "mydb2", "bar");
   doc = BCON_NEW ("xyz", BCON_INT32 (999));
   ret = mongoc_collection_insert_one (coll, doc, NULL /* opts */, *reply, error);
   if (!ret) {
      goto fail;
   }
   success = true;
fail:
   mongoc_collection_destroy (coll);
   bson_destroy (doc);
   return success;
}

This example highlights the key components of the transactions API. In particular, it uses the callback API. The callback API:

starts a transaction
executes the specified operations
commits the result (or aborts on error)

The callback API incorporates retry logic for certain errors. The server tries to rerun the transaction after a TransientTransactionError or UnknownTransactionCommitResult commit error.

Starting in MongoDB 6.2, the server does not retry the transaction if it receives a TransactionTooLargeForCache error.

Important

Use the MongoDB driver for your MongoDB version.
When using drivers, each operation in the transaction must pass the session to each operation.
Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
You can create collections in transactions implicitly or explicitly. See Create Collections and Indexes in a Transaction.

// The mongocxx::instance constructor and destructor initialize and shut down the driver,
// respectively. Therefore, a mongocxx::instance must be created before using the driver and
// must remain alive for as long as the driver is in use.
mongocxx::instance inst{};
// For a replica set, include the replica set name and a seedlist of the members in the URI
// string; e.g.
// uriString =
// 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'
// For a sharded cluster, connect to the mongos instances; e.g.
// uriString = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'
mongocxx::client client{mongocxx::uri{"mongodb://localhost/?replicaSet=repl0"}};
write_concern wc_majority{};
wc_majority.acknowledge_level(write_concern::level::k_majority);
read_concern rc_local{};
rc_local.acknowledge_level(read_concern::level::k_local);
read_preference rp_primary{};
rp_primary.mode(read_preference::read_mode::k_primary);
// Prereq: Create collections.
auto foo = client["mydb1"]["foo"];
auto bar = client["mydb2"]["bar"];
try {
    options::insert opts;
    opts.write_concern(wc_majority);
    foo.insert_one(make_document(kvp("abc", 0)), opts);
    bar.insert_one(make_document(kvp("xyz", 0)), opts);
} catch (const mongocxx::exception& e) {
    std::cout << "An exception occurred while inserting: " << e.what() << std::endl;
    return EXIT_FAILURE;
}
// Step 1: Define the callback that specifies the sequence of operations to perform inside the
// transactions.
client_session::with_transaction_cb callback = [&](client_session* session) {
    // Important::  You must pass the session to the operations.
    foo.insert_one(*session, make_document(kvp("abc", 1)));
    bar.insert_one(*session, make_document(kvp("xyz", 999)));
};
// Step 2: Start a client session
auto session = client.start_session();
// Step 3: Use with_transaction to start a transaction, execute the callback,
// and commit (or abort on error).
try {
    options::transaction opts;
    opts.write_concern(wc_majority);
    opts.read_concern(rc_local);
    opts.read_preference(rp_primary);
    session.with_transaction(callback, opts);
} catch (const mongocxx::exception& e) {
    std::cout << "An exception occurred: " << e.what() << std::endl;
    return EXIT_FAILURE;
}
return EXIT_SUCCESS;

This example highlights the key components of the transactions API. In particular, it uses the callback API. The callback API:

starts a transaction
executes the specified operations
commits the result (or aborts on error)

The callback API incorporates retry logic for certain errors. The server tries to rerun the transaction after a TransientTransactionError or UnknownTransactionCommitResult commit error.

Starting in MongoDB 6.2, the server does not retry the transaction if it receives a TransactionTooLargeForCache error.

Important

Use the MongoDB driver for your MongoDB version.
When using drivers, each operation in the transaction must pass the session to each operation.
Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
You can create collections in transactions implicitly or explicitly. See Create Collections and Indexes in a Transaction.

// For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
// string uri = "mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl";
// For a sharded cluster, connect to the mongos instances; e.g.
// string uri = "mongodb://mongos0.example.com:27017,mongos1.example.com:27017/";
var client = new MongoClient(connectionString);
// Prereq: Create collections.
var database1 = client.GetDatabase("mydb1");
var collection1 = database1.GetCollection<BsonDocument>("foo").WithWriteConcern(WriteConcern.WMajority);
collection1.InsertOne(new BsonDocument("abc", 0));
var database2 = client.GetDatabase("mydb2");
var collection2 = database2.GetCollection<BsonDocument>("bar").WithWriteConcern(WriteConcern.WMajority);
collection2.InsertOne(new BsonDocument("xyz", 0));
// Step 1: Start a client session.
using (var session = client.StartSession())
{
    // Step 2: Optional. Define options to use for the transaction.
    var transactionOptions = new TransactionOptions(
        writeConcern: WriteConcern.WMajority);
    // Step 3: Define the sequence of operations to perform inside the transactions
    var cancellationToken = CancellationToken.None; // normally a real token would be used
    result = session.WithTransaction(
        (s, ct) =>
        {
            try
            {
                collection1.InsertOne(s, new BsonDocument("abc", 1), cancellationToken: ct);
                collection2.InsertOne(s, new BsonDocument("xyz", 999), cancellationToken: ct);
            }
            catch (MongoWriteException)
            {
                // Do something in response to the exception
                throw; // NOTE: You must rethrow the exception otherwise an infinite loop can occur.
            }
            return "Inserted into collections in different databases";
        },
        transactionOptions,
        cancellationToken);
}

This example highlights the key components of the transactions API. In particular, it uses the callback API. The callback API:

starts a transaction
executes the specified operations
commits the result (or aborts on error)

The callback API incorporates retry logic for certain errors. The server tries to rerun the transaction after a TransientTransactionError or UnknownTransactionCommitResult commit error.

Starting in MongoDB 6.2, the server does not retry the transaction if it receives a TransactionTooLargeForCache error.

Important

Use the MongoDB driver for your MongoDB version.
When using drivers, each operation in the transaction must pass the session to each operation.
Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
You can create collections in transactions implicitly or explicitly. See Create Collections and Indexes in a Transaction.

// WithTransactionExample is an example of using the Session.WithTransaction function.
func WithTransactionExample(ctx context.Context) error {
	// For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
	// uri := "mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl"
	// For a sharded cluster, connect to the mongos instances; e.g.
	// uri := "mongodb://mongos0.example.com:27017,mongos1.example.com:27017/"
	uri := mtest.ClusterURI()
	clientOpts := options.Client().ApplyURI(uri)
	client, err := mongo.Connect(clientOpts)
	if err != nil {
		return err
	}
	defer func() { _ = client.Disconnect(ctx) }()
	// Prereq: Create collections.
	wcMajority := writeconcern.Majority()
	wcMajorityCollectionOpts := options.Collection().SetWriteConcern(wcMajority)
	fooColl := client.Database("mydb1").Collection("foo", wcMajorityCollectionOpts)
	barColl := client.Database("mydb1").Collection("bar", wcMajorityCollectionOpts)
	// Step 1: Define the callback that specifies the sequence of operations to perform inside the transaction.
	callback := func(sesctx context.Context) (interface{}, error) {
		// Important: You must pass sesctx as the Context parameter to the operations for them to be executed in the
		// transaction.
		if _, err := fooColl.InsertOne(sesctx, bson.D{{"abc", 1}}); err != nil {
			return nil, err
		}
		if _, err := barColl.InsertOne(sesctx, bson.D{{"xyz", 999}}); err != nil {
			return nil, err
		}
		return nil, nil
	}
	// Step 2: Start a session and run the callback using WithTransaction.
	session, err := client.StartSession()
	if err != nil {
		return err
	}
	defer session.EndSession(ctx)
	result, err := session.WithTransaction(ctx, callback)
	if err != nil {
		return err
	}
	log.Printf("result: %v\n", result)
	return nil
}

This example highlights the key components of the transactions API. In particular, it uses the callback API. The callback API:

starts a transaction
executes the specified operations
commits the result (or aborts on error)

The callback API incorporates retry logic for certain errors. The server tries to rerun the transaction after a TransientTransactionError or UnknownTransactionCommitResult commit error.

Starting in MongoDB 6.2, the server does not retry the transaction if it receives a TransactionTooLargeForCache error.

Important

Use the MongoDB driver for your MongoDB version.
When using drivers, each operation in the transaction must pass the session to each operation.
Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
You can create collections in transactions implicitly or explicitly. See Create Collections and Indexes in a Transaction.

/*
  For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
  String uri = "mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/admin?replicaSet=myRepl";
  For a sharded cluster, connect to the mongos instances.
  For example:
  String uri = "mongodb://mongos0.example.com:27017,mongos1.example.com:27017:27017/admin";
 */
final MongoClient client = MongoClients.create(uri);
/*
    Create collections.
 */
client.getDatabase("mydb1").getCollection("foo")
        .withWriteConcern(WriteConcern.MAJORITY).insertOne(new Document("abc", 0));
client.getDatabase("mydb2").getCollection("bar")
        .withWriteConcern(WriteConcern.MAJORITY).insertOne(new Document("xyz", 0));
/* Step 1: Start a client session. */
final ClientSession clientSession = client.startSession();
/* Step 2: Optional. Define options to use for the transaction. */
TransactionOptions txnOptions = TransactionOptions.builder()
        .readPreference(ReadPreference.primary())
        .readConcern(ReadConcern.LOCAL)
        .writeConcern(WriteConcern.MAJORITY)
        .build();
/* Step 3: Define the sequence of operations to perform inside the transactions. */
TransactionBody txnBody = new TransactionBody<String>() {
    public String execute() {
        MongoCollection<Document> coll1 = client.getDatabase("mydb1").getCollection("foo");
        MongoCollection<Document> coll2 = client.getDatabase("mydb2").getCollection("bar");
        /*
           Important:: You must pass the session to the operations.
         */
        coll1.insertOne(clientSession, new Document("abc", 1));
        coll2.insertOne(clientSession, new Document("xyz", 999));
        return "Inserted into collections in different databases";
    }
};
try {
    /*
       Step 4: Use .withTransaction() to start a transaction,
       execute the callback, and commit (or abort on error).
    */
    clientSession.withTransaction(txnBody, txnOptions);
} catch (RuntimeException e) {
    // some error handling
} finally {
    clientSession.close();
}

This example highlights the key components of the transactions API. In particular, it uses the callback API. The callback API:

starts a transaction
executes the specified operations
commits the result (or aborts on error)

The callback API incorporates retry logic for certain errors. The server tries to rerun the transaction after a TransientTransactionError or UnknownTransactionCommitResult commit error.

Starting in MongoDB 6.2, the server does not retry the transaction if it receives a TransactionTooLargeForCache error.

Important

Use the MongoDB driver for your MongoDB version.
When using drivers, each operation in the transaction must pass the session to each operation.
Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
You can create collections in transactions implicitly or explicitly. See Create Collections and Indexes in a Transaction.

# For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
# uriString = 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'
# For a sharded cluster, connect to the mongos instances; e.g.
# uriString = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'
client = AsyncIOMotorClient(uriString)
wc_majority = WriteConcern("majority", wtimeout=1000)
# Prereq: Create collections.
await client.get_database("mydb1", write_concern=wc_majority).foo.insert_one({"abc": 0})
await client.get_database("mydb2", write_concern=wc_majority).bar.insert_one({"xyz": 0})
# Step 1: Define the callback that specifies the sequence of operations to perform inside the transactions.
async def callback(my_session):
    collection_one = my_session.client.mydb1.foo
    collection_two = my_session.client.mydb2.bar
    # Important:: You must pass the session to the operations.
    await collection_one.insert_one({"abc": 1}, session=my_session)
    await collection_two.insert_one({"xyz": 999}, session=my_session)
# Step 2: Start a client session.
async with await client.start_session() as session:
    # Step 3: Use with_transaction to start a transaction, execute the callback, and commit (or abort on error).
    await session.with_transaction(
        callback,
        read_concern=ReadConcern("local"),
        write_concern=wc_majority,
        read_preference=ReadPreference.PRIMARY,
    )

This example highlights the key components of the transactions API. In particular, it uses the callback API. The callback API:

starts a transaction
executes the specified operations
commits the result (or aborts on error)

The callback API incorporates retry logic for certain errors. The server tries to rerun the transaction after a TransientTransactionError or UnknownTransactionCommitResult commit error.

Starting in MongoDB 6.2, the server does not retry the transaction if it receives a TransactionTooLargeForCache error.

Important

Use the MongoDB driver for your MongoDB version.
When using drivers, each operation in the transaction must pass the session to each operation.
Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
You can create collections in transactions implicitly or explicitly. See Create Collections and Indexes in a Transaction.

  // For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
  // const uri = 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'
  // For a sharded cluster, connect to the mongos instances; e.g.
  // const uri = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'
  const client = new MongoClient(uri);
  await client.connect();
  // Prereq: Create collections.
  await client
    .db('mydb1')
    .collection('foo')
    .insertOne({ abc: 0 }, { writeConcern: { w: 'majority' } });
  await client
    .db('mydb2')
    .collection('bar')
    .insertOne({ xyz: 0 }, { writeConcern: { w: 'majority' } });
  // Step 1: Start a Client Session
  const session = client.startSession();
  // Step 2: Optional. Define options to use for the transaction
  const transactionOptions = {
    readPreference: 'primary',
    readConcern: { level: 'local' },
    writeConcern: { w: 'majority' }
  };
  // Step 3: Use withTransaction to start a transaction, execute the callback, and commit (or abort on error)
  // Note: The callback for withTransaction MUST be async and/or return a Promise.
  try {
    await session.withTransaction(async () => {
      const coll1 = client.db('mydb1').collection('foo');
      const coll2 = client.db('mydb2').collection('bar');
      // Important:: You must pass the session to the operations
      await coll1.insertOne({ abc: 1 }, { session });
      await coll2.insertOne({ xyz: 999 }, { session });
    }, transactionOptions);
  } finally {
    await session.endSession();
    await client.close();
  }

This example highlights the key components of the transactions API. In particular, it uses the callback API. The callback API:

starts a transaction
executes the specified operations
commits the result (or aborts on error)

The callback API incorporates retry logic for certain errors. The server tries to rerun the transaction after a TransientTransactionError or UnknownTransactionCommitResult commit error.

Starting in MongoDB 6.2, the server does not retry the transaction if it receives a TransactionTooLargeForCache error.

Important

Use the MongoDB driver for your MongoDB version.
When using drivers, each operation in the transaction must pass the session to each operation.
Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
You can create collections in transactions implicitly or explicitly. See Create Collections and Indexes in a Transaction.

sub runTransactionWithRetry {
    my ( $txnFunc, $session ) = @_;
    LOOP: {
        eval {
            $txnFunc->($session); # performs transaction
        };
        if ( my $error = $@ ) {
            print("Transaction aborted-> Caught exception during transaction.\n");
            # If transient error, retry the whole transaction
            if ( $error->has_error_label("TransientTransactionError") ) {
                print("TransientTransactionError, retrying transaction ->..\n");
                redo LOOP;
            }
            else {
                die $error;
            }
        }
    }
    return;
}
sub commitWithRetry {
    my ($session) = @_;
    LOOP: {
        eval {
            $session->commit_transaction(); # Uses write concern set at transaction start.
            print("Transaction committed->\n");
        };
        if ( my $error = $@ ) {
            # Can retry commit
            if ( $error->has_error_label("UnknownTransactionCommitResult") ) {
                print("UnknownTransactionCommitResult, retrying commit operation ->..\n");
                redo LOOP;
            }
            else {
                print("Error during commit ->..\n");
                die $error;
            }
        }
    }
    return;
}
# Updates two collections in a transactions
sub updateEmployeeInfo {
    my ($session) = @_;
    my $employeesCollection = $session->client->ns("hr.employees");
    my $eventsCollection    = $session->client->ns("reporting.events");
    $session->start_transaction(
        {
            readConcern  => { level => "snapshot" },
            writeConcern => { w     => "majority" },
            readPreference => 'primary',
        }
    );
    eval {
        $employeesCollection->update_one(
            { employee => 3 }, { '$set' => { status => "Inactive" } },
            { session => $session},
        );
        $eventsCollection->insert_one(
            { employee => 3, status => { new => "Inactive", old => "Active" } },
            { session => $session},
        );
    };
    if ( my $error = $@ ) {
        print("Caught exception during transaction, aborting->\n");
        $session->abort_transaction();
        die $error;
    }
    commitWithRetry($session);
}
# Start a session
my $session = $client->start_session();
eval {
    runTransactionWithRetry(\&updateEmployeeInfo, $session);
};
if ( my $error = $@ ) {
    # Do something with error
}
$session->end_session();

This example highlights the key components of the transactions API. In particular, it uses the callback API. The callback API:

starts a transaction
executes the specified operations
commits the result (or aborts on error)

The callback API incorporates retry logic for certain errors. The server tries to rerun the transaction after a TransientTransactionError or UnknownTransactionCommitResult commit error.

Starting in MongoDB 6.2, the server does not retry the transaction if it receives a TransactionTooLargeForCache error.

Important

Use the MongoDB driver for your MongoDB version.
When using drivers, each operation in the transaction must pass the session to each operation.
Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
You can create collections in transactions implicitly or explicitly. See Create Collections and Indexes in a Transaction.

/*
 * For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
 * uriString = 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'
 * For a sharded cluster, connect to the mongos instances; e.g.
 * uriString = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'
 */
$client = new \MongoDB\Client($uriString);
// Prerequisite: Create collections.
$client->selectCollection(
    'mydb1',
    'foo',
    [
        'writeConcern' => new \MongoDB\Driver\WriteConcern(\MongoDB\Driver\WriteConcern::MAJORITY, 1000),
    ],
)->insertOne(['abc' => 0]);
$client->selectCollection(
    'mydb2',
    'bar',
    [
        'writeConcern' => new \MongoDB\Driver\WriteConcern(\MongoDB\Driver\WriteConcern::MAJORITY, 1000),
    ],
)->insertOne(['xyz' => 0]);
// Step 1: Define the callback that specifies the sequence of operations to perform inside the transactions.
$callback = function (\MongoDB\Driver\Session $session) use ($client): void {
    $client
        ->selectCollection('mydb1', 'foo')
        ->insertOne(['abc' => 1], ['session' => $session]);
    $client
        ->selectCollection('mydb2', 'bar')
        ->insertOne(['xyz' => 999], ['session' => $session]);
};
// Step 2: Start a client session.
$session = $client->startSession();
// Step 3: Use with_transaction to start a transaction, execute the callback, and commit (or abort on error).
\MongoDB\with_transaction($session, $callback);

This example highlights the key components of the transactions API. In particular, it uses the callback API. The callback API:

starts a transaction
executes the specified operations
commits the result (or aborts on error)

The callback API incorporates retry logic for certain errors. The server tries to rerun the transaction after a TransientTransactionError or UnknownTransactionCommitResult commit error.

Starting in MongoDB 6.2, the server does not retry the transaction if it receives a TransactionTooLargeForCache error.

Important

Use the MongoDB driver for your MongoDB version.
When using drivers, each operation in the transaction must pass the session to each operation.
Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
You can create collections in transactions implicitly or explicitly. See Create Collections and Indexes in a Transaction.

# For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
# uriString = 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'
# For a sharded cluster, connect to the mongos instances; e.g.
# uriString = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'
client = MongoClient(uriString)
wc_majority = WriteConcern("majority", wtimeout=1000)
# Prereq: Create collections.
client.get_database("mydb1", write_concern=wc_majority).foo.insert_one({"abc": 0})
client.get_database("mydb2", write_concern=wc_majority).bar.insert_one({"xyz": 0})
# Step 1: Define the callback that specifies the sequence of operations to perform inside the transactions.
def callback(session):
    collection_one = session.client.mydb1.foo
    collection_two = session.client.mydb2.bar
    # Important:: You must pass the session to the operations.
    collection_one.insert_one({"abc": 1}, session=session)
    collection_two.insert_one({"xyz": 999}, session=session)
# Step 2: Start a client session.
with client.start_session() as session:
    # Step 3: Use with_transaction to start a transaction, execute the callback, and commit (or abort on error).
    session.with_transaction(
        callback,
        read_concern=ReadConcern("local"),
        write_concern=wc_majority,
        read_preference=ReadPreference.PRIMARY,
    )

This example highlights the key components of the transactions API. In particular, it uses the callback API. The callback API:

starts a transaction
executes the specified operations
commits the result (or aborts on error)

The callback API incorporates retry logic for certain errors. The server tries to rerun the transaction after a TransientTransactionError or UnknownTransactionCommitResult commit error.

Starting in MongoDB 6.2, the server does not retry the transaction if it receives a TransactionTooLargeForCache error.

Important

Use the MongoDB driver for your MongoDB version.
When using drivers, each operation in the transaction must pass the session to each operation.
Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
You can create collections in transactions implicitly or explicitly. See Create Collections and Indexes in a Transaction.

# For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.
# uriString = 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'
# For a sharded cluster, connect to the mongos instances; e.g.
# uri_string = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'
client = Mongo::Client.new(uri_string, write_concern: {w: :majority, wtimeout: 1000})
# Prereq: Create collections.
client.use('mydb1')['foo'].insert_one(abc: 0)
client.use('mydb2')['bar'].insert_one(xyz: 0)
# Step 1: Define the callback that specifies the sequence of operations to perform inside the transactions.
callback = Proc.new do |my_session|
  collection_one = client.use('mydb1')['foo']
  collection_two = client.use('mydb2')['bar']
  # Important: You must pass the session to the operations.
  collection_one.insert_one({'abc': 1}, session: my_session)
  collection_two.insert_one({'xyz': 999}, session: my_session)
end
#. Step 2: Start a client session.
session = client.start_session
# Step 3: Use with_transaction to start a transaction, execute the callback, and commit (or abort on error).
session.with_transaction(
  read_concern: {level: :local},
  write_concern: {w: :majority, wtimeout: 1000},
  read: {mode: :primary},
  &callback)

This example highlights the key components of the transactions API. In particular, it uses the callback API. The callback API:

starts a transaction
executes the specified operations
commits the result (or aborts on error)

The callback API incorporates retry logic for certain errors. The server tries to rerun the transaction after a TransientTransactionError or UnknownTransactionCommitResult commit error.

Starting in MongoDB 6.2, the server does not retry the transaction if it receives a TransactionTooLargeForCache error.

Important

Use the MongoDB driver for your MongoDB version.
When using drivers, each operation in the transaction must pass the session to each operation.
Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
You can create collections in transactions implicitly or explicitly. See Create Collections and Indexes in a Transaction.

// For a replica set, include the replica set name and a seedlist of the members in the URI
// string; e.g. let uri = "mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?
// replicaSet=myRepl"; For a sharded cluster, connect to the mongos instances; e.g.
// let uri = "mongodb://mongos0.example.com:27017,mongos1.example.com:27017/";
let client = Client::with_uri_str(uri).await?;
// Prereq: Create collections. CRUD operations in transactions must be on existing collections.
client
    .database("mydb1")
    .collection::<Document>("foo")
    .insert_one(doc! { "abc": 0})
    .await?;
client
    .database("mydb2")
    .collection::<Document>("bar")
    .insert_one(doc! { "xyz": 0})
    .await?;
// Step 1: Define the callback that specifies the sequence of operations to perform inside the
// transaction.
async fn callback(session: &mut ClientSession) -> Result<()> {
    let collection_one = session
        .client()
        .database("mydb1")
        .collection::<Document>("foo");
    let collection_two = session
        .client()
        .database("mydb2")
        .collection::<Document>("bar");
    // Important: You must pass the session to the operations.
    collection_one
        .insert_one(doc! { "abc": 1 })
        .session(&mut *session)
        .await?;
    collection_two
        .insert_one(doc! { "xyz": 999 })
        .session(session)
        .await?;
    Ok(())
}
// Step 2: Start a client session.
let mut session = client.start_session().await?;
// Step 3: Use and_run to start a transaction, execute the callback, and commit (or
// abort on error).
session
    .start_transaction()
    .and_run((), |session, _| callback(session).boxed())
    .await?;

This example uses the core API. Because the core API does not incorporate retry logic for the TransientTransactionError or UnknownTransactionCommitResult commit errors, the example includes explicit logic to retry the transaction for these errors:

Important

Use the MongoDB driver for your MongoDB version.
When using drivers, each operation in the transaction must pass the session to each operation.
Operations in a transaction use transaction-level read concern, transaction-level write concern, and transaction-level read preference.
You can create collections in transactions implicitly or explicitly. See Create Collections and Indexes in a Transaction.

/*
 * Copyright 2008-present MongoDB, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.mongodb.scala
import org.mongodb.scala.model.{Filters, Updates}
import org.mongodb.scala.result.UpdateResult
import scala.concurrent.Await
import scala.concurrent.duration.Duration
//scalastyle:off magic.number
class DocumentationTransactionsExampleSpec extends RequiresMongoDBISpec {
  // Implicit functions that execute the Observable and return the results
  val waitDuration = Duration(5, "seconds")
  implicit class ObservableExecutor[T](observable: Observable[T]) {
    def execute(): Seq[T] = Await.result(observable.toFuture(), waitDuration)
  }
  implicit class SingleObservableExecutor[T](observable: SingleObservable[T]) {
    def execute(): T = Await.result(observable.toFuture(), waitDuration)
  }
  // end implicit functions
  "The Scala driver" should "be able to commit a transaction" in withClient { client =>
    assume(serverVersionAtLeast(List(4, 0, 0)) && !hasSingleHost())
    client.getDatabase("hr").drop().execute()
    client.getDatabase("hr").createCollection("employees").execute()
    client.getDatabase("hr").createCollection("events").execute()
    updateEmployeeInfoWithRetry(client).execute() should equal(Completed())
    client.getDatabase("hr").drop().execute() should equal(Completed())
  }
  def updateEmployeeInfo(database: MongoDatabase, observable: SingleObservable[ClientSession]): SingleObservable[ClientSession] = {
    observable.map(clientSession => {
      val employeesCollection = database.getCollection("employees")
      val eventsCollection = database.getCollection("events")
      val transactionOptions = TransactionOptions.builder()
        .readPreference(ReadPreference.primary())
        .readConcern(ReadConcern.SNAPSHOT)
        .writeConcern(WriteConcern.MAJORITY)
        .build()
      clientSession.startTransaction(transactionOptions)
      employeesCollection.updateOne(clientSession, Filters.eq("employee", 3), Updates.set("status", "Inactive"))
        .subscribe((res: UpdateResult) => println(res))
      eventsCollection.insertOne(clientSession, Document("employee" -> 3, "status" -> Document("new" -> "Inactive", "old" -> "Active")))
        .subscribe((res: Completed) => println(res))
      clientSession
    })
  }
  def commitAndRetry(observable: SingleObservable[Completed]): SingleObservable[Completed] = {
    observable.recoverWith({
      case e: MongoException if e.hasErrorLabel(MongoException.UNKNOWN_TRANSACTION_COMMIT_RESULT_LABEL) => {
        println("UnknownTransactionCommitResult, retrying commit operation ...")
        commitAndRetry(observable)
      }
      case e: Exception => {
        println(s"Exception during commit ...: $e")
        throw e
      }
    })
  }
  def runTransactionAndRetry(observable: SingleObservable[Completed]): SingleObservable[Completed] = {
    observable.recoverWith({
      case e: MongoException if e.hasErrorLabel(MongoException.TRANSIENT_TRANSACTION_ERROR_LABEL) => {
        println("TransientTransactionError, aborting transaction and retrying ...")
        runTransactionAndRetry(observable)
      }
    })
  }
  def updateEmployeeInfoWithRetry(client: MongoClient): SingleObservable[Completed] = {
    val database = client.getDatabase("hr")
    val updateEmployeeInfoObservable: Observable[ClientSession] = updateEmployeeInfo(database, client.startSession())
    val commitTransactionObservable: SingleObservable[Completed] =
      updateEmployeeInfoObservable.flatMap(clientSession => clientSession.commitTransaction())
    val commitAndRetryObservable: SingleObservable[Completed] = commitAndRetry(commitTransactionObservable)
    runTransactionAndRetry(commitAndRetryObservable)
  }
}

Tip

Transactions and Atomicity

For situations that require atomicity of reads and writes to multiple documents (in a single or multiple collections), MongoDB supports distributed transactions, including transactions on replica sets and sharded clusters.

Distributed transactions are atomic:

Transactions either apply all data changes or roll back the changes.
If a transaction commits, all data changes made in the transaction are saved and are visible outside of the transaction.
Until a transaction commits, the data changes made in the transaction are not visible outside the transaction.
However, when a transaction writes to multiple shards, not all outside read operations need to wait for the result of the committed transaction to be visible across the shards. For example, if a transaction is committed and write 1 is visible on shard A but write 2 is not yet visible on shard B, an outside read at read concern "local" can read the results of write 1 without seeing write 2.
When a transaction aborts, all data changes made in the transaction are discarded without ever becoming visible. For example, if any operation in the transaction fails, the transaction aborts and all data changes made in the transaction are discarded without ever becoming visible.

Important

In most cases, a distributed transaction incurs a greater performance cost over single document writes, and the availability of distributed transactions should not be a replacement for effective schema design. For many scenarios, the denormalized data model (embedded documents and arrays) will continue to be optimal for your data and use cases. That is, for many scenarios, modeling your data appropriately will minimize the need for distributed transactions.

For additional transactions usage considerations (such as runtime limit and oplog size limit), see also Production Considerations.

Tip

Transactions and Operations

Distributed transactions can be used across multiple operations, collections, databases, documents, and shards.

For transactions:

You can create collections and indexes in transactions. For details, see Create Collections and Indexes in a Transaction
The collections used in a transaction can be in different databases.
Note
You cannot create new collections in cross-shard write transactions. For example, if you write to an existing collection in one shard and implicitly create a collection in a different shard, MongoDB cannot perform both operations in the same transaction.
You cannot write to capped collections.
You cannot use read concern "snapshot" when reading from a capped collection. (Starting in MongoDB 5.0)
You cannot read/write to collections in the config, admin, or local databases.
You cannot write to system.* collections.
You cannot return the supported operation's query plan using explain or similar commands.

For cursors created outside of a transaction, you cannot call getMore inside the transaction.
For cursors created in a transaction, you cannot call getMore outside the transaction.

You cannot specify killCursors as the first operation in a transaction.

For a list of operations not supported in transactions, see Restricted Operations.

Tip

When creating or dropping a collection immediately before starting a transaction, if the collection is accessed within the transaction, issue the create or drop operation with write concern "majority" to ensure that the transaction can acquire the required locks.

Tip

Create Collections and Indexes in a Transaction

You can perform the following operations in a distributed transaction if the transaction is not a cross-shard write transaction:

Create collections.
Create indexes on new empty collections created earlier in the same transaction.

When creating a collection inside a transaction:

You can implicitly create a collection, such as with:
- an insert operation for a non-existent collection, or
- an update/findAndModify operation with upsert: true for a non-existent collection.
You can explicitly create a collection using the create command or its helper db.createCollection().

When creating an index inside a transaction [1], the index to create must be on either:

a non-existent collection. The collection is created as part of the operation.
a new empty collection created earlier in the same transaction.

[1]	You can also run `db.collection.createIndex()` and `db.collection.createIndexes()` on existing indexes to check for existence. These operations return successfully without creating the index.

Restrictions

You cannot create new collections in cross-shard write transactions. For example, if you write to an existing collection in one shard and implicitly create a collection in a different shard, MongoDB cannot perform both operations in the same transaction.
You cannot use the $graphLookup stage within a transaction while targeting a sharded collection.

For explicit creation of a collection or an index inside a transaction, the transaction read concern level must be "local".

To explicitly create collections and indexes, use the following commands and methods:

Command	Method
`create`	`db.createCollection()`
`createIndexes`	`db.collection.createIndex()` `db.collection.createIndexes()`

Tip

Count Operation

To perform a count operation within a transaction, use the $count aggregation stage or the $group (with a $sum expression) aggregation stage.

MongoDB drivers provide a collection-level API countDocuments(filter, options) as a helper method that uses the $group with a $sum expression to perform a count. The count() API is deprecated.

mongosh provides the db.collection.countDocuments() helper method that uses the $group with a $sum expression to perform a count.

Distinct Operation

To perform a distinct operation within a transaction:

For unsharded collections, you can use the db.collection.distinct() method/the distinct command as well as the aggregation pipeline with the $group stage.
For sharded collections, you cannot use the db.collection.distinct() method or the distinct command.
To find the distinct values for a sharded collection, use the aggregation pipeline with the $group stage instead. For example:
- Instead of db.coll.distinct("x"), use
  db.coll.aggregate([ { $group: { _id: null, distinctValues: { $addToSet: "$x" } } }, { $project: { _id: 0 } } ])
- Instead of db.coll.distinct("x", { status: "A" }), use:
  db.coll.aggregate([ { $match: { status: "A" } }, { $group: { _id: null, distinctValues: { $addToSet: "$x" } } }, { $project: { _id: 0 } } ])
The pipeline returns a cursor to a document:
```
{ "distinctValues" : [ 2, 3, 1 ] }
```
Iterate the cursor to access the results document.

Informational Operations

Informational commands, such as hello, buildInfo, connectionStatus (and their helper methods) are allowed in transactions; however, they cannot be the first operation in the transaction.

Restricted Operations

Changed in version 4.4.

The following operations are not allowed in transactions:

Creating new collections in cross-shard write transactions. For example, if you write to an existing collection in one shard and implicitly create a collection in a different shard, MongoDB cannot perform both operations in the same transaction.
Explicit creation of collections, e.g. db.createCollection() method, and indexes, e.g. db.collection.createIndexes() and db.collection.createIndex() methods, when using a read concern level other than "local".
The listCollections and listIndexes commands and their helper methods.
Other non-CRUD and non-informational operations, such as createUser, getParameter, count, etc. and their helpers.

Tip

Transactions and Sessions

Transactions are associated with a session.
You can have at most one open transaction at a time for a session.
When using the drivers, each operation in the transaction must be associated with the session. Refer to your driver specific documentation for details.
If a session ends and it has an open transaction, the transaction aborts.

Read Concern/Write Concern/Read Preference

Transactions and Read Preference

Operations in a transaction use the transaction-level read preference.

Using the drivers, you can set the transaction-level read preference at the transaction start:

If the transaction-level read preference is unset, the transaction uses the session-level read preference.
If transaction-level and the session-level read preference are unset, the transaction uses the client-level read preference. By default, the client-level read preference is primary.

Distributed transactions that contain read operations must use read preference primary. All operations in a given transaction must route to the same member.

Transactions and Read Concern

Operations in a transaction use the transaction-level read concern. This means a read concern set at the collection and database level is ignored inside the transaction.

You can set the transaction-level read concern at the transaction start.

If the transaction-level read concern is unset, the transaction-level read concern defaults to the session-level read concern.
If transaction-level and the session-level read concern are unset, the transaction-level read concern defaults to the client-level read concern. By default, the client-level read concern is "local" for reads on the primary. See also:
- Transactions and Read Preference
- Default MongoDB Read Concerns/Write Concerns

Transactions support the following read concern levels:

`"local"`

Read concern "local" returns the most recent data available from the node but can be rolled back.
For transactions on sharded cluster, "local" read concern cannot guarantee that the data is from the same snapshot view across the shards. If snapshot isolation is required, use "snapshot" read concern.
You can create collections and indexes inside a transaction. If explicitly creating a collection or an index, the transaction must use read concern "local". If you implicitly create a collection, you can use any of the read concerns available for transactions.

`"majority"`

If the transaction commits with write concern "majority", read concern "majority" returns data that has been acknowledged by a majority of the replica set members and can't be rolled back. Otherwise, read concern "majority" provides no guarantees that read operations read majority-committed data.
For transactions on sharded cluster, read concern "majority" can't guarantee that the data is from the same snapshot view across the shards. If snapshot isolation is required, use read concern "snapshot".

`"snapshot"`

Read concern "snapshot" returns data from a snapshot of majority committed data if the transaction commits with write concern "majority".
If the transaction does not use write concern "majority" for the commit, the "snapshot" read concern provides no guarantee that read operations used a snapshot of majority-committed data.
For transactions on sharded clusters, the "snapshot" view of the data is synchronized across shards.

Transactions and Write Concern

Transactions use the transaction-level write concern to commit the write operations. Write operations inside transactions must be run without an explicit write concern specification and use the default write concern. At commit time, the writes committed using the transaction-level write concern.

Tip

Don't explicitly set the write concern for the individual write operations inside a transaction. Setting write concerns for the individual write operations inside a transaction returns an error.

You can set the transaction-level write concern at the transaction start:

If the transaction-level write concern is unset, the transaction-level write concern defaults to the session-level write concern for the commit.
If the transaction-level write concern and the session-level write concern are unset, the transaction-level write concern defaults to the client-level write concern of:
- w: "majority" in MongoDB 5.0 and later, with differences for deployments containing arbiters. See Implicit Default Write Concern.
- w: 1

Tip

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.

The server parameter coordinateCommitReturnImmediatelyAfterPersistingDecision controls when transaction commit decisions are returned to the client.

The parameter was introduced in MongDB 5.0 with a default value of true. In MongoDB 6.1 the default value changes to false.

When coordinateCommitReturnImmediatelyAfterPersistingDecision is false, the shard transaction coordinator waits for all members to acknowledge a multi-document transaction commit before returning the commit decision to the client.

If you specify a "majority" write concern for a multi-document transaction and the transaction fails to replicate to the calculated majority of replica set members, then the transaction may not immediately roll back on replica set members. The replica set will be eventually consistent. A transaction is always applied or rolled back on all replica set members.

Regardless of the write concern specified for the transaction, the driver applies w: "majority" as the write concern when retrying commitTransaction.

General Information

The following sections describe additional considerations for transactions.

Production Considerations

For transactions in production environments, see Production Considerations. In addition, for sharded clusters, see Production Considerations (Sharded Clusters).

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.

Shard Configuration Restriction

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.

Diagnostics

To obtain transaction status and metrics, use the following methods:

Source	Returns
`db.serverStatus()` method `serverStatus` command	Returns transactions metrics.
`$currentOp` aggregation pipeline	Returns: `$currentOp.transaction` if an operation is part of a transaction. Information on inactive sessions that are holding locks as part of a transaction. `$currentOp.twoPhaseCommitCoordinator` metrics for sharded transactions that involes writes to multiple shards.
`db.currentOp()` method `currentOp` command	Returns: `currentOp.transaction` if an operation is part of a transaction. `currentOp.twoPhaseCommitCoordinator` metrics for sharded transactions that involes writes to multiple shards.
`mongod` and `mongos` log messages	Includes information on slow transactions (which are transactions that exceed the `operationProfiling.slowOpThresholdMs` threshold) in the `TXN` log component.

Feature Compatibility Version (fCV)

To use transactions, the featureCompatibilityVersion for all members of the deployment must be at least:

Deployment	Minimum `featureCompatibilityVersion`
Replica Set	`4.0`
Sharded Cluster	`4.2`

To check the fCV for a member, connect to the member and run the following command:

db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )

For more information, see the setFeatureCompatibilityVersion reference page.

Storage Engines

distributed transactions are supported on replica sets and sharded clusters where:

the primary uses the WiredTiger storage engine, and
the secondary members use either the WiredTiger storage engine or the in-memory storage engines.

Note

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.

Limit Critical Section Wait Time

Starting in MongoDB 5.2 (and 5.0.4):

When a query accesses a shard, a chunk migration or DDL operation may hold the critical section for the collection.
To limit the time a shard waits for a critical section within a transaction, use the metadataRefreshInTransactionMaxWaitBehindCritSecMS parameter.

Learn More

Back

Limitations

Drivers API

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Transactions API

Important

Important

Important

Important

Important

Important

Important

Important

Important

Important

Important

Important

Important

Tip

See also:

Transactions and Atomicity

Important

Tip

See also:

Transactions and Operations

Note

Tip

Tip

See also:

Create Collections and Indexes in a Transaction

Restrictions

Tip

See also:

Count Operation

Distinct Operation

Informational Operations

Restricted Operations

Tip

See also:

Transactions and Sessions

Read Concern/Write Concern/Read Preference

Transactions and Read Preference

Transactions and Read Concern

"local"

"majority"

"snapshot"

Transactions and Write Concern

Tip

Tip

See also:

w: 1

Important

w: "majority"

Note

General Information

Production Considerations

Arbiters

Shard Configuration Restriction

Note

Diagnostics

Feature Compatibility Version (fCV)

Storage Engines

Note

Limit Critical Section Wait Time

Learn More

Transactions

`"local"`

`"majority"`

`"snapshot"`

`w: 1`

`w: "majority"`