Session.commitTransaction()¶

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Definition
Behavior
Example

Definition¶

Session.commitTransaction()¶

New in version 4.0.

Saves the changes made by the operations in the multi-document transaction and ends the transaction.

Availability

In version 4.0, MongoDB supports multi-document transactions on replica sets.
In version 4.2, MongoDB introduces distributed transactions, which adds support for multi-document transactions on sharded clusters and incorporates the existing support for multi-document transactions on replica sets.

Behavior¶

Write Concern¶

When commiting the transaction, the session uses the write concern specified at the transaction start. See Session.startTransaction().

If you commit using "w: 1" write concern, your transaction can be rolled back during the failover process.

Atomicity¶

When a transaction commits, all data changes made in the transaction are saved and visible outside the transaction. That is, a transaction will not commit some of its changes while rolling back others.

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.

Retryable¶

If the commit operation encounters an error, MongoDB drivers retry the commit operation a single time regardless of whether retryWrites is set to false. For more information, see Transaction Error Handling.

Example¶

Consider a scenario where as changes are made to an employee’s record in the hr database, you want to ensure that the events collection in the reporting database are in sync with the hr changes. That is, you want to ensure that these writes are done as a single transaction, such that either both operations succeed or fail.

The employees collection in the hr database has the following documents:

copy

{ "_id" : ObjectId("5af0776263426f87dd69319a"), "employee" : 3, "name" : { "title" : "Mr.", "name" : "Iba Ochs" }, "status" : "Active", "department" : "ABC" }
{ "_id" : ObjectId("5af0776263426f87dd693198"), "employee" : 1, "name" : { "title" : "Miss", "name" : "Ann Thrope" }, "status" : "Active", "department" : "ABC" }
{ "_id" : ObjectId("5af0776263426f87dd693199"), "employee" : 2, "name" : { "title" : "Mrs.", "name" : "Eppie Delta" }, "status" : "Active", "department" : "XYZ" }

The events collection in the reporting database has the following documents:

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{ "_id" : ObjectId("5af07daa051d92f02462644a"), "employee" : 1, "status" : { "new" : "Active", "old" : null }, "department" : { "new" : "ABC", "old" : null } }
{ "_id" : ObjectId("5af07daa051d92f02462644b"), "employee" : 2, "status" : { "new" : "Active", "old" : null }, "department" : { "new" : "XYZ", "old" : null } }
{ "_id" : ObjectId("5af07daa051d92f02462644c"), "employee" : 3, "status" : { "new" : "Active", "old" : null }, "department" : { "new" : "ABC", "old" : null } }

The following example opens a transaction, updates an employee’s status to Inactive in the employees status and inserts a corresponding document to the events collection, and commits the two operations as a single transaction.

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// Runs the txnFunc and retries if TransientTransactionError encountered

function runTransactionWithRetry(txnFunc, session) {
    while (true) {
        try {
            txnFunc(session);  // performs transaction
            break;
        } catch (error) {
            // If transient error, retry the whole transaction
            if ( error.hasOwnProperty("errorLabels") && error.errorLabels.includes("TransientTransactionError")  ) {
                print("TransientTransactionError, retrying transaction ...");
                continue;
            } else {
                throw error;
            }
        }
    }
}

// Retries commit if UnknownTransactionCommitResult encountered

function commitWithRetry(session) {
    while (true) {
        try {
            session.commitTransaction(); // Uses write concern set at transaction start.
            print("Transaction committed.");
            break;
        } catch (error) {
            // Can retry commit
            if (error.hasOwnProperty("errorLabels") && error.errorLabels.includes("UnknownTransactionCommitResult") ) {
                print("UnknownTransactionCommitResult, retrying commit operation ...");
                continue;
            } else {
                print("Error during commit ...");
                throw error;
            }
       }
    }
}

// Updates two collections in a transactions

function updateEmployeeInfo(session) {
    employeesCollection = session.getDatabase("hr").employees;
    eventsCollection = session.getDatabase("reporting").events;

    session.startTransaction( { readConcern: { level: "snapshot" }, writeConcern: { w: "majority" } } );

    try{
        employeesCollection.updateOne( { employee: 3 }, { $set: { status: "Inactive" } } );
        eventsCollection.insertOne( { employee: 3, status: { new: "Inactive", old: "Active" } } );
    } catch (error) {
        print("Caught exception during transaction, aborting.");
        session.abortTransaction();
        throw error;
    }

    commitWithRetry(session);
}

// Start a session.
session = db.getMongo().startSession( { readPreference: { mode: "primary" } } );

try{
   runTransactionWithRetry(updateEmployeeInfo, session);
} catch (error) {
   // Do something with error
} finally {
   session.endSession();
}