MongoDB::ClientSession - MongoDB session and transaction management


version v2.2.2


    my $session = $client->start_session( $options );

    # use session in operations
    my $result = $collection->find( { id => 1 }, { session => $session } );

    # use sessions for transactions
    if ( $ok ) {
    else {


This class encapsulates an active session for use with the current client. Sessions support is new with MongoDB 3.6, and can be used in replica set and sharded MongoDB clusters.

Explicit and Implicit Sessions

If you specifically apply a session to an operation, then the operation will be performed with that session id. If you do not provide a session for an operation, and the server supports sessions, then an implicit session will be created and used for this operation.

The only exception to this is for unacknowledged writes - the driver will not provide an implicit session for this, and if you provide a session then the driver will raise an error.


During cursors, if a session is not provided then an implicit session will be created which is then used for the lifetime of the cursor. If you provide a session, then note that ending the session and then continuing to use the cursor will raise an error.

Thread Safety

NOTE: Per threads documentation, use of Perl threads is discouraged by the maintainers of Perl and the MongoDB Perl driver does not test or provide support for use with threads.

Sessions are NOT thread safe, and should only be used by one thread at a time. Using a session across multiple threads is unsupported and unexpected issues and errors may occur. Note that the driver does not check for multi-threaded use.


A session may be associated with at most one open transaction (on MongoDB 4.0+). For detailed instructions on how to use transactions with drivers, see the MongoDB manual page: Transactions.



The client this session was created using. Sessions may only be used with the client that created them.


Stores the last received $clusterTime for the client session. This is an opaque value, to set it use the advance_cluster_time function.


Options provided for this particular session. Available options include:

  • causalConsistency - If true, will enable causalConsistency for this session. For more information, see MongoDB documentation on Causal Consistency. Note that causalConsistency does not apply for unacknowledged writes. Defaults to true.

  • defaultTransactionOptions - Options to use by default for transactions created with this session. If when creating a transaction, none or only some of the transaction options are defined, these options will be used as a fallback. Defaults to inheriting from the parent client. See "start_transaction" for available options.


The last operation time. This is updated when an operation is performed during this session, or when "advance_operation_time" is called. Used for causal consistency.



The session id for this particular session. This should be considered an opaque value. If end_session has been called, this returns undef.


    my $cluster_time = $session->get_latest_cluster_time;

Returns the latest cluster time, when compared with this session's recorded cluster time and the main client cluster time. If neither is defined, returns undef.


    $session->advance_cluster_time( $cluster_time );

Update the $clusterTime for this session. Stores the value in "cluster_time". If the cluster time provided is more recent than the sessions current cluster time, then the session will be updated to this provided value.

Setting the $clusterTime with a manually crafted value may cause a server error. It is recommended to only use $clusterTime values retrieved from database calls.


    $session->advance_operation_time( $operation_time );

Update the "operation_time" for this session. If the value provided is more recent than the sessions current operation time, then the session will be updated to this provided value.

Setting operation_time with a manually crafted value may cause a server error. It is recommended to only use an operation_time retrieved from another session or directly from a database call.


    $session->start_transaction( $options );

Start a transaction in this session. If a transaction is already in progress or if the driver can detect that the client is connected to a topology that does not support transactions, this method will throw an error.

A hash reference of options may be provided. Valid keys include:

  • readConcern - The read concern to use for the first command in this transaction. If not defined here or in the defaultTransactionOptions in "options", will inherit from the parent client.

  • writeConcern - The write concern to use for committing or aborting this transaction. As per readConcern, if not defined here then the value defined in defaultTransactionOptions will be used, or the parent client if not defined.

  • readPreference - The read preference to use for all read operations in this transaction. If not defined, then will inherit from defaultTransactionOptions or from the parent client. This value will override all other read preferences set in any subsequent commands inside this transaction.

  • maxCommitTimeMS - The maxCommitTimeMS specifies a cumulative time limit in milliseconds for processing operations on the cursor. MongoDB interrupts the operation at the earliest following interrupt point.



Commit the current transaction. This will use the writeConcern set on this transaction.

If called when no transaction is in progress, then this method will throw an error.

If the commit operation encounters an error, an error is thrown. If the error is a transient commit error, the error object will have a label containing "UnknownTransactionCommitResult" as an element and the commit operation can be retried. This can be checked via the has_error_label:

    LOOP: {
        eval {
        if ( my $error = $@ ) {
            if ( $error->has_error_label("UnknownTransactionCommitResult") ) {
                redo LOOP;
            else {
                die $error;



Aborts the current transaction. If no transaction is in progress, then this method will throw an error. Otherwise, this method will suppress all other errors (including network and database errors).



Close this particular session and release the session ID for reuse or recycling. If a transaction is in progress, it will be aborted. Has no effect after calling for the first time.

This will be called automatically by the object destructor.


    $session->with_transaction($callback, $options);

Execute a callback in a transaction.

This method starts a transaction on this session, executes $callback, and then commits the transaction, returning the return value of the $callback. The $callback will be executed at least once.

If the $callback throws an error, the transaction will be aborted. If less than 120 seconds have passed since calling with_transaction, and the error has a TransientTransactionError label, the transaction will be restarted and the callback will be executed again. Otherwise, the error will be thrown.

If the $callback succeeds, then the transaction will be committed. If an error is thrown from committing the transaction, and it is less than 120 seconds since calling with_transaction, then:

  • If the error has a TransientTransactionError label, the transaction will be restarted.

  • If the error has an UnknownTransactionCommitResult label, and is not a MaxTimeMSExpired error, then the commit will be retried.

If the $callback aborts or commits the transaction, no other actions are taken and the return value of the $callback is returned.

The callback is called with the first (and only) argument being the session, after starting the transaction:

    $session->with_transaction( sub {
        # this is the same session as used for with_transaction
        my $cb_session = shift;
    }, $options);

To pass arbitrary arguments to the $callback, wrap your callback in a coderef:

    $session->with_transaction(sub { $callback->($session, $foo, ...) }, $options);

Warning: you must either use the provided session within the callback, or otherwise pass the session in use to the callback. You must pass the $session as an option to all database operations that need to be included in the transaction.

Warning: The $callback can be called multiple times, so it is recommended to make it idempotent.

A hash reference of options may be provided. these are the same as for "start_transaction".


  • David Golden <>

  • Rassi <>

  • Mike Friedman <>

  • Kristina Chodorow <>

  • Florian Ragwitz <>


This software is Copyright (c) 2020 by MongoDB, Inc.

This is free software, licensed under:

  The Apache License, Version 2.0, January 2004