NAME

MongoDB::ClientSession - MongoDB session and transaction management

VERSION

version v2.0.0

SYNOPSIS

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

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

    # use sessions for transactions
    $session->start_transaction;
    ...
    if ( $ok ) {
        $session->commit_transaction;
    }
    else {
        $session->abort_transaction;
    }

DESCRIPTION

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.

Cursors

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

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.

Transactions

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.

ATTRIBUTES

client

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

cluster_time

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

options

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.

operation_time

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.

METHODS

session_id

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

get_latest_cluster_time

    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.

advance_cluster_time

    $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 reccomended to only use $clusterTime values retrieved from database calls.

advance_operation_time

    $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 retreived from another session or directly from a database call.

start_transaction

    $session->start_transaction;
    $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.

commit_transaction

    $session->commit_transaction;

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 {
            $session->commit_transaction;
        };
        if ( my $error = $@ ) {
            if ( $error->has_error_label("UnknownTransactionCommitResult") ) {
                redo LOOP;
            }
            else {
                die $error;
            }
        }
    }

abort_transaction

    $session->abort_transaction;

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).

end_session

    $session->end_session;

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.

AUTHORS

• David Golden <david@mongodb.com>

• Rassi <rassi@mongodb.com>

• Mike Friedman <friedo@friedo.com>

• Kristina Chodorow <k.chodorow@gmail.com>

• Florian Ragwitz <rafl@debian.org>

COPYRIGHT AND LICENSE

This software is Copyright (c) 2018 by MongoDB, Inc.

This is free software, licensed under:

  The Apache License, Version 2.0, January 2004

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