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Starts a session for the connection. mongo assigns the session ID to commands associated with the session.


mongo Shell Method

This page documents a mongo method. This is not the documentation for database commands or language-specific drivers, such as Node.js. To use the database command, see the startSession command.

For MongoDB API drivers, refer to the language-specific MongoDB driver documentation.

The startSession() method can take a document with session options. The options available are:


Boolean. Enables or disables causal consistency for the session. Mongo.startSession() enables causalConsistency by default.

After starting a session, you cannot modify its causalConsistency setting.


The session may have causal consistency enabled even though the Mongo() connection object may have causal consistency disabled or vice versa. To set causal consistency on the connection object, see Mongo.setCausalConsistency().


Document. Specifies the read concern.

To modify the setting after starting a session, see Session.getOptions().setReadConcern().


Document. Specifies the read preference.

The readPreference document contains the mode field and the optional tags field:

{ mode: <string>, tags: <array> }

To modify the setting after starting a session, see Session.getOptions().setReadPreference().


Boolean. Enables or disables the ability to retry writes upon encountering transient network errors.

If you start the mongo shell with the --retryWrites option, retryWrites is enabled by default for Mongo.startSession().

After starting a session, you cannot modify its retryWrites setting.


Document. Specifies the write concern.

To modify the setting after starting a session, see Session.getOptions().setWriteConcern().

The following starts a session with causal consistency and retryable writes enabled on the Mongo() connection object associated with the mongo shell's global db variable:

db = db.getMongo().startSession({retryWrites: true, causalConsistency: true}).getDatabase(db.getName());
← Mongo.setSecondaryOk()

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