# mongoc_client_start_session()

## Synopsis

    mongoc_client_session_t *
    mongoc_client_start_session (mongoc_client_t *client,
                                 mongoc_session_opt_t *opts,
                                 bson_error_t *error)

Create a session for a sequence of operations.

Start a session with mongoc_client_start_session(), use the session for a sequence of operations and multi-document transactions, then free it with mongoc_client_session_destroy(). Any mongoc_cursor_t or mongoc_change_stream_t using a session must be destroyed before the session, and a session must be destroyed before the mongoc_client_t it came from.

By default, sessions are causally consistent. To disable causal consistency, before starting a session create a mongoc_session_opt_t with mongoc_session_opts_new() and call mongoc_session_opts_set_causal_consistency(), then free the struct with mongoc_session_opts_destroy().

Unacknowledged writes are prohibited with sessions.

A mongoc_client_session_t must be used by only one thread at a time. Due to session pooling, mongoc_client_start_session() may return a session that has been idle for some time and is about to be closed after its idle timeout. Use the session within one minute of acquiring it to refresh the session and avoid a timeout.

## Parameters

- `client`: A mongoc_client_t.

- `opts`: An optional mongoc_session_opt_t.

- `error`: A `bson_error_t`.

## Returns

If successful, this function returns a newly allocated mongoc_client_session_t that should be freed with mongoc_client_session_destroy() when no longer in use. On error, returns NULL and sets `error`.

## Errors

This function can fail if the driver is not built with crypto support, if `opts` is misconfigured, or if the session is configured with options that the server does not support.

See also

mongoc_client_session_abort_transaction()
mongoc_client_session_advance_cluster_time()
mongoc_client_session_advance_operation_time()
mongoc_client_session_commit_transaction()
mongoc_client_session_destroy()
mongoc_client_session_get_client()
mongoc_client_session_get_cluster_time()
mongoc_client_session_get_lsid()
mongoc_client_session_get_operation_time()
mongoc_client_session_get_opts()
mongoc_client_session_get_server_id()
mongoc_client_session_start_transaction()
mongoc_client_session_t
mongoc_client_start_session()
mongoc_session_opts_clone()
mongoc_session_opts_destroy()
mongoc_session_opts_get_causal_consistency()
mongoc_session_opts_get_default_transaction_opts()
mongoc_session_opts_get_transaction_opts()
mongoc_session_opts_new()
mongoc_session_opts_set_causal_consistency()
mongoc_session_opts_set_default_transaction_opts()
mongoc_transaction_opts_clone()
mongoc_transaction_opts_destroy()
mongoc_transaction_opts_get_max_commit_time_ms()
mongoc_transaction_opts_get_read_concern()
mongoc_transaction_opts_get_read_prefs()
mongoc_transaction_opts_get_write_concern()
mongoc_transaction_opts_new()
mongoc_transaction_opts_set_max_commit_time_ms()
mongoc_transaction_opts_set_read_concern()
mongoc_transaction_opts_set_read_prefs()
mongoc_transaction_opts_set_write_concern()