# mongoc_session_opts_set_causal_consistency()

## Synopsis

    void
    mongoc_session_opts_set_causal_consistency (mongoc_session_opt_t *opts,
                                                bool causal_consistency);

Configure causal consistency in a session. If true (the default), each operation in the session will be causally ordered after the previous read or write operation. Set to false to disable causal consistency. See the MongoDB Manual Entry for Causal Consistency.

Causal consistency and snapshot reads are mutually exclusive. Attempting to set both to true will result in an error. See mongoc_session_opts_set_snapshot().

Unacknowledged writes are not causally consistent. If you execute a write operation with a mongoc_write_concern_t on which you have called mongoc_write_concern_set_w() with a value of 0, the write does not participate in causal consistency.

## Parameters

- `opts`: A mongoc_session_opt_t.

- `causal_consistency`: True or false.

## Example

    mongoc_client_t *client;
    mongoc_session_opt_t *session_opts;
    mongoc_client_session_t *client_session;
    mongoc_collection_t *collection;
    bson_t insert_opts = BSON_INITIALIZER;
    bson_t *doc;
    bson_error_t error;
    bool r;

    client = mongoc_client_new ("mongodb://example/?appname=session-opts-example");
    mongoc_client_set_error_api (client, 2);

    session_opts = mongoc_session_opts_new ();
    mongoc_session_opts_set_causal_consistency (session_opts, false);
    client_session = mongoc_client_start_session (client, session_opts, &error);
    mongoc_session_opts_destroy (session_opts);

    if (!client_session) {
       fprintf (stderr, "Failed to start session: %s\n", error.message);
       abort ();
    }

    collection = mongoc_client_get_collection (client, "test", "collection");
    doc = BCON_NEW ("_id", BCON_INT32 (1));
    r = mongoc_client_session_append (client_session, &insert_opts, NULL);
    if (!r) {
       fprintf (stderr, "mongoc_client_session_append failed: %s\n", error.message);
       abort ();
    }

    r = mongoc_collection_insert_one (
       collection, doc, &insert_opts, NULL /* reply */, &error);

    if (!r) {
       fprintf (stderr, "Insert failed: %s\n", error.message);
       abort ();
    }

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()