Start a session with mongoc_client_start_session(), use the session for a sequence of operations and

       MongoDB, Inc

       2009-present, MongoDB, Inc.

1.30.4                                            Jun 08, 2025                        MONGOC_CLIENT_SESSION_T(3)

       example-session.c

          /* gcc example-session.c -o example-session \
           *     $(pkg-config --cflags --libs libmongoc-1.0) */

          /* ./example-session [CONNECTION_STRING] */

          #include <stdio.h>
          #include <mongoc/mongoc.h>

          int
          main (int argc, char *argv[])
          {
             int exit_code = EXIT_FAILURE;

             mongoc_client_t *client = NULL;
             const char *uri_string = "mongodb://127.0.0.1/?appname=session-example";
             mongoc_uri_t *uri = NULL;
             mongoc_client_session_t *client_session = NULL;
             mongoc_collection_t *collection = NULL;
             bson_error_t error;
             bson_t *selector = NULL;
             bson_t *update = NULL;
             bson_t *update_opts = NULL;
             bson_t *find_opts = NULL;
             mongoc_read_prefs_t *secondary = NULL;
             mongoc_cursor_t *cursor = NULL;
             const bson_t *doc;
             char *str;
             bool r;

             mongoc_init ();

             if (argc > 1) {
                uri_string = argv[1];
             }

             uri = mongoc_uri_new_with_error (uri_string, &error);
             if (!uri) {
                fprintf (stderr,
                         "failed to parse URI: %s\n"
                         "error message:       %s\n",
                         uri_string,
                         error.message);
                goto done;
             }

             client = mongoc_client_new_from_uri (uri);
             if (!client) {
                goto done;
             }

             mongoc_client_set_error_api (client, 2);

             /* pass NULL for options - by default the session is causally consistent */
             client_session = mongoc_client_start_session (client, NULL, &error);
             if (!client_session) {
                fprintf (stderr, "Failed to start session: %s\n", error.message);
                goto done;
             }

             collection = mongoc_client_get_collection (client, "test", "collection");
             selector = BCON_NEW ("_id", BCON_INT32 (1));
             update = BCON_NEW ("$inc", "{", "x", BCON_INT32 (1), "}");
             update_opts = bson_new ();
             if (!mongoc_client_session_append (client_session, update_opts, &error)) {
                fprintf (stderr, "Could not add session to opts: %s\n", error.message);
                goto done;
             }

             r = mongoc_collection_update_one (collection, selector, update, update_opts, NULL /* reply */, &error);

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

             bson_destroy (selector);
             selector = BCON_NEW ("_id", BCON_INT32 (1));
             secondary = mongoc_read_prefs_new (MONGOC_READ_SECONDARY);

             find_opts = BCON_NEW ("maxTimeMS", BCON_INT32 (2000));
             if (!mongoc_client_session_append (client_session, find_opts, &error)) {
                fprintf (stderr, "Could not add session to opts: %s\n", error.message);
                goto done;
             }

             /* read from secondary. since we're in a causally consistent session, the
              * data is guaranteed to reflect the update we did on the primary. the query
              * blocks waiting for the secondary to catch up, if necessary, or times out
              * and fails after 2000 ms.
              */
             cursor = mongoc_collection_find_with_opts (collection, selector, find_opts, secondary);

             while (mongoc_cursor_next (cursor, &doc)) {
                str = bson_as_relaxed_extended_json (doc, NULL);
                fprintf (stdout, "%s\n", str);
                bson_free (str);
             }

             if (mongoc_cursor_error (cursor, &error)) {
                fprintf (stderr, "Cursor Failure: %s\n", error.message);
                goto done;
             }

             exit_code = EXIT_SUCCESS;

          done:
             if (find_opts) {
                bson_destroy (find_opts);
             }
             if (update) {
                bson_destroy (update);
             }
             if (selector) {
                bson_destroy (selector);
             }
             if (update_opts) {
                bson_destroy (update_opts);
             }
             if (secondary) {
                mongoc_read_prefs_destroy (secondary);
             }
             /* destroy cursor, collection, session before the client they came from */
             if (cursor) {
                mongoc_cursor_destroy (cursor);
             }
             if (collection) {
                mongoc_collection_destroy (collection);
             }
             if (client_session) {
                mongoc_client_session_destroy (client_session);
             }
             if (uri) {
                mongoc_uri_destroy (uri);
             }
             if (client) {
                mongoc_client_destroy (client);
             }

             mongoc_cleanup ();

             return exit_code;
          }

       A  mongoc_client_session_t is only usable in the parent process after a fork. The child process must call
       mongoc_client_reset() on the client field.

       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 causallyconsistent. 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.