SYNOPSIS

       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.

FORK SAFETY

       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.

EXAMPLE

       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_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;
          }

AUTHOR

       MongoDB, Inc

COPYRIGHT

       2017-present, MongoDB, Inc