NAME

       mongoc_client_session_t - mongoc_client_session_t

       Use  a  session  for a sequence of operations, optionally with causal consistency. See the
       MongoDB Manual Entry for Causal Consistency.

SYNOPSIS

       Start a session with mongoc_client_start_session,  use  the  session  for  a  sequence  of
       operations,  then  free  it  with mongoc_client_session_destroy(). A session must be freed
       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 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.

       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.

EXAMPLE

       example-session.c.INDENT 0.0

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

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

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

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

             mongoc_client_t *client;
             mongoc_client_session_t *client_session = NULL;
             mongoc_collection_t *collection = NULL;
             const char *uristr = "mongodb://127.0.0.1/?appname=session-example";
             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) {
                uristr = argv[1];
             }

             client = mongoc_client_new (uristr);

             if (!client) {
                fprintf (stderr, "Failed to parse URI.\n");
                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);

             bson_destroy (selector);
             mongoc_read_prefs_destroy (secondary);
             bson_destroy (find_opts);

             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 (client) {
                mongoc_client_destroy (client);
             }

             mongoc_cleanup ();

             return exit_code;
          }

AUTHOR

       MongoDB, Inc

COPYRIGHT

       2018, MongoDB, Inc