Provided by: libmongoc-doc_1.15.0-1_all bug


       mongoc_client_session_start_transaction - mongoc_client_session_start_transaction()


          mongoc_client_session_start_transaction (mongoc_client_session_t *session,
                                                   const mongoc_transaction_opt_t *opts,
                                                   bson_error_t *error);

       Start  a  multi-document  transaction  for  all  following operations in this session. Any
       options      provided      in      opts       override       options       passed       to
       mongoc_session_opts_set_default_transaction_opts,   and   options   inherited   from   the
       mongoc_client_t. The opts argument is copied and can be freed after calling this function.

       The  transaction  must  be  completed  with  mongoc_client_session_commit_transaction   or
       mongoc_client_session_abort_transaction.   An  in-progress  transaction  is  automatically
       aborted by mongoc_client_session_destroy.


       · session: A mongoc_client_session_t.

       · opts: A mongoc_transaction_opt_t or NULL.

       · error: An optional location for a bson_error_t or NULL.


       Returns true if the transaction was started. Returns false and sets  error  if  there  are
       invalid arguments, such as a session with a transaction already in progress.


       The  following  example  demonstrates  how  to  use  error  labels  to  reliably execute a
       multi-document transaction despite network errors and other transient failures.


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

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

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

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

             mongoc_client_t *client = NULL;
             mongoc_database_t *database = NULL;
             mongoc_collection_t *collection = NULL;
             mongoc_client_session_t *session = NULL;
             mongoc_session_opt_t *session_opts = NULL;
             mongoc_transaction_opt_t *default_txn_opts = NULL;
             mongoc_transaction_opt_t *txn_opts = NULL;
             mongoc_read_concern_t *read_concern = NULL;
             mongoc_write_concern_t *write_concern = NULL;
             const char *uri_string = "mongodb://";
             mongoc_uri_t *uri;
             bson_error_t error;
             bson_t *doc = NULL;
             bson_t *insert_opts = NULL;
             int32_t i;
             int64_t start;
             bson_t reply = BSON_INITIALIZER;
             char *reply_json;
             bool r;

             mongoc_init ();

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

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

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

             mongoc_client_set_error_api (client, 2);
             database = mongoc_client_get_database (client, "example-transaction");

             /* inserting into a nonexistent collection normally creates it, but a
              * collection can't be created in a transaction; create it now */
             collection =
                mongoc_database_create_collection (database, "collection", NULL, &error);

             if (!collection) {
                /* code 48 is NamespaceExists, see error_codes.err in mongodb source */
                if (error.code == 48) {
                   collection = mongoc_database_get_collection (database, "collection");
                } else {
                   MONGOC_ERROR ("Failed to create collection: %s", error.message);
                   goto done;

             /* a transaction's read preferences, read concern, and write concern can be
              * set on the client, on the default transaction options, or when starting
              * the transaction. for the sake of this example, set read concern on the
              * default transaction options. */
             default_txn_opts = mongoc_transaction_opts_new ();
             read_concern = mongoc_read_concern_new ();
             mongoc_read_concern_set_level (read_concern, "snapshot");
             mongoc_transaction_opts_set_read_concern (default_txn_opts, read_concern);
             session_opts = mongoc_session_opts_new ();
             mongoc_session_opts_set_default_transaction_opts (session_opts,

             session = mongoc_client_start_session (client, session_opts, &error);
             if (!session) {
                MONGOC_ERROR ("Failed to start session: %s", error.message);
                goto done;

             /* in this example, set write concern when starting the transaction */
             txn_opts = mongoc_transaction_opts_new ();
             write_concern = mongoc_write_concern_new ();
             mongoc_write_concern_set_wmajority (write_concern, 1000 /* wtimeout */);
             mongoc_transaction_opts_set_write_concern (txn_opts, write_concern);

             insert_opts = bson_new ();
             if (!mongoc_client_session_append (session, insert_opts, &error)) {
                MONGOC_ERROR ("Could not add session to opts: %s", error.message);
                goto done;

             r = mongoc_client_session_start_transaction (session, txn_opts, &error);
             if (!r) {
                MONGOC_ERROR ("Failed to start transaction: %s", error.message);
                goto done;

             /* insert two documents - on error, retry the whole transaction */
             for (i = 0; i < 2; i++) {
                doc = BCON_NEW ("_id", BCON_INT32 (i));
                bson_destroy (&reply);
                r = mongoc_collection_insert_one (
                   collection, doc, insert_opts, &reply, &error);

                bson_destroy (doc);

                if (!r) {
                   MONGOC_ERROR ("Insert failed: %s", error.message);
                   mongoc_client_session_abort_transaction (session, NULL);

                   /* a network error, primary failover, or other temporary error in a
                    * transaction includes {"errorLabels": ["TransientTransactionError"]},
                    * meaning that trying the entire transaction again may succeed
                   if (mongoc_error_has_label (&reply, "TransientTransactionError")) {
                      goto retry_transaction;

                   goto done;

                reply_json = bson_as_json (&reply, NULL);
                printf ("%s\n", reply_json);
                bson_free (reply_json);

             /* in case of transient errors, retry for 5 seconds to commit transaction */
             start = bson_get_monotonic_time ();
             while (bson_get_monotonic_time () - start < 5 * 1000 * 1000) {
                bson_destroy (&reply);
                r = mongoc_client_session_commit_transaction (session, &reply, &error);
                if (r) {
                   /* success */
                } else {
                   MONGOC_ERROR ("Warning: commit failed: %s", error.message);
                   if (mongoc_error_has_label (&reply, "TransientTransactionError")) {
                      goto retry_transaction;
                   } else if (mongoc_error_has_label (&reply,
                                                      "UnknownTransactionCommitResult")) {
                      /* try again to commit */

                   /* unrecoverable error trying to commit */

             exit_code = EXIT_SUCCESS;

             bson_destroy (&reply);
             bson_destroy (insert_opts);
             mongoc_write_concern_destroy (write_concern);
             mongoc_read_concern_destroy (read_concern);
             mongoc_transaction_opts_destroy (txn_opts);
             mongoc_transaction_opts_destroy (default_txn_opts);
             mongoc_client_session_destroy (session);
             mongoc_collection_destroy (collection);
             mongoc_database_destroy (database);
             mongoc_uri_destroy (uri);
             mongoc_client_destroy (client);

             mongoc_cleanup ();

             return exit_code;


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