Provided by: libmongoc-doc_2.2.1-1_all 
SYNOPSIS
bool
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() <>.
If a command inside of the transaction fails, it may cause the transaction on the server to be aborted.
An attempt to commit such transaction will be rejected with NoSuchTransaction error.
PARAMETERS
• 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.
RETURN
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.
EXAMPLE
The following example demonstrates how to use error labels <#error-labels> to reliably execute a
multi-document transaction despite network errors and other transient failures.
example-transaction.c
/* gcc example-transaction.c -o example-transaction \
* $(pkg-config --cflags --libs libmongoc-1.0) */
/* ./example-transaction [CONNECTION_STRING] */
#include <mongoc/mongoc.h>
#include <stdio.h>
int
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://127.0.0.1/?appname=transaction-example";
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",
uri_string,
error.message);
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, default_txn_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;
}
retry_transaction:
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_relaxed_extended_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 */
break;
} 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 */
continue;
}
/* unrecoverable error trying to commit */
break;
}
}
exit_code = EXIT_SUCCESS;
done:
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;
}
Author
MongoDB, Inc
Copyright
2009-present, MongoDB, Inc.
2.2.1 Dec 11, 2025 MONGOC_CLIENT_SESSION_START_TRANSACTION(3)