mongoc_client_session_with_transaction - Man Page

bool
mongoc_client_session_with_transaction (mongoc_client_session_t *session,
                                        mongoc_client_session_with_transaction_cb_t cb,
                                        const mongoc_transaction_opt_t *opts,
                                        void *ctx,
                                        bson_t *reply,
                                        bson_error_t *error);

This method will start a new transaction on session, run cb, and then commit the transaction. If it cannot commit the transaction, the entire sequence may be retried, and cb may be run multiple times. ctx will be passed to cb each time it is called.

This method has an internal time limit of 120 seconds, and will retry until that time limit is reached. This timeout is not configurable.

cb should not attempt to start new transactions, but should simply run operations meant to be contained within a transaction. The cb does not need to commit transactions; this is handled by the mongoc_client_session_with_transaction(). If cb does commit or abort a transaction, however, this method will return without taking further action.

If a command inside cb fails, it may cause the transaction on the server to be aborted. This situation is normally handled transparently by the driver. However, if the application does not return that error from cb, the driver will not be able to determine whether the transaction was aborted or not. The driver will then retry cb indefinitely. To avoid this situation, the application MUST NOT silently handle errors within cb. If the application needs to handle errors within cb, it MUST return them after doing so.

The parameter reply is initialized even upon failure to simplify memory management.

• session: A mongoc_client_session_t.
• cb: A mongoc_client_session_with_transaction_cb_t callback, which will run inside of a new transaction on the session. See example below.
• opts: An optional mongoc_transaction_opt_t.
• ctx: A void*. This user-provided data will be passed to cb.
• reply: A maybe-NULL pointer to overwritable storage for a bson_t to contain the results.
• error: An optional location for a bson_error_t or NULL.

Returns true if the transaction was completed successfully.  Otherwise, returns false in case of failure.  In cases of failure error will also be set, except if the passed-in cb fails without setting error.  If a non-NULL reply is passed in, reply will be set to the value of the last server response, except if the passed-in cb fails without setting a reply.

Use with_transaction() to run a callback within a transaction

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

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

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

/*
 * We pass this context object to mongoc_client_session_with_transaction() along
 * with our callback function. The context object will be passed to our callback
 * function when it runs, so we can access it.
 */
typedef struct {
   mongoc_collection_t *collection;
   bson_t *insert_opts;
} ctx_t;

/*
 * We pass this method as the callback to
 * mongoc_client_session_with_transaction(). The insert that this method
 * performs will happen inside of a new transaction.
 */
bool
create_and_insert_doc (mongoc_client_session_t *session,
                       void *ctx,
                       bson_t **reply, /* out param for our server reply */
                       bson_error_t *error)
{
   /*
    * mongoc_collection_insert_one requires an uninitialized, stack-allocated
    * bson_t to receive the update result
    */
   bson_t local_reply;
   bson_t *doc = NULL;
   ctx_t *data = NULL;
   bool retval;

   /*
    * Create a new bson document - { id: 1 }
    */
   doc = BCON_NEW ("_id", BCON_INT32 (1));

   printf ("Running the user-defined callback in a newly created transaction...\n");
   data = (ctx_t *) ctx;
   retval = mongoc_collection_insert_one (data->collection, doc, data->insert_opts, &local_reply, error);

   /*
    * To return to the mongoc_client_session_with_transaction() method, set
    * *reply to a new copy of our local_reply before destroying it.
    */
   *reply = bson_copy (&local_reply);
   bson_destroy (&local_reply);

   bson_destroy (doc);
   return retval;
}

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

   mongoc_uri_t *uri = NULL;
   const char *uri_string = "mongodb://127.0.0.1/?appname=with-txn-cb-example";
   mongoc_client_t *client = NULL;
   mongoc_database_t *database = NULL;
   mongoc_collection_t *collection = NULL;
   mongoc_client_session_t *session = NULL;
   bson_t *insert_opts = NULL;
   bson_t reply;
   ctx_t ctx;
   char *str;
   bson_error_t error;

   /*
    * Required to initialize libmongoc's internals
    */
   mongoc_init ();

   /*
    * Optionally get MongoDB URI from command line
    */
   if (argc > 1) {
      uri_string = argv[1];
   }

   /*
    * Safely create a MongoDB URI object from the given string
    */
   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;
   }

   /*
    * Create a new client instance
    */
   client = mongoc_client_new_from_uri (uri);
   if (!client) {
      goto done;
   }

   mongoc_client_set_error_api (client, 2);

   /*
    * Get a handle on the database "example-with-txn-cb"
    */
   database = mongoc_client_get_database (client, "example-with-txn-cb");

   /*
    * 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;
      }
   }

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

   /*
    * Append a logical session id to command options
    */
   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;
   }

   ctx.collection = collection;
   ctx.insert_opts = insert_opts;

   /*
    * This method will start a new transaction on session, run our callback
    * function, i.e., &create_and_insert_doc, passing &ctx as an argument and
    * commit the transaction.
    */
   if (!mongoc_client_session_with_transaction (session, &create_and_insert_doc, NULL, &ctx, &reply, &error)) {
      MONGOC_ERROR ("Insert failed: %s", error.message);
      goto done;
   }

   str = bson_as_relaxed_extended_json (&reply, NULL);
   printf ("%s\n", str);

   exit_code = EXIT_SUCCESS;

done:
   bson_free (str);
   bson_destroy (&reply);
   bson_destroy (insert_opts);
   mongoc_client_session_destroy (session);
   mongoc_collection_destroy (collection);
   mongoc_database_destroy (database);
   mongoc_client_destroy (client);
   mongoc_uri_destroy (uri);

   mongoc_cleanup ();

   return exit_code;
}

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