mongoc_client_read_command_with_opts()

Synopsis

bool
mongoc_client_read_command_with_opts (mongoc_client_t *client,
                                      const char *db_name,
                                      const bson_t *command,
                                      const mongoc_read_prefs_t *read_prefs,
                                      const bson_t *opts,
                                      bson_t *reply,
                                      bson_error_t *error);

Execute a command on the server, applying logic that is specific to commands that read, and taking the MongoDB server version into account. To send a raw command to the server without any of this logic, use mongoc_client_command_simple().

Use this function for commands that read such as “count” or “distinct”.

Read preferences, read concern, and collation can be overridden by various sources. In a transaction, read concern and write concern are prohibited in opts and the read preference must be primary or NULL. The highest-priority sources for these options are listed first in the following table. No write concern is applied.

Read Preferences Read Concern Collation
read_prefs opts opts
Transaction Transaction  
client    

See the example for transactions and for the “distinct” command with opts.

reply is always initialized, and must be freed with bson_destroy().

This function is considered a retryable read operation. Upon a transient error (a network error, errors due to replica set failover, etc.) the operation is safely retried once. If retryreads is false in the URI (see mongoc_uri_t) the retry behavior does not apply.

Retry logic occurs regardless of the underlying command. Retrying mapReduce has the potential for degraded performance. Retrying a getMore command has the potential to miss results. For those commands, use generic command helpers (like mongoc_client_command_with_opts()) instead.

Parameters

  • client: A mongoc_client_t.
  • db_name: The name of the database to run the command on.
  • command: A bson_t containing the command specification.
  • read_prefs: An optional mongoc_read_prefs_t.
  • opts: A bson_t containing additional options.
  • reply: A location for the resulting document.
  • error: An optional location for a bson_error_t or NULL.

opts may be NULL or a BSON document with additional command options:

Consult the MongoDB Manual entry on Database Commands for each command’s arguments.

Errors

Errors are propagated via the error parameter.

Returns

Returns true if successful. Returns false and sets error if there are invalid arguments or a server or network error.

Example

example-command-with-opts.c
/*

Demonstrates how to prepare options for mongoc_client_read_command_with_opts and
mongoc_client_write_command_with_opts. First it calls "cloneCollectionAsCapped"
command with "writeConcern" option, then "distinct" command with "collation" and
"readConcern" options,

Start a MongoDB 3.4 replica set with --enableMajorityReadConcern and insert two
documents:

$ mongo
MongoDB Enterprise replset:PRIMARY> db.my_collection.insert({x: 1, y: "One"})
WriteResult({ "nInserted" : 1 })
MongoDB Enterprise replset:PRIMARY> db.my_collection.insert({x: 2, y: "Two"})
WriteResult({ "nInserted" : 1 })

Build and run the example:

gcc example-command-with-opts.c -o example-command-with-opts $(pkg-config
--cflags --libs libmongoc-1.0)
./example-command-with-opts [CONNECTION_STRING]
cloneCollectionAsCapped: { "ok" : 1 }
distinct: { "values" : [ 1, 2 ], "ok" : 1 }

*/

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

int
main (int argc, char *argv[])
{
   mongoc_client_t *client;
   const char *uri_string = "mongodb://127.0.0.1/?appname=client-example";
   mongoc_uri_t *uri;
   bson_t *cmd;
   bson_t *opts;
   mongoc_write_concern_t *write_concern;
   mongoc_read_prefs_t *read_prefs;
   mongoc_read_concern_t *read_concern;
   bson_t reply;
   bson_error_t error;
   char *json;

   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);
      return EXIT_FAILURE;
   }

   client = mongoc_client_new_from_uri (uri);
   if (!client) {
      return EXIT_FAILURE;
   }

   mongoc_client_set_error_api (client, 2);

   cmd = BCON_NEW ("cloneCollectionAsCapped",
                   BCON_UTF8 ("my_collection"),
                   "toCollection",
                   BCON_UTF8 ("my_capped_collection"),
                   "size",
                   BCON_INT64 (1024 * 1024));

   /* include write concern "majority" in command options */
   write_concern = mongoc_write_concern_new ();
   mongoc_write_concern_set_wmajority (write_concern, 10000 /* wtimeoutMS */);
   opts = bson_new ();
   mongoc_write_concern_append (write_concern, opts);

   if (mongoc_client_write_command_with_opts (
          client, "test", cmd, opts, &reply, &error)) {
      json = bson_as_canonical_extended_json (&reply, NULL);
      printf ("cloneCollectionAsCapped: %s\n", json);
      bson_free (json);
   } else {
      fprintf (stderr, "cloneCollectionAsCapped: %s\n", error.message);
   }

   bson_free (cmd);
   bson_free (opts);

   /* distinct values of "x" in "my_collection" where "y" sorts after "one" */
   cmd = BCON_NEW ("distinct",
                   BCON_UTF8 ("my_collection"),
                   "key",
                   BCON_UTF8 ("x"),
                   "query",
                   "{",
                   "y",
                   "{",
                   "$gt",
                   BCON_UTF8 ("one"),
                   "}",
                   "}");

   read_prefs = mongoc_read_prefs_new (MONGOC_READ_SECONDARY);

   /* "One" normally sorts before "one"; make "One" sort after "one" */
   opts = BCON_NEW ("collation",
                    "{",
                    "locale",
                    BCON_UTF8 ("en_US"),
                    "caseFirst",
                    BCON_UTF8 ("lower"),
                    "}");

   /* add a read concern to "opts" */
   read_concern = mongoc_read_concern_new ();
   mongoc_read_concern_set_level (read_concern,
                                  MONGOC_READ_CONCERN_LEVEL_MAJORITY);

   mongoc_read_concern_append (read_concern, opts);

   if (mongoc_client_read_command_with_opts (
          client, "test", cmd, read_prefs, opts, &reply, &error)) {
      json = bson_as_canonical_extended_json (&reply, NULL);
      printf ("distinct: %s\n", json);
      bson_free (json);
   } else {
      fprintf (stderr, "distinct: %s\n", error.message);
   }

   bson_destroy (cmd);
   bson_destroy (opts);
   bson_destroy (&reply);
   mongoc_read_prefs_destroy (read_prefs);
   mongoc_read_concern_destroy (read_concern);
   mongoc_write_concern_destroy (write_concern);
   mongoc_uri_destroy (uri);
   mongoc_client_destroy (client);

   mongoc_cleanup ();

   return EXIT_SUCCESS;
}