# 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:

- `readConcern`: Construct a mongoc_read_concern_t and use mongoc_read_concern_append() to add the read concern to `opts`. See the example code for mongoc_client_read_command_with_opts(). Read concern requires MongoDB 3.2 or later, otherwise an error is returned.
- `sessionId`: First, construct a mongoc_client_session_t with mongoc_client_start_session(). You can begin a transaction with mongoc_client_session_start_transaction(), optionally with a mongoc_transaction_opt_t that overrides the options inherited from `client`, and use mongoc_client_session_append() to add the session to `opts`. See the example code for mongoc_client_session_t.
- `collation`: Configure textual comparisons. See Setting Collation Order, and the MongoDB Manual entry on Collation. Collation requires MongoDB 3.2 or later, otherwise an error is returned.
- `serverId`: To target a specific server, include an int32 “serverId” field. Obtain the id by calling mongoc_client_select_server(), then mongoc_server_description_id() on its return value.

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;
    }