Advanced Connections¶

The following guide contains information specific to certain types of MongoDB configurations.

For an example of connecting to a simple standalone server, see the Tutorial. To establish a connection with authentication options enabled, see the Authentication page.

Connecting to a Replica Set¶

Connecting to a replica set is much like connecting to a standalone MongoDB server. Simply specify the replica set name using the ?replicaSet=myreplset URI option.

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

int
main (int argc, char *argv[])
{
   mongoc_client_t *client;

   mongoc_init ();

   /* Create our MongoDB Client */
   client = mongoc_client_new (
      "mongodb://host01:27017,host02:27017,host03:27017/?replicaSet=myreplset");

   /* Do some work */
   /* TODO */

   /* Clean up */
   mongoc_client_destroy (client);
   mongoc_cleanup ();

   return 0;
}

Tip

Multiple hostnames can be specified in the MongoDB connection string URI, with a comma separating hosts in the seed list.

It is recommended to use a seed list of members of the replica set to allow the driver to connect to any node.

Connecting to a Sharded Cluster¶

To connect to a sharded cluster, specify the mongos nodes the client should connect to. The C Driver will automatically detect that it has connected to a mongos sharding server.

If more than one hostname is specified, a seed list will be created to attempt failover between the mongos instances.

Warning

Specifying the replicaSet parameter when connecting to a mongos sharding server is invalid.

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

int
main (int argc, char *argv[])
{
   mongoc_client_t *client;

   mongoc_init ();

   /* Create our MongoDB Client */
   client = mongoc_client_new ("mongodb://myshard01:27017/");

   /* Do something with client ... */

   /* Free the client */
   mongoc_client_destroy (client);

   mongoc_cleanup ();

   return 0;
}

Connecting to an IPv6 Address¶

The MongoDB C Driver will automatically resolve IPv6 addresses from host names. However, to specify an IPv6 address directly, wrap the address in [].

mongoc_uri_t *uri = mongoc_uri_new ("mongodb://[::1]:27017");

Connecting with IPv4 and IPv6¶

If connecting to a hostname that has both IPv4 and IPv6 DNS records, the behavior follows RFC-6555. A connection to the IPv6 address is attempted first. If IPv6 fails, then a connection is attempted to the IPv4 address. If the connection attempt to IPv6 does not complete within 250ms, then IPv4 is tried in parallel. Whichever succeeds connection first cancels the other. The successful DNS result is cached for 10 minutes.

As a consequence, attempts to connect to a mongod only listening on IPv4 may be delayed if there are both A (IPv4) and AAAA (IPv6) DNS records associated with the host.

To avoid a delay, configure hostnames to match the MongoDB configuration. That is, only create an A record if the mongod is only listening on IPv4.

Connecting to a UNIX Domain Socket¶

On UNIX-like systems, the C Driver can connect directly to a MongoDB server using a UNIX domain socket. Pass the URL-encoded path to the socket, which must be suffixed with .sock. For example, to connect to a domain socket at /tmp/mongodb-27017.sock:

mongoc_uri_t *uri = mongoc_uri_new ("mongodb://%2Ftmp%2Fmongodb-27017.sock");

Include username and password like so:

mongoc_uri_t *uri = mongoc_uri_new ("mongodb://user:pass@%2Ftmp%2Fmongodb-27017.sock");

Connecting to a server over SSL¶

These are instructions for configuring TLS/SSL connections.

To run a server locally (on port 27017, for example):

$ mongod --port 27017 --sslMode requireSSL --sslPEMKeyFile server.pem --sslCAFile ca.pem

Add /?ssl=true to the end of a client URI.

mongoc_client_t *client = NULL;
client = mongoc_client_new ("mongodb://localhost:27017/?ssl=true");

MongoDB requires client certificates by default, unless the --sslAllowConnectionsWithoutCertificates is provided. The C Driver can be configured to present a client certificate using a mongoc_ssl_opt_t:

const mongoc_ssl_opt_t *ssl_default = mongoc_ssl_opt_get_default ();
mongoc_ssl_opt_t ssl_opts = { 0 };

/* optionally copy in a custom trust directory or file; otherwise the default is used. */
memcpy (&ssl_opts, ssl_default, sizeof ssl_opts);
ssl_opts.pem_file = "client.pem"

mongoc_client_set_ssl_opts (client, &ssl_opts);

The client certificate provided by pem_file must be issued by one of the server trusted Certificate Authorities listed in --sslCAFile, or issued by a CA in the native certificate store on the server when omitted.

To verify the server certificate against a specific CA, provide a PEM armored file with a CA certificate, or concatenated list of CA certificates using the ca_file option, or c_rehash directory structure of CAs, pointed to using the ca_dir option. When no ca_file or ca_dir is provided, the driver will use CAs provided by the native platform certificate store.

See mongoc_ssl_opt_t for more information on the various SSL related options.

Compressing data to and from MongoDB¶

MongoDB 3.4 added Snappy compression support, zlib compression in 3.6, and zstd compression in 4.2. To enable compression support the client must be configured with which compressors to use:

mongoc_client_t *client = NULL;
client = mongoc_client_new ("mongodb://localhost:27017/?compressors=snappy,zlib,zstd");

The compressors option specifies the priority order of compressors the client wants to use. Messages are compressed if the client and server share any compressors in common.

Note that the compressor used by the server might not be the same compressor as the client used. For example, if the client uses the connection string compressors=zlib,snappy the client will use zlib compression to send data (if possible), but the server might still reply using snappy, depending on how the server was configured.

The driver must be built with zlib and/or snappy and/or zstd support to enable compression support, any unknown (or not compiled in) compressor value will be ignored. Note: to build with zstd requires cmake 3.12 or higher.

Additional Connection Options¶

The full list of connection options can be found in the mongoc_uri_t docs.

Certain socket/connection related options are not configurable:

Option	Description	Value
SO_KEEPALIVE	TCP Keep Alive	Enabled
TCP_KEEPIDLE	How long a connection needs to remain idle before TCP starts sending keepalive probes	300 seconds
TCP_KEEPINTVL	The time in seconds between TCP probes	10 seconds
TCP_KEEPCNT	How many probes to send, without acknowledgement, before dropping the connection	9 probes
TCP_NODELAY	Send packets as soon as possible or buffer small packets (Nagle algorithm)	Enabled (no buffering)