typedef struct _mongoc_uri_t mongoc_uri_t;


mongoc_uri_t provides an abstraction on top of the MongoDB connection URI format. It provides standardized parsing as well as convenience methods for extracting useful information such as replica hosts or authorization information.

See Connection String URI Reference on the MongoDB website for more information.


mongodb[+srv]://                             <1>
   [username:password@]                      <2>
   host1                                     <3>
   [:port1]                                  <4>
   [,host2[:port2],...[,hostN[:portN]]]      <5>
   [/[database]                              <6>
   [?options]]                               <7>
  1. “mongodb” is the specifier of the MongoDB protocol. Use “mongodb+srv” with a single service name in place of “host1” to specify the initial list of servers with an SRV record.
  2. An optional username and password.
  3. The only required part of the uri. This specifies either a hostname, IPv4 address, IPv6 address enclosed in “[” and “]”, or UNIX domain socket.
  4. An optional port number. Defaults to :27017.
  5. Extra optional hosts and ports. You would specify multiple hosts, for example, for connections to replica sets.
  6. The name of the database to authenticate if the connection string includes authentication credentials. If /database is not specified and the connection string includes credentials, defaults to the ‘admin’ database.
  7. Connection specific options.


Option names are case-insensitive. Do not repeat the same option (e.g. “mongodb://localhost/db?opt=value1&OPT=value2”) since this may have unexpected results.

The MongoDB C Driver exposes constants for each supported connection option. These constants make it easier to discover connection options, but their string values can be used as well.

For example, the following calls are equal.

uri = mongoc_uri_new ("mongodb://localhost/?" MONGOC_URI_APPNAME "=applicationName");
uri = mongoc_uri_new ("mongodb://localhost/?appname=applicationName");
uri = mongoc_uri_new ("mongodb://localhost/?appName=applicationName");

Replica Set Example

To describe a connection to a replica set named ‘test’ with the following mongod hosts:

  • on port 27017
  • on port 2500

You would use a connection string that resembles the following.


SRV Example

If you have configured an SRV record with a name like “” whose records are a list of one or more MongoDB server hostnames, use a connection string like this:

uri = mongoc_uri_new ("mongodb+srv://");

The driver prefixes the service name with “_mongodb._tcp.”, then performs a DNS SRV query to resolve the service name to one or more hostnames. If this query succeeds, the driver performs a DNS TXT query on the service name (without the “_mongodb._tcp” prefix) for additional URI options configured as TXT records.

On Unix, the MongoDB C Driver relies on libresolv to look up SRV and TXT records. If libresolv is unavailable, then using a “mongodb+srv” URI will cause an error. If your libresolv lacks res_nsearch then the driver will fall back to res_search, which is not thread-safe.

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.

Connection Options

Constant Key Description
MONGOC_URI_RETRYREADS retryreads If “true” and the server is a MongoDB 3.6+ standalone, replica set, or sharded cluster, the driver safely retries a read that failed due to a network error or replica set failover.
MONGOC_URI_RETRYWRITES retrywrites If “true” and the server is a MongoDB 3.6+ replica set or sharded cluster, the driver safely retries a write that failed due to a network error or replica set failover. Only inserts, updates of single documents, or deletes of single documents are retried.
MONGOC_URI_APPNAME appname The client application name. This value is used by MongoDB when it logs connection information and profile information, such as slow queries.
MONGOC_URI_TLS tls {true|false}, indicating if TLS must be used. (See also mongoc_client_set_ssl_opts() and mongoc_client_pool_set_ssl_opts().)
MONGOC_URI_COMPRESSORS compressors Comma separated list of compressors, if any, to use to compress the wire protocol messages. Snappy, zlib, and zstd are optional build time dependencies, and enable the “snappy”, “zlib”, and “zstd” values respectively. Defaults to empty (no compressors).
MONGOC_URI_CONNECTTIMEOUTMS connecttimeoutms This setting applies to new server connections. It is also used as the socket timeout for server discovery and monitoring operations. The default is 10,000 ms (10 seconds).
MONGOC_URI_SOCKETTIMEOUTMS sockettimeoutms The time in milliseconds to attempt to send or receive on a socket before the attempt times out. The default is 300,000 (5 minutes).
MONGOC_URI_REPLICASET replicaset The name of the Replica Set that the driver should connect to.
MONGOC_URI_ZLIBCOMPRESSIONLEVEL zlibcompressionlevel When the MONGOC_URI_COMPRESSORS includes “zlib” this options configures the zlib compression level, when the zlib compressor is used to compress client data.

Setting any of the *timeoutMS options above to 0 will be interpreted as “use the default value”.

Authentication Options

Constant Key Description
MONGOC_URI_AUTHMECHANISM authmechanism Specifies the mechanism to use when authenticating as the provided user. See Authentication for supported values.
MONGOC_URI_AUTHMECHANISMPROPERTIES authmechanismproperties Certain authentication mechanisms have additional options that can be configured. These options should be provided as comma separated option_key:option_value pair and provided as authMechanismProperties.
MONGOC_URI_AUTHSOURCE authsource The authSource defines the database that should be used to authenticate to. It is unnecessary to provide this option the database name is the same as the database used in the URI.

Mechanism Properties

Constant Key Description
MONGOC_URI_CANONICALIZEHOSTNAME canonicalizehostname Use the canonical hostname of the service, rather than its configured alias, when authenticating with Cyrus-SASL Kerberos.
MONGOC_URI_GSSAPISERVICENAME gssapiservicename Use alternative service name. The default is mongodb.

TLS Options

Constant Key Description
MONGOC_URI_TLSCERTIFICATEKEYFILE tlscertificatekeyfile Path to PEM formatted Private Key, with its Public Certificate concatenated at the end.
MONGOC_URI_TLSCERTIFICATEKEYPASSWORD tlscertificatekeypassword The password, if any, to use to unlock encrypted Private Key.
MONGOC_URI_TLSCERTIFICATEAUTHORITYFILE tlscertificateauthorityfile One, or a bundle of, Certificate Authorities whom should be considered to be trusted.
MONGOC_URI_TLSALLOWINVALIDCERTIFICATES tlsallowinvalidcertificates Accept and ignore certificate verification errors (e.g. untrusted issuer, expired, etc etc)
MONGOC_URI_TLSALLOWINVALIDHOSTNAMES tlsallowinvalidhostnames Ignore hostname verification of the certificate (e.g. Man In The Middle, using valid certificate, but issued for another hostname)
MONGOC_URI_TLSINSECURE tlsinsecure {true|false}, indicating if insecure TLS options should be used. Currently this implies MONGOC_URI_TLSALLOWINVALIDCERTIFICATES and MONGOC_URI_TLSALLOWINVALIDHOSTNAMES.

See mongoc_ssl_opt_t for details about these options and about building libmongoc with SSL support.

Deprecated SSL Options

The following options have been deprecated and may be removed from future releases of libmongoc.

Constant Key Deprecated For Key

Server Discovery, Monitoring, and Selection Options

Clients in a mongoc_client_pool_t share a topology scanner that runs on a background thread. The thread wakes every heartbeatFrequencyMS (default 10 seconds) to scan all MongoDB servers in parallel. Whenever an application operation requires a server that is not known–for example, if there is no known primary and your application attempts an insert–the thread rescans all servers every half-second. In this situation the pooled client waits up to serverSelectionTimeoutMS (default 30 seconds) for the thread to find a server suitable for the operation, then returns an error with domain MONGOC_ERROR_SERVER_SELECTION.

Technically, the total time an operation may wait while a pooled client scans the topology is controlled both by serverSelectionTimeoutMS and connectTimeoutMS. The longest wait occurs if the last scan begins just at the end of the selection timeout, and a slow or down server requires the full connection timeout before the client gives up.

A non-pooled client is single-threaded. Every heartbeatFrequencyMS, it blocks the next application operation while it does a parallel scan. This scan takes as long as needed to check the slowest server: roughly connectTimeoutMS. Therefore the default heartbeatFrequencyMS for single-threaded clients is greater than for pooled clients: 60 seconds.

By default, single-threaded (non-pooled) clients scan only once when an operation requires a server that is not known. If you attempt an insert and there is no known primary, the client checks all servers once trying to find it, then succeeds or returns an error with domain MONGOC_ERROR_SERVER_SELECTION. But if you set serverSelectionTryOnce to “false”, the single-threaded client loops, checking all servers every half-second, until serverSelectionTimeoutMS.

The total time an operation may wait for a single-threaded client to scan the topology is determined by connectTimeoutMS in the try-once case, or serverSelectionTimeoutMS and connectTimeoutMS if serverSelectionTryOnce is set “false”.

Constant Key Description
MONGOC_URI_HEARTBEATFREQUENCYMS heartbeatfrequencyms The interval between server monitoring checks. Defaults to 10,000ms (10 seconds) in pooled (multi-threaded) mode, 60,000ms (60 seconds) in non-pooled mode (single-threaded).
MONGOC_URI_SERVERSELECTIONTIMEOUTMS serverselectiontimeoutms A timeout in milliseconds to block for server selection before throwing an exception. The default is 30,0000ms (30 seconds).
MONGOC_URI_SERVERSELECTIONTRYONCE serverselectiontryonce If “true”, the driver scans the topology exactly once after server selection fails, then either selects a server or returns an error. If it is false, then the driver repeatedly searches for a suitable server for up to serverSelectionTimeoutMS milliseconds (pausing a half second between attempts). The default for serverSelectionTryOnce is “false” for pooled clients, otherwise “true”. Pooled clients ignore serverSelectionTryOnce; they signal the thread to rescan the topology every half-second until serverSelectionTimeoutMS expires.
MONGOC_URI_SOCKETCHECKINTERVALMS socketcheckintervalms Only applies to single threaded clients. If a socket has not been used within this time, its connection is checked with a quick “isMaster” call before it is used again. Defaults to 5,000ms (5 seconds).

Setting any of the *TimeoutMS options above to 0 will be interpreted as “use the default value”.

Connection Pool Options

These options govern the behavior of a mongoc_client_pool_t. They are ignored by a non-pooled mongoc_client_t.

Constant Key Description
MONGOC_URI_MAXPOOLSIZE maxpoolsize The maximum number of clients created by a mongoc_client_pool_t total (both in the pool and checked out). The default value is 100. Once it is reached, mongoc_client_pool_pop() blocks until another thread pushes a client.
MONGOC_URI_MINPOOLSIZE minpoolsize Deprecated. This option’s behavior does not match its name, and its actual behavior will likely hurt performance.
MONGOC_URI_MAXIDLETIMEMS maxidletimems Not implemented.
MONGOC_URI_WAITQUEUEMULTIPLE waitqueuemultiple Not implemented.
MONGOC_URI_WAITQUEUETIMEOUTMS waitqueuetimeoutms Not implemented.

Write Concern Options

Constant Key Description

Determines the write concern (guarantee). Valid values:

  • 0 = The driver will not acknowledge write operations but will pass or handle any network and socket errors that it receives to the client. If you disable write concern but enable the getLastError command’s w option, w overrides the w option.
  • 1 = Provides basic acknowledgement of write operations. By specifying 1, you require that a standalone mongod instance, or the primary for replica sets, acknowledge all write operations. For drivers released after the default write concern change, this is the default write concern setting.
  • majority = For replica sets, if you specify the special majority value to w option, write operations will only return successfully after a majority of the configured replica set members have acknowledged the write operation.
  • n = For replica sets, if you specify a number n greater than 1, operations with this write concern return only after n members of the set have acknowledged the write. If you set n to a number that is greater than the number of available set members or members that hold data, MongoDB will wait, potentially indefinitely, for these members to become available.
  • tags = For replica sets, you can specify a tag set to require that all members of the set that have these tags configured return confirmation of the write operation.
MONGOC_URI_WTIMEOUTMS wtimeoutms The time in milliseconds to wait for replication to succeed, as specified in the w option, before timing out. When wtimeoutMS is 0, write operations will never time out.

Controls whether write operations will wait until the mongod acknowledges the write operations and commits the data to the on disk journal.

  • true = Enables journal commit acknowledgement write concern. Equivalent to specifying the getLastError command with the j option enabled.
  • false = Does not require that mongod commit write operations to the journal before acknowledging the write operation. This is the default option for the journal parameter.

Read Concern Options

Constant Key Description
MONGOC_URI_READCONCERNLEVEL readconcernlevel The level of isolation for read operations. If the level is left unspecified, the server default will be used. See readConcern in the MongoDB Manual for details.

Read Preference Options

When connected to a replica set, the driver chooses which member to query using the read preference:

  1. Choose members whose type matches “readPreference”.
  2. From these, if there are any tags sets configured, choose members matching the first tag set. If there are none, fall back to the next tag set and so on, until some members are chosen or the tag sets are exhausted.
  3. From the chosen servers, distribute queries randomly among the server with the fastest round-trip times. These include the server with the fastest time and any whose round-trip time is no more than “localThresholdMS” slower.
Constant Key Description

Specifies the replica set read preference for this connection. This setting overrides any slaveOk value. The read preference values are the following:

  • primary (default)
  • primaryPreferred
  • secondary
  • secondaryPreferred
  • nearest
MONGOC_URI_READPREFERENCETAGS readpreferencetags A representation of a tag set. See also Tag Sets.
MONGOC_URI_LOCALTHRESHOLDMS localthresholdms How far to distribute queries, beyond the server with the fastest round-trip time. By default, only servers within 15ms of the fastest round-trip time receive queries.
MONGOC_URI_MAXSTALENESSSECONDS maxstalenessseconds The maximum replication lag, in wall clock time, that a secondary can suffer and still be eligible. The smallest allowed value for maxStalenessSeconds is 90 seconds.


When connecting to more than one mongos, libmongoc’s localThresholdMS applies only to the selection of mongos servers. The threshold for selecting among replica set members in shards is controlled by the mongos’s localThreshold command line option.

Legacy Options

For historical reasons, the following options are available. They should however not be used.

Constant Key Description
MONGOC_URI_SAFE safe {true|false} Same as w={1|0}
MONGOC_URI_SLAVEOK slaveok When set, same as readPreference=secondaryPreferred