Skip to content


Switch branches/tags

Name already in use

A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch?

Latest commit


Git stats


Failed to load latest commit information.
Latest commit message
Commit time

This is the MongoDB driver for Erlang.

Run tests Enot


Add this repo as the dependency: Rebar

{deps, [
  {mongodb, ".*",
   {git, "git://", {tag, "<Latest tag>"}}}

DEPS = mongodb
dep_mongodb = git <Latest tag>

Where Latest tag is the latest tag from github.


If you want to use it from command line - download and compile the application:

$ git clone git:// mongodb
$ cd mongodb
$ make

You will need erlang 18+ and make installed.

Starting and using the api

Start all applications, needed by mongodb

> application:ensure_all_started (mongodb).

Important: mongoc API was changed in 3.0.0. mc_cursor API was changed in 3.0.0.

This driver has two api modules - mc_worker_api and mongo_api. mc_worker_api works directly with one connection, while all mongo_api interfaces refer to mongoc pool. Although mongoc is not stable for now you should use it if you have shard and need to determine mongo topology. If you are choosing between using mongos and using mongo shard with mongo_api - prefer mongos and use mc_worker_api.

mc_worker_api -- direct connection client


To connect to a database test on mongodb server listening on localhost:27017 (or any address & port of your choosing) use mc_worker_api:connect/1.

> Database = <<"test">>.
> {ok, Connection} = mc_worker_api:connect ([{database, Database}]).

mc_worker_api:connect returns {error, Reason} if it failed to connect. See arguments you can pass in mc_worker_api.erl type spec:

-type arg() :: {database, database()}
| {login, binary()}
| {password, binary()}
| {w_mode, write_mode()}
| {r_mode, read_mode()}
| {host, list()}
| {port, integer()}
| {register, atom() | fun()}
| {next_req_fun, fun()}.

To connect mc_worker in your supervised pool, use mc_worker:start_link/1 instead and pass all args to it.

safe, along with {safe, GetLastErrorParams} and unsafe, are write-modes. Safe mode makes a getLastError request after every write in the sequence. If the reply says it failed then the rest of the sequence is aborted and returns {failure, {write_failure, Reason}}, or {failure, not_master} when connected to a slave. An example write failure is attempting to insert a duplicate key that is indexed to be unique. Alternatively, unsafe mode issues every write without a confirmation, so if a write fails you won't know about it and remaining operations will be executed. This is unsafe but faster because you there is no round-trip delay.

master, along with slave_ok, are read-modes. master means every query in the sequence must read fresh data (from a master/primary server). If the connected server is not a master then the first read will fail, the remaining operations will be aborted, and mongo:do will return {failure, not_master}. slave_ok means every query is allowed to read stale data from a slave/secondary (fresh data from a master is fine too).

Read-modes only apply to the deprecated mc_worker_api query commands. Pass a readopts map, like #{<<"mode">> => <<"primary">>} to an mc_worker_api function like find_one(Conn, Coll, Selector, [{readopts, #{<<"mode">> => <<"primary">>}}]) for the new API.

If you set {register, Name} option - mc_worker process will be registered on this Name, or you can pass function fun(pid()), which it runs with self pid. If you set {login, Login} and {password, Password} options - mc_worker will try to authenticate to the database.

next_req_fun is a function caller every time, when worker sends request to database. It can be use to optimise pool usage. When you use poolboy transaction (or mongoc transaction, which use poolboy transaction) - mc_worker sends request to database and do nothing, waiting for reply. You can use {next_req_fun, fun() -> poolboy:checkin(?DBPOOL, self()) end} to make workers return to pool as soon as they finish request. When response from database comes back - it will be saved in mc_worker msgbox. Msgbox will be processed just before the next call to mc_worker. Notice, that poolboy's pool should be created with {strategy, fifo} to make uniform usage of pool workers.


After you connected to your database - you can carry out write operations, such as insert, update and delete:

> Collection = <<"test">>.
> mc_worker_api:insert(Connection, Collection, [
                                                   #{<<"name">> => <<"Yankees">>,
                                                     <<"home">> => #{<<"city">> => <<"New York">>, <<"state">> => <<"NY">>},
                                                     <<"league">> => <<"American">>},
                                                   #{<<"name">> => <<"Mets">>,
                                                     <<"home">> => #{<<"city">> => <<"New York">>, <<"state">> => <<"NY">>},
                                                     <<"league">> => <<"National">>},
                                                   #{<<"name">> => <<"Phillies">>,
                                                     <<"home">> => #{<<"city">> => <<"Philadelphia">>, <<"state">> => <<"PA">>},
                                                     <<"league">> => <<"National">>},
                                                   #{<<"name">> => <<"Red Sox">>,
                                                     <<"home">>=> #{<<"city">> => <<"Boston">>, <<"state">> => <<"MA">>},
                                                     <<"league">> => <<"American">>}

An insert example (from mongo_SUITE test module). Connection is your Connection, got from mc_worker_api:connect, Collection is your collection name, Doc is something, you want to save. Doc will be returned, if insert succeeded. If Doc doesn't contains _id field - an updated Doc will be returned - with automatically generated '_id' fields. If error occurred - Connection will fall.

> mc_worker_api:delete(Connection, Collection, Selector).

Delete example. Connection is your Connection, Collection - is a collection you want to clean. Selector is the rules for cleaning. If you want to clean everything - pass empty {}. You can also use maps instead bson documents:

> Collection = <<"test">>.
> mc_worker_api:insert(Connection, Collection, #{<<"name">> => <<"Yankees">>, <<"home">> =>
  #{<<"city">> => <<"New York">>, <<"state">> => <<"NY">>}, <<"league">> => <<"American">>}),


To call read operations use find, find_one:

> {ok, Cursor} = mc_worker_api:find(Connection, Collection, Selector)

All params similar to delete. The difference between find and find_one is in return. Find_one just returns your result, while find returns you a Cursor - special process' pid. You can query data through the process with the help of `mc_cursor' module.

> Result = mc_cursor:next(Cursor),
> mc_cursor:close(Cursor),

Important! Do not forget to close cursors after using them! mc_cursor:rest closes the cursor automatically.

To search for params - specify Selector:

mc_worker_api:find_one(Connection, Collection, #{<<"key">> => <<"123">>}).

will return one document from collection Collection with key == <<"123">>.

mc_worker_api:find_one(Connection, Collection, #{<<"key">> => <<"123">>, <<"value">> => <<"built_in">>}).

will return one document from collection Collection with key == <<"123">> and value == <<"built_in">>. Tuples {<<"key">>, <<"123">>} in first example and {<<"key">>, <<"123">>, <<"value">>, <<"built_in">>} are selectors.

For filtering result - use Projector:

mc_worker_api:find_one(Connection, Collection, {}, #{projector => #{<<"value">> => true}).

will return one document from collection Collection with fetching only _id and value.

mc_worker_api:find_one(Connection, Collection, {}, #{projector => #{<<"key">> => false, <<"value">> => false}}).

will return your data without key and value params. If there is no other data - only _id will be returned.


To add or update field in document - use mc_worker_api:update function with $set param. This updates selected fields:

Command = #{<<"$set">> => #{
    <<"quantity">> => 500,
    <<"details">> => #{<<"model">> => "14Q3", <<"make">> => "xyz"},
    <<"tags">> => ["coats", "outerwear", "clothing"]
mc_worker_api:update(Connection, Collection, #{<<"_id">> => 100}, Command),

This will add new field expired, if there is no such field, and set it to true.

Command = #{<<"$set">> => #{<<"expired">> => true}},
mc_worker_api:update(Connection, Collection, #{<<"_id">> => 100}, Command),

This will update fields in nested documents.

Command = #{<<"$set">> => #{<<"details.make">> => "zzz"}},
mc_worker_api:update(Connection, Collection, #{<<"_id">> => 100}, Command),

This will update elements in array.

Command = #{<<"$set">> => #{
    <<"tags.1">> => "rain gear",
    <<"ratings.0.rating">> => 2
mc_worker_api:update(Connection, Collection, #{'_id' => 100}, Command),

For result of executing this functions - see mongo_SUITE update test.

Creating indexes

To create indexes - use mc_worker_api:ensure_index/3 command:

mc_worker_api:ensure_index(Connection, Collection, #{<<"key">> => #{<<"index">> => 1}}).  %simple
mc_worker_api:ensure_index(Connection, Collection, #{<<"key">> => #{<<"index">> => 1}, <<"name">> => <<"MyI">>}).  %advanced
mc_worker_api:ensure_index(Connection, Collection, #{<<"key">> => #{<<"index">> => 1}, <<"name">> => <<"MyI">>, <<"unique">> => true, <<"dropDups">> => true}).  %full

ensure_index takes mc_worker' pid or atom name as first parameter, collection, where to create index, as second parameter and bson document with index specification - as third parameter. In index specification one can set all or only some parameters. If index specification is not full - it is automatically filled with values: name, Name, unique, false, dropDups, false, where Name is index's key.


This driver does not provide helper functions for commands. Use mc_worker_api:command directly and refer to the MongoDB documentation for how to issue raw commands.


To authenticate use function mc_worker_api:connect, or mc_worker:start_link([...{login, <<"login">>}, {password, <<"password">>}...]


By default timeout for all connections to connection gen_server is infinity. If you found problems with it - you can modify timeout. To modify it just add mc_worker_call_timeout with new value to your applications's env config.

Timeout for operations with cursors may be explicitly passed to mc_cursor:next/2, mc_cursor:take/3, mc_cursor:rest/2, and mc_cursor:foldl/5 functions, by default used value of cursor_timeout from application config, or infinity if cursor_timeout not specified.


If you need simple pool - use modified Poolboy, which is included in this app's deps. As a worker module use mc_worker_api. If you need pool to mongo shard with determining topology - use mongo_api for automatic topology discovery and monitoring. It uses poolboy inside.

mongoc -- client with automatic MongoDB topology discovery and monitoring

You can use mongo_api.erl for easy working with mongoc.


For opening a connection to a MongoDB server you can call mongoc:connect/3:

{ok, Topology} = mongoc:connect(Seed, Options, WorkerOptions)

Where Seed contains information about host names and ports to connect and info about topology of MongoDB deployment.

So you can pass just a hostname with port (or tuple with single key) for connection to a single server deployment:

SingleSeed = "hostname:27017",
SingleSeedTuple = { single, "hostname:27017" }

If you want to connect to a replica set ReplicaSetName use this format of the Seed value:

ReplicaSeed = { rs, <<"ReplicaSetName">>, [ "hostname1:port1", "hostname2:port2"] }

To connect to a sharded cluster of mongos:

ShardedSeed = { sharded,  ["hostname1:port1", "hostname2:port2"] }

And if you want your MongoDB deployment metadata to be auto-discovered use the unknown type in the Seed tuple:

AutoDiscoveredSeed = { unknown,  ["hostname1:port1", "hostname2:port2"] }

mongoc topology Options

    { name,  Name },    % Name should be used for mongoc pool to be registered with
    { register,  Name },    % Name should be used for mongoc topology process to be registered with

    { pool_size, 5 }, % pool size on start
    { max_overflow, 10 },	% number of overflow workers be created, when all workers from pool are busy
    { overflow_ttl, 1000 }, % number of milliseconds for overflow workers to stay in pool before terminating
    { overflow_check_period, 1000 }, % overflow_ttl check period for workers (in milliseconds)

    { localThresholdMS, 1000 }, % secondaries only which RTTs fit in window from lower RTT to lower RTT + localThresholdMS could be selected for handling user's requests

    { connectTimeoutMS, 20000 },
    { socketTimeoutMS, 100 },

    { serverSelectionTimeoutMS, 30000 }, % max time appropriate server should be select by
    { waitQueueTimeoutMS, 1000 }, % max time for waiting worker to be available in the pool

    { heartbeatFrequencyMS, 10000 },    %  delay between Topology rescans
    { minHeartbeatFrequencyMS, 1000 },

    { rp_mode, primary }, % default ReadPreference mode - primary, secondary, primaryPreferred, secondaryPreferred, nearest

    { rp_tags, [{tag,1}] }, % tags that servers shoul be tagged by for becoming candidates for server selection  (may be an empty list)

mongoc WorkerOptions (as described in mongo Connecting chapter)

-type arg() :: {database, database()}
| {login, binary()}
| {password, binary()}
| {w_mode, write_mode()}.

More Documentation

API Docs - Documentation generated from source code comments