Brod - Apache Kafka Erlang client library

Brod is an erlang implementation of the Apache Kafka protocol, providing support for both producers and consumers.

Why "brod"? http://en.wikipedia.org/wiki/Max_Brod

Features

• Supports Apache Kafka v0.8+
• Robust producer implementation supporting in-flight requests and asynchronous acknowledgements
• Both consumer and producer handle leader re-election and other cluster disturbances internally
• Opens max 1 tcp connection to a broker per "brod_client", one can create more clients if needed
• Producer: will start to batch automatically when number of unacknowledged (in flight) requests exceeds configurable maximum
• Producer: will try to re-send buffered messages on common errors like "Not a leader for partition", errors are resolved automatically by refreshing metadata
• Simple consumer: The poller, has a configurable "prefetch count" - it will continue sending fetch requests as long as total number of unprocessed messages (not message-sets) is less than "prefetch count"
• Group subscriber: Support for consumer groups with options to have Kafka as offset storage or a custom one
• Topic subscriber: Subscribe on messages from all or selected topic partitions without using consumer groups
• Pick latest supported version when sending requests to kafka.

Missing features

• lz4 compression & decompression
• new 0.10.1.0 create/delete topic api

Building and testing

make
make test-env t # requires docker-composer in place

Working With Kafka 0.9.x or Earlier

Make sure {query_api_versions, false} exists in client config. This is because ApiVersionRequest was introduced in kafka 0.10, sending such request to older version brokers will cause connection failure.

e.g. in sys.config:

[{brod,
   [ { clients
     , [ { brod_client_1 %% registered name
         , [ { endpoints, [{"localhost", 9092}]}
           , { query_api_versions, false} %% <---------- here
           ]}]}]}]

Quick Demo

Assuming kafka is running at localhost:9092 and there is a topic named brod-test.

Below code snippet is copied from Erlang shell with some non-important printouts trimmed.

> rr(brod).
> {ok, _} = application:ensure_all_started(brod).
> KafkaBootstrapEndpoints = [{"localhost", 9092}].
> Topic = <<"brod-test">>.
> Partition = 0.
> ok = brod:start_client(KafkaBootstrapEndpoints, client1).
> ok = brod:start_producer(client1, Topic, _ProducerConfig = []).
> ok = brod:produce_sync(client1, Topic, Partition, <<"key1">>, <<"value1">>).
> ok = brod:produce_sync(client1, Topic, Partition, <<"key2">>, <<"value2">>).
> SubscriberCallbackFun =
    fun(Partition, Msg, ShellPid = CallbackState) ->
      ShellPid ! Msg,
      {ok, ack, CallbackState}
    end.
> Receive = fun() -> receive Msg -> Msg after 1000 -> timeout end end.
> brod_topic_subscriber:start_link(client1, Topic, Partitions=[Partition],
                                   _ConsumerConfig=[{begin_offset, earliest}],
                                   _CommittdOffsets=[], message, SubscriberCallbackFun,
                                   _CallbackState=self()).
> Receive().
#kafka_message{offset = 0,magic_byte = 0,attributes = 0,
               key = <<"key1">>,value = <<"value1">>,crc = 1978725405}
> Receive().
#kafka_message{offset = 1,magic_byte = 0,attributes = 0,
               key = <<"key2">>,value = <<"value2">>,crc = 1964753830}
> {ok, CallRef} = brod:produce(client1, Topic, Partition, <<"key3">>, <<"value3">>).
> #brod_produce_reply{ call_ref = CallRef,
                       result   = brod_produce_req_acked
                     } = Receive().
> Receive().
#kafka_message{offset = 2,magic_byte = 0,attributes = 0,
               key = <<"key3">>,value = <<"value3">>,crc = -1013830416}

Overview

Brod supervision (and process link) tree.

Clients

A brod_client in brod is a gen_server responsible for establishing and maintaining tcp sockets connecting to kafka brokers. It also manages per-topic-partition producer and consumer processes under two-level supervision trees.

Start clients by default

You may include client configs in sys.config have them started by default (by application controller)

Example of configuration (for sys.config):

[{brod,
   [ { clients
     , [ { brod_client_1 %% registered name
         , [ { endpoints, [{"localhost", 9092}]}
           , { reconnect_cool_down_seconds, 10} %% socket error recovery
           ]
         }
       ]
     }
     %% start another client for another kafka cluster
     %% or if you think it's necessary to start another set of tcp connections
   ]
}]

Start brod client on demand

You may also call brod:start_client/1,2,3 to start a client on demand, which will be added to brod supervision tree.

ClientConfig = [{reconnect_cool_down_seconds, 10}],
ok = brod:start_client([{"localhost", 9092}], brod_client_1, ClientConfig).

Producers

Auto start producer with default producer config

Put below configs to client config in sys.config or app env:

{auto_start_producers, true}
{default_producer_config, []}

Start a producer on demand

brod:start_producer(_Client         = brod_client_1,
                    _Topic          = <<"brod-test-topic-1">>,
                    _ProducerConfig = []).

Produce to a known topic-partition:

{ok, CallRef} =
  brod:produce(_Client    = brod_client_1,
               _Topic     = <<"brod-test-topic-1">>,
               _Partition = 0
               _Key       = <<"some-key">>
               _Value     = <<"some-value">>),

%% just to illustrate what message to expect
receive
  #brod_produce_reply{ call_ref = CallRef
                     , result   = brod_produce_req_acked
                     } ->
    ok
after 5000 ->
  erlang:exit(timeout)
end.

Synchronized produce request

Block calling process until Kafka confirmed the message:

{ok, CallRef} =
  brod:produce(_Client    = brod_client_1,
               _Topic     = <<"brod-test-topic-1">>,
               _Partition = 0
               _Key       = <<"some-key">>
               _Value     = <<"some-value">>),
brod:sync_produce_request(CallRef).

or the same in one call:

brod:produce_sync(_Client    = brod_client_1,
                  _Topic     = <<"brod-test-topic-1">>,
                  _Partition = 0
                  _Key       = <<"some-key">>
                  _Value     = <<"some-value">>).

Produce with random partitioner

Client = brod_client_1,
Topic  = <<"brod-test-topic-1">>,
PartitionFun = fun(_Topic, PartitionsCount, _Key, _Value) ->
                   {ok, crypto:rand_uniform(0, PartitionsCount)}
               end,
{ok, CallRef} = brod:produce(Client, Topic, PartitionFun, Key, Value).

Produce a batch of (maybe nested) Key-Value list

%% The top-level key is used for partitioning
%% and nested keys are discarded.
%% Nested messages are serialized into a message set to the same partition.
brod:produce(_Client    = brod_client_1,
             _Topic     = <<"brod-test-topic-1">>,
             _Partition = MyPartitionerFun
             _Key       = KeyUsedForPartitioning
             _Value     = [ {<<"k1", <<"v1">>}
                          , {<<"k2", <<"v2">>}
                          , { _KeyDiscarded = <<>>
                            , [ {<<"k3">>, <<"v3">>}
                              , {<<"k4">>, <<"v4">>}
                              ]}
                          ]).

Produce message create time.

brod:produce(Client, Topic, Partition, _Key = <<>>,
             _Value = [{Ts1, Key1, Value1}, {Ts2, Key2, Value2}]).

Handle acks from kafka

Unless brod:produce_sync was called, callers of brod:produce should expect a message of below pattern for each produce call. Add -include_lib("brod/include/brod.hrl"). to use the record.

#brod_produce_reply{ call_ref = CallRef %% returned from brod:produce
                   , result   = brod_produce_req_acked
                   }

NOTE: If required_acks is set to 0 in producer config, kafka will NOT ack the requests, and the reply message is sent back to caller immediately after the message has been sent to the socket process.

In case the brod:produce caller is a process like gen_server which receives ALL messages, the callers should keep the call references in its looping state and match the replies against them when received. Otherwise brod:sync_produce_request/1 can be used to block-wait for acks.

NOTE: The replies are only strictly ordered per-partition. i.e. if the caller is producing to two or more partitions, it may receive replies ordered differently than in which order brod:produce API was called.

Consumers

Kafka consumers work in poll mode. In brod, brod_consumer is the poller, which is constantly asking for more data from the kafka node which is a leader for the given partition.

By subscribing to brod_consumer a process should receive the polled message sets (not individual messages) into its mailbox.

In brod, we have so far implemented two different subscribers (brod_topic_subscriber and brod_group_subscriber), hopefully covered most of the common use cases.

For maximum flexibility, an applications may implement their own per-partition subscriber.

Below diagrams illustrate 3 examples of how subscriber processes may work with brod_consumer.

Partition subscriber

This gives the best flexibility as the per-partition subscribers work directly with per-partition pollers.

The messages are delivered to subscribers in message sets (batches), not individual messages, (however the subscribers are allowed to ack individual offsets).

Topic subscriber (brod_topic_subscriber)

A topic subscriber provides the easiest way to receive and process messages from ALL partitions of a given topic. See brod_demo_cg_collector and brod_demo_topic_subscriber for example.

Users may choose to implement the brod_topic_subscriber behaviour callbacks in a module, or simply provide an anonymous callback function to have the individual messages processed.

Group subscriber (brod_group_subscriber)

Similar to topic subscriber, the brod_group_subscriber behaviour callbacks are to be implemented to process individual messages. See brod_demo_group_subscriber_koc and brod_demo_group_subscriber_loc for example.

A group subscriber is started by giving a set of topics, some (maybe none, or maybe all) of the partitions in the topic set will be assigned to it, then the subscriber should subscribe to ALL the assigned partitions.

Users may also choose to implement the brod_group_member behaviour (callbacks for brod_group_coordinator) for a different group subscriber (e.g. spawn one subscriber per partition), see brucke for example.

Example of group consumer which commits offsets to Kafka

-module(my_subscriber).
-include_lib("brod/include/brod.hrl"). %% needed for the #kafka_message record definition

-export([start/1]).
-export([init/2, handle_message/4]). %% callback api

%% brod_group_subscriber behaviour callback
init(_GroupId, _Arg) -> {ok, []}.

%% brod_group_subscriber behaviour callback
handle_message(_Topic, Partition, Message, State) ->
  #kafka_message{ offset = Offset
                , key   = Key
                , value = Value
                } = Message,
  error_logger:info_msg("~p ~p: offset:~w key:~s value:~s\n",
                        [self(), Partition, Offset, Key, Value]),
  {ok, ack, State}.

%% @doc The brod client identified ClientId should have been started
%% either by configured in sys.config and started as a part of brod application
%% or started by brod:start_client/3
%% @end
-spec start(brod:client_id()) -> {ok, pid()}.
start(ClientId) ->
  Topic  = <<"brod-test-topic-1">>,
  %% commit offsets to kafka every 5 seconds
  GroupConfig = [{offset_commit_policy, commit_to_kafka_v2},
                 {offset_commit_interval_seconds, 5}
                ],
  GroupId = <<"my-unique-group-id-shared-by-all-members">>,
  ConsumerConfig = [{begin_offset, earliest}],
  brod:start_link_group_subscriber(ClientId, GroupId, [Topic],
                                   GroupConfig, ConsumerConfig,
                                   _CallbackModule  = ?MODULE,
                                   _CallbackInitArg = []).

Authentication support

brod supports SASL PLAIN authentication mechanism out of the box. To use it add {sasl, {plain, Username, Password}} to client config. Also, brod has authentication plugins support. Authentication callback module should implement brod_auth_backend behaviour. Auth function spec:

auth(Host :: string(), Sock :: gen_tcp:socket() | ssl:sslsocket(),
     Mod :: gen_tcp | ssl, ClientId :: binary(),
     Timeout :: pos_integer(), SaslOpts :: term()) ->
        ok | {error, Reason :: term()}

If authentication is successful - callback function should return an atom ok, otherwise - error tuple with reason description. For example, you can use brod_gssapi' plugin (https://github.com/ElMaxo/brod_gssapi) for SASL GSSAPI authentication. To use it - add it as dependency to your top level project that uses brod. Then add {sasl, {callback, brod_gssapi, {gssapi, Keytab, Principal}}}` to client config. Keytab should be the keytab file path, and Principal should be a byte-list or binary string.

See also: https://github.com/klarna/brod/wiki/SASL-gssapi-(kerberos)-authentication

Other API to play with/inspect kafka

These functions open a connetion to kafka cluster, send a request, await response and then close the connection.

Hosts = [{"localhost", 9092}].
Topic = <<"topic">>.
Partition = 0.
brod:get_metadata(Hosts).
brod:get_metadata(Hosts, [Topic]).
brod:get_offsets(Hosts, Topic, Partition).
brod:fetch(Hosts, Topic, Partition, 1).

brod-cli: A command line tool to interact with Kafka

This will build a self-contained binary with brod application

make rel
./_rel/brod/bin/brod --help

Disclaimer: This script is NOT designed for use cases where fault-tolerance is a hard requirement. As it may crash when e.g. kafka cluster is temporarily unreachable, or (for fetch command) when the parition leader migrates to another broker in the cluster.

Start an Erlang shell with brod started

./_rel/brod/bin/brod-i

brod-cli examples:

Fetch and print metadata

./scripts/brod meta -b localhost

Produce a Message

./scripts/brod send -b localhost -t test-topic -p 0 -k "key" -v "value"

Fetch a Message

./scripts/brod fetch -b localhost -t test-topic -p 0 --fmt 'io:format("offset=~p, ts=~p, key=~s, value=~s\n", [Offset, Ts, Key, Value])'

Bound variables to be used in --fmt expression:

• Offset: Message offset
• Key: Kafka key
• Value: Kafka Value
• CRC: Message CRC
• TsType: Timestamp type either create or append
• Ts: Timestamp, -1 as no value

Stream Messages to Kafka

Send README.md to kafka one line per kafka message

./scripts/brod pipe -b localhost:9092 -t test-topic -p 0 -s @./README.md

Resolve Offset

./scripts/brod offset -b localhost:9092 -t test-topic -p 0

List or Describe Groups

# List all groups
./scripts/brod groups -b localhost:9092

# Describe groups
./scripts/brod groups -b localhost:9092 --ids group-1,group-2

Display Committed Offsets

# all topics
./scripts/brod commits -b localhost:9092 --id the-group-id --describe

# a specific topic
./scripts/brod commits -b localhost:9092 --id the-group-id --describe --topic topic-name

Commit Offsets

NOTE: This feature is designed for force overwriting commits, not for regular use of offset commit.

# Commit 'latest' offsets of all partitions with 2 days retention
./scripts/brod commits -b localhost:9092 --id the-group-id --topic topic-name --offsets latest --retention 2d

# Commit offset=100 for partition 0 and 200 for partition 1
./scripts/brod commits -b localhost:9092 --id the-group-id --topic topic-name --offsets "0:100,1:200"

# Use --retention 0 to delete commits (may linger in kafka before cleaner does its job)
./scripts/brod commits -b localhost:9092 --id the-group-id --topic topic-name --offsets latest --retention 0

# Try join an active consumer group using 'range' protocol and steal one partition assignment then commit offset=10000
./scripts/brod commits -b localhost:9092 -i the-group-id -t topic-name -o "0:10000" --protocol range