Apache Kafka client library for Erlang/Elixir
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README.md

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Brod - Apache Kafka Client for Erlang/Elixir

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.
  • Direct APIs for message send/fetch and cluster inspection/management without having to start clients/producers/consumers.
  • A escriptized command-line tool for message send/fetch and cluster inspection/management.

Building and testing

make compile
make test-env t # requires docker-compose 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 test-topic.

Start Erlang shell by make compile; erl -pa _build/default/lib/*/ebin, then paste lines below into shell:

rr(brod),
{ok, _} = application:ensure_all_started(brod),
KafkaBootstrapEndpoints = [{"localhost", 9092}],
Topic = <<"test-topic">>,
Partition = 0,
ok = brod:start_client(KafkaBootstrapEndpoints, client1),
ok = brod:start_producer(client1, Topic, _ProducerConfig = []),
{ok, FirstOffset} = brod:produce_sync_offset(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, FirstOffset}],
                                 _CommittdOffsets=[], message, SubscriberCallbackFun,
                                 _CallbackState=self()),
AckCb = fun(Partition, BaseOffset) -> io:format(user, "\nProduced to partition ~p at base-offset ~p\n", [Partition, BaseOffset]) end,
ok = brod:produce_cb(client1, Topic, Partition, <<>>, [{<<"key3">>, <<"value3">>}], AckCb).
Receive().
Receive().
{ok, {_, [Msg]}} = brod:fetch(KafkaBootstrapEndpoints, Topic, Partition, FirstOffset + 2), Msg.

Example outputs:

#kafka_message{offset = 0,key = <<"key1">>,
               value = <<"value1">>,ts_type = create,ts = 1531995555085,
               headers = []}
#kafka_message{offset = 1,key = <<"key2">>,
               value = <<"value2">>,ts_type = create,ts = 1531995555107,
               headers = []}
Produced to partition 0 at base-offset 406
#kafka_message{offset = 2,key = <<"key3">>,
               value = <<"value3">>,ts_type = create,ts = 1531995555129,
               headers = []}

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).

Extra socket options could be passed as {extra_sock_opts, ExtraSockOpts}, e.g.

ExtraSockOpts = [{sndbuf, 1024*1024}],
ok = brod:start_client([{"localhost", 9092}], brod_client_1, [{extra_sock_opts, ExtraSockOpts}]).

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 = []).

Supported Message Input Format

Brod supports below produce APIs:

  • brod:produce: Async produce with ack message sent back to caller.
  • brod:produce_cb: Async produce with a callback evaluated when ack is received.
  • brod:produce_sync: Sync produce returns ok.
  • brod:produce_sync_offset: Sync produce returns {ok, BaseOffset}.

The Value arg in these APIs can be:

  • binary(): One single message
  • {brod:msg_ts(), binary()}: One single message with its create-time timestamp
  • #{ts => brod:msg_ts(), value => binary(), headers => [{_, _}]}: One single message. If this map does not have a key field, the Key argument is used.
  • [{K, V} | {T, K, V}]: A batch, where V could be a nested list of such representation.
  • [#{key => K, value => V, ts => T, headers => [{_, _}]}]: A batch.

When Value is a batch, the Key argument is only used as partitioner input. All messages are unified into a batch format of below spec: [#{key => K, value => V, ts => T, headers => [{_, _}]}]. ts field is dropped for kafka prior to version 0.10 (produce API version 0, magic version 0) headers field is dropped for kafka prior to version 0.11 (produce API version 0-2, magic version 0-1)

Synchronized Produce API

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

Or 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).

Produce One Message and Receive Its Offset in Kafka

Client = brod_client_1,
Topic  = <<"brod-test-topic-1">>,
{ok, Offset} = brod:produce_sync_offset(Client, Topic, 0, <<>>, <<"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 = brod:produce_sync(Client, Topic, PartitionFun, Key, Value).

Produce a Batch

brod:produce(_Client    = brod_client_1,
             _Topic     = <<"brod-test-topic-1">>,
             _Partition = MyPartitionerFun
             _Key       = KeyUsedForPartitioning
             _Value     = [ #{key => "k1" value => "v1", headers = [{"foo", "bar"}]}
                          , #{key => "k2" value => "v2"}
                          ]).

Handle Acks from Kafka as Messages

For async produce APIs brod:produce/3 and brod:produce/5, the caller should expect a message of below pattern for each produce call.

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

Add -include_lib("brod/include/brod.hrl"). to use the record.

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: If required_acks is set to none 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.

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.

Handle Acks from Kafka in Callback Function

Async APIs brod:produce_cb/4 and brod:produce_cb/6 allow callers to provided a callback function to handle acknowledgements from kafka. In this case, the caller may want to monitor the producer process because then they know that the callbacks will not be evaluated if the producer is 'DOWN', and there is perhaps a need for retry.

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, SCRAM-SHA-256 and SCRAM-SHA-512 authentication mechanisms out of the box. To use it, add {sasl, {Mechanism, Username, Password}} or {sasl, {Mechanism, File}} to client config. Where Mechanism is plain | scram_sha_256 | scram_sha_512, and File is the path to a text file which contains two lines, first line for username and second line for password

Also, brod has authentication plugins support with {sasl, {callback, Module, Opts}} in client config. 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:resolve_offset(Hosts, Topic, Partition).

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

This will build a self-contained binary with brod application

make brod-cli
_build/brod_cli/rel/brod/bin/brod -h

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.

brod-cli examples (with alias brod=_build/brod_cli/rel/brod/bin/brod):

Fetch and print metadata

brod meta -b localhost

Produce a Message

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

Fetch a Message

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
  • 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

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

Resolve Offset

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

List or Describe Groups

# List all groups
brod groups -b localhost:9092

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

Display Committed Offsets

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

# a specific topic
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
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
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)
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
brod commits -b localhost:9092 -i the-group-id -t topic-name -o "0:10000" --protocol range

TODOs

  • HTML tagged EDoc
  • Support scram-sasl in brod-cli
  • lz4 compression & decompression
  • Create/delete topic APIs
  • Transactional produce APIs