NOTICE

This lib is a fork of the upstream repo disclosed by EMQ Technologies Co., Ltd. GitHub/emqx

Wolff

Kafka's publisher. See why the name

How is it different from brod

More resilient to network and Kafka disturbances

With replayq_dir producer config set to a directory, wolff will queue pending messages on disk so it can survive from message loss in case of application, network or kafka disturbances.

In case of producer restart, messages queued on disk are replayed towards kafka, however, async callback functions are not evaluated upon acknowledgements received from kafka for replayed messages.

More flexible connection management

wolff provides per_partition and per_broker connection management strategy. In case of per_partition strategy, wolff establishes one TCP connection per-partition leader. brod however, only establishes connections per-broker, that is, if two partition leaders happen to reside on the same broker, they will have to share the same TCP connection.

There is still a lack of benchmarking to tell the difference of how performant per_partition is though.

Auto partition count refresh

wolff periodically refreshes topic metata to discover partition increase and automatically rebalance the partitioner.

Example Code

Sync Produce

application:ensure_all_started(wolff).
ClientCfg = #{}.
{ok, Client} = wolff:ensure_supervised_client(<<"client-1">>, [{"localhost", 9092}], ClientCfg).
ProducerCfg = #{replayq_dir => "/tmp/wolff-replayq-1"}.
{ok, Producers} = wolff:start_producers(Client, <<"test-topic">>, ProducerCfg).
Msg = #{key => <<"key">>, value => <<"value">>}.
{Partition, BaseOffset} = wolff:send_sync(Producers, [Msg], 3000).
io:format(user, "\nmessage produced to partition ~p at offset ~p\n",
          [Partition, BaseOffset]).
ok = wolff:stop_producers(Producers).
ok = wolff:stop_client(Client).

Async Produce with Callback

application:ensure_all_started(wolff).
ClientCfg = #{}.
{ok, Client} = wolff:ensure_supervised_client(<<"client-2">>, [{"localhost", 9092}], ClientCfg).
ProducerCfg = #{replayq_dir => "/tmp/wolff-replayq-2"}.
{ok, Producers} = wolff:start_producers(Client, <<"test-topic">>, ProducerCfg).
Msg = #{headers => [{<<"foo">>, <<"bar">>}], key => <<"key">>, value => <<"value">>}.
Self = self().
AckFun = fun(Partition, BaseOffset) ->
            io:format(user, "\nmessage produced to partition ~p at offset ~p\n",
                      [Partition, BaseOffset]),
            ok
         end.
wolff:send(Producers, [Msg], AckFun).

For upgrade safety, it's recommended to avoid using anonymous function as ack callback. In production code, the caller should provide a {fun module:handle_ack/3, [ExtraArg]} for AckFun. The handle_ack function should expect first two args as Partition and BaseOffset, and the third arg ExtraArg is served back to the caller. For example.

-export([handle_ack/3]).

send(...) ->
  wolff:send(Producers, Messages, {fun ?MODULE/handle_ack, [self()]}).

handle_ack(Partition, Offset, Caller) ->
  Caller ! {kafka_acked, Partition, Offset},
  ok. % must return ok

Supervised Producers

application:ensure_all_started(wolff).
Client = <<"client-1">>.
ClientCfg = #{}.
{ok, _ClientPid} = wolff:ensure_supervised_client(Client, [{"localhost", 9092}], ClientCfg).
ProducerCfg = #{replayq_dir => "/tmp/wolff-replayq-3"}.
{ok, Producers} = wolff:ensure_supervised_producers(Client, <<"test-topic">>, ProducerCfg).
Msg = #{headers => [{<<"foo">>, <<"bar">>}], key => <<"key">>, value => <<"value">>}.
Self = self().
AckFun = fun(Partition, BaseOffset) ->
            io:format(user, "\nmessage produced to partition ~p at offset ~p\n",
                      [Partition, BaseOffset]),
            ok
         end.
wolff:send(Producers, [Msg], AckFun).

Client Config

• reg_name register the client process to a name. e.g. {local, client_1}

• connection_strategy: default per_partition, can also be per_broker. This is to configure how client manages connections: one connection per-partition or one connection per-broker. per_partition may give better throughput, but it could be quite exhausting for both beam and kafka cluster when there is a great number of partitions

• min_metadata_refresh_interval: default 1000 (milliseconds). This is to avoid excessive metadata refresh and partition leader reconnect when a lot of connections restart around the same moment. Also, when kafka partition leader broker is down, it usually takes a few seconds to get a new leader elacted, hence it is a good idea to have a delay before trying to reconnect.

• Connection level configs are merged into wolff client config, including: iconnect_timeout, client_id, extra_sock_opts, query_api_versions, request_timeout, sasl and ssl. Ref: kpro_connection.erl

Producer Config

• replayq_dir: Base directory for replayq to store messages on disk. If this config entry if missing or set to undefined, replayq works in a mem-only manner. i.e. messages are not queued on disk -- in such case, the send or send_sync API callers are responsible for possible message loss in case of application, network or kafka disturbances. For instance, in the wolff:send API caller may trap_exit then react on parition-producer worker pid's 'EXIT' message to issue a retry after restarting the producer.

• replayq_seg_bytes: default=10MB, replayq segment size.

• required_acks: all_isr, leader_only or none, see kafka_protocol lib doc.

• ack_timeout: default=10000. Timeout leader wait for replicas before reply to producer.

• max_batch_bytes: Most number of bytes to collect into a produce request. NOTE: This is only a rough estimation of the batch size towards kafka, NOT the exact max allowed message size configured in kafka.

• max_linger_ms: Age in milliseconds a batch can stay in queue when the connection is idle (as in no pending acks from kafka). Default=0 (as in send immediately).

• max_send_ahead: Number of batches to be sent ahead without receiving ack for the last request. Must be 0 if messages must be delivered in strict order.

• compression: no_compression (default) snappy or gzip.

• partitioner: default random. other possible values are: roundrobin: Starts from partition 0, next partition to use is saved in caller's process dictionary. first_key_dispatch: erlang:phash2(Key) rem PartitionCount where Key is the key field of the first message in the batch to produce. fun((PartitionCount, [msg()]) -> partition()): Caller defined callback. partition(): Caller specified exact partition.

• name: defaul=wolff_producers Atom used to register producer manager process when starting producers under wolff's supervision tree. It is also used as the ets table name for producer worker lookup.

• partition_count_refresh_interval_seconds: default=300 (5 minutes) Non-negative integer to refresh topic metadata in order to auto-discover newly added partitions. Set 0 to disable auto-discover.

How to Test

Start Kafka in docker containers from dokcer-compose.yml in this repo.

docker-compose up -d

License

Apache License Version 2.0