vonnegut

Vonnegut is a append-only log that follows the file format and API of Kafka 1.0. The server can be run standalone, with 1 or more chains each with 1 or more replicas, or as part of another Erlang release which can talk to it directly.

Each chain is responsible for a range of the topic space. A read or write to a topic requires finding what chain the topic belongs to and then making a request to the head, in the case of a write, or the tail, in the case of a read.

Configuration

Server

A node in a chain can discover other nodes within the chain through DNS SRV record queries. The replicas configuration tells vonnegut node how many other nodes it needs to connect to to form the required chain length to ack writes.

{vonnegut, [{chain, [{name, "chain-1"},
                     {discovery, {srv, "chain-1.service.cluster.local"}},
                     {replicas, "2"},
                     {port, 5555}]}
            ]}

Client

Clients start a pool of connections to the head and tail of each chain. Chains are found through DNS queries against endpoints:

{vonnegut, [{client, [{endpoints, [{"chain-1.service.cluster.local", 5555}]}]}]}

Erlang Interface

A local interface can be used to create, read and write topics.

$ rebar3 shell
1> vg:create_topic(<<"test_topic">>).
2> vg:write(<<"test_topic">>, [<<"some log message">>, <<"more log message">>]).
3> vg:fetch(<<"test_topic">>).
{ok,#{high_water_mark => 1,partition => 0,
      record_batches =>
          [#{headers => [],key => <<>>,offset => 1,sequence_number => 1,
             timestamp => 1517613646458,value => <<"more log message">>},
           #{headers => [],key => <<>>,offset => 0,sequence_number => 0,
             timestamp => 1517613646458,
             value => <<"some log message">>}]}}

By default index and log files will be written to ./data:

$ ls data/test_topic-0/
00000000000000000000.index  00000000000000000000.log

Kafkaesque Client

$ rebar3 shell
1> vg_client_pool:start().
ok
2> vg_client:produce(<<"my-topic-2">>, [<<"message 1">>, <<"message 2">>]).
{ok,1}
3> vg_client:fetch(<<"my-topic-2">>).
{ok,#{<<"test_topic-2">> =>
          #{0 =>
                #{error_code => 0,high_water_mark => 1,
                  record_batches =>
                      [#{headers => [],key => <<>>,offset => 1,
                         sequence_number => 1,timestamp => 1517616861441,
                         value => <<"message 2">>},
                       #{headers => [],key => <<>>,offset => 0,
                         sequence_number => 0,timestamp => 1517616861441,
                         value => <<"message 1">>}],
                  record_batches_size => 95}}}}

Running Tests

The tests require opening thousands of files and so may require increasing the limit per process on your system with:

$ ulimit -n 63536

Tests also require a nodename:

$ rebar3 ct