THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
Introduction
This document covers the protocol implemented in Kafka 0.8. It is meant to give a readable guide to the protocol that covers the available requests, their binary format, and the proper way to make use of them in a client. It is aimed at making it easier to implement a client. This document assumes you understand the basic design and terminology described here.
Preliminaries
Kafka uses a binary protocol over TCP. This protocol defines all apis as request response message pairs. All messages are size delimited and are made up of the following primitive:
Fixed Width Primitives
int8, int16, int32, int64, uint8, uint16, uint32, uint64 - Integers with the given precision (in bits) stored in big endian order. unitXX variants are unsigned and have double the range.
Variable Length Primitives
bytes16, bytes32, string16 - These types consist of a signed integer giving a length N followed by N bytes of content. -1 indicates null. bytes16 and string16 use a two byte int16 size and bytes32 uses a four byte int32 size. string16 is identical in format to bytes16 but the bytes should be interpreted as UTF8 encoded characters.
Arrays
It is often useful to repeat some structure multiple times. This will always be encoded as a unit size containing the length N followed by N repetitions of the structure. In the BNF grammars below we will show an array of a structure foo as [foo].
Client Requests
The Kafka protocol is extremely simple. There are only four client requests APIs.
- Metadata - Describes the currently available brokers, their host and port information, and gives information about which broker hosts which partitions.
- Send - Send messages to a broker
- Fetch - Fetch messages from a broker, one which fetches data, one which gets cluster metadata, and one which gets offset information about a topic.
- Offsets - Get information about the available offsets for a given topic partition.
Each of these will be described in great detail below.
Request Response Structure
There are a number of common fields shared by many or all requests. I will repeat these in each BNF, but only describe their usage here:
RequestOrResponse => Size (Request | Response) Request => RequestId VersionId ClientId RequestMessage Response => VersionId ResponseMessage Size => uint32 RequestId => uint16 VersionId => uint16 ClientId => string RequestMessage => MetadataRequest | ProduceRequest | FetchRequest | OffsetRequest ResponseMessage => MetadataResponse | ProduceResponse | FetchResponse | OffsetResponse
The above BNF gives the grammar for all requests and responses.
Field |
Purpose |
|---|---|
Size |
This is the total size of the remainder of the message. A client can always read a complete response from a socket by first reading this four byte size which will contain a non-negative integer N and then reading the N remaining bytes in the response. |
RequestId |
This request id indicates which API is being invoked. Each API has a numeric code given in the table below. |
VersionId |
This is the version of the request or response format. Each request and response is versioned independently to allow evolution and to allow clients to check compatibility. |
ClientId |
This is a string specified by the client that indicates |
RequestMessage, ResponseMessage |
The individual RequestMessage and ResponseMessage formats are given with their documentation below. |
Notes on reading the BNF
The BNFs below give an exact context free grammar for the request and response binary format. For each API I will give the request and response together followed by all the sub-definitions. The BNF is intentionally not compact in order to give human-readable name (for example I define a production for ErrorCode even though it is just an int16 in order to give it a symbolic name). As always in a BNF a sequence of productions indicates concatenation, so the MetadataRequest given below would be a sequence of bytes containing first a VersionId, then a ClientId, and then an array of TopicNames (each of which has its own definition). Productions are always given in camel case and primitive types in lower case. When there are multiple possible productions these are separated with '|' and may be inclosed in parenthesis for grouping.
Message sets
One structure common to both the produce and fetch requests is the message set format. A message in kafka is a key-value pair with a small amount of associated metadata. A message set is just a sequence of messages with offset and size information. This format happens to be used both for the on-disk storage on the broker and the on-the-wire format.
A message set is also the unit of compression in Kafka, and we allow messages to recursively contain compressed message sets to allow batch compression.
MessageSet => [Offset MessageSize Message] Offset => int64 MessageSize => int32 Message => Crc MagicByte Attributes Key Value Crc => int32 MagicByte => int8 Attributes => int8 Key => bytes32 Value => bytes32
Field |
Description |
|---|---|
Offset |
This is the offset used in kafka as the log sequence number. When the producer is sending messages it doesn't actually know the offset and can fill in any any value here it likes. |
Crc |
The CRC is the CRC32 of the remainder of the message bytes. This is used to check the integrity of the message on the broker and consumer. |
MagicByte |
This is a version id used to allow backwards compatible evolution of the message binary format. |
Attributes |
This byte holds metadata attributes about the message. In particular the last 3 bits contain the compression codec used for the message. |
Key |
The key is an optional message key that was used for partition assignment. The key can be null. |
Value |
The value is the actual message contents as an opaque byte array. Kafka supports recursive messages in which case this may itself contain a message set. |
Metadata API
This API answers the question "who has what data and where are they?". Specifically this request will tell for each topic how many partitions it has, which brokers currently host each of these partitions, and which of these is the master. Since Kafka is a partitioned system requests need to be directed to the appropriate server--the one currently acting as the master for the partition you want to interact with. Since cluster membership in Kafka is dynamic, you can't just give all the clients a config file with all the brokers (some of them may be down, or partitions may have moved); instead you need to ask the cluster about its current state at run time. Hence the first thing a client needs to do when it connects is ask, "where is everyone?" using this metadata API.
This is the only request that can be made to any server without regard to partition ownership and all servers will give the same answer (disregarding timing differences). Fetch and produce requests always interact with particular partitions, and sending these to the wrong broker will result in an invalid metadata error. The client is expected to cache the cluster metadata locally, using it to direct requests to the correct hosts, until it gets an invalid metadata error or can't reach a particular broker, at which point it should fetch the metadata again and update its cache.
This presents a bit of a catch-22, since the only way to find out which Kafka servers exists is to ask a Kafka server, so how can a client ever connect the first time? To do this a client should take a "bootstrap urls" configuration from which it can find out the list of currently available servers. Importantly this need not contain all the servers in the cluster, maybe just two or three for redundancy. The client should try each of these until it finds one it can connect to. This will ensure that even if one of the bootstrap servers is down the client can still fetch the cluster metadata.
For deployment you may not want to hardcode such a list and may prefer to rely on dns or a VIP or something like that to find a bootstap server.
So the lifecycle of most clients looks something like this:
- Cycle through a list of bootstrap kafka urls until we find one we can connect to. Fetch cluster metadata.
- Process fetch or produce requests, directing them to the appropriate broker based on the topic/partitions they send to or fetch from.
- If we get an appropriate error, refresh the metadata and try again.
Metadata Request And Response Format
The client sends a request for metadata. Since there may be many topics the client can give an optional list of topic names in order to only return metadata for a subset of topics.
The metdata returned is at the partition level, but grouped together by topic for convenience and to avoid redundancy. For each partition the metadata contains the information for the leader as well as for all the replicas and the list of replicas that are currently in-sync.
MetadataRequest => [TopicName] MetadataResponse => [Metadata] TopicName => string Metadata => TopicName [PartitionMetadata] TopicErrorCode PartitionMetadata => PartitionId Leader [Replicas] [Isr] PartitionErrorCode Replica => Broker Isr => Broker Broker => NodeId CreatorId Host Port NodeId => int32 CreatorId => string Host => string Port => int32 VersionId => int16 ClientId => string
Notes: Replicas should contain the broker information for the relevant brokers. The leader and isr should just be ids.
Produce API
The produce API is used to send message sets to the server. For efficiency it allows sending message sets intended for many topic partitions in a single request.
The produce API uses the generic message set format, but since no offset has been assigned to the messages at the time of the send the producer is free to fill in this field in any way it likes.
ProduceRequest =>
Interaction With The Server
Some Common Philosophical Questions
Some people have asked why we don't use HTTP. There are a number of reasons, the best is that client implementors can make use of some of the more advanced TCP features--the ability to multiplex requests, the ability to simultaneously poll many connections, etc. We have also found HTTP libraries in many languages to be surprisingly shabby.
Others have asked if maybe we shouldn't support many different protocols. Prior experience with this was that it makes it very hard to add and test new features if they have to be ported across many protocol implementations. Our feeling is that most users don't really see multiple protocols as a feature, they just want a good reliable client in the language of their choice.
Another question is why we don't adopt XMPP, STOMP, AMQP or an existing protocol. The answer to this varies by protocol, but in general the problem is that the protocol does determine large parts of the implementation and we couldn't do what we are doing if we didn't have control over the protocol. Our belief is that it is possible to do better than existing messaging systems have in providing a truly distributed messaging system, and to do this we need to build something that works different.
A final question is why we don't use a system like Protocol Buffers or Thrift to define our messages. These packages excel at helping you to managing lots and lots of serialized messages. However we have only a few messages. Support across languages is somewhat spotty (depending on the package). Finally the mapping between binary log format and wire protocol is something we manage somewhat carefully and this would not be possible with these systems. Finally we prefer the style of versioning APIs explicitly and checking this to inferring new values as nulls as it allows more nuanced control of compatibility.