THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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| Code Block | ||||
|---|---|---|---|---|
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| REQUEST_LENGTH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| REQUEST_TYPE | TOPIC_LENGTH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
/ TOPIC (variable length) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PARTITION |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
REQUEST_LENGTH = int32 // Length in bytes of entire request (excluding this field)
REQUEST_TYPE = int16 // See table below
TOPIC_LENGTH = int16 // Length in bytes of the topic name
TOPIC = String // Topic name, ASCII, not null terminated
// This becomes the name of a directory on the broker, so no
// chars that would be illegal on the filesystem.
PARTITION = int32 // Partition to act on. Number of available partitions is
// controlled by broker config. Partition numbering
// starts at 0.
============ ===== =======================================================
REQUEST_TYPE VALUE DEFINITION
============ ===== =======================================================
PRODUCE 0 Send a group of messages to a topic and partition.
FETCH 1 Fetch a group of messages from a topic and partition.
MULTIFETCH 2 Multiple FETCH requests, chained together
MULTIPRODUCE 3 Multiple PRODUCE requests, chained together
OFFSETS 4 Find offsets before a certain time (this can be a bit
misleading, please read the details of this request).
============ ===== =======================================================
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Very similar to the Request-Header is the multi-request header used for requesting more than one topic-partition combo at a time. Either for multi-produce, or multi-fetch.
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0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RESPONSEREQUEST_LENGTH | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ERROR_CODEREQUEST_TYPE | TOPICPARTITION_COUNT | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ RESPONSE_LENGTH = int32 // -+-+-+-+-+-+-+-+-+ REQUEST_LENGTH = int32 // Length in bytes of entire responserequest (excluding this field) REQUEST_TYPE ERROR_CODE = int16 // See table below. above TOPICPARTITION_COUNT = int16 // number of unique topic-partition combos in this request |
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0 1 ================ ===== =================================================== ERROR_CODE VALUE DEFINITION ================ ===== =================================================== Unknown 2 -1 Unknown Error NoError3 0 1 2 3 4 5 6 7 8 9 0 Success OffsetOutOfRange 1 2 3 4 Offset5 requested6 is7 no8 longer9 available on the server InvalidMessage 2 A message you sent failed its checksum and is corrupt. WrongPartition 3 You tried to access a partition that doesn't exist 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RESPONSE_LENGTH (was not between 0 and (num_partitions - 1)). InvalidFetchSize 4 The size you requested for fetching is smaller than | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ERROR_CODE | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ RESPONSE_LENGTH = int32 // Length in bytes theof messageentire you're trying to fetch.response (excluding this field) ERROR_CODE = int16 // See table below. ================ ===== =================================================== |
FIXME: Add tests to verify all these codes.
FIXME: Check that there weren't more codes added in 0.7.
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ERROR_CODE 0 VALUE DEFINITION ================ 1 ===== =================================================== Unknown -1 2Unknown Error NoError 0 3 Success OffsetOutOfRange 0 1 2Offset 3requested 4is 5no 6longer 7available 8on 9the 0server 1 2InvalidMessage 3 4 5 6 7 82 9 0 1 2A 3message 4you 5sent 6failed 7its 8checksum 9and 0is 1corrupt. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | WrongPartition 3 You tried to access a partition that doesn't exist LENGTH (was not between 0 and (num_partitions - 1)). InvalidFetchSize 4 The size you requested for fetching is smaller |than +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MAGIC | the message you're trying to fetch. ================ ===== =================================================== |
FIXME: Add tests to verify all these codes.
FIXME: Check that there weren't more codes added in 0.7.
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0 1 CHECKSUM | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | CHECKSUM (cont.)| 2 PAYLOAD /3 +-+-+-+-+-+-+-+-+-+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | / / LENGTH PAYLOAD (cont.) /| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ LENGTH| = int32 //MAGIC Length in bytes of entire message (excluding| this field) MAGIC = int8 // 0 is the only valid value CHECKSUM = int32 // CRC32 checksum of the PAYLOAD PAYLOAD = Bytes[] // Message content |
The offsets to request messages are just byte offsets. To find the offset of the next message, take the offset of this message (that you made in the request), and add LENGTH + 4 bytes (length of this message + 4 byte header to represent the length of this message).
Starting with version 0.7, Kafka added an extra field for compression:
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0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | CHECKSUM (cont.)| 1 PAYLOAD 2 / +-+-+-+-+-+-+-+-+-+ 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | / / LENGTH PAYLOAD (cont.) |/ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |LENGTH = int32 MAGIC// Length in bytes of entire message |(excluding this COMPRESSIONfield) |MAGIC = int8 // 0 is the CHECKSUMonly valid value CHECKSUM = int32 // CRC32 checksum | of the PAYLOAD PAYLOAD = Bytes[] // Message content |
The offsets to request messages are just byte offsets. To find the offset of the next message, take the offset of this message (that you made in the request), and add LENGTH + 4 bytes (length of this message + 4 byte header to represent the length of this message).
Starting with version 0.7, Kafka added an extra field for compression:
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0 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | CHECKSUM (cont.) | PAYLOAD1 / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2 3 0 / 1 2 /3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 PAYLOAD (cont.) /8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ LENGTH| = int32 // Length in bytes of entire message (excluding this field) MAGIC = int8 // 0 = COMPRESSION attribute byte does not exist (v0.6 and below) LENGTH // 1 = COMPRESSION attribute byte exists (v0.7 and above) COMPRESSION = int8 // 0 =| none; 1 = gzip; 2 = snappy; +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MAGIC | COMPRESSION | // Only exists at all if MAGIC == 1 CHECKSUM CHECKSUM = int32 // CRC32 checksum of the PAYLOAD PAYLOAD = Bytes[] // Message content |
Note that compression is end-to-end. Meaning that the Producer is responsible for sending the compressed payload, it's stored compressed on the broker, and the Consumer is responsible for decompressing it. Gzip gives better compression ratio, snappy gives faster performance.
Let's look at what compressed messages act like:
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+-+-+| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | CHECKSUM (cont.) CP1 | CP2 PAYLOAD | CP3 | | M1 | M2 | M3 | M4... | M12 | M13 | M14... | M26 | M27 | M28 ... | / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
In this scenario, let's say that M1, M2, etc. represent complete, uncompressed messages (including headers) that the user of your library wants to send. What your driver needs to do is take M1, M2... up to some predetermined number/size, concatenate them together, and then compress them using gzip or snappy. The result (CP1 in this case) becomes the PAYLOAD for the compressed message CM1 that your library will send to Kafka.
It also means that we have to be careful about calculating the offsets. To Kafka, M1, M2, don't really exist. It only sees the CM1 you send. So when you make calculations for the offset you can fetch next, you have to make sure you're doing it on the boundaries of the compressed messages, not the inner messages.
FIXME: Haven't implemented compression yet, need to verify this is correct.
Interactions
Produce
To produce messages from the driver and send to Kafka, use the following format:
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0 / / PAYLOAD (cont.) / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ LENGTH = int32 // Length in bytes of entire message (excluding this field) MAGIC = int8 // 0 = COMPRESSION attribute byte does not exist (v0.6 and below) // 1 = COMPRESSION attribute byte exists (v0.7 and above) COMPRESSION = int8 // 0 = none; 1 = gzip; 2 = snappy; 1 // Only exists at all if MAGIC == 1 2 CHECKSUM = int32 // CRC32 checksum of the PAYLOAD PAYLOAD = Bytes[] // 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +Message content |
Note that compression is end-to-end. Meaning that the Producer is responsible for sending the compressed payload, it's stored compressed on the broker, and the Consumer is responsible for decompressing it. Gzip gives better compression ratio, snappy gives faster performance.
Let's look at what compressed messages act like:
| Code Block | ||
|---|---|---|
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /| CP1 | REQUEST HEADER CP2 | CP3 / / | | M1 | M2 | M3 | M4... | M12 | M13 | M14... | M26 | M27 | M28 /... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
In this scenario, let's say that M1, M2, etc. represent complete, uncompressed messages (including headers) that the user of your library wants to send. What your driver needs to do is take M1, M2... up to some predetermined number/size, concatenate them together, and then compress them using gzip or snappy. The result (CP1 in this case) becomes the PAYLOAD for the compressed message CM1 that your library will send to Kafka.
It also means that we have to be careful about calculating the offsets. To Kafka, M1, M2, don't really exist. It only sees the CM1 you send. So when you make calculations for the offset you can fetch next, you have to make sure you're doing it on the boundaries of the compressed messages, not the inner messages.
FIXME: Haven't implemented compression yet, need to verify this is correct.
Interactions
Produce
To produce messages from the driver and send to Kafka, use the following format:
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0 | MESSAGES_LENGTH | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / 1 2 /3 / 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 MESSAGES7 8 9 /0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ MESSAGES_LENGTH = int32 // Length in bytes of the MESSAGES section MESSAGES = Collection of MESSAGES (see above) |
There is no response to a PRODUCE Request. There is currently no way to tell if the produce was successful or not. This is being worked.
Multi-Produce
The multi-produce request has a different header, with the (topic-length/topic/message_length/messages) repeated many times.
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Here is the general format of the multi-produce request / REQUEST HEADER / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / | MULTI-PRODUCE HEADER MESSAGES_LENGTH /| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | / TOPIC-PARTION/MESSAGES (n times | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Multi-Produce Header 0 1 / / 2 MESSAGES 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | REQUESTMESSAGES_LENGTH = int32 // Length in bytes of the MESSAGES section MESSAGES = Collection of MESSAGES (see above) |
There is no response to a PRODUCE Request. There is currently no way to tell if the produce was successful or not. This is being worked.
Multi-Produce
The multi-produce request has a different header, with the (topic-length/topic/message_length/messages) repeated many times.
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Here is the general format of the multi-produce request, see multi-request header above. +- | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / | REQUEST_TYPEMULTI-REQUEST HEADER | TOPICPARTITION_COUNT |/ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | REQUEST_LENGTH = int32 // Length in bytes of entire request (excluding this field) REQUEST_TYPE TOPIC-PARTION/MESSAGES (n times) = int16 // See table below TOPICPARTITION_COUNT = int16 // number of unique topic-partition combos in this request| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Per Topic-Partition (repeated n times) 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TOPIC_LENGTH | TOPIC (variable length) / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PARTITION | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MESSAGES_LENGTH | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / MESSAGES / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The TOPIC_LENGTH, TOPIC, PARTITION, MESSAGES_LENGTH are documented above for size. |
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- If you ask the broker for up to 300K worth of messages from a given topic and partition, it will send you the appropriate headers followed by a 300K chunk worth of the message log. If 300K ends in the middle of a message, you get half a message at the end. If it ends halfway through a message header, you get a broken header. This is not an error, this is Kafka pushing complexity outward to the driver to make the broker simple and fast.
- Kafka stores its messages in log files of a configurable size (512MB by default) called segments. A fetch of messages will not cross the segment boundary to read from multiple files. So if you ask for a fetch of 300K's worth of messages and the offset you give is such that there's only one message at the end of that segment file, then you will get just one message back. The next time you call fetch with the following offset, you'll get a full set of messages from the next segment file. Basically, don't make any assumptions about how many messages are remaining from how many you got in the last fetch.
Multi-Fetch
FIXME: Haven't implemented this to verify yet.
- the following offset, you'll get a full set of messages from the next segment file. Basically, don't make any assumptions about how many messages are remaining from how many you got in the last fetch.
Multi-Fetch
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ MULTI-REQUEST HEADER /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOPIC-PARTION-FETCH-REQUEST (n times ) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
REQUEST_HEADER = See MULTI_REQUEST_HEADER above
OFFSET = int64 // Offset in topic and partition to start from
MAX_SIZE = int32 // MAX_SIZE of the message set to return
The TOPIC_LENGTH, TOPIC, PARTITION, MESSAGES_LENGTH are documented above for size.
Per Topic-Partition-Fetch- Request (repeated n times)
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOPIC_LENGTH | TOPIC (variable length) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PARTITION |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OFFSET |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ MAX_SIZE /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
Offsets
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ REQUEST HEADER /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TIME |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAX_NUMBER (of OFFSETS) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TIME = int64 // Milliseconds since UNIX Epoch.
// -1 = LATEST
// -2 = EARLIEST
MAX_NUMBER = int32 // Return up to this many offsets
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0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / REQUESTRESPONSE HEADER / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NUMBER_OFFSETS | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / OFFSETS (0 or more) / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NUMBER_OFFSETS = int32 // How many offsets are being returned OFFSETS = int64[] // List of offsets |
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Kafka relies on ZooKeeper in order to coordinate multiple brokers and consumers. If you're unfamiliar with ZooKeeper, just think of it as a server that allows you to atomically create nodes in a tree, assign values to those nodes, and sign up for notifications when a node or its children get modified. Nodes can be either be permanent or ephemeral, the latter meaning that the nodes will disappear if the process that created them disconnects (after some timeout delay).unmigrated-wiki-markup
While creating the nodes we care about, you'll often need to create the intermediate nodes that they are children of. For instance, since offsets are stored at {{/consumers/\[consumer_group\]/offsets/\[topic\]/\[broker_id\]-\[partition_id\]}}, something has to create {{/consumers}}, {{/consumers/\[consumer_group\]}}, etc. All nodes have values associated with them in ZooKeeper, even if Kafka doesn't use them for anything. To make debugging easier, the value that should be stored at an intermediate node is the ID of the node's creator. In practice that means that the first Consumer you create will need to make this skeleton structure and store its ID as the value for {{/consumers}}, {{/consumers/\[consumer_group\]}}, etc.
ZooKeeper has Java and C libraries, and can be run as a cluster.
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Role | ZooKeeper Path | Type | Data Description | |||
|---|---|---|---|---|---|---|
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="06129f08-84ab-46d0-be40-89a9513015b8"><ac:plain-text-body><![CDATA[ | ID Registry |
| Ephemeral | String in the format of "creator:host:port" of the broker. | ]]></ac:plain-text-body></ac:structured-macro> | <ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="6e1b2253-cf49-4235-8c19-6c0980b79f91"><ac:plain-text-body><![CDATA[:port" of the broker. |
Topic Registry |
| Ephemeral | Number of partitions that topic has on that Broker. | ]]></ac:plain-text-body></ac:structured-macro> |
So let's take the example of the following hypothetical broker:
...
- Broker IDs don't have to be sequential, but they do have to be integers. They are a config setting, and not randomly generated. If a Kafka server goes offline for some reason and comes back an hour later, it should be reconnecting with the same Broker ID.
- The ZooKeeper hierarchy puts individual brokers under topics because Producers and Consumers will want to put a watch on a specific topic node, to get notifications when new brokers enter or leave the pool.
- The Broker's description is formatted such that it's
creator:host:port. The host will also up as part of the creator because of the version of UUID that Kafka's using, but don't rely on that behavior. Always split on ":" and extract the host that will be the second element. - These nodes are ephemeral, so if the Broker crashes or is disconnected from the network, it will automatically be removed. But this removal is not instantaneous, and it might show up for a few seconds. This can cause errors when a broker crashes and is restarted, and subsequently tries to re-create its still existent Broker ID registry node.
Producer
Reads:
Wiki Markup /brokers/topics/\[topic\]/\[0..N\]}}, so that it knows what Broker IDs are available for this topic, and how many partitions they have.unmigrated-wiki-markup- {{
/brokers/ids/\[0..N\]}}, to find the address of the Brokers, so it knows how to connect to them.
Watches:
...
- {{
/brokers/topics/\[topic\]}}, so that it knows when Brokers enter and exit the pool. /brokers/ids, so that it can update the Broker addresses in case you bring down a Broker and bring it back up under a different IP/port.
...
- A Producer is created for a topic.unmigrated-wiki-markuptopic.
- The Producer reads the Broker-created nodes in {{
/brokers/ids/\[0..N\]}} and sets up an internal mapping of Broker IDs => Kafka connections. Wiki Markup /brokers/topics/\[topic\]/\[0..N\]}} to find the number of partitions it can send to for each Broker.- The Producer takes every Broker+Partition combination and puts them in an internal list.
- When a Producer is asked to send a message set, it picks from one of it's Broker+Partition combinations, looks up the appropriate Broker address, and sends the message set to that Broker, for that topic and partition. The precise mechanism for choosing a destination is undefined, but debugging would probably be easier if you ordered them by Broker+Partition (e.g. "0-3") and used a hash function to pick the index you wanted to send to. You could also just make it randomly choose.
...
The latter is actually extremely common, which brings us to the only tricky part about Producers – dealing with new topics.
Creating New Topics
...
Topics are not pre-determined. You create them just by sending a new message to Kafka for that topic. So let's say you have a number of Brokers that have joined the pool and don't list themselves in {{/brokers/topics/\[topic\]/\[0..N\]}} for the topic you're interested in. They haven't done so because those topics don't exist on those Brokers yet. But our Producer knows the Brokers themselves exist, because they are in the Broker registry at {{/brokers/ids/\[0..N\]}}. We definitely need to send messages to them, but what partitions are safe to send to? Brokers can be configured differently from each other and topics can be configured on an individual basis, so there's no way to infer the definitive answer by looking at what's in ZooKeeper.
Wiki Markup /brokers/topics/\[topic\]}}, and you'll add the new Broker+Partitions to your destination pool.
Consumer
FIXME: Go over all the registration stuff that needs to happen.
...
- All Consumers in a ConsuerGroup will come to a consensus as to who is consuming what.
- Each Broker+Topic+Partition combination is consumed by one and only one Consumer, even if it means that some Consumers don't get anything at all.
Wiki Markup - Consumers are sorted by their Consumer IDs. If there are three Consumers, two Brokers, and three partitions in each, the split might look like:
Wiki Markup Wiki Markup - Consumer B: \ [0-2, 1-0\]
Wiki Markup
- If the distribution can't be even and some Consumers must have more partitions than others, the extra partitions always go to the earlier consumers on the list. So you could have a distribution like 4-4-4-4 or 5-5-4-4, but never 4-4-4-5 or 4-5-4-4.