Page Comparison

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Row Key Format

concat(topic, generation, publish_timestamp, seq_id)

• topic
  
  • String representation of the topic name
• generation
  • 4 bytes int representing the generation of this topic. Generation increase by one when at topic creation time.
• publish_timestamp
  
  • 8 bytes time in milliseconds
• seq_id
  
  • 2 bytes sequence ID. It is for uniquely identifying messages published in the same millisecond.
  • The 2 bytes gives us 64M messages per second per topic, which is well over the limit we need

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Row Key Format

concat(topic, generation, transaction_write_pointer, write_timestamp, p_seq_id)

• topic
  • String representation of the topic name
• generation
  • 4 bytes int representing the generation of this topic. Generation increase by one when at topic creation time.
• transaction_write_pointer
  • 8 bytes transaction write pointer of the transaction that the message was published from
• write_timestamp
  • 8 bytes time in milliseconds of the time when the message was persisted
  • This is for ordering messages published in the same transaction
• p_seq_id
  • 2 bytes sequence ID. It is for uniquely identifying messages published in the same millisecond.

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With the row key format described above, each message in a topic is uniquely identified by an a message ID in this format:

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The publishing client is responsible for telling the messaging system to rollback published messages (by Message IDs). To rollback, simply delete mark the corresponding entries in the Message Table and the Payload Table. Since we encoded enough information in the Message ID, it should be just a scan and delete of the Payload Table, without the need of reading from the Message Table.

Consuming

The general principle of consuming messages is to scan the Message Table in ascending order. When it encounters an entry with the payload stored in the Payload Table, scan the Payload Table using the transaction write pointer as the prefix.

Non-transactional

This is the low latency choice since it will never get blocked by uncommitted messages. However, the side effect of that is it will see uncommitted messages. In case the uncommitted messages are rollbacked, a non-transactional consumer will see more messages than the transactional consumer, although the ordering of committed messages will remain the same.

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1. The row represents a message and the payload is the value in the p column
2. Message ID is generated from the row key as concat(publish_time, seq_id, 0L, 0)

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as invalid (invert the sign of Transaction write ptr). We don't have to do anything to the entries in the payload table (and let the TTL of the topic delete the entries). This is because we can simply skip reading entries in message table while transactionally reading messages and thus we won't touch the payload table. This gives us couple of benefits - i) rollback will be faster since we don't have to delete entries in payload table ii) non-transactionally consuming messages will give us the repeatable results back as long as the entries have not been removed by the expiration of TTL.

Consuming

The general principle of consuming messages is to scan the Message Table in ascending order. When it encounters an entry with the payload stored in the Payload Table, scan the Payload Table using the transaction write pointer as the prefix.

Non-transactional

This is the low latency choice since it will never get blocked by uncommitted messages. However, the side effect of that is it will see uncommitted messages. In case the uncommitted messages are rollbacked, a non-transactional consumer will see more messages than the transactional consumer, although the ordering of committed messages will remain the same.

1. Scan the Message Table, with an optional start Message ID or timestamp, which can either be inclusive or exclusive
2. Depending on whether the row has the payload column (the p column), the handling is different:
   1. With the payload column (the p column)
      1. The row represents a message and the payload is the value in the p column
      2. Message ID is generated from the row key in the Message Table and the row key in the Payload Table as concat(publish_time, seq_id, write_timestamp, p_seq_id)

Transactional

...

1. 1. 1. as concat(publish_time, seq_id, 0L, 0)
   2. Without the payload column (the p column): the transaction column (the t column) must exist
      1. Scan the Payload Table with prefix concat(topic, transaction_write_pointer), where the transaction_write_pointer is the value of the t column in the Message Table
      2. Each row encountered during the scanning is a message and the payload is the value in the p column
      3. Message ID is generated from the row key in the Message Table and the row key in the Payload Table as concat(publish_time, seq_id, write_timestamp, p_seq_id)

Transactional

Transactional consumption basically follow the same procedures as the non-transactional one, with the addition that it will stop at the first uncommitted message when scanning the Message Table. The transaction information comes from the client and it is the client's responsibility to open a new transaction in order to get a new snapshot of committed messages in the messaging system. This will increase the latency of message consumption, but with the technique described above for message publishing, this latency should be minimal; in the range of less than a second.

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For authentication, it is pretty much the same as all other intra-CDAP RESTful calls. For authorization, we can do it at the topic level.

APIs

RESTful

Tables used for store

• MessageTable

• PayloadTable

• MetadataTable 

APIs

RESTful

Base URL: /v1/namespaces/<ns-id> (since REST API is internal)

Create Topic

• Request method and URI
  • PUT [base_url]/topics/[topic]
• Request body
  • Can be empty
  • If provided, it is a JSON object containing topic properties

    • e.g. {"ttl" : [ttl-in-seconds]}

Publish

Consume

Programmatic

...

• Response
  • 200 OK if the topic was created successfully
  • 409 CONFLICT if a topic of the same name exists already
  • 400 BAD REQUEST if the given TTL is invalid

Update Topic

• Request method and URI
  • PUT [base_url]/topics/[topic]/properties

• Request body
  • JSON object containing topic properties. Note that this call will replace all the existing properties.
    

    • e.g. {"ttl" : [ttl-in-seconds]}

• Response
  • 200 OK if the topic properties were updated successfully
  • 404 NOT FOUND if the topic is not present
  • 400 BAD REQUEST if the properties were not correct

Get Topic Properties

• Request method and URI
  • GET [base_url]/topics/[topic]

• Request body
  • JSON object containing the topic name and its properties.
    

    • e.g. {"name" : "topic1", "properties" : { "ttl" : "123231" } }

• Response
  • 200 OK
  • 404 NOT FOUND if the topic is not present

List Topics

• Request method and URI
  • GET [base_url]/topics

• Request body
  • JSON object containing a list of topic name under the namespace
    

    • e.g. [ "topic1", "topic2" ]

• Response
  • 200 OK

Delete Topic

• Request method and URI
  • DELETE [base_url]/topics/[topic]

• Response
  • 200 OK if the topic was deleted successfully
  • 404 NOT FOUND if the topic is not present

Publish Message

There are two separate endpoints to support different publishing cases as described above. They both share the same request format.

• Request Body Schema

  Code Block
  language js
  { "type" : "record", "name" : "PublishRequest", "fields" : [ { "name" : "transactionWritePointer", "type" : [ "long", "null" ] }, { "name" : "messages", "type" : { "type" : "array", "items" : "bytes" } } ] }

• Request body
  • Can be Avro binary or JSON, based on the request - Content-Type: application/json or avro/binary

  • Schema Fields:

    1. messages - Contains an array of byte arrays that correspond to messages

    2. transactionWritePointer - Corresponds to a transaction write pointer.

Store Messages to Payload Table 

• Request method and URI

  • POST [base_url]/topics/[topic]/store

• Request body should contain transactionWritePointer and messages

• Response:
  404 NOT FOUND if the topic is not present
  200 OK if message is persisted

Publish Messages to Message Table 

• Request method and URI

  • POST [base_url]/topics/[topic]/publish

• Request

  • Request can optionally contain transactionWritePointer. If store calls were made previously with the same transactionWritePointer, the messages array should be empty.

  • This call can be preceded by multiple calls to the 'store' endpoint. If it is preceded by calls to store with same transactionWritePointer, then this endpoint should be called with an empty messages field. If it is not preceded by store calls with same transactionWritePointer, then this call will store the messages to the MessageTable
    directly.
  • If the call does not contain a transactionWritePointer, then the messages are stored non-transactionally (ie, without a tx write ptr).

• Response

  404 NOT FOUND if topic is not present
  200 OK if messages are persisted
  400 BAD REQUEST if the {{transactionWritePointer}} is null and the messages field is empty. Messages field can be empty if the transactionWritePointer is provided

  Response Body:

  • If the publish is non-transactional, the response body is empty

  • If the publish is transactional, the response body will contains the information needed for rollback with the following schema

    Code Block
    language js
    { "type" : "record", "name" : "PublishResponse", "fields" : [ { "name" : "transactionWritePointer", "type" : [ "long", "null" ] }, { "name" : "startTimestamp", "type" : "long" }, { "name" : "startSequenceId", "type" : "int" }, { "name" : "endTimestamp", "type" : "long" }, { "name" : "endSequenceId", "type" : "int" } ] }

  • The client shouldn't need to parse the response body, but rather treats it as opaque bytes. On rollback, simply use it as the request body.

Rollback Transactionally published messages 

• Request method and URI

  • POST [base_url]/topics/[topic]/rollback

• Request Body

  • Use the response body as is from the publish call above

• Response:

  404 NOT FOUND if topic is not present
  200 OK if messages are rolled back

Consume Message

• Request method and URI
  • POST [base_url]/topics/[topic]/poll


• Request Schema

  Code Block
  language js
  { "type" : "record", "name" : "ConsumeRequest", "fields" : [ { "name" : "startFrom", "type" : [ "bytes", "long", "null" ] }, { "name" : "inclusive", "type" : "boolean", "default" : true }, { "name" : "limit", "type" : [ "int", "null" ] }, { "name" : "transaction", "type" : [ "bytes", "null" ] } ] }

• Response Schema (will be in JSON or Avro binary based on the request Content-Type)

  Code Block
  language js
  { "type" : "array", "items" : { "type" : "record", "name": "Message", "fields": [ { "name" : "id", "type" : "bytes" }, { "name" : "payload", "type" : "bytes" } ] } }

• Request body
• Can be Avro binary or JSON, based on the request - Content-Type: application/json or avro/binary

• Schema Fields:
  i) startFrom - can be bytes in which case it is considered as messageId, can be long in which case it is considered as timestamp
  ii) inclusive - boolean field that says whether the messageId/timestamp should be inclusive
  iii) limit - max number of responses to return [an hard limit will be set by the TransactionServer which will be a cConf property]
  iv) transaction -  serialized bytes (TransactionCodec) of the transaction object

• Response
  404 NOT FOUND if topic is not present
  200 OK Response body contains an Avro binary or JSON object based on the request Content-Type with the response schema

Data Cleanup In Tables

Various cleanup of data needs to be performed. And the clean up strategy varies based on the underlying storage - LevelDB vs HBase. The matrix below describes the combinations and types of cleanup we need to do and how it is going to be performed.