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The recommendations here are under development and may change before implementation.

Overview

The intent of the REPLICA READ (also known as CMD_GET_REPLICA in the server) operation is to allow a client to perform retrieval operations only against one or more replicas. This would be an inconsistent read. While it could be used for nearly any purpose, the only common use case is expected to be in the event of failures when a known inconsistent read is okay.

Request


  Byte/     0       |       1       |       2       |       3       |
     /              |               |               |               |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
    +---------------+---------------+---------------+---------------+
   0|  0x80         |  0x83         |  0x00         |  0x05         |
    +-----------    +-----------    +-----------    +-----------    +
   4|  0x04         |  0x00         |  0x00         |  0x00         |
    +-----------    +-----------    +-----------    +-----------    +
   8|  0x00         |  0x00         |  0x00         |  0x09         |
    +-----------    +-----------    +-----------    +-----------    +
  12|  0x00         |  0x00         |  0x00         |  0x00         |
    +-----------    +-----------    +-----------    +-----------    +
  16|  0x00         |  0x00         |  0x00         |  0x00         |
    +-----------    +-----------    +-----------    +-----------    +
  20|  0x00         |  0x00         |  0x00         |  0x00         |
    +---------------+---------------+---------------+---------------+
  24|  0x66 ('f')   |  0x6f ('o')   |  0x6f ('o')   |
    +---------------+---------------+---------------+

Field        (offset) (value)
Magic        (0)    : 0x80 (PROTOCOL_BINARY_REQ)
Opcode       (1)    : 0x83
Key length   (2,3)  : 0x0003 (3)
Extra length (0)    : 0x00
Data type    (5)    : 0x00
vbucket      (6,7)  : 0x0000 (0)
Total body   (8-11) : 0x00000003 (3)
Opaque       (12-15): 0x00000000
CAS          (16-23): 0x0000000000000000
Key          (24-26): The textual string "foo"

Request


   Byte/     0       |       1       |       2       |       3       |
      /              |               |               |               |
     |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
     +---------------+---------------+---------------+---------------+
    0| 0x81          | 0x83          | 0x00          | 0x00          |
     +---------------+---------------+---------------+---------------+
    4| 0x04          | 0x00          | 0x00          | 0x00          |
     +---------------+---------------+---------------+---------------+
    8| 0x00          | 0x00          | 0x00          | 0x09          |
     +---------------+---------------+---------------+---------------+
   12| 0x00          | 0x00          | 0x00          | 0x00          |
     +---------------+---------------+---------------+---------------+
   16| 0x00          | 0x00          | 0x00          | 0x00          |
     +---------------+---------------+---------------+---------------+
   20| 0x00          | 0x00          | 0x00          | 0x01          |
     +---------------+---------------+---------------+---------------+
   24| 0xde          | 0xad          | 0xbe          | 0xef          |
     +---------------+---------------+---------------+---------------+
   28| 0x57 ('W')    | 0x6f ('o')    | 0x72 ('r')    | 0x6c ('l')    |
     +---------------+---------------+---------------+---------------+
   32| 0x64 ('d')    |
     +---------------+


Field         (offset) (value)
 Magic        (0)    : 0x81 (PROTOCOL_BINARY_RES)
 Opcode       (1)    : 0x83
 Key length   (2,3)  : 0x0000
 Extra length (4)    : 0x04
 Data type    (5)    : 0x00
 Status       (6,7)  : 0x0000
 Total body   (8-11) : 0x00000009
 Opaque       (12-15): 0x00000000
 CAS          (16-23): 0x0000000000000001
 Extras              :
   Flags      (24-27): 0xdeadbeef
 Key                 : None
 Value        (28-32): The textual string "World"

Client API

Java

Note: not sure if we like this yet...


GetFuture resf;
boolean isReplicaRead;

try { 
  resf = cbc.asyncGet("foo");
  isReplicaRead = false;
} catch (TimeoutException ex) {
  // uhoh, something went wrong, server isn't there!
  resf = cbc.asyncReplicaGet("foo");
  isReplicaRead = true;
} finally {

  // do something useful
}

.NET

PHP

Ruby

res = nil
is_replica_read = false

begin
  res = cbc.get("foo")
rescue Couchbase::Error::Timeout => ex
  res = cbc.get("foo", :replica => true)
  is_replica_read = true
ensure
  # do something useful
end

Recommended Implementation

These recommendations are preliminary, and have not been reviewed.

The replica read command is intentionally simple. It would attempt to iterate through all of the replicas as designated by the configuration supplied by the cluster in order, trying to get the specified key.

Implementation Constraints

REPLICA READ is a binary protocol only operation. It could be implemented in ASCII, but that would require changes in moxi and additional clients.

Implementation Questions

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