~drizzle-trunk/drizzle/development

.. Drizzle Client & Protocol Library

.. Copyright (C) 2008 Eric Day (eday@oddments.org)

.. All rights reserved.

.. Use and distribution licensed under the BSD license.  See

.. the COPYING.BSD file in the root source directory for full text.

Drizzle Protocol

================

`This is currently a proposed draft as of November 29, 2008`

The Drizzle protocol works over TCP, UDP, and Unix Domain Sockets

(UDS, also known as IPC sockets), although there are limitations when

using UDP (this is discussed below). In the case of TCP and UDS,

a connection is made, a command is sent, and a response loop is

started. Socket communication ends when either side closes the

connection or a QUIT command is issued.

TCP and UDS communications will be full duplex. This means that as

the client is sending a command, it is possible for the server to

report an error before the sending of data completes. This allows

the server to do preliminary checks (table exists, authentication,

...) before a request is completely sent so the client may abort. This

will primarily be used for large requests (INSERTing large BLOBs).

TCP and UDS communications will also allow for pipe-lining of requests

and concurrent command execution. This means a client does not need

to wait for a command to finish before a new command is sent. It is

even possible a later command issued will complete and have a result

before an earlier command. Result packets may be interleaved so a

client issuing concurrent commands must be able to parse results

concurrently.

UDP sockets are supported to allow small, fast updates for

applications such as statistical gathering. Since UDP does not

guarantee delivery, this method should not be used for applications

that require reliable transport. When using UDP, the authentication

packet (if needed) and command packet are bundled into a single UDP

packet and sent. This puts a limitation on the size of the request

being made, and this limit can be different between network hosts. The

absolute limit is 65,507 bytes (28 bytes used for IPv4 and UDP

headers), but again, this can depend on the network hosts. Responses

are optional when issuing UDP commands, and this preference is

specified in the handshake packet.

All sizes given throughout this document are in bytes. Byte order

for all multi-byte binary objects such as lengths and mutli-byte

bit-fields are packed little-endian.

Packet Sequence Overview

------------------------

The sequence of packets for a simple connection and command that

responds with an OK packet:

C: Command

S: OK

The sequence of packets for a simple connection and query command

with results:

C: Command

S: OK

S: Fields (optional, multiple packets)

S: Rows (multiple packets)

S: EOF

When authentication is required for a command, the server will ask

for it. For example:

C: Command

S: Authentication Required

C: Authentication Credentials

S: OK    

S: Fields

S: Rows

S: EOF

The server will use the most recent credential information when

processing subsequent commands.

If a client wishes to multiplex commands on a single connection,

it can do so using the command identifiers. Here is an example of

how the packets could be ordered, but this will largely depend on

the servers ability to process the commands concurrently and the

processing time for each command.

C: Command (Command ID=1)

C: Command (Command ID=2)

S: OK (Command ID=2)

S: Field (Command ID=2)

S: OK (Command ID=1)

S: Fields (Command ID=2)

S: Rows (Command ID=2)

S: EOF

As you can see, the commands may be executed with results generated

in any order, and the packet containing the results may be interleaved.

Length Encoding

---------------

Some lengths used within the protocol packets are length encoded. This

means the size of the length field will vary between 1 and 9 bytes,

and is determined by the value of the first byte.

0-252 - Actual length

253   - NULL value (only applicable in row results)

254   - Following 8 bytes contain actual length

255   - Depends on context, usually signifies end

Packets

-------

Packets consist of two layers. The first is meant to be small,

simple, and have just enough information for fast router and proxy

processing. It consists of a fixed-size part, along with a variable

sized client id (explained later), a series of chunked data, followed

by a checksum at the end. The chunked transfer encoding allows for

not having to pre-compute the packet data length before sending,

and support packets of any size. It also allows for a large packet

to be aborted gracefully (without having to close the connection)

in the event of an error.

   +-------------------------------------------------------------------------+

   +                                  32 Bits                                +

   +-------------------------------------------------------------------------+

   +-----+----------------+----------------+---------------------------------+

   |   0 | Magic          | Protocol       | Command ID                      |

   +-----+----------------+----------------+---------------------------------+

   +-----+---------------------------------+---------------------------------+

   |  32 | Command / Result Code           | Client ID Length                |

   +-----+---------------------------------+---------------------------------+

   +-----+---------------------------------+---------------------------------+

   |  64 | Client ID (optional, variable length)                             |

   +-----+---------------------------------+---------------------------------+

   +-----+---------------------------------+---------------------------------+

   | 64+ | Chunk Length and Value Pairs (optional, variable length)          |

   +-----+---------------------------------+---------------------------------+

   +-----+---------------------------------+---------------------------------+

   + 64+ | Chunk Length = 0                |                                 |

   +-----+---------------------------------+---------------------------------+

   +-----+---------------------------------+---------------------------------+

   | 80+ | Checksum                                                          |

   +-----+---------------------------------+---------------------------------+

The first part of a packet is:

1-byte Magic number, the value should be 0x44.

1-byte Protocol version, currently 1.

2-byte Command ID. This is a unique number among all other queries

       currently being executed on the connection. The client is

       responsible for choosing a unique number while generating a

       command packet, and all response packets associated with that

       command must have the same command ID. Once a command has been

       completed, the client may reuse the ID.

2-byte Command/result code. For commands, this may be:

       1  ECHO

         The entire packet is simply echoed back to the caller.

       2  SET

         Set protocol options.

       3  QUERY

         Execute query.

       4  QUERY_RO

         Same as QUERY, but hints that this is a read-only

         query. This is only useful for routers/proxies who may want

         to redirect the request to a read slave.

       Result codes may be:

       1  OK

         Single packet success response. No data associated

         with the result besides parameters.

       2  ERROR

         Single packet error response.

       3  DATA

         Start of a multi-packet result set.

       3  DATA_END

         Mark the end of a series of data packets. This is

         useful so a low level router or proxy can know when a

         response is complete without inspecting the contents of

         the packets.

2-byte Client ID length.

X-byte Client ID (length is value of client ID length).

The client ID is there for the client and routers/proxies to use. The server

treats this as opaque data, and will only preserve it to send

in responses. This can be used as a sharding key, to keep

state information in a proxy, or any other use.

Next, zero or more chunks are given, terminated by a chunk length of

0. Each chunk consist of a length and then that amount of data.

2-byte Chunk length

X-byte Chunk (length is value of chunk length)

After the the chunk length of 0 is given, a checksum value is given

that was computed for the entire packet.

4-byte Checksum

The second layer of the protocol is encapsulated inside of the

chunked encoding. This consists of zero or more packet parameters,

an end of parameter marker, followed by an optional data set that is

given until the end of a packet (or the end of all chunks).

Packet Parameters

-----------------

Packet parameter names are defined in a global namespace, although

not all parameters are relevant for all packet types. Parameters are

enumerated, and the name is specified with a 1-byte value representing

the enumerated name. Each packet parameter may have a value associated

with it, and each parameter defines the size and how that value is

given. The list of possible packet parameters are:

0   END_OF_PARAMETERS - Marks the end of a parameter list.

Parameters used for setting options:

1   AUTH              - 1-byte value with authentication mechanism

                        to use. Possible values are:

                        0 - None.

                        1 - MD5 on user and password.

                        2 - 3-way handshake.

2   CHECKSUM          - 1-byte value with preferred checksum

                        type. Possible values are:

                        0 - None.

                        1 - CRC32

3   COMPRESSION       - 1-byte value with preferred compression

                        type. Possible values are:

                        0 - None.

                        1 - zlib.

                        2 - bzip2.

4   FIELD_ENCODING    - 1-byte value with preferred field encoding

                        type. Possible values are:

                        0 - String.

                        1 - Native.

5   FIELD_INFO        - 1-byte value to determine if field information

                        should be sent. Possible values are:

                        0 - None.

                        1 - Send field info.

(6-63 Reserved for future options that can be set)

Parameters used in responses:

64  STATUS            - 4-byte bit field.

65  NUM_ROWS_AFFECTED - Length-encoded count of rows affected.

66  NUM_ROWS_SCANNED  - Length-encoded count of rows scanned.

67  NUM_WARNINGS      - Length-encoded count of warnings encountered.

68  INSERT_ID         - Last insert ID.

69  ERROR_CODE        - 4-byte error code.

70  ERROR_STRING      - Length-encoded string.

71  SQL_STATE         - Length-encoded string.

72  NUM_FIELDS        - 4-byte integer.

73  FIELD_START       - No value, starts a new set of field parameters.

74  FIELD_TYPE        - 2-byte enumerated type.

75  FIELD_LENGTH      - Length-encoded value.

76  FIELD_FLAGS       - 4-byte bit-field.

77  DB_NAME           - Length-encoded string.

78  TABLE_NAME        - Length-encoded string.

79  ORIG_TABLE_NAME   - Length-encoded string.

80  FIELD_NAME        - Length-encoded string.

81  ORIG_FIELD_NAME   - Length-encoded string.

82  DEFAULT_VALUE     - Length-encoded string.

(83-255 Reserved for future responses parameters)

"Length-encoded string" means a length-encoded value, followed by a

string of that length.

Command

-------

Inside of the chunked data, command packets consist of zero or more

parameters depending on which options are being set, followed by

a end of parameter marker, and then all data until the end of the

chunks are considered arguments for the command. For a QUERY, this

will be the actual query to run.

OK/ERROR

--------

The server responds with an OK or ERROR if no row data is given. A

list of parameters may follow, and the marked with an end of parameter

value.

DATA

----

A data packet consists of a series of parameters, followed by the end

of parameter, and then a series of length-encoded values holding field

values. The NUM_FIELDS parameter must be given before any values, as

this indicates when a start of a new row happens. The field values may

either be in string format or native data type, depending on the value

of FIELD_ENCODING.

There may be multiple rows inside of a single DATA result packet. In

the case of large result sets, the result should be split into multiple

DATA packets since other concurrent commands on the connection will

block if a single large packet is sent. By breaking resulting rows

into multiple DATA packets, other commands are then allowed to send

interleaved response packets.