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The HandlerSocket protocol

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Basic syntax

- The HandlerSocket protocol is line-based. Each line ends with LF(0x0a).
- Each line consists a concatenation of tokens separated by HT(0x09).
- A token is either NULL or an encoded string. Note that you need to
  distinguish NULL from an empty string, as most DBMs does so.
- NULL is expressed as a single NUL(0x00).
- An encoded string is a string with the following encoding rules.
  	- Characters in the range [0x10 - 0xff] are not encoded.
	- A character in the range [0x00 - 0x0f] is prefixed by 0x01 and
	  shifted by 0x40. For example, 0x03 is encoded as 0x01 0x43.
- Note that a string can be empty. A continuation of 0x09 0x09 means that
  there is an empty string between them. A continuation of 0x09 0x0a means
  that there is an empty string at the end of the line.

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Request and Response

- The HandlerSocket protocol is a simple request/response protocol. After a
  connection is established, the client side sends a request, and then the
  server side sends a response.
- A request/response consists of a single line.
- Requests can be pipelined; That is, you can send multiple requests (ie.
  lines) at one time, and receive responses for them at one time.

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'open_index' request

The 'open_index' request has the following syntax.

    P <indexid> <dbname> <tablename> <indexname> <columns>

- <indexid> is a number in decimal.
- <dbname>, <tablename>, and <indexname> are strings. To open the primary
  key, use PRIMARY as <indexname>.
- <columns> is a comma-separated list of column names.

Once an 'open_index' request is issued, the HandlerSocket plugin opens the
specified index and keep it open until the client connection is closed. Each
open index is identified by <indexid>. If <indexid> is already open, the old
open index is closed. You can open the same combination of <dbname>
<tablename> <indexname> multple times, possibly with different <columns>.
For efficiency, keep <indexid> small as far as possible.

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Getting data

The 'find' request has the following syntax.

    <indexid> <op> <vlen> <v1> ... <vn> <limit> <offset>

- <indexid> is a number. This number must be an <indexid> specified by a
  'open_index' request executed previously on the same connection.
- <op> specifies the comparison operation to use. The current version of
  HandlerSocket supports '=', '>', '>=', '<', and '<='.
- <vlen> indicates the length of the trailing parameters <v1> ... <vn>. This
  must be smaller than or equal to the number of index columns specified by
  specified by the corresponding 'open_index' request.
- <v1> ... <vn> specify the index column values to fetch.
- <limit> and <offset> are numbers. These parameters can be omitted. When
  omitted, it works as if 1 and 0 are specified.

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Updating/Deleting data

The 'find_modify' request has the following syntax.

    <indexid> <op> <vlen> <v1> ... <vn> <limit> <offset> <mop> <m1> ... <mk>

- <mop> is either 'U' (update) or 'D' (delete).
- <m1> ... <mk> specifies the column values to set. The length of <m1> ...
  <mk> must be smaller than or equal to the length of <columns> specified by
  the corresponding 'open_index' request. If <mop> is 'D', these parameters
  are ignored.

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Inserting data

The 'insert' request has the following syntax.

    <indexid> '+' <vlen> <v1> ... <vn>

- <vlen> indicates the length of the trailing parameters <v1> ... <vn>. This
  must be smaller than or equal to the length of <columns> specified by the
  corresponding 'open_index' request.
- <v1> ... <vn> specify the column values to set. For columns not in
  <columns>, the default values for each column are set.

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Response syntax

HandlerSocket returns a response of the following syntax for each request.

    <errorcode> <numcolumns> <r1> ... <rn>

- <errorcode> indicates whether the request has successfully executed or not.
  '0' means success. Non-zero means an error.
- <numcolumns> indicates the number of columns of the result set.
- <r1> ... <rn> is the result set. The length of <r1> ... <rn> is always a
  multiple of <numcolumns>. It is possible that <r1> ... <rn> is empty.

If <errorcode> is non-zero, <numcolumns> is always 1 and <r1> indicates a
human-readable error message, though sometimes <r1> is not provided.

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Response for 'open_index'

If 'open_index' is succeeded, HandlerSocket returns a line of the following
syntax.

    0 1

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Response for 'find'

If 'find' is succeeded, HandlerSocket returns a line of the following
syntax.

   0 <numcolumns> <r1> ... <rn>

- <numcolumns> always equals to the length of <columns> of the corresponding
  'open_index' request.
- <r1> ... <rn> is the result set. If N rows are found, the length of <r1>
  ... <rn> becomes ( <numcolumns> * N ).

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Response for 'find_modify'

If 'find_modify' is succeeded, HandlerSocket returns a line of the following
syntax.

   0 1 <nummod>

- <nummod> is the number of modified rows.

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Response for 'insert'

If 'insert' is succeeded, HanderSocket returns a line of the following
syntax.

   0 1