Remote Backend Protocol

This document describes version 22 of the protocol used by Xapian's remote backend. The protocol assumes a reliable two-way connection across which arbitrary data can be sent - this could be provided by a TCP socket for example (as it is with xapian-tcpsrv), but any such connection could be used. For example, you could used xapian-progsrv across an ssh connection, or even a custom server across a suitable serial connection.

All messages start with a single byte identifying code. A message from client to server has a MSG_XXX identifying code, while a message from server to client has a REPLY_XXX identifying code (but note that a reply might not actually be in response to a message - REPLY_GREETING isn't - and some messages result in multiple replies).

The identifying code is followed by the encoded length of the contents followed by the contents themselves.

Inside the contents, strings are general passed as an encoded length followed by the string data (this is indicated below by L<...>) except when the string is the last or only thing in the contents in which case we know the length because we know the length of the contents so we don't need to explicitly specify it.

Integers are encoded using the same encoding used for string lengths (indicated by I<...> below).

Floating pointing values are passed using a bit packed encoding of the sign and exponent and a base-256 encoding of the mantissa which avoids any rounding issues (assuming that both machines have FLT_RADIX equal to some power of 2). This is indicated by F<...> below.

Boolean values are passed as a single byte which is the ASCII character value for 0 or 1. This is indicated by B<...> below.

Server greeting and statistics

The protocol version is simply passed as a single byte ('\x09' for version 9).

Exception

If an unknown exception is caught by the server, this message is sent but with empty contents.

This message can be sent at any point - the serialised exception is unserialised by the client and thrown. The server and client both abort any current sequence of messages.

All Terms

Term Exists

Term Frequency

Collection Frequency

Document

Document Length

Keep Alive

Reopen

Query

docid order is '0', '1' or '2'.

sort by is '0', '1', '2' or '3'.

Termlist

Shut Down

No reply is sent - this message signals that the client has ended the session.

Update

Only useful for a WritableDatabase (since the same statistics are sent when the connection is initiated in the REPLY_GREETING and they don't change if the database can't change).

Add document

Delete document

Replace document

Cancel

Flush