This document describes version 30.5 of the protocol used by Xapian's remote backend. Clients and servers must support matching major protocol versions and the client's minor protocol version must be the same or lower. This means that for a minor protocol version change, you can upgrade first servers and then clients and everything should work during the upgrades.
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 generally 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.
REPLY_GREETING <protocol major version> <protocol minor version> I<db doc count> I<last docid> B<has positions?> F<db average length>
The protocol major and minor versions are passed as a single byte each (e.g.
'\x1e\x01' for version 30.1). The server and client must
understand the same protocol major version, and the server protocol minor
version must be greater than or equal to that of the client (this means that
the server understands newer MSG_XXX, but will only send newer
REPLY_YYY in response to an appropriate client message.
REPLY_EXCEPTION <serialised Xapian::Error object>
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.
MSG_ALLTERMS
REPLY_ALLTERMS I<term freq> L<term name>
...
REPLY_DONE
MSG_TERMEXISTS <term name>
REPLY_TERMEXISTS or REPLY_TERMDOESNTEXIST
MSG_TERMFREQ <term name>
REPLY_TERMFREQ I<term freq>
MSG_COLLFREQ <term name>
REPLY_COLLFREQ I<collection freq>
MSG_DOCUMENT I<document id>
REPLY_DOCDATA L<document data>
REPLY_VALUE I<value no> <value>
...
REPLY_DONE
MSG_DOCLENGTH I<document id>
REPLY_DOCLENGTH F<document length>
MSG_KEEPALIVE
REPLY_DONE
MSG_REOPEN
REPLY_UPDATE I<db doc count> I<last docid> B<has positions?> F<db average length>
The reply is the same as for MSG_UPDATE.
MSG_QUERY L<serialised Xapian::Query object>
I<query length> I<collapse key number> <docid order>
I<sort key number> <sort by> B<sort value forward>
<percent cutoff> F<weight cutoff> <serialised Xapian::Weight object> <serialised Xapian::RSet object>
REPLY_STATS <serialised Stats object>
MSG_GETMSET I<first> I<max items> I<check at least>
<serialised global Stats object>
REPLY_RESULTS <serialised Xapian::MSet object> <percent_factor>
(Instead of MSG_GETMSET and REPLY_RESULTS, clients running protocol 30.3 or 30.4 have:
MSG_GETMSET_PRE_30_5 I<first> I<max items> I<check at least>
<serialised global Stats object>
REPLY_RESULTS_PRE_30_5 <serialised Xapian::MSet object>
and clients running protocol 30.2 or earlier send:
MSG_GETMSET_PRE_30_3 I<first> I<max items>
<serialised global Stats object>
REPLY_RESULTS_PRE_30_5 <serialised Xapian::MSet object>
).
docid order is '0', '1' or '2'.
sort by is '0', '1', '2' or '3'.
MSG_TERMLIST I<document id>
REPLY_DOCLENGTH F<document length>
REPLY_TERMLIST I<wdf> I<term freq> L<term name>
...
REPLY_DONE
MSG_POSITIONLIST I<document id> <term name>
REPLY_POSITIONLIST I<termpos delta - 1>
...
REPLY_DONE
Since positions must be strictly monotonically increasing, we encode (pos - lastpos - 1) so that small differences between large position values can still be encoded compactly. The first position is encoded as its true value.
MSG_POSTLIST <term name>
REPLY_POSTLISTSTART I<termfreq> I<collfreq>
REPLY_POSTLISTITEM I<docid delta - 1> I<wdf> F<document length>
...
REPLY_DONE
Since document IDs in postlists must be strictly monotonically increasing, we encode (docid - lastdocid - 1) so that small differences between large document IDs can still be encoded compactly. The first document ID is encoded as its true value - 1 (since document IDs are always > 0).
MSG_SHUTDOWN
No reply is sent - this message signals that the client has ended the session.
MSG_UPDATE
REPLY_UPDATE I<db doc count> I<last docid> B<has positions?> F<db average length>
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).
MSG_ADDDOCUMENT <serialised Xapian::Document object>
REPLY_ADDDOCUMENT I<document id>
MSG_DELETEDOCUMENT I<document id>
REPLY_DONE
MSG_DELETEDOCUMENT_PRE_30_2 I<document id>
MSG_DELETEDOCUMENTTERM <term name>
MSG_REPLACEDOCUMENT I<document id> <serialised Xapian::Document object>
MSG_REPLACEDOCUMENTTERM L<term name> <serialised Xapian::Document object>
MSG_CANCEL
MSG_FLUSH
REPLY_DONE