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+---
+layout: page
+title: Spectrum 2
+---
+
+## Spectrum backend protocol
+
+Spectrum 2 communicates with backend using protocol based on [Google Protocol Buffers](http://code.google.com/p/protobuf/). This protocol has libraries for many languages, so backend can be written practically in any language.
+
+This document describes how to implement new backend in different language than C++.
+
+### Backend execution
+
+When XMPP user wants to login to legacy network, Spectrum 2 executes particular backend according to configuration. Typically it executes backend with following arguments:
+
+	/usr/bin/backend --host localhost --port 31640 --service.backend_id=1 "/etc/spectrum2/transports/sample_client.cfg"
+
+As the first thing after start, backed should connect Spectrum 2 main instance according to `--host` and `--port` argument. `--service.backend_id` is ID of backend and the last argument is always path to config file. Note that Spectrum 2 can pass more
+arguments to backend and backend should ignore them.
+
+### The protocol
+
+When connection betwen backend and Spectrum 2 is establish, Spectrum 2 starts communicating with the backend.
+
+The protocol is defined in in [include/transport/protocol.proto](https://github.com/hanzz/libtransport/blob/master/include/transport/protocol.proto) file. Usually there is tool for your language to compile this .proto file into source module which can be later used to serialize/deserialize structures in your language and make packets from them.
+
+Once you serialize particular Protocol Buffer structure, you can send it in following format:
+
+	| size | serialized Protocol Buffer structure |
+
+* size - 4 bytes long size of "serialized Protocol Buffer structure" in network encoding (as returned by htonl() function - see "man htonl")
+
+### Receiving PING message
+
+First message sent by Spectrum 2 is always PING. Backend has to response this message with PONG.
+
+Your backend will receive 4 bytes in network encoding. You should parse those 4 bytes to number representation and store it into variable N. Then you should read N bytes from the socket to receive serialized Protobuf message called WrapperMessage. Use the
+module generated from protocol.proto to parse it.
+
+WrapperMessage has two fields, type and payload:
+
+* type - Type of the real message stored in the payload field.
+* payload - Real message wrapped in this WrapperMessage. This is again serialized Protobuf message and can be parsed using the module generated from protocol.proto
+
+In our case, the type is TYPE_PING and the payload is not defined. You should now answer the PING message
+
+### Answering PING message
+
+PING message has to be answered with PONG. To Answer this message, just generate WrapperMessage using the module generated from protocol.proto and set the type to TYPE_PONG. Now serialie the message, prepend it with its size as defined above and send it to Spectrum 2.
+
+
+