Spectrum 2 is an XMPP transport/gateway and XMPP server. It allows XMPP users to communicate with their friends who are using one of the supported networks. It supports a wide range of different networks such as ICQ, XMPP (Jabber, GTalk), AIM, MSN, Facebook, Twitter, Gadu-Gadu, IRC and SIMPLE. Spectrum is written in C++ and uses the Swiften XMPP library and different libraries for legacy networks. Spectrum 2 is open source and released under the GNU GPL2+ license.

h2. Gateway mode and server mode

Spectrum 2 can work in two modes: Gateway mode and Server mode. This chapter describes differences betweeen those two modes. If you want to find out how to configure Spectrum 2 to run in gateway or server mode, read the Configuration part of this documentation.

h3. Gateway mode

Gateway mode represents the classic way how XMPP gateway works. You have to configure an external XMPP server (like Prosody or Ejabberd) to serve the subdomain you want to use for Spectrum 2 (for example "icq.domain.tld"). Spectrum 2 in gateway mode then connects the XMPP server as its component and users are able to find out "icq.domain.tld" in Service Discovery, register it and use it.

*Advantages:*
* Users can use more legacy networks using single XMPP account (and using single TCP connection).
* It's easy to extend existing XMPP servers using gateway mode.

*Disadvantages:*
* Passwords are stored (even in encrypted form) on server.
* Roster (contact list) synchronization can be problematic, because it depends on the client user uses. This can be improved by usage of Remote Roster protoXEP.
* You have to setup XMPP server and use database even if you only want to use Spectrum 2 as a tool to connect legacy networks using XMPP protocol.

h3. Server mode

In server mode, Spectrum 2 behaves as standalone server. User then logins legacy networks by logging XMPP account like this one: "my_msn_name%hotmail.com@msn.domain.tld".

*Advantages:*
* Passwords are not stored on server.
* Roster synchronization is easy, because Spectrum 2 acts as normal server.
* If you want to use Spectrum 2 as wrapper between different networks, you don't need database or Jabber server as another layer.
* Using SRV records you can easily run Spectrum 2 on different machines to scale it.

*Disadvantages:*
* Clients have to support more accounts to connect more legacy networks (Therefore they will need have to use more TCP connections).

h2. Spectrum 2 instance, Spectrum 2 main process and backends

This chapter describes differences between Spectrum 2 instance, Spectrum 2 main process and Spectrum 2 backends.

h3. Spectrum 2 instance

In server mode, Spectrum 2 instance is single XMPP server. In gateway mode, it is single XMPP gateway. We can say these statements about Spectrum 2 instance:

* One instance is defined by one configuration file (By default stored in /etc/spectrum2/transports/).
* Spectrum 2 instance is represented to end user by its Jabber ID (for example icq.domain.tld).
* Internally every Spectrum 2 instance consists of Spectrum 2 main process and Spectrum 2 backends.
* One instance can run just one type of Spectrum 2 backend.

h3. Spectrum 2 main process

Spectrum 2 main process is the main process of Spectrum 2 instance.

* Spectrum 2 main process is responsible for running Spectrum 2 backends and routing users' requests to proper backend.
* By default the logs of Spectrum 2 main process are stored in /var/log/spectrum2/<jid>/spectrum2.log.

h3. Spectrum 2 backend

Spectrum 2 backend is special application run by Spectrum 2 main process. The goal of Spectrum 2 backend is to handle users's sessions. Spectrum 2 main process can handle more Spectrum 2 backends, but all messages from single user are always handled by the same backend.

One Spectrum 2 instance can run only one type of Spectrum 2 backend.