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Protozilla: pipes, protocols, and P2P in Mozilla

(If you were directed to this page from a recent Slashdot discussion on Protozilla, you may want to check out a posting on the Protozilla newsgroup that addresses some of issues raised in that discussion.)

Protozilla is a browser add-on that makes it very easy to implement protocols in Mozilla (or Netscape 6.x). It is not a traditional browser plugin, but may be described as a "socket adapter", like the kind that you may carry around with your laptop when you travel internationally. With Protozilla, implementing a new protocol becomes as easy as typing a one line URL prefix, or dragging-and-dropping a file. Protozilla enables any CGI program or Unix/MSDOS command line program to instantly become a Mozilla "plugin" with little or no modification. These external programs, which may be written in any language, communicate with Mozilla through their standard input/output and the environment variables.

Before you begin to wonder why anyone would want to do this, here are some applications: interprocess communication, client-side CGI, protocol handlers, and peer-to-peer applications. (See the Examples and Documentation pages for more information on using Protozilla.)

Here is a brief technical overview of Protozilla: Mozilla communicates with the outside world using TCP sockets and multi-threaded asynchronous I/O. What Protozilla does is to execute an external program as a separate process, but makes it appear to Mozilla like a TCP socket. Protozilla communicates with the process using pipes and synchronous I/O, but uses asynchronous I/O to communicate with the rest of Mozilla. The core portion of Protozilla, which deals with pipes and IPC, is an XPCOM component written in C++. It provides an interface called nsIPipeTransport similar to the nsISocketTransport interface for sockets. The rest of Protozilla, dealing with protocols, is implemented as a Javascript component. (At present, the C++ portion of Protozilla has been tested on Unix/Linux and Windows platforms only.)

Of course, an out-of-process implementation of a protocol is not going to be as efficient as a multi-threaded, in-process implementation. But there is a trade-off between efficiency and ease of implementation. Protozilla is better suited for experimental protocol implementations, rather than for mature protocols. Out-of-process CGI execution is considered a *bad* thing on an HTTP server with a large number of users, but it is not as serious a performance issue on a client that is serving just one user. An out-of-process implementation does have one advantage though--if an experimental protocol implementation hangs or crashes, it will not drag the rest of the browser down with it!

The protozilla project can be contacted through the mailing list or the member list.
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