Eventmachine Based Ftp Server

The short version

I’ve released a demo of an FTP server for ruby, built on the EventMachine library.

Check it out here: http://github.com/yob/em-ftpd/tree

The full version

Inside our little web 2.0 bubble we all get very excited about shiny things like REST, web services and open JSON APIs, so it’s easy to forget in the business world with many trading partners and legacy systems, a technology that works tends to hang around for an awfully long time.

In the supply chain world, the most common way I’ve encountered for electronic trading between businesses (submitting orders, receiving invoices, etc) is over good ‘ole FTP.

Yes, it’s old and insecure. However it’s also simple, well understood, has plenty of library support and is now embedded in inventory management software all over the place. You work with what you’ve got.

For a few years I’ve maintained a custom FTP server for a client. It accepts uploaded orders from their customers, translates them to a format the clients system can understand and saves a copy to a outbox folder. Every few hours the clients inventory software connects and downloads the orders directly into their system. It’s hardly a high traffic site, maybe 20 transfers a day, all small text files.

The server was a small ruby daemon, based on the ftpd.rb script published by Chris Wanstrath, and it performed admirably. Recently I was asked to add some new features, and I realised over the years the code had turned into 800 lines of spaghetti. A rewrite was in order.

At the same time a few fellow Australian Rubyist’s had been playing around with EventMachine, a library that claims to help produce highly scalable network code. Scalability wasn’t my issue, but the examples I could see looked alarmingly clean and simple for network socket based applications. Best of all, there was no need to involve threads, the arch nemesis of maintainable code.

Clean and simple was the goal of my rewrite. Bingo.

It took a little while to come to a working solution, mainly because I found few public EventMachine samples that indicated how to open multiple sockets and share data between them. It turned out to be possible and my rewritten server went into production a few days ago.

There was no point asking this client to release their new server as open source, it’s fairly specific to their needs. However I was keen to provide a public demo of an EventMachine based FTP server, so I’ve published a new demo script on github: http://github.com/yob/em-ftpd/tree

It’s not particularly useful as a real FTP server - it hard codes the authentication tokens and provides a simulated directory structure. Hopefully it proves interesting or useful to someone out there though - as a basis for their own custom FTP server or some other multi-socket protocol.