What I Learned Writing a Dropbox Clone - Part 1 - Introduction

  commit 03764fc
  Author: Rob Hoelz <***@*****.**>
  Date:   Fri Mar 25 11:14:52 2011 -0500
      First commit
      I should really commit earlier on my projects...

Nearly four years ago, I made the above commit on a new project I'd just started. I had been using Dropbox for a little bit, and I liked the idea, but I favor free software solutions for services, particularly when my personal data (like files) are concerned. I looked around to see if anyone had written an FOSS Dropbox clone, and the offerings in that space were very meager (just SparkleShare, and I think maybe a nascent version of OwnCloud ). I had some misgivings about some of SparkleShare's design, and OwnCloud didn't offer the same features I needed, so I decided to go ahead and write my own. Because the project was an effort to distance myself from cloud products, I decided to call it Sahara Sync (the Sahara desert having few clouds). 1)

After nearly a year of development, I had something that mostly worked; it had a lot of automated tests, and I could drop a text file in to the sync directory (called the Sandbox, get it?), but wouldn't properly handle things like Vim saving a file and transporting it to other machines, for some odd reason. The test suite would occasionally reveal odd timing bugs or race conditions that were troublesome to find and fix.


This is the first of a multi-part series about what I learned while writing Sahara Sync (go figure). Before I get into the technical things I learned, I would like to discuss the overall design I had in mind for Sahara Sync.

The two main players were called hostd (host daemon) and clientd (client daemon). I avoided the term “server”, because the client daemon was also a sort of server, and I wanted to clarify that the host daemon was hosting your data. The host daemon offered up a RESTy API so that clients could upload changes and those changes would be broadcast to other clients. The host daemon was meant to be kept very simple; any additional features (eg. encryption of files before they are saved to the data store, history, etc) would be provided through plugins. The client daemon is responsible for detecting changes in the sync directory, responding to changes from other clients via the host daemon, and keeping those in sync. Here's a little graphic illustrating the flow of information:

Sahara Sync Information Flow

The host was written with decentralization in mind, so that you could more easily break away from a single instance of hostd. The reasoning behind this was you could run a hostd on a different machine if you wanted to use Sahara Sync to share files, but your normal hostd instance was not available. I had plans for clientd to be able to act like hostd to service this use case. I also had plans for hostd to behave as a client to other hostds, so you and your friends could share files between your own Sahara Sync instances, forming a federated network.

Another design decision that I made was to simplify clientd and decouple it from what I saw as optional functionality. There would be a notion of clientd plugins, for features like encryption, but orthogonal to that were what I called components (named after components in XMPP). Components existed in external processes, and would augment client's functionality. For example, a component could offer file manager integration, or a systray icon, or Growl notifications. Since they would be communicating with the clientd process via some form of IPC, they could also be written in any language.

The Sahara Sync Wiki contains a lot more of the ideas I had, if you're interested in reading about them.

In the next post, I'll cover what I learned about filesystem operations that POSIX-style operating systems provide, and what guarantees you can depend on when using them.

  1. 1) I actually bought the domain names for both saharasync.net and saharasuite.net; the latter was intended for other personal cloud tools I thought about writing that would compliment Sahara Sync. I had the idea of turning it into a company as well.