Tuesday 26 August 2014

Private Source Maps

Since the launch of source maps, you have been asking for a way to support source maps without having to make all your code public. Today I'm glad to announce private source map support in Errorception.

Rather than explain how the feature works, I shall take you through the thought process behind private source maps support. It has taken 2 months to fine-tune the experience. I was actively working with a bunch of people in a private beta for this feature. (Thanks to you fine folk. You are awesome!)

A quick primer: Source map files, generated by your minifier, contains a mapping between your original source code and your minfied code. Your source map files are linked to from within your minified code, using a //#sourceMappingUrl=... comment in your minified code, inserted by your minifier. The source map file doesn't contain the original source code itself — just the mappings. Instead, the source map file links to your source code, which a consumer (like Errorception, or your browser's dev tools) will have to fetch separately.

The First Cut

When I had launched source map support two months ago, I had already written the code for private source maps too. The way I intended it to work was that I'd ask you to upload your source map files and your source code to Errorception as part of your deploy script. I wasn't sure how everyone would take to the idea of making an API call from within their deploy script, so I decided to start a private beta and work with a few select people directly to see what their experience would be.

However, everyone disliked the idea of making API calls from their deploy scripts. Every single one of them! Here's the big reasons:

  • It introduces a remote network dependency in the deploy script. This means that the deployment's success or failure depends on a third-party service (Errorception in this case), and on the network. This simply isn't a good idea.
  • If, for some reason, the API call fails, it will cause inconsistency in data at Errorception's end. It's not just that source maps won't work, it's worse than that — source maps will point to the wrong location in your files! That's misleading, and actively inhibits easy debugging. Not good at all.
  • Making API calls isn't really the simplest of things to do, especially when you have to do it from a deploy script. You'd first have to prepare your bundle by zipping up your files (which is likely to be a pretty big zip), then send it across to Errorception using some HTTP client, after you've figured out how to do multipart uploads. To deal with failures, you'd also have to implement some kind of retry mechanism. It doesn't have to be this complicated.
  • As an aside, this is also tremendously wasteful. Unless your errors are all over your code-base, Errorception doesn't need all your files at all. The bulk of the zip you'll have uploaded won't be useful for debugging at all, but there's no way to know which parts are needed beforehand.

Errorception's Crawler

Instead of asking you to upload your source maps and source code to Errorception, we decided that you could instead upload the files to your own web server/CDN. This is much more easy to do, considering that your deploy script already does this with your built code. It also eliminates the external network request to Errorception at deploy-time, which makes your deployment script simpler and far more reliable.

Errorception already has a crawler that crawls your site to get the required JavaScript files, needed both for the code view and for source maps support. This crawler is actually pretty powerful, and has evolved quite a bit within its few months of existence. It has baked into it a couple of really cool ideas to ensure consistency of data. This ensures that Errorception can always show you the correct version of the file that caused the error, even if the file has since changed with newer deployments. It is also resilient to network failures, retrying failed requests intelligently, and retroactively updating existing errors with the file. (Building this has been pretty crazy engineering-wise — heck, I even had to create a versioned file store to manage file versions correctly!)

Let's say you have an error in one of your script files. Errorception's crawler parses your JavaScript file to look for the //#sourceMappingUrl pragma comment. If it finds this comment, it already has everything it needs to crawl your source map files and your source code. This is how public source maps work already.

However, many people would rather not have that //#sourceMappingUrl comment in their code. That's because this comment is the one link to all of your code, and will let anyone with a browser access the original unminified source code.

Private Source Maps

If this //#sourceMappingUrl comment is removed from your minified file, your source maps are now effectively private. This is because no one can know where you've put your files if there isn't a link pointing them to them. HTTP doesn't have any discovery mechanism built in, and a secret path is just as unguessable as a password, since no one else knows the secret. (This assumes that you don't have directory listing turned on.)

So, this is how private source maps work in Errorception: You specify a secret folder name in Errorception's Settings > Source Maps. This secret folder should be as unguessable as you would want a password to be. Then, modify your build/deploy script such that Errorception can find your source map on your web server by constructing a path that incorporates this secret folder. (More about this below.) Once the crawler gets your source map file, it has everything it needs to figure its way about your code.


Here's how Errorception uses your secret folder to discover your source map file: Let's say an error occurred in your script at http://example.com/script.js, and you've specified your secret folder to be deadbeef, Errorception will look for the source map at http://example.com/deadbeef/script.js.map. That is, it looks inside a secret folder (which is expected to be a sibling of the script file), for a file that has the same name as the script file with a .map appended to it.

To give you another example, if the error was in http://example.com/a/b/c/script.js and you specify your secret folder to be secret, Errorception will look for the source map at http://example.com/a/b/c/secret/script.js.map.

All of this sounds complicated, but it really isn't. In fact, in most cases, it will simply be one or two lines in your deploy script — to strip the //#sourceMappingUrl comment, and to copy your source map files and original source code to the secret folder. Doing stuff like copying and modifying files is exactly what deploy scripts are good at, so it plays to the strengths of the deploy script too.

But this isn't really private at all

Yes, in a sense, this is really only security by obscurity. However, it is security by obscurity in the same way that passwords are security by obscurity. As with good passwords, a good folder name would be just as unguessable. Since it is impossible to discover anything over HTTP if you don't explicitly link to it, unwanted access to your source code should be near-impossible.

That said, I can see how you might be worried that all your files are still public. I'm open to consider even more stringent security, if you like. Feel free to get in touch. However, like I said, you shouldn't have to worry about it in the first place.

Also, Errorception turned three last week. Drink one for Errorception! Cheers!