Vaurien: The Chaos Proxy

I had a need to test out a program's behavior when its backend web server returned errors. Unit testing would have been difficult in this case, because I specifically wanted to test the program's macro result when encountering, say, 10% server response errors.

After searching around, I came across Vaurien, the Chaos TCP Proxy. It's seriously cool. As its name suggests, it's a TCP proxy server that you can route a local connection through to a backend server. In TCP mode, it can delay packets, insert bad data, or drop packets, testing the socks (pun intended) off your application.

It also supports the application-level protocols: HTTP, Memcache, MySQL, Redis, and SMTP, and its architecture is very modular, so it's easy to plug in new protocols.

For example, here's how I can run it in HTTP mode:

$ vaurien --protocol http --proxy 127.0.0.1:8888 \
          --backend www.google.com:80 --behavior 50:error

This says to run an HTTP proxy that connects to google.com on the backend and return a 5xx HTTP error code 50% of the time. Testing it out with curl:

$ curl --head -H "Host: www.google.com" http://127.0.0.1:8888/
HTTP/1.1 200 OK
...

$ curl --head -H "Host: www.google.com" http://127.0.0.1:8888/
HTTP/1.1 500 Internal Server Error
Content-Type: text/html; charset=UTF-8

$ curl --head -H "Host: www.google.com" http://127.0.0.1:8888/
HTTP/1.1 502 Bad Gateway
Content-Type: text/html; charset=UTF-8

Filed this away as a super useful tool.

Git Branches by Date

I tend to create a huge number of branches in my git repositories, but I have a bad habit of not cleaning them up once I'm finished with them.

I found a nice command from an answer on StackOverflow that allows you to sort branches by date. I modified it slightly to also show the date when printing the branch information:

$ git for-each-ref --sort=-committerdate refs/heads/ --format='%(committerdate) %09 %(refname:short)'
Mon Jan 14 23:46:15 2013 +0000   keyfile-seek-rebase
Sat Jan 12 02:09:04 2013 +0000   perfdiag-mbit-fix
Fri Jan 11 17:38:13 2013 -0800   keyfile-seek
Thu Jan 10 01:05:43 2013 +0000   master

You can also set the date format to be relative (or other possibilities, see man git-for-each-ref):

$ git for-each-ref --sort=-committerdate refs/heads/ --format='%(committerdate:relative) %09 %(refname:short)'
3 days ago   keyfile-seek-rebase
6 days ago   perfdiag-mbit-fix
6 days ago   keyfile-seek
8 days ago   master

I added it to my .gitconfig file as an alias:

[alias]
    branchdates = for-each-ref --sort=-committerdate refs/heads/ --format='%(committerdate:relative) %09 %(refname:short)'

That allows me to just type git branchdates and get a nice listing of my local branches by date.

Google Cloud Storage Signed URLs

Google Cloud Storage has a feature called Signed URLs, which allows you to use your private key file to authorize access to a specific operation to a third-party.

Putting all the bits together to create a properly signed URL can be a bit tricky, so I wrote a Python example that we just open-sourced in a repository called storage-signedurls-python. It demonstrates signing a PUT, GET, and DELETE request to Cloud Storage.

The example uses the awesome requests module for its HTTP operations and PyCrypto for its RSA signing methods.