A reverse proxy that allows scaling down a cloud service to zero servers without sacrificing (eventual) availability.

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Pullcord will be a reverse proxy for cloud-based web apps that will allow the servers the web apps run on to be turned off when not in use. Pullcord should be nimble enough to run on the smallest cloud servers available, but it will be able to quickly spin up much larger servers running much bulkier web apps. Once traffic to these larger servers has stopped for a specified period of time, these other servers will be spun back down. Pullcord will be able to perform these duties for multiple web apps simultaneously, so there is no need to have more than one Pullcord server, and there is no need to consolidate multiple web apps onto a single server (in fact, Pullcord will likely work better if each web app is on its own server, as it will still be possible to spin up multiple servers if there are dependencies between the web apps).

For more information, email Charlie.

Acceptance Testing

To aide in acceptance testing, first run the tests and build the container from a clean copy of the codebase:

make clean container

Then run the pullcord container with the default config:

docker run \
	-d \
	--name pullcord-acceptance \
	-p \

If you would like to try the example login config instead (the user/password is admin):

docker run \
	-d \
	--name pullcord-acceptance \
	-p \
	pullcord --config example/login.json

Now visit your local pullcord instance.

To follow along with the logs:

docker logs -f pullcord-acceptance

To clean up and collect logs when finished:

docker kill pullcord-acceptance || echo "Container already killed, continuing."

docker logs pullcord-acceptance > pullcord-acceptance-`date +%s`.log

docker rm pullcord-acceptance

To try a different config, you could set a different ${PULLCORD_CONFIG_PATH} in this command (be sure it evaluates to a full path):

docker run \
	-d \
	--name pullcord-acceptance \
	-p \
	-v `dirname ${PULLCORD_CONFIG_PATH}`:/config \
	pullcord --config /config/`basename ${PULLCORD_CONFIG_PATH}`

Common make targets

Just clean up any lingering out-of-date artifacts:

make clean

Just run tests:

make test

Just build binary (will run tests if they have not yet been run):



make all

Just build container (will build binary if needed):

make container

The Main Problem

Over the years, Stuph Labs has used various web apps and other software daemons for our side projects (i.e. Gitolite, Trac, OpenVPN, SFTP, etc.), but this has required a server to be running all the time despite the fact that we'd only use this server for a few random hours a month. At least now that cloud computing has become more popular, we are no longer restricted to choosing between expensive dedicated servers or Seriously over-provisioned and inflexible shared hosting. However, manually going into the various cloud consoles and turning servers on and off is a hassle at best, and even when another potential user is sufficiently trusted that they are given copies of the administrative credentials (often in an insecure way to begin with), it is unrealistic to think that such a user would log in to start and stop these servers as needed when doing so requires that they use an unfamiliar and very complicated interface that they realize is full of buttons that, if accidentally pressed, could incur extreme costs in a very short span of time. As a result, just as before modern cloud computing was an option, we have often resorted to either eating the hefty cost of a properly equipped server that is only used 1% of the time, or else using a seriously under-powered server that causes a great deal of frustration to the users and is still only used 1% of the time. One of the things that modern cloud computing has given us is the ability to quickly, easily, and automatically scale from as little as one server up to thousands in a short amount of time, and then almost as quickly scale the number of servers back down again. While this has enabled regularly utilized services to have a server footprint that more accurately matches their needs (and thus save a tremendous amount of money without sacrificing availability or performance), the same cannot be said of very lightly utilized services at this time.

The Secondary Problem

There are a variety of reasons we may install pieces of software at some point, but there are also many reasons we may choose not to update these pieces of software (perhaps we are trying to decide on which version of the software to use elsewhere, perhaps we are trying to test the scope and ease of exploitation of a known vulnerability, or perhaps we are just too busy/lazy to get around to updating every single piece of software we aren't sold on yet anyway). While most all of the software that we wouldn't want to update would not be used for legitimate data we would care about, we are certainly aware that data leakage is by no means the only thing one should worry about when it comes to information security. As a result, we often either spend a frustrating amount of time updating software we haven't decided if we care about at all, or else we choose not to install the software in the first place for fear of running into this very predicament. Today it is common to install such pieces of software in virtual machines either manually or using some tool like Vagrant, but setting up such services in a way that many people can test them over the internet is tedious, error-prone, and depends on an always on system with reliable internet access, at which point we are right back to the very reasons we use external hosting in the first place. We could just have a beefy server that is always on and running VM host software, but it is a waste of such a machine if it is only occasionally used, and it wouldn't be able to scale if you wanted to run very many of these services at once for even a short time.

The (Possible) Solution

It should be possible for Pullcord to sit on one small server and launch other much more powerful cloud servers which host the desired pieces of software. Once the desired pieces of software have gone unused for a period of time, the cloud servers will be automatically turned back off. While it may take a few minutes for these pieces of software to first become available, they will not feel sluggish at all once they are running. Furthermore, the total cost should remain low since these cloud servers were only used for a short time. Also, if Pullcord is used as a proxy for these servers, then the potentially vulnerable pieces of software would only be exposed to properly authenticated and authorized users of the Pullcord service.

Initial Design Considerations

The proposed solution would have some tedious aspects (i.e. an HTTP proxying mechanism, a cookie handler, etc.), but the internal complexity should be relatively low, and so a minimalist design with modular functionality would likely be the best choice. At this point (which is admittedly after some initial development work), it would appear that this solution could be split into a few largely distinct components: the remote services monitoring system, the remote service launching/destructing (trigger) system, the user authentication system, the proxying system, and the configuration system.

Initial Development Considerations

Many programming languages would be acceptable for this project, but I chose to use Go as it seemed well-suited for the task, and as such this seemed like a good opportunity to try out the language on a larger project than the original Go tutorial. There is a Go library called Falcore which could prove useful. Given the likely low algorithmic complexity involved with solving this problem, it should be possible to develop a solution using many minimal changes in an iterative development technique. This also has the advantage of lending the process to test-driven development techniques. However, it is important to continuously update the documentation, something which many developers (including myself) have often been bad at. By adding some tests to the continuous integration process to check that both code coverage and the documentation ratio doesn't drop below a certain threshold, it should be possible to keep me honest.