# On The Difficulty Of Conjuring Up A Dryad

In which deploys are made boring.

When we started building Skyliner, our goal was to make deploys on AWS safe, reliable, and easy. To accomplish this at scale, we made some key design and implementation decisions. In this post, I’ll tell you what those decisions were, why they work, and how we built a system which is reliable enough to even deploy itself.

## Use A Finite-State Machine

Our earliest major design decision was to model the Skyliner deploy process as a Finite-State Machine (FSM), with transitions from one state to another associated with specific conditions and actions. For example, a deploy in the rollout-wait state will check the newly-launched instances of the deploy. If the instances are up and running, the deploy is advanced via rollout-ok to the evaluate-wait state. If the instances have failed to launch, the deploy is advanced via rollout-failed to the rollback state. If the instances are still launching, the deploy is kept in the rollout-wait state via rollout-in-progress.

Using an FSM allows us not just to exhaustively determine that all possible states of a deploy are handled, but also to decompose our own code into small, comprehensively-tested, state-specific functions. It also allows us to extract state management as first-class concern; unlike many deploy tools which keep this state in memory, we store an append-only history of deploy states. As a result, our code is reentrant: it can be interrupted at any time and safely resumed later. If one of our servers crashes another one can seamlessly take its place with no disruption in service.

## Use A Reliable Coordinator

Unlike some deploy tools which require a single “master” server to coordinate deploys, Skyliner uses Amazon’s Simple Queue Service (SQS)—a highly-available, scalable, reliable message queue service—as a distributed clock to advance each deploy’s state.

SQS has a very robust model for dealing with failures: when a consumer polls the server for a new message, it specifies a visibility timeout — a period during which that message will not be visible to other workers. If the consumer successfully processes the message, it deletes it from the queue. If the consumer crashes, the visibility timeout elapses and the message becomes visible to another consumer. Similarly, when sending a message to a queue, one can specify a delay — a period of time during which that message will not be visible to any consumer. We use the delay and the visibility timeouts to create “ticks” for deploys.

When a deploy is started, we send an SQS message with the deploy ID, environment, etc. using a delay of e.g. 10 seconds. After 10 seconds, it becomes visible to a Skyliner background thread, which receives it using a visibility timeout of e.g. 10 seconds. The thread looks up the deploy’s current state and takes any appropriate action to advance it. If the deploy has finished, the thread deletes the message from the queue. Otherwise, the message is left in the queue to reappear after another 10 seconds has passed.

If the instance handling the deploy crashes, the deploy’s message becomes visible after another 10 seconds, and another instance will receive it and advance the deploy.

## Use Blue-Green Deploys

Skyliner uses blue-green deploys. Instead of modifying servers in-place and hoping nothing goes wrong, we leverage EC2’s elasticity and launch an entirely new set of instances running the new version of your software. If the new version passes healthchecks, we roll forward by terminating the old instances. If the new version doesn’t come up cleanly, we roll back by terminating the new instances.

As a result, deploys on Skyliner are:

1. Reliable. Both rolling forward and backward requires only the termination of EC2 instances—no coordinating system package upgrades and downgrades, no modifying local Git checkouts or switching symlinks.

2. Safe. At no point in time is your application’s capacity reduced, so there’s no need to wait until a scheduled maintenance period to deploy.

3. Fast. Compared to other safe deploy strategies, Skyliner doesn’t have to reduce the rollout rate in order to maintain capacity. In a traditional data center, the set of servers an application can run on is typically fixed, and in order to maintain a minimum capacity, deploys are usually done in a “rolling” fashion, either making changes one server at a time or in small batches. Consequently, rolling deploys take $\mathcal{O}(\frac{N}{M})$ time for $N$ total servers and batches of size $M$. Blue-green deploys, on the other hand, can perform all their operations in parallel, and take $\Omega(1)$ time—only as long as the worst-case time of any single instance.

## Skyliner On Skyliner

The end result of these decisions is that several times a day, we can use Skyliner to deploy a new version of itself. When a build is finished, we use the old version to start the deploy, which launches EC2 instances with the new version of Skyliner. For a brief moment, both are running simultaneously until a background thread rolls the deploy forward and begins the termination of the EC2 instances running the old version. As those shut down, the instances running the new version oversee the cleanup of the deploy which brought them into existence.