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Why the Replicated Stateful Set Operator Exists

As we all know, the accepted best practice for cloud applications is to create stateless services. However, some legacy workflows don't fit this model and may be non-trivial to rewrite or replace. Additionally, many applications will eventually end up persisting something somewhere, be it to a database, a key- value store, or a disk. Some past OSP summit speakers have advocated that databases and key-value stores be treated like filesystems and exist as a separate cluster to Kubernetes. While attractive in that we get to continue using existing best practices for recovering after lights out, this approach misses out on the benefits of containerisation and one-stop management of the application stack.

Other community members simplify the architecture by advocating a single copy of mysql, relying on Kubernetes restarts and shared storage to solve the problem. At best this leads to long recovery times for large databases, at worst the current lack of fencing could lead to stalled recovery or disk corruption.

So we want replication to avoid long recovery times but what about lights-out recovery?

Some rare applications will allow you start copies in any order and do the reconciliation for you, however most require the most up-to-date copy to start first.

It seems apparent that Stateful Sets were originally created for this use case, at first blush it looks like pod 0 will always start first and by virtue of being the last to stop, will have the most recent copy of the database. Unfortunately due to the lack of a feedback mechanism in the scheduler, it is trivial to arrange failure scenarios that allow pod 0 to fall behind - particularly during scale down events. Additionally, like the single-copy- architecture, the current lack of fencing can lead to stalled recovery and an inability to scale up or down in bare metal Kubernetes deployment.

More recently, the CoreOS folks successfully showed that some replicated applications can be modelled with their operator pattern.

The etcd and Prometheus implementations are based on ephemeral storage, requiring periodic backups to (currently) AWS and a full restore after lights- out. Apart from the reliance on AWS for cold starts (possible security and/or compliance issues), databases (particularly those in OSP) can grow rather large resulting in extended recovery times.

The Replicated Stateful Set Operator is therefor a new operator based on Stateful Sets and persistent storage. Using Stateful Sets allows us to reuse core functionality for much of the upgrade and restart logic (the CoreOS folks implemented their own), allowing the operator to focus on figuring out which copy has the most recent version of the data and arranging it to start first.

To do this, we allow the admin to specify hooks for:

  • detecting an application's sequence number seeding the application (first
  • primary) starting subsequent primaries stopping the application

During a lights-out recovery, the operator will:

  • wait for the Stateful Set to launch the application's containers (CMD for the container should not actually start the application)

  • use Kubernetes APIs to execute the sequence number hook on each container (an integer response to stdout is required)

  • determine the container with the largest value, and use Kubernetes APIs to execute the seed hook there

  • use Kubernetes APIs to execute the primary hook on the remaining copies

To make life easier for container authors, we automatically create an internal service so that the peers are addressable as '{pod_name}.{service_name}' and the list of active primaries is passed to the primary hook on the command line.

With an eye to the future, we also allow for status checks and the ability to have copies that are not primaries, whatever that may mean to the app (perhaps read-only replicas?).