CRL Scheduler

The CRL Scheduler is the k8s scheduler compiled with a custom scheduling plugin.

Additional plugin examples can be found here.

It is currently compiled into the scheduler for kubernetes 1.16.

The Issue

Deploying a statefulset across multiple availability zones may encounter some strange issues when changing the number of replicas.

Consider the following deployment of a statefulset with 4 replicas spread across 3 zones.

A	B	C
0	2	3
1

Suppose the number of replicas gets scaled down to 3

A	B	C
0	2
1

We've now lost the availability zone C.

Suppose we scale down to 1 replica and then back up to 4, again.

A	B	C
0	1	2
3

The scheduling of these pods is non-deterministic (Especially if nodes are being added and removed while the number of replicas change).

Pods 1, 2, and 3 are now unschedulable as their PVs (which are not removed by the statefulset controller) are still in the previous zones.

This scheduler plugin "locks" statefulset pods to specific zones bases on their ordinal, giving us deterministic scheduling at a zonal level.

# Build the docker image tagging as the current commit
make build

# Tag the image with todays date and push to GCR
make release