Automatic K8s Pod Placement to Match External Service Zones

I wanted to share something I've worked a bit to solve regarding Kubernetes: its scheduler has no awareness of the network topology for external services that workloads communicate with. If a pod talks to a database (e.g AWS RDS), K8s does not know it should schedule it in the same AZ as the database. If placed in the wrong AZ, it leads to unnecessary cross-AZ network traffic, adding latency (and costs $).

I've made a tool I've called "Automatic Zone Placement", which automatically aligns Pod placements with their external dependencies.

Testing shows that placing the pod in the same AZ resulted in a ~175-375% performance increase. Measured with small, frequent SQL requests. It's not really that strange, same AZ latency is much lower than cross-AZ. Lower latency = increased performance.

The tool has two components:

1) A lightweight lookup service: A dependency-free Python service that takes a domain name (e.g., your RDS endpoint) and resolves its IP to a specific AZ.

2 ) A Kyverno mutating webhook: This policy intercepts pod creation requests. If a pod has a specific annotation, the webhook calls the lookup service and injects the required nodeAffinity to schedule the pod onto a node in the correct AZ.

The goal is to make this an automatic process, the alternative is to manually add a nodeAffinity spec to your workloads. But resources moves between AZ, e.g. during maintenance events for RDS instances. I built this with AWS services in mind, the concept is generic enough to be used for on-premise clusters to make scheduling decisions based on rack, row, or data center properties.

I'd love some feedback on this, happy to answer questions :)

How about, don't use Kubernetes? The lack of control over where the workload runs is a problem caused by Kubernetes. If you deploy an application as e.g. systemd services, you can pick the optimal host for the workload, and it will not suddenly jump around.

> To gather zone information, use this command ...

Why couldn't most of this information be gathered by lookup service itself? A point could be made about excessive IAM, but a simple case of RDS reader residing in a given AZ could be easily handled by simply listing the subnets and finding where a given IP belongs.

Kyverno requirement makes it limited. There is no "automatic-zone-placement-disabled" function in case someone wants to temporarily disable zone placement but not remove the label. How do we handle RDS Zone changing after workload scheduling? No automatic look up of IPs and Zones. What if we only have one node in specific zone? Are we willing to handle EC2 failure or should we trigger scale out?

Most people should start with a single-zone setup and just accept that there's a risk associated with zone failure. If you have a single-zone setup, you have a node group in that one zone, you have the managed database in the same zone, and you're done. Zone-wide failure is extremely rare in practice and you would be surprised at the number (and size of) companies that run single-zone production setups to save on cloud bills. Just write the zone label selector into the node affinity section by hand, you don't need a fancy admission webhook if you want to reduce chance's factor.

If you decide that you want to handle the additional complexity of supporting failover in case of zone failure, the easiest approach is to just setup another node group in the secondary zone. If the primary zone fails, manually scale up the node pool in the secondary zone. Kubernetes will automatically schedule all the pods on the scaled up node pool (remember: primary zone failure, no healthy nodes in the primary zone), and you're done.

If you want to handle zone failover completely automatically, this tool represents additional cost, because it forces you to have nodes running in the secondary zone during normal usage. Hopefully you are not running a completely empty, redundant set of service VMs in normal operation, because that would be a colossal waste of money. So you are presuming that, when RDS automatically fails over to zone b to account for zone a failure, that you will certainly be able to scale up a full scale production environment in zone b as well, in spite of nearly every other AWS customer attempting more or less the same strategy; roughly half of zone a traffic will spill over to zone b, roughly half to zone c, minus all the traffic that is zone-locked to a (e.g. single-zone databases without failover mechanisms). That is a big assumption to make and you run a serious risk of not getting sufficient capacity in what was basically an arbitrarily chosen zone (chosen without context on whether there is sufficient capacity for the rest of your workloads) and being caught with zonal mismatches and not knowing what to do. You very well might need to failover to another region entirely to get sufficient capacity to handle your full workload.

If you are both cost- and latency-sensitive to stick to a single zone, you're likely much better off coming up with a migration plan, writing an automated runbook/script to handle it, and testing it on gamedays.