# Limitations and known issues¶

Shipper is just software, and all software has limits. Here are the highlights for Shipper currently. Some of these are not principal problems, just shortcuts that we took while building Shipper.

## Chart restrictions¶

Shipper expects a few properties to be true about the Chart it is rolling out. We hope to loosen or remove most of these restrictions over time.

### Only Deployments¶

The Chart must have exactly one Deployment object. The name of the Deployment should be templated with {{.Release.Name}}. The Deployment object should have apiVersion: apps/v1.

Shipper cannot yet perform roll outs for StatefulSets, HorizontalPodAutoscalers, or bare ReplicaSets. These objects can be present in the Chart, but Shipper only knows how to manipulate Deployment objects to scale capacity over the course of a rollout.

### Services¶

The Chart must contain either:

• exactly one Service, or
• exactly one Service labeled with the label shipper-lb: production.

The name of the Service should be fixed: either a literal in the Chart template, or a value which does not change from release to release.

The Service should have a selector which matches the application, not a single release. A Service with release: {{ .Release.Name }} as part of the Service selector will cause Shipper to error, as it will not be able to balance traffic between multiple Releases.

If you cannot modify the Chart you’re rolling out, you can ask Shipper to remove the release selector from the Service selector by adding the enable-helm-release-workaround: true label to your Application. This workaround helps make Charts created with helm create work out of the box.

Shipper uses Kubernetes’ built-in mechanism for shifting traffic: labeling Pods to add or remove them to a Service’s selector. This means you don’t need any special support in your Kubernetes clusters, but it has several drawbacks.

We hope to mitigate these by adding support for service mesh providers as traffic shifting backends.

### Pod-based traffic shifting¶

Traffic shifting happens at the granularity of Pods, not requests. While Shipper’s interface specifes a traffic weight, small fleets of Pods may find that their actual weight differs significantly from the one they requested.

### New Pods don’t get traffic if Shipper is not working¶

Shipper adds the shipper-traffic-status: enabled label to Pods after they start. This allows Shipper to correctly manage the number of Pods exposed to traffic. However, if a Pod is deleted and Shipper is not currently running or cannot contact the cluster, the new Pod spawned by the ReplicaSet will not get traffic until Shipper is working again.

The primary issue is that we cannot “cork” a successfully completed rollout by adding the traffic label to the Deployment or ReplicaSet without triggering a native Deployment-based rollout. We could solve this by working directly with ReplicaSets instead of Deployments, but that’s probably working against the grain of the ecosystem (most charts contain Deployments).

## Lock-step rollouts¶

Shipper is good at making sure that all clusters involved in a rollout are in the same state. It does this by ensuring that all clusters are in the correct state before marking a rollout step as complete.

However, this means that Shipper cannot perform cluster-by-cluster rollouts, like first kube-us-east1-a, then kube-eu-west2-b. Our “federation” layer supports this, but we have not yet designed the extension to our strategy language to describe this kind of rollout.

This cluster-by-cluster strategy is important when limiting traffic or capacity exposure to a new change is not enough to mitigate risk: for example, perhaps the new version will change a cluster-local schema once it starts running.