In-place Update is one of the key features provided by OpenKruise.
Workloads that support in-place update:
Advanced StatefulSet and
Advanced DaemonSet re-use the same code package
./pkg/util/inplaceupdate and have similar behaviours of in-place update. In this article, we would like to introduce the usage and workflow of them.
Note that the in-place update workflow of
SidecarSet is a little different from the other workloads, such as it will not set Pod to not-ready before update. So the things we talk below do not totally go for
What is in-place update?
Once we are going to update image in a existing Pod, look at the comparation between Recreate and InPlace Update:
In ReCreate way we have to delete the old Pod and create a new Pod:
- Pod name and uid all changed, because they are totally different Pod objects (such as Deployment update)
- Or Pod name may not change but uid changed, because they are still different Pod objects, althrough re-use the same name (such as StatefulSet update)
- Node name of the Pod changed, because the new Pod is almost impossible to be scheduled to the previous node.
- Pod IP changed, because the new Pod is almost impossible to be allocated the previous IP.
But for InPlace way we can re-use the Pod object but only modify the fields in it, so that:
- Avoid additional cost of scheduling, allocating IP, allocating and mounting volumes
- Faster image pulling, because of we can re-use most of image layers pulled by the old image and only to pull several new layers
- When a container is in-place updating, the other containers in Pod will not be affected and remain running.
The update type in Kruise workloads is named
InPlaceIfPossible, which tells Kruise to update Pods in-place as possible, and it should go back to ReCreate Update if impossible.
What changes does it consider to be possilble to in-place update?
spec.template.metadata.*in workloads, such as labels and annotations, Kruise will only update the metadata to existing Pods without recreate them.
spec.template.spec.containers[x].imagein workloads, Kruise will in-place update the container image in Pods without recreate them.
- Since Kruise v1.0 (including v1.0 alpha/beta), update
spec.template.metadata.labels/annotationsand there exists container env from the changed labels/annotations, Kruise will in-place update them to renew the env value in containers.
Otherwise, the changes to other fields such as
spec.template.spec.containers[x].resources will go back to ReCreate Update.
You can see the whole workflow of in-place update below (you may need to right click and open it in a new tab):
To use InPlace Update for env from metadata, you have to enable
kruise-daemon (defaults to be enabled) and
InPlaceUpdateEnvFromMetadata feature-gate when install or upgrade Kruise chart.
Note that if you have some nodes of virtual-kubelet type, kruise-daemon may not work on them and in-place update for env from metadata will not be executed.