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Gardener Scheduler

The Gardener Scheduler is in essence a controller that watches newly created shoots and assigns a seed cluster to them. Conceptually, the task of the Gardener Scheduler is very similar to the task of the Kubernetes Scheduler: finding a seed for a shoot instead of a node for a pod.

Either the scheduling strategy or the shoot cluster purpose hereby determines how the scheduler is operating. The following sections explain the configuration and flow in greater detail.

Why is the Gardener Scheduler needed?

1. Decoupling

Previously, an admission plugin in the Gardener API server conducted the scheduling decisions. This implies changes to the API server whenever adjustments of the scheduling are needed. Decoupling the API server and the scheduler comes with greater flexibility to develop these components independently from each other.

2. Extensibility

It should be possible to easily extend and tweak the scheduler in the future. Possibly, similar to the Kubernetes scheduler, hooks could be provided which influence the scheduling decisions. It should be also possible to completely replace the standard Gardener Scheduler with a custom implementation.

Configuration

The Gardener Scheduler configuration has to be supplied on startup. It is a mandatory and also the only available flag. Here is an example scheduler configuration.

Most of the configuration options are the same as in the Gardener Controller Manager (leader election, client connection, …). However, the Gardener Scheduler on the other hand does not need a TLS configuration, because there are currently no webhooks configurable.

The scheduling strategy is defined in the candidateDeterminationStrategy and can have the possible values SameRegion and MinimalDistance. The SameRegion strategy is the default strategy.

1. Same Region strategy

The Gardener Scheduler reads the spec.provider.type and .spec.region fields from the Shoot resource. It tries to find a Seed that has the identical .spec.provider.type and .spec.provider.region fields set. If it cannot find a suitable Seed, it adds an event to the shoot stating, that it is unschedulable.

2. Minimal Distance strategy

As a first step, the SameRegion strategy is being executed. If no seed in the same region could be found, the scheduler uses a lexicographical approach to determine a suitable seed cluster. This leverages the fact that most cloud providers (except from Azure) use geographically aligned region names. The scheduler takes into consideration the region names of all available seeds in the cluster of the desired infrastructure and picks the regions that match lexicographically the best (starting from the left letter to right letter of the region name). E.g. if the shoots wants a cluster in AWS eu-north-1, the scheduler picks all Seeds in region AWS eu-central-1, because at least the continent “eu-“ matches (even better with region instances like AWS ap-southeast-1 and AWS ap-southeast-2).

In the last step, the scheduler picks the one seed having the least shoots currently deployed.

In order to put the scheduling decision into effect, the scheduler sends an update request for the Shoot resource to the API server. After validation, the Gardener Aggregated API server updates the shoot to have the spec.seedName field set. Subsequently, the Gardenlet picks up and starts to create the cluster on the specified seed.

Special handling based on shoot cluster purpose

Every shoot cluster can have a purpose that describes what the cluster is used for, and also influences how the cluster is setup (see this document for more information).

In case the shoot has the testing purpose then the scheduler only reads the .spec.provider.type from the Shoot resource and tries to find a Seed that has the identical .spec.provider.type. The region does not matter, i.e., testing shoots may also be scheduled on a seed in a complete different region if it is better for balancing the whole Gardener system.

Filtering to determine the best candidate

The section above has explained which strategies are used to determine the potential seed candidates. Once this list has been computed the scheduler tries to find the best one out of them to which, eventually, the shoot gets assigned to. It filters out Seeds

  • whose networks have intersections with the Shoot's networks (due to the VPN connectivity between seeds and shoots their networks must be disjoint)
  • that are tainted with the seed.gardener.cloud/disable-dns taint (only if the shoot specifies a DNS domain or does not use the unmanaged DNS provider)
  • whose labels don’t match the .spec.seedSelector field of the CloudProfile that is used in the Shoot (there might be multiple environments for the same provider type, e.g., you might have multiple OpenStack systems connected to Gardener)

After this filtering process the least utilized seed, i.e., the one with the least number of shoot control planes, will be the winner and written to the .spec.seedName field of the Shoot.

seedSelector field in the Shoot specification

Similar to the .spec.nodeSelector field in Pods, the Shoot specification has an optional .spec.seedSelector field. It allows the user to provide a label selector that must match the labels of Seeds in order to be scheduled to one of them. The labels on Seeds are usually controlled by Gardener administrators/operators - end users cannot add arbitrary labels themselves. If provided, the Gardener Scheduler will only consider those seeds as “suitable” whose labels match those provided in the .spec.seedSelector of the Shoot.

Failure to determine a suitable seed

In case the scheduler fails to find a suitable seed, the operation is being retried with an exponential backoff - starting with the retrySyncPeriod (default of 15s).

Current Limitation / Future Plans

  • Azure has unfortunately a geographically non-hierarchical naming pattern and does not start with the continent. This is the reason why we will exchange the implementation of the MinimalRegion strategy with a more suitable one in the future.
  • Currently, shoots can only scheduled to seeds from the same cloud provider (spec.provider.type), however that is not a technical limitation and might be changed in the future.