Scheduling and Orchestration

This section provides an overview of the scheduling and orchestration mechanisms used in IronCore's Infrastructure as a Service (IaaS) layer. It covers the concepts of Pools, poollets, and the IronCore Runtime Interface (IRI), which together enable efficient resource management and allocation.

Pools and Classes

The core concept in IronCore's scheduling architecture is the resource Pool. A Pool is announced by a poollet which represents an entity onto which resources can be scheduled. The announcement of a Pool resource is done by a poollet, which also provides in the Pool status the AvailableMachineClasses a Pool supports. A Class in this context represents a list of resource-specific capabilities that a Pool can provide, such as CPU, memory, and storage.

Pools and Classes are defined for all major resource types in IronCore, including compute and storage. Resources in the networking API have no Pool concept, as they are not scheduled but rather provided on-demand by the network related components. The details are described in the networking section.

An example definition of a Pool from the compute API group (MachinePool) is shown below:

apiVersion: compute.ironcore.dev/v1alpha1
kind: MachinePool
metadata:
  name: machinepool-sample
spec:
  providerID: ironcore://shared
status:
  availableMachineClasses:
    - name: machineclass-sample

The corresponding MachineClass defines the capabilities of the pool, such as CPU and memory:

apiVersion: compute.ironcore.dev/v1alpha1
kind: MachineClass
metadata:
  name: machineclass-sample
capabilities:
  cpu: 4
  memory: 16Gi

Scheduling

If a Machine or Volume resource has been created, the IronCore scheduler will look for a suitable Pool which supports the defined MachineClass or VolumeClass. If a suitable Pool is found, the scheduler will set the .spec.machinePoolRef or .spec.volumePoolRef field of the Machine or Volume resource to the name of the Pool. This reference indicates the poollet responsible for the announced Pool that something needs to be done, such as creating a Machine or Volume resource.

The current schedule implementation works on a best-effort basis, meaning that it will try to find a suitable Pool and assign the correct Pool but it does not guarantee that the Machine or Volume will be created. A new Reservation based scheduling mechanism which is described in this enhancement proposal should provide a more robust scheduling mechanism in the future.

Poollets

The poollets responsibilities besides announcing the Pool are manifold and are depicted in the diagram below:

1. Ensuring prerequisites are met: In the example of a Machine, the creation of a Machine should only be proceeded with, if e.g. the root Volume of the Machine is available.
2. Resource Creation: The poollet is responsible for creating the actual resources in the underlying infrastructure, such as virtual machines or storage volumes.
3. Status Propagation: The poollet updates the status of the Machine resource (e.g. Running, Failed, etc.) based on the state of the underlying resources it manages.

The key role in managing the resources is defined in the IronCore Runtime Interface (IRI), which provides a well-defined gRPC interface for each resource group.

This architecture with resources scheduled on Pools and poollets acting as the resource managers by invoking a backend interface API corresponds to the same model Kubernetes is using with Kubelet announcing Nodes and Pods being scheduled on Nodes. The Kubelet here interact via the Container Runtime Interface (CRI) with the container runtime, to manifest the actual instance of a Pod.