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Jan 7, 2025
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Add notes about uniform controllers #4

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16 changes: 15 additions & 1 deletion designs/multi-cluster.md
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Expand Up @@ -106,7 +106,9 @@ The `ctrl.Manager` _SHOULD_ be extended to get an optional `cluster.Provider` vi
// pkg/cluster

// Provider defines methods to retrieve clusters by name. The provider is
// responsible for discovering and managing the lifecycle of each cluster.
// responsible for discovering and managing the lifecycle of each cluster,
// and to engage or disengage clusters with the manager the provider is
// run against.
//
// Example: A Cluster API provider would be responsible for discovering and
// managing clusters that are backed by Cluster API resources, which can live
Expand Down Expand Up @@ -196,6 +198,8 @@ clusters with non-empty name "provider clusters" or "enganged clusters", while
the embedded cluster of the manager is called the "default cluster" or "hub
cluster".

### Cluster-Aware Request

To provide information about the source cluster of a request, a new type
`reconcile.ClusterAwareRequest` _SHOULD_ be added:

Expand All @@ -214,11 +218,21 @@ Given that an empty cluster name represents the "default cluster", a `reconcile.
can be used as `request` type even for controllers that do not have an active cluster provider.
The cluster name will simply be an empty string, which is compatible with calls to `mgr.GetCluster`.

**Note:** controller-runtime must provide this cluster-aware request type to
allow writing *uniform* multi-cluster controllers (see goals), i.e. controllers
that both work as single-cluster and multi-cluster controllers against arbitrary
cluster providers. Think of generic CNCF projects like cert-manager wanting to
support multi-cluster setups generically without forking the codebase.

### BYO Request Type

Instead of using the new `reconcile.ClusterAwareRequest`, implementations _CAN_ also bring their
own request type through the generics support in `Typed*` types (`request comparable`).

**Note:** these kind of controllers won't be uniform in the sense of compatibility
with arbitrary cluster providers, but for use-cases that are tiedly integrated
with specific cluster providers, this might be useful.

Optionally, a passed `TypedEventHandler` will be duplicated per engaged cluster if they
fullfil the following interface:

Expand Down
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