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Merge pull request #3765 from robscott/topology-hints-1-27-updates
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KEP-2433 Topology Aware Hints: Adding SameZone heuristic and other tweaks
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2 changes: 0 additions & 2 deletions keps/prod-readiness/sig-network/2433.yaml
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Expand Up @@ -3,5 +3,3 @@ alpha:
approver: "@wojtek-t"
beta:
approver: "@wojtek-t"
stable:
approver: "@wojtek-t"
170 changes: 110 additions & 60 deletions keps/sig-network/2433-topology-aware-hints/README.md
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Expand Up @@ -8,19 +8,22 @@
- [Proposal](#proposal)
- [Risks and Mitigations](#risks-and-mitigations)
- [Design Details](#design-details)
- [Assumptions](#assumptions)
- [Identifying Zones](#identifying-zones)
- [Excluding Control Plane Nodes](#excluding-control-plane-nodes)
- [Configuration](#configuration)
- [Interoperability](#interoperability)
- [Feature Gate](#feature-gate)
- [Interoperability](#interoperability)
- [Feature Gate](#feature-gate)
- [API](#api)
- [Future API Expansion](#future-api-expansion)
- [Kube-Proxy](#kube-proxy)
- [EndpointSlice Controller](#endpointslice-controller)
- [Heuristics](#heuristics)
- [Proportional CPU Heuristic](#proportional-cpu-heuristic)
- [Assumptions](#assumptions)
- [Identifying Zones](#identifying-zones)
- [Excluding Control Plane Nodes](#excluding-control-plane-nodes)
- [Example](#example)
- [Overload](#overload)
- [Handling Node Updates](#handling-node-updates)
- [Additional Heuristics](#additional-heuristics)
- [Future Expansion](#future-expansion)
- [Test Plan](#test-plan)
- [Unit tests](#unit-tests)
Expand Down Expand Up @@ -94,6 +97,7 @@ Kubernetes clusters are increasingly deployed in multi-zone environments.
Network traffic is routed randomly to any endpoint matching a Service. Some
users might want the traffic to stay in the same zone for the following
reasons:

- Cost savings: Keeping traffic within a zone can limit cross-zone networking
costs.
- Performance: Traffic within a zone usually has less latency and bandwidth
Expand Down Expand Up @@ -125,10 +129,19 @@ for most use cases.
- Ensuring that Pods are distributed evenly across zones.

## Proposal
This KEP describes two related concepts:

1. A way to express the heuristic you'd like to use for Topology Aware Routing.
2. A new Hints field in EndpointSlices that can be used to enable certain
topology heuristics.

When this feature is enabled, the EndpointSlice controller will be updated to
provide hints for each endpoint. These hints will initially be limited to a
single zone per-endpoint. Kube-Proxy will then use these hints to filter the
For now, the only heuristic proposed relies on hints so these concepts are
closely tied. It is important to note that that may not be the case for future
heuristics.

When a heuristic that depends on Hints is chosen, the EndpointSlice controller
will populate hints for each endpoint. These hints will initially be limited to
a single zone per-endpoint. Kube-Proxy will then use these hints to filter the
endpoints they should route to.

For example, for a Service with 3 endpoints, the EndpointSlice controller may
Expand Down Expand Up @@ -178,43 +191,16 @@ with a new Service annotation.

## Design Details

### Assumptions

- Incoming traffic is proportional to the number of allocatable CPU cores in a
zone. Although this is an imperfect metric, it is the best available way of
predicting how much traffic will be received in a zone. If we are unable to
derive the number of allocatable cores in a zone we will fall back to the
number of nodes in that zone.
- Service capacity is proportional to the number of endpoints in a zone. This
assumes that each endpoint has equivalent capacity. Although this is not
always true, it usually is. We can explore ways to deal with variable capacity
endpoints in the future.

### Identifying Zones

The EndpointSlice controller reads the standard `topology.kubernetes.io/zone`
label on Nodes to determine which zone a Pod is running in. Kube-Proxy would be
updated to read the same information to identify which zone it is running in.

### Excluding Control Plane Nodes

Any Nodes with the following labels (set to any value) will be excluded when
calculating allocatable cores in a zone:

* `node-role.kubernetes.io/control-plane`
* `node-role.kubernetes.io/master`

### Configuration

A new `service.kubernetes.io/topology-aware-routing` annotation can be used to
enable or disable Topology Aware Routing (and by extension, hints) for a
Service. This may be set to "Auto" or "Disabled". Any other value is treated as
"Disabled".
A new `service.kubernetes.io/topology-mode` annotation can be used to enable or
disable Topology Aware Routing heuristics for a Service.

The previous `service.kubernetes.io/topology-aware-hints` annotation will
continue to be supported as a means of configuring this feature.
continue to be supported as a means of configuring this feature for both "Auto"
and "Disabled" values. New values will only be supported by the new annotation.

#### Interoperability
### Interoperability

Topology hints will be ignored if the TopologyKeys field has at least one entry.
This field is deprecated and will be removed soon.
Expand All @@ -225,7 +211,7 @@ topology was enabled, external traffic would be routed using the
ExternalTrafficPolicy configuration while internal traffic would be routed with
topology.

#### Feature Gate
### Feature Gate

This functionality will be guarded by the `TopologyAwareHints` feature gate.
This gate also interacts with 2 other feature gates:
Expand Down Expand Up @@ -290,7 +276,6 @@ conditions are true:

- Kube-Proxy is able to determine the zone it is running within (likely based
on node labels).
- The annotation is set to `Auto`.
- At least one endpoint for the Service has a hint pointing to the zone
Kube-Proxy is running within.
- All endpoints for the Service have zone hints.
Expand All @@ -304,17 +289,56 @@ and disabled states. Without this fallback, endpoints could easily get
overloaded as hints were being added or removed from some EndpointSlices but
had not yet propagated to all of them.

Note: Some future heuristics may not rely on hints and could instead be
implemented directly by kube-proxy.

### EndpointSlice Controller

When the `TopologyAwareHints` feature gate is enabled and the annotation is set
to `Auto` for a Service, the EndpointSlice controller will add hints to
EndpointSlices. These hints will indicate where an endpoint should be consumed
by proxy implementations to enable topology aware routing.
to `Auto` or `ProportionalByCore` for a Service, the EndpointSlice controller
will add hints to EndpointSlices. These hints will indicate where an endpoint
should be consumed by proxy implementations to enable topology aware routing.

## Heuristics

This KEP starts with the following heuristics:

| Heuristic Name | Description |
|-|-|
| Auto | EndpointSlice controller and/or underlying dataplane can choose the heuristic used. |
| ProportionalByCore | Endpoints will be allocated to each zone proportionally, based on the allocatable Node CPU cores in each zone. |

In the future, additional heuristics may be added. Until that point, "Auto" will
be the only configurable value. In most clusters, that will translate to
`ProportionalByCore` unless the underlying dataplane has a better approach
available.

The EndpointSlice controller will determine how many endpoints should be
available for each zone based on the proportion of CPU cores in each zone. If
it is not possible to determine the number CPU cores, 1 core per node will be
assumed for calculations.
### Proportional CPU Heuristic
#### Assumptions

- Incoming traffic is proportional to the number of allocatable CPU cores in a
zone. Although this is an imperfect metric, it is the best available way of
predicting how much traffic will be received in a zone. If we are unable to
derive the number of allocatable cores in a zone we will fall back to the
number of nodes in that zone.
- Service capacity is proportional to the number of endpoints in a zone. This
assumes that each endpoint has equivalent capacity. Although this is not
always true, it usually is. We can explore ways to deal with variable capacity
endpoints in the future.

#### Identifying Zones

The EndpointSlice controller reads the standard `topology.kubernetes.io/zone`
label on Nodes to determine which zone a Pod is running in. Kube-Proxy would be
updated to read the same information to identify which zone it is running in.

#### Excluding Control Plane Nodes

Any Nodes with the following labels (set to any value) will be excluded when
calculating allocatable cores in a zone:

* `node-role.kubernetes.io/control-plane`
* `node-role.kubernetes.io/master`

#### Example

Expand Down Expand Up @@ -369,12 +393,20 @@ of the following scenarios:
2. A new Node results in a Service that is able to achieve an endpoint
distribution below 20% for the first time.

### Additional Heuristics
To enable additional heuristics to be added in the future, we will:

1. Remove the requirement in kube-proxy that the hints annotation must be set to
a known value on the associated Service before the values of EndpointSlice
hints will be considered.
2. Ensure the EndpointSlice controller TopologyCache provides an interface that
simplifies adding additional heuristics in the future.

### Future Expansion

In the future we may expand this functionality if needed. This could include:

- A new `RequireZone` algorithm that would keep endpoints in EndpointSlices for
the same zone they are in.
- As described above, additional heuristics may be added in the future.
- A new option to specify a minimum threshold for the `Auto` (PreferZone)
approach.
- Support for region based hints.
Expand Down Expand Up @@ -467,6 +499,16 @@ EndpointSliceSyncs = metrics.NewCounterVec(
[]string{"result"}, // either "success" or "failure"
)
// EndpointSliceHints tracks the number of endpoints that have hints assigned.
EndpointSliceEndpointsWithHints = metrics.NewGaugeVec(
&metrics.CounterOpts{
Subsystem: EndpointSliceSubsystem,
Name: "endpoints_with_hints",
Help: "Number of endpoints that have hints assigned",
StabilityLevel: metrics.ALPHA,
},
[]string{"result"}, // either "Auto" or "SameZone"
)
```

### Events
Expand All @@ -490,7 +532,7 @@ feature.

#### Sample Events

| Type | Reason | Message |
| Type | Reason | Message |
|-|-|-|
| Normal | TopologyAwareRoutingEnabled | Topology Aware Routing has been enabled |
| Normal | TopologyAwareRoutingDisabled | Topology Aware Routing configuration was removed |
Expand Down Expand Up @@ -532,11 +574,17 @@ completeness.
disabled.
- Ensure that existing Topology Hints e2e test runs as a presubmit if any code
changes in kube-proxy or the EndpointSlice controller.
- Topology Hints e2e tests will graduate to conformance tests.
- Autoscaling and Scheduling SIGs have a plan to provide zone aware autoscaling
(and scheduling) that allows users to proportionally distribute endpoints
across zones.

**Note on Conformance Tests:**
It's worth noting that conformance tests are intentionally out of scope for this
KEP. We want to provide flexibility for underlying dataplanes to provide
improved topology aware routing options. As the name suggests, "hints" can be
useful when implementing topology aware routing, but we do not want them to be
considered a strict requirement.

### Version Skew Strategy
This KEP requires updates to both the EndpointSlice Controller and kube-proxy.
Thus there could be two potential version skew scenarios:
Expand All @@ -559,6 +607,7 @@ enabled even if the annotation has been set on the Service.
- [x] Feature gate (also fill in values in `kep.yaml`)
- Feature gate name: TopologyAwareHints
- Components depending on the feature gate:
- kube-apiserver
- kube-controller-manager
- kube-proxy

Expand All @@ -575,13 +624,14 @@ enabled even if the annotation has been set on the Service.
EndpointSlices for Services that have this feature enabled.

* **Are there any tests for feature enablement/disablement?**
Per Service enablement and disablement is covered in depth by unit tests. As a
prerequisite for graduation to GA, we will also add the following:

- Test coverage in EndpointSlice strategy to ensure that the Hints field is
dropped when the feature gate is not enabled.
- Test coverage in EndpointSlice controller for the transition from enabled to
disabled.
Enablement is covered by a variety of tests:

* Per Service enablement and disablement in EndpointSlice Controller. [(Unit
Tests.)](https://github.com/kubernetes/kubernetes/blob/468ce5918377ab4d4e3180b4fd33fdd2bdb16ec9/pkg/controller/endpointslice/reconciler_test.go#L1641-L1907)
* Hints field is dropped when feature gate is off. [(Strategy Unit
Tests.)](https://github.com/kubernetes/kubernetes/blob/468ce5918377ab4d4e3180b4fd33fdd2bdb16ec9/pkg/registry/discovery/endpointslice/strategy_test.go)
* TODO before GA: Test coverage in EndpointSlice controller for the transition
from enabled to disabled.

### Rollout, Upgrade and Rollback Planning

Expand Down
6 changes: 3 additions & 3 deletions keps/sig-network/2433-topology-aware-hints/kep.yaml
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Expand Up @@ -23,18 +23,18 @@ replaces:
- "github.com/kubernetes/enhancements/tree/master/keps/sig-network/536-topology-aware-routing"

# The target maturity stage in the current dev cycle for this KEP.
stage: stable
stage: beta

# The most recent milestone for which work toward delivery of this KEP has been
# done. This can be the current (upcoming) milestone, if it is being actively
# worked on.
latest-milestone: "v1.26"
latest-milestone: "v1.27"

# The milestone at which this feature was, or is targeted to be, at each stage.
milestone:
alpha: "v1.21"
beta: "v1.23"
stable: "v1.26"
stable: "v1.28"

# The following PRR answers are required at alpha release
# List the feature gate name and the components for which it must be enabled
Expand Down

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