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In a Knative application, traffic can be managed by creating a traffic split. A traffic split is configured as part of a route, which is managed by a Knative service.

Traffic management for a Knative application

Configuring a route allows requests to be sent to different revisions of a service. This routing is determined by the traffic spec of the Service object.

A traffic spec declaration consists of one or more revisions, each responsible for handling a portion of the overall traffic. The percentages of traffic routed to each revision must add up to 100%, which is ensured by a Knative validation.

The revisions specified in a traffic spec can either be a fixed, named revision, or can point to the “latest” revision, which tracks the head of the list of all revisions for the service. The "latest" revision is a type of floating reference that updates if a new revision is created. Each revision can have a tag attached that creates an additional access URL for that revision.

The traffic spec can be modified by:

  • Editing the YAML of a Service object directly.

  • Using the kn CLI --traffic flag.

  • Using the OpenShift Container Platform web console.

When you create a Knative service, it does not have any default traffic spec settings.

Traffic spec examples

The following example shows a traffic spec where 100% of traffic is routed to the latest revision of the service. Under status, you can see the name of the latest revision that latestRevision resolves to:

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: example-service
  namespace: default
spec:
...
  traffic:
  - latestRevision: true
    percent: 100
status:
  ...
  traffic:
  - percent: 100
    revisionName: example-service

The following example shows a traffic spec where 100% of traffic is routed to the revision tagged as current, and the name of that revision is specified as example-service. The revision tagged as latest is kept available, even though no traffic is routed to it:

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: example-service
  namespace: default
spec:
...
  traffic:
  - tag: current
    revisionName: example-service
    percent: 100
  - tag: latest
    latestRevision: true
    percent: 0

The following example shows how the list of revisions in the traffic spec can be extended so that traffic is split between multiple revisions. This example sends 50% of traffic to the revision tagged as current, and 50% of traffic to the revision tagged as candidate. The revision tagged as latest is kept available, even though no traffic is routed to it:

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: example-service
  namespace: default
spec:
...
  traffic:
  - tag: current
    revisionName: example-service-1
    percent: 50
  - tag: candidate
    revisionName: example-service-2
    percent: 50
  - tag: latest
    latestRevision: true
    percent: 0

Knative CLI traffic management flags

The kn CLI supports traffic operations on the traffic block of a service as part of the kn service update command.

The following table displays a summary of traffic splitting flags, value formats, and the operation the flag performs. The Repetition column denotes whether repeating the particular value of flag is allowed in a kn service update command.

Flag Value(s) Operation Repetition

--traffic

RevisionName=Percent

Gives Percent traffic to RevisionName

Yes

--traffic

Tag=Percent

Gives Percent traffic to the revision having Tag

Yes

--traffic

@latest=Percent

Gives Percent traffic to the latest ready revision

No

--tag

RevisionName=Tag

Gives Tag to RevisionName

Yes

--tag

@latest=Tag

Gives Tag to the latest ready revision

No

--untag

Tag

Removes Tag from revision

Yes

Multiple flags and order precedence

All traffic-related flags can be specified using a single kn service update command. kn