Sequence

Sequence CRD provides a way to define an in-order list of functions that will be invoked. Each step can modify, filter or create a new kind of an event. Sequence creates Channels and Subscriptions under the hood.

Usage

Sequence Spec

Sequence has three parts for the Spec:

  1. Steps which defines the in-order list of Subscribers, aka, which functions are executed in the listed order. These are specified using the eventingv1alpha1.SubscriberSpec just like you would when creating Subscription. Each step should be Callable.
  2. ChannelTemplate defines the Template which will be used to create Channels between the steps.
  3. Reply (Optional) Reference to where the results of the final step in the sequence are sent to.

Sequence Status

Sequence has four parts for the Status:

  1. Conditions which detail the overall Status of the Sequence object
  2. ChannelStatuses which convey the Status of underlying Channel resources that are created as part of this Sequence. It is an array and each Status corresponds to the Step number, so the first entry in the array is the Status of the Channel before the first Step.
  3. SubscriptionStatuses which convey the Status of underlying Subscription resources that are created as part of this Sequence. It is an array and each Status corresponds to the Step number, so the first entry in the array is the Subscription which is created to wire the first channel to the first step in the Steps array.
  4. AddressStatus which is exposed so that Sequence can be used where Addressable can be used. Sending to this address will target the Channel which is fronting the first Step in the Sequence.

Examples

For each of these examples below, we’ll use CronJobSource as the source of events.

We also use a very simple transformer which performs very trivial transformation of the incoming events to demonstrate they have passed through each stage.

Sequence with no reply (terminal last Step)

For the first example, we’ll use a 3 Step Sequence that is wired directly into the CronJobSource. Each of the steps simply tacks on “- Handled by ”, for example the first Step in the Sequence will take the incoming message and append “- Handled by 0” to the incoming message.

Sequence with reply (last Step produces output)

For the next example, we’ll use the same 3 Step Sequence that is wired directly into the CronJobSource. Each of the steps simply tacks on “- Handled by ”, for example the first Step in the Sequence will take the incoming message and append “- Handled by 0” to the incoming message.

The only difference is that we’ll use the Subscriber.Spec.Reply field to wire the output of the last Step to an event display pod.

Chaining Sequences together

For the next example, we’ll use the same 3 Step Sequence that is wired directly into the CronJobSource. Each of the steps simply tacks on “- Handled by ”, for example the first Step in the Sequence will take the incoming message and append “- Handled by 0” to the incoming message.

The only difference is that we’ll use the Subscriber.Spec.Reply field to wire the output of the last Step to another Sequence that does the smae message modifications as the first pipeline (with different steps however).

Using Sequence with Broker/Trigger model

You can also create a Trigger which targets Sequence. This time we’ll wire CronJobSource to send events to a Broker and then we’ll have the Sequence emit the resulting Events back into the Broker so that the results of the Sequence can be observed by other Triggers.