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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 Channel
s and Subscription
s under the hood.
Usage
Sequence Spec
Sequence has three parts for the Spec:
Steps
which defines the in-order list ofSubscriber
s, aka, which functions are executed in the listed order. These are specified using themessaging.v1.SubscriberSpec
just like you would when creatingSubscription
. Each step should beAddressable
.ChannelTemplate
defines the Template which will be used to createChannel
s between the steps.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:
- Conditions which detail the overall Status of the Sequence object
- 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 theChannel
before the first Step. - 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 theSubscription
which is created to wire the first channel to the first step in theSteps
array. - 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
PingSource
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 PingSource
. Each of the steps simply tacks on “- Handled by
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 PingSource
. Each of the steps simply tacks on “- Handled
by 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 PingSource
. Each of the steps simply tacks on “- Handled
by 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 same 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
PingSource
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 Trigger
s.
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