Understanding Kubernetes’ tools/cache package: part 11—Towards a Kubernetes controller CDI 2.0 portable extension

In part 10, we laid out what we need to do to make a simple programming model available to an end user who wants to be notified of Kubernetes events in the same style as Go programmers using the tools/cache package.  This series starts with part 0.

Here we’ll look at implementing a CDI 2.0 portable extension that enables this lean programming model.

From part 10, we sketched out the idea of pairing an observer method with a producer method, using the same qualifiers on both the producer method’s return type and on the observer method’s event type.  Linked in this way, you now have the ability to build boilerplate machinery in between the Listable-and-VersionWatchable made by the producer method and the event observed by the observer method, and the end user doesn’t have to know about or invoke any of it: her events “just work” assuming this extension is present, opaquely, at runtime.

This post will make use of the microbean-kubernetes-controller framework, the Java Kubernetes controller framework built up over the course of this series (you can start at the beginning if you like).  This framework is CDI-independent.  We’ll adapt it to CDI 2.0 so end users don’t have to deal with any boilerplate.

As discussed, our portable extension will have to:

  • find beans that have Listable and VersionWatchable among their bean types
  • find observer methods observing Event objects that contain Kubernetes resources in them
  • ensure that for any given observer method whose event parameter is qualified with a set of qualifier annotations, there exists a producer method (or any other kind of CDI bean, actually) with the right kind of type that is also qualified by those same qualifier annotations (or no event delivery will take place)
  • have some way of recognizing that arbitrary qualifier annotations defined by the user are those that should be used to identify Kubernetes events—i.e. some way of meta-qualifying a qualifier annotation that appears on relevant producer methods and relevant event parameters

We can do all these things thanks to the well-defined lifecycle mandated by CDI.

First, we’ll sketch out a meta-qualifier annotation named KubernetesEventSelector.  The intent of this meta-annotation will be that if you put it on a qualifier annotation that you create, then your qualifier annotation will be treated as a qualifier that is trying to say something specifically about sets of Kubernetes events.

Here’s what that might look like:

Now as an end user if I define my own qualifier annotation and annotate it with KubernetesEventSelector:

…I will have expressed my intent that when we “see” this annotation:

@AllConfigMapEvents

…we are talking about choosing Kubernetes events in some handwavy manner, and not something else.

So now our as-yet-to-be-written portable extension has at least the theoretical means to find event parameters and beans and see if they’re qualified with qualifiers that, in turn, are annotated with @KubernetesEventSelector.  Those will be the only things of interest to this extension.

Next, we’ll take advantage of the CDI lifecycle and the prescribed order of things and recognize that bean processing happens before observer method processing.  So let’s start with looking for those beans that have Listable and VersionWatchable among their bean types.

We’re going to run into a problem right away.  We would like to look for beans whose bean types are of type X where X is some type that extends Listable and VersionWatchable, but otherwise we don’t really care what X actually is.  But producer methods must return concrete types without type variables in them.

What we’ll do instead is look for the most general type in the fabric8 Kubernetes model hierarchy that implements or extends both Listable and VersionWatchable and deal with that.  That type happens to be Operation.  We’ll keep track during bean processing of all Beans that match these types.  That means it doesn’t matter if the bean in question is a producer method, a producer field, a managed bean, a synthetic bean or any of the other kinds of beans you can cause to come into existence in a CDI world: they’ll all be accounted for.

Now, obviously we’re not actually interested in all beans whose bean types include Operation.  We’re interested in those that have been explicitly “tagged” with the KubernetesEventSelector annotation.

It would be nice if there were a convenience method of sorts in CDI that would let you ask a bean for its qualifiers but would restrict the returned set to only those qualifiers that, in turn, are meta-qualified with something else (like KubernetesEventSelector).  Unfortunately, there is not, so we’ll have to do this ourselves.

So, refined: we’ll keep track of Beans with Operation (and hence by definition Listable and VersionWatchable) among their bean types, and with KubernetesEventSelector-qualified qualifiers.  We’ll keep track of those annotations, too.

Once we do this, we’ll have some machinery that can now pick out sources for raw materials that we’ll need to feed to a Reflector.

But obviously we don’t really want to do any of this if there aren’t any observer methods around.  Fortunately but also unfortunately, bean processing happens before observer method processing, so we will do this work, and then discard it if we don’t find any relevant observer methods.

What’s a relevant observer method?  In this case it’s an observer method whose (a) AbstractEvent-typed event parameter is (b) qualified by one or more qualifier annotations that are themselves (c) meta-qualified by KubernetesEventSelector, and that (d) “matches” an equivalent set of qualifiers on an Operation-typed bean type.  This means that indirectly the observer method will “watch” for what the producer method is indirectly providing access to.

Here’s an example observer method:

Note the usage of the @AllConfigMapEvents qualifier we defined earlier in this blog post, and note that the event parameter so qualified is an Event.  Let’s say we found a “matching” producer method:

Note the Operation return type and the matching qualifier-meta-qualified-with-KubernetesEventSelector that decorates it (@AllConfigMapEvents).  Do you see how it “matches” the observer method’s event parameter qualifier (also @AllConfigMapEvents)?  This is the linkage that connects this producer method with the observer method that observes events that will result from the producer method’s existence.

So once we have a mapping from a Bean to a set of relevant qualifier annotations, then we can see if, during observer method processing, we’re talking about the first observer method that should cause our boilerplate machinery to get invoked.  If there are many observer methods that “match” a relevant producer method, that’s fine; that just indicates that the very same Kubernetes event might be processed by many observers.  The point is, before we jump through all the hoops to set up lists and watches and so on, we should make sure that someone is actually going to take advantage of our work.

So once we “get a match”, we can build the boilerplate for all matches of the same type, once.

For each distinct match, we’ll need a new Controller.  The controller will need an instance of whatever the producer method is making (some kind of Operation, which is by definition both a Listable and a VersionWatchable).

The Controller, in turn, will need some kind of Map representing Kubernetes objects we “know about”.  That Map will be empty to start with, since we don’t know about any Kubernetes objects yet.  But if, down the road, a deletion gets skipped (maybe due to a watch timing out or a disconnect), we need to retroactively fire a delete if, say, the Kubernetes list of relevant objects is “less than” our “known objects”, and we’ll need to know what object was actually deleted.

The Controller will also need some kind of Consumer (a siphon) that will slurp up EventQueues and dispatch their events in order without blocking threads.  That Consumer will also need to update that very same Map of “known objects” as it does so.

Finally, that Consumer of EventQueues will also need a mechanism to link it to the native CDI event broadcast mechanism which will take care of actually dispatching events to observer methods.

Whew!  Let’s do the easy boring stuff first.

The first lifecycle event that looks good is ProcessBean.  If we could process all beans whose types are Operation here, in one place, regardless of whether the bean in question is a producer method, managed bean, etc. that would be great.  As it turns out for various obscure reasons, we can’t.  So we’ll implement a lifecycle event observer method for each type of bean, and route all their stuff to a common private method for processing:

And the processPotentialEventSelectorBean method that these all effectively delegate to might look like this:

…where the Annotations utility method can be found elsewhere, and where the getOperationType() method might look like this:

So at the end of all this we’ll have a Map, eventSelectorBeans, that has as its keys Sets of Annotation instances that have KubernetesEventSelector somewhere “on” them, and has as its values Beans that are qualified with those annotations.

Now we’ll need to route all observer method processing to something similar (fortunately, this is a great deal simpler).  Recall that we are guaranteed by contract that observer method processing happens after discovering beans, so by this point we already have knowledge of any producer methods or managed beans “making” Operation instances of various kinds:

(Above, we also keep track of whether the observer method in question is synchronous or asynchronous.)

Finally, we can clean up a little bit after bean discovery is all done:

At the end of all of this bean discovery, we have a Set of Bean instances stored in a beans instance variable that (a) “make” Operation instances, (b) are appropriately meta-qualified to declare their interest in taking part in Kubernetes event production, and (c) have at least one observer method interested in their effects.

Let’s build some controllers and start them when the user’s program comes up!

We’ll begin by observing the initialization of the application scope, which is exactly equal to the start of a CDI application.

When this happens, our beans instance variable will contain some of the materials to build a Controller.  For each Bean in there, we’ll “instantiate” it (by way of a beanManager.getReference() call, create a new Controller, hook that Controller up to the right kind of EventDistributor, start the Controller and keep track of it for later cleanup:

In the above code example, you’ll see a reference to a CDIEventDistributor.  That private class is a Consumer of events that simply fires them using the existing CDI broadcasting mechanism:

Finally, when the user’s application stops, we should clean up after ourselves.  We observe the imminent destruction of the application scope and close everything we started up:

The net effect is that if you put this portable extension and its dependencies on the classpath of your CDI 2.0 application and author a producer method and observer method pair (and nothing else), your CDI application will receive Kubernetes events and should behave semantically like any other kind of Kubernetes controller.

The fledgling project housing all this code is microbean-kubernetes-controller-cdi and is available on Github under the Apache 2.0 license.

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2 thoughts on “Understanding Kubernetes’ tools/cache package: part 11—Towards a Kubernetes controller CDI 2.0 portable extension

  1. Pingback: Understanding Kubernetes’ tools/cache package: part 10—Designing Kubernetes controllers in CDI 2.0 | Blame Laird

  2. Pingback: KubeWeekly #125 – KubeWeekly

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