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This adds a series of FAQs. A decent amount of this should be migrated to the KB book.
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# FAQ | ||
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### Q: How do I know which type of object a controller references? | ||
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**A**: Each controller should only reconcile one object type. Other | ||
affected objects should be mapped to a single type of root object, using | ||
the `EnqueueRequestForOwner` or `EnqueueRequestsFromMapFunc` event | ||
handlers, and potentially indicies. Then, your Reconcile method should | ||
attempt to reconcile *all* state for that given root objects. | ||
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### Q: How do I have different logic in my reconciler for different types of events (e.g. create, update, delete)? | ||
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**A**: You should not. Reconcile functions should be idempotent, and | ||
should always reconcile state by reading all the state it needs, then | ||
writing updates. This allows your reconciler to correctly respond to | ||
generic events, adjust to skipped or coalesced events, and easily deal | ||
with application startup. The controller will enqueue reconcile requests | ||
for both old and new objects if a mapping changes, but it's your | ||
responsibility to make sure you have enough information to be able clean | ||
up state that's no longer referenced. | ||
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### Q: My cache might be stale if I read from a cache! How should I deal with that? | ||
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**A**: There are several different approaches that can be taken, depending | ||
on your situation. | ||
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- When you can, take advantage of optimistic locking: use deterministic | ||
names for objects you create, so that the Kubernetes API server will | ||
warn you if the object already exists. May controllers in Kubernetes | ||
take this approach: the StatefulSet controller appends a specific number | ||
to each pod that it creates, while the Deployment controller hashes the | ||
pod template spec and appends that. | ||
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- In the few cases when you cannot take advantage of deterministic names | ||
(e.g. when using generateName), it may | ||
be useful in to track which actions you took, and assume that they need | ||
to be repeated if they don't occur after a given time (e.g. using | ||
a requeue result). This is what the ReplicaSet controller does. | ||
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In general, write your controller with the assumption that information | ||
will eventually be correct, but may be slightly out of date. Make sure | ||
that your reconcile function enforces the entire state of the world each | ||
time it runs. If none of this works for you, you can always construct | ||
a client that reads directly from the API server, but this is generally | ||
considered to be a last resort, and the two approaches above should | ||
generally cover most circumstances. | ||
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### Q: Where's the fake client? How do I use it? | ||
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**A**: The fake client | ||
[exists](https://godoc.org/sigs.k8s.io/controller-runtime/pkg/client/fake), | ||
but we generally reccomend using | ||
[envtest.Environment](https://godoc.org/sigs.k8s.io/controller-runtime/pkg/envtest#Environment) | ||
to test against a real API server. In our experience, tests using fake | ||
clients gradually re-implement poorly-written impressions of a real API | ||
server, which leads to hard-to-maintain, complex test code. | ||
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### Q: How should I write tests? Any suggestions for getting started? | ||
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- Use the aforementioned | ||
[envtest.Environment](https://godoc.org/sigs.k8s.io/controller-runtime/pkg/envtest#Environment) | ||
to spin up a real API server instead of trying to mock one out. | ||
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- Structure your tests to check the that the state of the world is as you | ||
expect it, *not* that a particular set of API calls were made, when | ||
working with Kubernetes APIs. This will allow you to more easily | ||
refactor and improve the internals of your controllers without changing | ||
your tests. | ||
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- Remember that any time you're interacting with the API server, changes | ||
may have some delay between write time and reconcile time. | ||
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### Q: What are these errors about no Kind being registered for a type? | ||
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**A**: You're probably missing a fully-set-up Scheme. Schemes record the | ||
mapping between Go types and group-version-kinds in Kubernetes. In | ||
general, your application should have its own Scheme containing the types | ||
from the API groups that it needs (be they Kubernetes types or your own). | ||
See the [scheme builder | ||
docs](https://godoc.org/sigs.k8s.io/controller-runtime/pkg/scheme) for | ||
more information. |
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