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Merge pull request #34980 from MrHohn/addon-manager-readme

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Updated addon manager READMEs

Updates addon-manager's README. Based on the pre-condition that the addon manager keeps current "reconciled" pattern instead of "fire-once".

@mikedanese
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cluster/addons/README.md
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# Cluster add-ons
Cluster add-ons are Services and Replication Controllers (with pods) that are
Cluster add-ons are resources like Services and Deployments (with pods) that are
shipped with the Kubernetes binaries and are considered an inherent part of the
Kubernetes clusters. The add-ons are visible through the API (they can be listed
using ```kubectl```), but manipulation of these objects is discouraged because
the system will bring them back to the original state, in particular:
* if an add-on is stopped, it will be restarted automatically
* if an add-on is rolling-updated (for Replication Controllers), the system will stop the new version and
start the old one again (or perform rolling update to the old version, in the
future).
Kubernetes clusters. The add-ons are visible through the API (they can be listed using
`kubectl`), but direct manipulation of these objects through Apiserver is discouraged
because the system will bring them back to the original state, in particular:
- If an add-on is deleted, it will be recreated automatically.
- If an add-on is updated through Apiserver, it will be reconfigured to the state given by
the supplied fields in the initial config. Though it is fine to modify a field that was
unspecified.
On the cluster, the add-ons are kept in ```/etc/kubernetes/addons``` on the master node, in yaml files
(json is not supported at the moment). A system daemon periodically checks if
the contents of this directory is consistent with the add-on objects on the API
server. If any difference is spotted, the system updates the API objects
accordingly. (Limitation: for now, the system compares only the names of objects
in the directory and on the API server. So changes in parameters may not be
noticed). So the only persistent way to make changes in add-ons is to update the
manifests on the master server. But still, users are discouraged to do it
on their own - they should rather wait for a new release of
Kubernetes that will also contain new versions of add-ons.
On the cluster, the add-ons are kept in `/etc/kubernetes/addons` on the master node, in
yaml / json files. The addon manager periodically `kubectl apply`s the contents of this
directory. Any legit modification would be reflected on the API objects accordingly.
Particularly, rolling-update for deployments is now supported.
Each add-on must specify the following label: ```kubernetes.io/cluster-service: true```.
Yaml files that do not define this label will be ignored.
Each add-on must specify the following label: `kubernetes.io/cluster-service: true`.
Config files that do not define this label will be ignored. For those resources
exist in `kube-system` namespace but not in `/etc/kubernetes/addons`, addon manager
will attempt to remove them if they are attached with this label. Currently the other
usage of `kubernetes.io/cluster-service` is for `kubectl cluster-info` command to recognize
these cluster services.
The naming convention for Replication Controllers is
```<basename>-<version>```, where ```<basename>``` is the same in consecutive
versions and ```<version>``` changes when the component is updated
(```<version>``` must not contain ```-```). For instance,
```heapster-controller-v1``` and ```heapster-controller-12``` are the
same controllers with two different versions, while ```heapster-controller-v1```
and ```heapster-newcontroller-12``` are treated as two different applications.
When a new version of a Replication Controller add-on is found, the system will
stop the old (current) replication controller and start the new one
(in the future, rolling update will be performed).
For services, the naming scheme is just ```<basename>``` (with empty version number)
because we do not expect the service name to change in consecutive versions (and
rolling-update of services does not exist).
The suggested naming for most types of resources is just `<basename>` (with no version
number) because we do not expect the resource name to change. But resources like `Pod`
, `ReplicationController` and `DaemonSet` are exceptional. As `Pod` updates may not change
fields other than `containers[*].image` or `spec.activeDeadlineSeconds` and may not add or
remove containers, it may not be sufficient during a major update. For `ReplicationController`,
most of the modifications would be legit, but the underlying pods would not got re-created
automatically. `DaemonSet` has similar problem as the `ReplicationController`. In these
cases, the suggested naming is `<basename>-<version>`. When version changes, the system will
delete the old one and create the new one (order not guaranteed).
# Add-on update procedure
To update add-ons, just update the contents of ```/etc/kubernetes/addons```
To update add-ons, just update the contents of `/etc/kubernetes/addons`
directory with the desired definition of add-ons. Then the system will take care
of:
1. Removing the objects from the API server whose manifest was removed.
1. This is done for add-ons in the system that do not have a manifest file with the
same basename
1. Creating objects from new manifests
1. This is done for manifests that do not correspond to existing API objects
with the same basename
1. Updating objects whose basename is the same, but whose versions changed.
1. The update is currently performed by removing the old object and creating
the new one. In the future, rolling update of replication controllers will
be implemented to keep the add-on services up and running during update of add-on
pods.
1. Note that this cannot happen for Services as their version is always empty.
Note that in order to run the updator script, python is required on the machine.
For OS distros that don't have python installed, a python container will be used.
- Removing objects from the API server whose manifest was removed.
- Creating objects from new manifests
- Updating objects whose fields are legally changed.
# Cooperating with Horizontal / Vertical Auto-Scaling
As all cluster add-ons will be reconciled to the original state given by the initial config.
In order to make Horizontal / Vertical Auto-scaling functional, the related fields in config should
be left unset. More specifically, leave `replicas` in `ReplicationController` / `Deployment`
/ `ReplicaSet` unset for Horizontal Scaling, and leave `resources` for container unset for Vertical
Scaling. The periodical update won't include these specs, which will be managed by Horizontal / Vertical
Auto-scaler.
[![Analytics](https://kubernetes-site.appspot.com/UA-36037335-10/GitHub/cluster/addons/README.md?pixel)]()

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cluster/addons/addon-manager/README.md
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### addon-manager
The `addon-manager` periodically checks for Kubernetes manifest changes in the `/etc/kubernetes/addons` directory,
and when there's a new or changed addon, the `addon-manager` automatically `kubectl create`s it.
The `addon-manager` periodically `kubectl apply`s the Kubernetes manifest in the `/etc/kubernetes/addons` directory,
and handles any added / updated / deleted addon.
It supports `ReplicationControllers`, `Deployments`, `DaemonSets`, `ConfigMaps`, `Services`, `PersistentVolumes` and
`PersistentVolumeClaims`.
It supports all types of resource.
The `addon-manager` is built for multiple architectures.