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This article mainly explains "how to use apiserver's list-watch". Interested friends may wish to have a look at it. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn how to use apiserver's list-watch.

0. Requirements of list-watch

The figure above is a typical Pod creation process, in which every time kubectl creates a ReplicaSet object, controller-manager gets the latest ReplicaSet object through list-watch and executes its own logic to create the Pod object. The same is true of several other components, Scheduler/Kubelet, which learn about changes and deal with them through list-watch. This is the processor code for the component:

C.NodeLister.Store, c.nodePopulator = framework.NewInformer (c.createNodeLW (),... (1) & api.Node {},... (2) 0 ... (3) framework.ResourceEventHandlerFuncs {... (4) AddFunc: c.addNodeToCache,... (5) UpdateFunc: c.updateNodeInCache DeleteFunc: c.deleteNodeFromCache,},)

Where (1) is the list-watch function, and (4) (5) is the entry of the corresponding event trigger operation.

The list-watch operation needs to do a few things:

The component initiates an watch request to the apiserver instead of the etcd, and subscribes when the component starts, telling apiserver what data needs to be changed. Watch is a typical publish-subscribe model.

Watch requests made by components to apiserver can be conditional. For example, scheduler wants to watch all unscheduled Pod, that is, Pod that satisfies Pod.destNode= "", to schedule operations, while kubelet only cares about the Pod list on its nodes. The watch initiated by apiserver to etcd is unconditional, only knows that some data has been changed or created or deleted, but cannot filter the specific value. That is, conditional filtering of object data must be done on the apiserver side rather than the ETCD side.

List is a means to make up for the failure of watch and the data is too obsolete. For more information, please see the detailed explanation of list-watch-based Kubernetes Asynchronous event handling Framework-client side. List itself is a simple list operation, like other apiserver add, delete and modify operations, no longer describe the details.

1. API processing of watch

Since watch itself is an API of http restful provided by apiserver, read its code in the way of API. As described in the article apiserver's basic function implementation, let's look at its code.

The key to dealing with API registration code pkg/apiserver/api_installer.go

Func (a * APIInstaller) registerResourceHandlers (path string, storage rest.Storage,.) Lister, isLister: = storage. (rest.Lister) watcher, isWatcher: = storage. (rest.Watcher). (1). Case "LIST": / / List all resources of a kind. (2) doc: = "list objects of kind" + kind if hasSubresource {doc = "list" + subresource + "of objects of kind" + kind} handler: = metrics.InstrumentRouteFunc (action.Verb, resource, ListResource (lister, watcher, reqScope, false A.minRequestTimeout). (3)

A rest.Storage object is converted to watcher and lister objects

The entry for providing list and watch services is the same, and GET / pods?watch=true is used in the API interface to distinguish between list and watch.

The API handling function is done by combining lister and watcher with ListResource ().

So let's take a look at the specific implementation of ListResource (), / pkg/apiserver/resthandler.go.

Func ListResource (r rest.Lister, rw rest.Watcher,... {... If (opts.Watch | | forceWatch) & & rw! = nil {watcher, err: = rw.Watch (ctx, & opts). (1). ServeWatch (watcher, scope, req, res, timeout) return} result, err: = r.List (ctx, & opts)... (2) write (http.StatusOK, scope.Kind.GroupVersion (), scope.Serializer, result, w, req.Request)

Every time a url request from watch comes over, rw.Watch () is called to create a watcher. Well, the name here is repeated with the name of the layer above, but we can tell it apart and use serveWatch () to process the request. It is important that the life cycle of an watcher is requested by each http.

List is another branch here, which is handled separately from watch and can be ignored.

The code of the procedure serveWatch () that responds to the http request is in / pkg/apiserver/watch.go

Func serveWatch (watcher watch.Interface... {server.ServeHTTP (res.ResponseWriter, req.Request)} func (s * WatchServer) ServeHTTP (w http.ResponseWriter, req * http.Request) {for {select {case event, ok: =

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