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contributors/design-proposals/aggregated-api-servers.md

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+# Aggregated API Servers
+
+## Abstract
+
+We want to divide the single monolithic API server into multiple aggregated
+servers. Anyone should be able to write their own aggregated API server to expose APIs they want.
+Cluster admins should be able to expose new APIs at runtime by bringing up new
+aggregated servers.
+
+## Motivation
+
+* Extensibility: We want to allow community members to write their own API
+  servers to expose APIs they want. Cluster admins should be able to use these
+  servers without having to require any change in the core kubernetes
+  repository.
+* Unblock new APIs from core kubernetes team review: A lot of new API proposals
+  are currently blocked on review from the core kubernetes team. By allowing
+  developers to expose their APIs as a separate server and enabling the cluster
+  admin to use it without any change to the core kubernetes repository, we
+  unblock these APIs.
+* Place for staging experimental APIs: New APIs can be developed in separate
+  aggregated servers, and installed only by those willing to take the risk of
+  installing an experimental API. One they are stable, it is then easy to
+  package them up for installation in other clusters.
+* Ensure that new APIs follow kubernetes conventions: Without the mechanism
+  proposed here, community members might be forced to roll their own thing which
+  may or may not follow kubernetes conventions.
+
+## Goal
+
+* Developers should be able to write their own API server and cluster admins
+  should be able to add them to their cluster, exposing new APIs at runtime. All
+  of this should not require any change to the core kubernetes API server.
+* These new APIs should be seamless extension of the core kubernetes APIs (ex:
+  they should be operated upon via kubectl).
+
+## Non Goals
+
+The following are related but are not the goals of this specific proposal:
+* Make it easy to write a kubernetes API server.
+
+## High Level Architecture
+
+There will be a new component in the cluster, `kube-aggregator`, which has these
+responsibilities:
+* Provide an API for registering API servers.
+* Summarize discovery information from all the servers.
+* Proxy client requests to individual servers.
+
+The reverse proxy is provided for convenience. Clients can discover server URLs
+using the summarized discovery information and contact them directly. Simple
+clients can always use the proxy and don't need to know that under the hood
+multiple apiservers are running.
+
+Wording note: When we say "API servers" we really mean groups of apiservers,
+since any individual apiserver is horizontally replicatable. Similarly,
+kube-aggregator itself is horizontally replicatable.
+
+## Operational configurations
+
+There are two configurations in which it makes sense to run `kube-aggregator`.
+ 1. In **test mode**/**single-user mode**. An individual developer who wants to test
+    their own apiserver could run their own private copy of `kube-aggregator`,
+    configured such that only they can interact with it. This allows for testing
+    both `kube-aggregator` and any custom apiservers without the potential for
+    causing any collateral damage in the rest of the cluster. Unfortunately, in
+    this configuration, `kube-aggregator`'s built in proxy will lack the client
+    cert that allows it to perform authentication that the rest of the cluster
+    will trust, so its functionality will be somewhat limited.
+ 2. In **gateway mode**. Each cluster should run kube-aggregator as the official
+    gateway to the cluster, where it aggregates all of the apiservers the cluster
+    administer wishes to provide. This configuration is the intended usage of
+    `kube-aggregator`.
+
+### Constraints
+
+* Unique API group versions across servers: Each API server (and groups of servers, in HA)
+  should expose unique API group versions. So, for example, you can serve
+  `api.mycompany.com/v1` from one apiserver and the replacement
+  `api.mycompany.com/v2` from another apiserver while you update clients. But
+  you can't serve `api.mycompany.com/v1/frobbers` and
+  `api.mcompany.com/v1/grobinators` from different apiservers. This restriction
+  allows us to limit the scope of `kube-aggregator` to a managable level.
+* Follow API conventions: APIs exposed by every API server should adhere to [kubernetes API
+  conventions](../devel/api-conventions.md).
+* Support discovery API: Each API server should support the kubernetes discovery API
+  (list the suported groupVersions at `/apis` and list the supported resources
+  at `/apis/<groupVersion>/`)
+* No bootstrap problem: The core kubernetes apiserver must not depend on any
+  other aggregated server to come up. Non-core apiservers may use other non-core
+  apiservers, but must not fail in their absence.
+
+## Component Dependency Order
+
+`kube-aggregator` is not part of the core `kube-apiserver`.
+The dependency order (for the cluster gateway configuration) looks like this:
+ 1. `etcd`
+ 2. `kube-apiserver`
+ 3. core scheduler, kubelet, service proxy (enough stuff to create a pod, run it on a node, and find it via service)
+ 4. `kubernetes-aggregator` as a pod/service - default summarizer and proxy
+ 5. controllers
+ 6. other API servers and their controllers
+ 7. clients, web consoles, etc
+
+Nothing below the `kubernetes-aggregator` can rely on the aggregator or proxy
+being present.  `kubernetes-aggregator` should be runnable as a pod backing a
+service in a well-known location.  Something like `api.kube-public.svc` or
+similar seems appropriate since we'll want to allow network traffic to it from
+every other namespace in the cluster.  We recommend using a dedicated namespace,
+since compromise of that namespace will expose the entire cluster: the
+proxy has the power to act as any user against any API server.
+
+## Implementation Details
+
+### Summarizing discovery information
+
+We can have a very simple Go program to summarize discovery information from all
+servers. Cluster admins will register each aggregated API server (its baseURL and swagger
+spec path) with the proxy. The proxy will summarize the list of all group versions
+exposed by all registered API servers with their individual URLs at `/apis`.
+
+### Reverse proxy
+
+We can use any standard reverse proxy server like nginx or extend the same Go program that
+summarizes discovery information to act as reverse proxy for all aggregated servers.
+
+Cluster admins are also free to use any of the multiple open source API management tools
+(for example, there is [Kong](https://getkong.org/), which is written in lua and there is
+[Tyk](https://tyk.io/), which is written in Go). These API management tools
+provide a lot more functionality like: rate-limiting, caching, logging,
+transformations and authentication.
+In future, we can also use ingress. That will give cluster admins the flexibility to
+easily swap out the ingress controller by a Go reverse proxy, nginx, haproxy
+or any other solution they might want.
+
+`kubernetes-aggregator` uses a simple proxy implementation alongside its discovery information
+which supports connection upgrade (for `exec`, `attach`, etc) and runs with delegated
+authentication and authorization against the core `kube-apiserver`.  As a proxy, it adds
+complete user information, including user, groups, and "extra" for backing API servers.
+
+### Storage
+
+Each API server is responsible for storing their resources. They can have their
+own etcd or can use kubernetes server's etcd using [third party
+resources](../design/extending-api.md#adding-custom-resources-to-the-kubernetes-api-server).
+
+### Health check
+
+Kubernetes server's `/api/v1/componentstatuses` will continue to report status
+of master components that it depends on (scheduler and various controllers).
+Since clients have access to server URLs, they can use that to do
+health check of individual servers.
+In future, if a global health check is required, we can expose a health check
+endpoint in the proxy that will report the status of all aggregated api servers
+in the cluster.
+
+### Auth
+
+Since the actual server which serves client's request can be opaque to the client,
+all API servers need to have homogeneous authentication and authorisation mechanisms.
+All API servers will handle authn and authz for their resources themselves.
+The current authentication infrastructure allows token authentication delegation to the
+core `kube-apiserver` and trust of an authentication proxy, which can be fullfilled by
+`kubernetes-aggregator`.
+
+#### Server Role Bootstrapping
+
+External API servers will often have to provide roles for the resources they
+provide to other API servers in the cluster.  This will usually be RBAC
+clusterroles, RBAC clusterrolebindings, and apiaggregation types to describe
+their API server.  The external API server should *never* attempt to
+self-register these since the power to mutate those resources provides the
+power to destroy the cluster.  Instead, there are two paths:
+ 1. the easy path - In this flow, the API server supports a `/bootstrap/<group>` endpoint
+    which provides the resources that can be piped to a `kubectl create -f` command a cluster-admin
+    can use those endpoints to prime other servers.
+ 2. the reliable path - In a production cluster, you generally want to know, audit, and
+    track the resources required to make your cluster work.  In these scenarios, you want
+    to have the API resource list ahead of time.  API server authors can provide a template.
+
+Nothing stops an external API server from supporting both.
+
+### kubectl
+
+kubectl will talk to `kube-aggregator`'s discovery endpoint and use the discovery API to
+figure out the operations and resources supported in the cluster.
+We will need to make kubectl truly generic. Right now, a lot of operations
+(like get, describe) are hardcoded in the binary for all resources. A future
+proposal will provide details on moving those operations to server.
+
+Note that it is possible for kubectl to talk to individual servers directly in
+which case proxy will not be required at all, but this requires a bit more logic
+in kubectl. We can do this in future, if desired.
+
+### Handling global policies
+
+Now that we have resources spread across multiple API servers, we need to
+be careful to ensure that global policies (limit ranges, resource quotas, etc) are enforced.
+Future proposals will improve how this is done across the cluster.
+
+#### Namespaces
+
+When a namespaced resource is created in any of the aggregated server, that
+server first needs to check with the kubernetes server that:
+
+* The namespace exists.
+* User has authorization to create resources in that namespace.
+* Resource quota for the namespace is not exceeded.
+
+To prevent race conditions, the kubernetes server might need to expose an atomic
+API for all these operations.
+
+While deleting a namespace, kubernetes server needs to ensure that resources in
+that namespace maintained by other servers are deleted as well. We can do this
+using resource [finalizers](../design/namespaces.md#finalizers). Each server
+will add themselves in the set of finalizers before they create a resource in
+the corresponding namespace and delete all their resources in that namespace,
+whenever it is to be deleted (kubernetes API server already has this code, we
+will refactor it into a library to enable reuse).
+
+Future proposal will talk about this in more detail and provide a better
+mechanism.
+
+#### Limit ranges and resource quotas
+
+kubernetes server maintains [resource quotas](../admin/resourcequota/README.md) and
+[limit ranges](../admin/limitrange/README.md) for all resources.
+Aggregated servers will need to check with the kubernetes server before creating any
+resource.
+
+## Methods for running on hosted kubernetes clusters
+
+Where "hosted" means the cluster users have very limited or no permissions to
+change the control plane installation, for example on GKE, where it is managed
+by Google. There are three ways of running on such a cluster:.
+
+ 1. `kube-aggregator` will run in the single-user / test configuration on any
+    installation of Kubernetes, even if the user starting it only has permissions
+    in one namespace.
+ 2. Just like 1 above, if all of the users can agree on a location, then they
+    can make a public namespace and run a copy of `kube-aggregator` in that
+    namespace for everyone. The downside of running like this is that none of the
+    cluster components (controllers, nodes, etc) would be going through this
+    kube-aggregator.
+ 3. The hosted cluster provider can integrate `kube-aggregator` into the
+    cluster. This is the best configuration, but it may take a quarter or two after
+    `kube-aggregator` is ready to go for providers to complete this integration.
+
+## Alternatives
+
+There were other alternatives that we had discussed.
+
+* Instead of adding a proxy in front, let the core kubernetes server provide an
+  API for other servers to register themselves. It can also provide a discovery
+  API which the clients can use to discover other servers and then talk to them
+  directly. But this would have required another server API a lot of client logic as well.
+* Validating aggregated servers: We can validate new servers when they are registered
+  with the proxy, or keep validating them at regular intervals, or validate
+  them only when explicitly requested, or not validate at all.
+  We decided that the proxy will just assume that all the servers are valid
+  (conform to our api conventions). In future, we can provide conformance tests.
+
+## Future Work
+
+* Validate servers: We should have some conformance tests that validate that the
+  servers follow kubernetes api-conventions.
+* Provide centralised auth service: It is very hard to ensure homogeneous auth
+  across multiple aggregated servers, especially in case of hosted clusters
+  (where different people control the different servers). We can fix it by
+  providing a centralised authentication and authorization service which all of
+  the servers can use.
+
+
+
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