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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, 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 replicable. Similarly, kube-aggregator itself is horizontally replicable.

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. The kube-apiserver will embed the kube-aggregator component and it will function as the official gateway to the cluster, where it aggregates all of the apiservers the cluster administer wishes to provide.

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.mycompany.com/v1/grobinators from different apiservers. This restriction allows us to limit the scope of kube-aggregator to a manageable level.
  • Follow API conventions: APIs exposed by every API server should adhere to kubernetes API conventions.
  • Support discovery API: Each API server should support the kubernetes discovery API (list the supported 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, which is written in lua and there is Tyk, 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.

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 fulfilled 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. 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 and limit ranges 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.