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Federation

Note: Federation V1, the current Kubernetes federation API which reuses the Kubernetes API resources ‘as is’, is currently considered alpha for many of its features, and there is no clear path to evolve the API to GA. However, there is a Federation V2 effort in progress to implement a dedicated federation API apart from the Kubernetes API. The details can be found at sig-multicluster community page.

This page explains why and how to manage multiple Kubernetes clusters using federation.

Why federation
Setting up federation
API resources
Cascading deletion
Scope of a single cluster
Selecting the right number of clusters
What's next

Why federation

Federation makes it easy to manage multiple clusters. It does so by providing 2 major building blocks:

Sync resources across clusters: Federation provides the ability to keep resources in multiple clusters in sync. For example, you can ensure that the same deployment exists in multiple clusters.
Cross cluster discovery: Federation provides the ability to auto-configure DNS servers and load balancers with backends from all clusters. For example, you can ensure that a global VIP or DNS record can be used to access backends from multiple clusters.

Some other use cases that federation enables are:

High Availability: By spreading load across clusters and auto configuring DNS servers and load balancers, federation minimises the impact of cluster failure.
Avoiding provider lock-in: By making it easier to migrate applications across clusters, federation prevents cluster provider lock-in.

Federation is not helpful unless you have multiple clusters. Some of the reasons why you might want multiple clusters are:

Low latency: Having clusters in multiple regions minimises latency by serving users from the cluster that is closest to them.
Fault isolation: It might be better to have multiple small clusters rather than a single large cluster for fault isolation (for example: multiple clusters in different availability zones of a cloud provider).
Scalability: There are scalability limits to a single kubernetes cluster (this should not be the case for most users. For more details: Kubernetes Scaling and Performance Goals).
Hybrid cloud: You can have multiple clusters on different cloud providers or on-premises data centers.

Caveats

While there are a lot of attractive use cases for federation, there are also some caveats:

Increased network bandwidth and cost: The federation control plane watches all clusters to ensure that the current state is as expected. This can lead to significant network cost if the clusters are running in different regions on a cloud provider or on different cloud providers.
Reduced cross cluster isolation: A bug in the federation control plane can impact all clusters. This is mitigated by keeping the logic in federation control plane to a minimum. It mostly delegates to the control plane in kubernetes clusters whenever it can. The design and implementation also errs on the side of safety and avoiding multi-cluster outage.
Maturity: The federation project is relatively new and is not very mature. Not all resources are available and many are still alpha. Issue 88 enumerates known issues with the system that the team is busy solving.

Hybrid cloud capabilities

Federations of Kubernetes Clusters can include clusters running in different cloud providers (e.g. Google Cloud, AWS), and on-premises (e.g. on OpenStack). Kubefed is the recommended way to deploy federated clusters.

Thereafter, your API resources can span different clusters and cloud providers.

Setting up federation

To be able to federate multiple clusters, you first need to set up a federation control plane. Follow the setup guide to set up the federation control plane.

API resources

Once you have the control plane set up, you can start creating federation API resources. The following guides explain some of the resources in detail:

The API reference docs list all the resources supported by federation apiserver.

Cascading deletion

Kubernetes version 1.6 includes support for cascading deletion of federated resources. With cascading deletion, when you delete a resource from the federation control plane, you also delete the corresponding resources in all underlying clusters.

Cascading deletion is not enabled by default when using the REST API. To enable it, set the option DeleteOptions.orphanDependents=false when you delete a resource from the federation control plane using the REST API. Using kubectl delete enables cascading deletion by default. You can disable it by running kubectl delete --cascade=false

Note: Kubernetes version 1.5 included cascading deletion support for a subset of federation resources.

Scope of a single cluster

On IaaS providers such as Google Compute Engine or Amazon Web Services, a VM exists in a zone or availability zone. We suggest that all the VMs in a Kubernetes cluster should be in the same availability zone, because:

compared to having a single global Kubernetes cluster, there are fewer single-points of failure.
compared to a cluster that spans availability zones, it is easier to reason about the availability properties of a single-zone cluster.
when the Kubernetes developers are designing the system (e.g. making assumptions about latency, bandwidth, or correlated failures) they are assuming all the machines are in a single data center, or otherwise closely connected.

It is recommended to run fewer clusters with more VMs per availability zone; but it is possible to run multiple clusters per availability zones.

Reasons to prefer fewer clusters per availability zone are:

improved bin packing of Pods in some cases with more nodes in one cluster (less resource fragmentation).
reduced operational overhead (though the advantage is diminished as ops tooling and processes mature).
reduced costs for per-cluster fixed resource costs, e.g. apiserver VMs (but small as a percentage of overall cluster cost for medium to large clusters).

Reasons to have multiple clusters include:

strict security policies requiring isolation of one class of work from another (but, see Partitioning Clusters below).
test clusters to canary new Kubernetes releases or other cluster software.

Selecting the right number of clusters

The selection of the number of Kubernetes clusters may be a relatively static choice, only revisited occasionally. By contrast, the number of nodes in a cluster and the number of pods in a service may change frequently according to load and growth.

To pick the number of clusters, first, decide which regions you need to be in to have adequate latency to all your end users, for services that will run on Kubernetes (if you use a Content Distribution Network, the latency requirements for the CDN-hosted content need not be considered). Legal issues might influence this as well. For example, a company with a global customer base might decide to have clusters in US, EU, AP, and SA regions. Call the number of regions to be in R.

Second, decide how many clusters should be able to be unavailable at the same time, while still being available. Call the number that can be unavailable U. If you are not sure, then 1 is a fine choice.

If it is allowable for load-balancing to direct traffic to any region in the event of a cluster failure, then you need at least the larger of R or U + 1 clusters. If it is not (e.g. you want to ensure low latency for all users in the event of a cluster failure), then you need to have R * (U + 1) clusters (U + 1 in each of R regions). In any case, try to put each cluster in a different zone.

Finally, if any of your clusters would need more than the maximum recommended number of nodes for a Kubernetes cluster, then you may need even more clusters. Kubernetes v1.3 supports clusters up to 1000 nodes in size. Kubernetes v1.8 supports clusters up to 5000 nodes. See Building Large Clusters for more guidance.

What's next

Learn more about the Federation proposal.
See this setup guide for cluster federation.
See this Kubecon2016 talk on federation
See this Kubecon2017 Europe update on federation

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