In a previous post we introduced the combination of Flux and the Cluster API to enable cluster management with GitOps. The result is you can declaratively define your clusters and perform operations on the clusters all via Git pull request, as you would the workloads on the clusters.
But what happens if you are building a large multi-cloud and/or hybrid platform? In this post we discuss this type of scenario and also introduce an advanced deployment pattern that we call “master-of-masters” or MoM.
Multi-cloud / Hybrid Scenarios with GitOps and Cluster API
Multi-cloud/hybrid are exactly the types of scenarios that the Cluster API was designed to excel at. Infrastructure providers, such as the AWS provider (CAPA) or vSphere provider (CAPV), provision infrastructure in a target environment. Depending on which provider is used, the infrastructure can be in a public or private cloud and thanks to providers like CAPV, they can be provisioned on-premise as well (although CAPV isn’t limited to on-premise).
There are a few things to note in a hybrid or multi-cloud scenario:
- The Management Cluster (a.k.a control plane cluster) must:
- Hold credentials for each of the target environments for use by Cluster API infrastructure providers.
- Have sufficient network connectivity for the Cluster API providers to be able to provision their infrastructure and the requirements are different for each provider.
- Every Workload Cluster (a.k.a. tenant cluster) is managed by the management cluster and the CAPI resources that it contains.
For example, if we are targeting on-premise vSphere, AWS (multiple accounts) and Azure:
There is nothing inherently wrong with the above approach. It is the simplest way to achieve multi-cloud and/or hybrid type scenarios with Cluster API and Flux.
A note about security
Since the management cluster holds the credentials for a number of target environments, it does represent an area of potential risk. If the management cluster is compromised then access to all the tenant clusters is possible. Additionally, every cluster is managed through the same management cluster, and depending on your business, you might see this as a single point of failure or an insufficient separation of concerns with regard to the cluster ownership. For example, perhaps each engineering team has their own AWS account.
What is Master-of-masters?
Master-of-master (MoM) is an advanced implementation pattern targeted at multi-cloud (public and/or private) and hybrid scenarios and it provides:
- Tenant clusters for a target environment to be only managed from within that target environment.
- Ability to deploy environment/zone/account level services. For instance, an aggregated prometheus that collects metrics from every tenant cluster in a particular environment.
- Greater segregation between cluster definitions for target environments.
- Target environments to run detached.
With the “master-of-masters” pattern there is a "master management cluster" in each target environment which looks after the tenant clusters for that target environment. The master management clusters are themselves provisioned by another management cluster that we call the “master-of-masters (MoM) Cluster”:
- The “master-of-masters” (MoM) cluster is responsible for provisioning the master management clusters in each of the target environments. It needs the Cluster API and providers installed for the target environments beforehand. The MoM cluster also requires credentials and network connectivity for the Cluster API providers to do their job.
An option is to make this cluster ephemeral (perhaps using KIND or k3s) and only create it when a new target environment is needed. In this scenario, the cluster would be spun up for a short period of time and the new master management cluster provisioned and then the cluster would be torn down
- The “masters repo’ holds the cluster definitions for the master management clusters. This could be the raw YAML files or you could also use a HelmRelease. A Flux instance running in the MoM cluster will automatically apply either the manifest or the Helm chart.
- Credentials are required for each target environment. These credentials can either be permanent or you can use short-lived credentials. To use short-lived credentials, secrets can be injected into the cluster from Vault (using secrets engines) for use by the Cluster API providers.
- Each target environment contains a “master management cluster”. It's the master management cluster’s responsibility to provision any tenant clusters within that environment. To do this it needs only the Cluster API and the provider for that environment (i.e. capz, capv). There will also be at least 1 instance of Flux deployed to monitor the “Cloud/DC Master Repo”
- The “Cloud/DC Master Repo” holds the following:
- Cluster definitions for the tenant clusters for that target environment.
- Application definitions for services that should run centrally in that target environment. For example, you might want to run log or metric aggregation on the management cluster for all tenants within that environment.
- The provider for that environment would need credentials for that environment only. You should be able to take advantage of things like IAM Roles for EC2 Instances (iamInstanceProfile property in CAPA) so that explicit credentials aren’t needed.
Note: The concept of an ephemeral management cluster is new. It assumes that the Cluster API providers are idempotent and can resume from where they left off. This has been tested with the AWS provider (CAPA) but we haven’t tested all the providers yet.
In this post we introduced the “master-of-masters” pattern for Flux and Cluster API to aid various multi-cloud and hybrid scenarios when using GitOps for cluster management. The pattern demonstrates additional architectures that you can take advantage of when adopting GitOps for Cluster Management. One size never fits all and the above patterns can have many variations to meet your specific requirements.
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