Multi-Tenant Kubernetes Cluster with Percona Operators

There are cases where multiple teams, customers, or applications run in the same Kubernetes cluster. Such an environment is called multi-tenant and requires some preparation and management. Multi-tenant Kubernetes deployment allows you to utilize the economy of scale model on various levels:

• Smaller compute footprint – one control plane, dense container deployments
• Ease of management – one cluster, not hundreds

In this blog post, we are going to review multi-tenancy best practices, recommendations and see how Percona Operators can be deployed and managed in such Kubernetes clusters.

Multi-Tenancy

Generic

Multi-tenancy usually means a lot of Pods and workloads in a single cluster. You should always remember that there are certain limits when designing your infrastructure. For vanilla Kubernetes, these limits are quite high and hard to reach:

• 5000 nodes
• 10 000 namespaces
• 150 000 pods

Managed Kubernetes services have their own limits that you should keep in mind. For example, GKE allows a maximum of 110 Pods per node on a standard cluster and only 32 on GKE Autopilot nodes.

The older AWS EKS CNI plugin was limiting the number of Pods per node to the number of IP addresses EC2 can have. With the prefix assignment enabled in CNI, you are still going to hit a limit of 110 pods per node.

Namespaces

Kubernetes Namespaces provides a mechanism for isolating groups of resources within a single cluster. The scope of k8s objects can either be cluster scope or namespace scoped. Objects which are accessible across all the namespaces like ClusterRole are cluster scoped and those which are accessible only in a single namespace like Deployments are namespace scoped.

Deploying a database with Percona Operators creates pods that are namespace scoped. This provides interesting opportunities to run workloads on different namespaces for different teams, projects, and potentially, customers too. 

Example: Percona Operator for MongoDB and Percona Server for MongoDB can be run on two different namespaces by adding namespace metadata fields. Snippets are as follows:

Suggestions:

1. Avoid using the standard namespaces like kube-system or default.
2. It’s always better to run independent workloads on different namespaces unless there is a specific requirement to do it in a shared namespace.

Namespaces can be used per team, per application environment, or any other logical structure that fits the use case.

Resources

The biggest problem in any multi-tenant environment is this – how can we ensure that a single bad apple doesn’t spoil the whole bunch of apples?

ResourceQuotas

Thanks to Resource Quotas, we can restrict the resource utilization of namespaces. ResourceQuotas also allows you to restrict the number of k8s objects which can be created in a namespace. 

Example of the YAML manifest with resource quotas:

Please refer to the Resource Quotas documentation and apply quotas that are required for your use case.

If resource quotas are applied to a namespace, it is required to set containers’ requests and limits, otherwise, you are going to have an error similar to the following:

All Percona Operators provide the capability to fine-tune the requests and limits. The following example sets CPU and memory requests for Percona XtraDB Cluster containers:

LimitRange

With ResourceQuotas we can control the cumulative resources in the namespaces but if we want to enforce constraints on individual Kubernetes objects, LimitRange is a useful option. 

For example, if Team 1,2,3 are provided a namespace to run workloads, ResourceQuota will ensure that none of the team can exceed the quotas allocated and over-utilize the cluster… but what if a badly configured workload (say an operator run from team 1 with higher priority class) is utilizing all the resources allocated to the team?

LimitRange can be used to enforce resources like compute, memory, ephemeral storage, storage with PVC. The example below highlights some of the possibilities.

Suggestions:

1. When it’s feasible, apply ResourceQuotas and LimitRanges to the namespaces where the Percona operator is running. This ensures that tenants are not overutilizing the cluster.
2. Set alerts to monitor objects and usage of resources in namespaces. Automation of ResourceQuotas changes may also be useful in some scenarios.
3. It is advisable to use a buffer on maximum expected utilization before setting the ResourceQuotas.
4. Set LimitRanges to ensure workloads are not overutilizing resources in individual namespaces.

Roles and Security

Kubernetes provides several modes to authorize an API request. Role-Based access control is a popular way for authorization. There are four important objects to provide access:

Subjects in the RoleBinding/ClusterRoleBinding can be users, groups, or service accounts. Every pod running in the cluster will have an identity and a service account attached (“default” service account in the same namespace will be attached if not explicitly specified). Permissions granted to the service account with RoleBinding/ClusterRoleBinding dictate the access that pods will have. 

Going by the best policy of least privileges, it’s always advisable to use Roles with the least set of permissions and bind it to a service account with RoleBinding. This service account can be used to run the operator or custom resource to ensure proper access and also restrict the blast radius.

Avoid granting cluster-level access unless there is a strong use case to do it.

Example: RBAC in MongoDB Operator uses Role and RoleBinding restricting access to a single namespace for the service account. The same service account is used for both CustomResource and the Operator. 

Network Policies

Network isolation provides additional security to applications and customers in a multi-tenant environment. Network policies are Kubernetes resources that allow you to control the traffic between Pods, CIDR blocks, and network endpoints, but the most common approach is to control the traffic between namespaces:

Most Container Network Interface (CNI) plugins support the implementation of network policies, however, if they don’t and NetworkPolicy is created, the resource is silently ignored. For example, AWS CNI does not support network policies, but AWS EKS can run Calico CNI which does.

It is a good approach to follow the least privilege approach, whereby default traffic is denied and access is granted granularly:

Allow traffic from Pods in namespace app1 to namespace app1-db:

Policy Enforcement

In a multi-tenant environment, policy enforcement plays a key role. Policy enforcement ensures that k8s objects pass the required quality gates set by administrators/teams. Some examples of policy enforcement could be:

1. All the workloads have proper labels 
2. Proper network policies are set for DB
3. Unsafe configurations are not allowed (Example)
4. Backups are always enabled (Example)

The K8s ecosystem offers a wide range of options to achieve this. Some of them are listed below:

1. Open Policy Agent (OPA) is a CNCF graduated project which gives a high-level declarative language to author and enforces policies across k8s objects. (Examples from Google and OPA repo can be helpful)
2. Mutating Webhooks can be used to modify API calls before it reaches the API server. This can be used to set required properties for k8s objects. (Example: mutating webhook to add NetworkPolicy for Pods created in production namespaces)
3. Validating Webhooks can be used to check if k8s API follows the required policy, any API which doesn’t follow the policy will be rejected. (Example: validating webhook to ensure huge pages of 1GB is not used in the pod )

Cluster-Wide

Percona Operator for MySQL and Percona Operator for PostgreSQL both support cluster-wide mode which allows single Operator to deploy and manage databases across multiple namespaces (support for cluster-wide mode in Percona Operator for MongoDB is on the roadmap). Is also possible to have an Operator per namespace:

For example, a single deployment of Percona Operator for MySQL can monitor multiple namespaces in cluster-wide mode. The use can specify them in WATCH_NAMESPACE environment variable in the cw-bundle.yaml file:

In a multi-tenant environment, it depends on the amount of freedom you want to give to the tenants. Usually when the tenants are highly trusted (for instance internal teams), then it is fine to choose namespace-scoped deployment, where each team can deploy and manage the Operator themselves.

Conclusion

It is important to remember that Kubernetes is not a multi-tenant system out of the box. Various levels of isolation were described in this blog post that would help you to run your applications and databases securely and ensure operational stability. 

We encourage you to try out our Operators:

• For MySQL (Github, Docs, Jira)
• For MongoDB (Github, Docs, Jira)
• For PostgreSQL (Github, Docs, Jira)

CONTRIBUTING.md in every repository is there for those of you who want to contribute your ideas, code, and docs.

For general questions please raise the topic in the community forum.

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