Kubernetes vs Docker

Options for installing CHT applications

To deploy the CHT, you should use the technology you are most familiar and comfortable with. It is possible to deploy 20 CHT instances without using Kubernetes. Conversely, if a hosting organisation uses Kubernetes and intends to deploy a single instance of the CHT, that’s also fine.

Which one to choose?

There is no one-size-fits-all solution for all deployments, and every solution comes with its advantages and disadvantages.

Docker is suitable for most CHT deployments use cases. Use Kubernetes for multi-tenant or specific hardware constrained deployments, such as bare-metal servers with a low core-count.

For more details on the research behind why Docker and single-node CouchDB are preferable for CHT deployments, see this forum post. Results from this experiment show no clear advantage for CouchDB clustering, as the performance for view indexing and replication is the same when using a CouchDB cluster vs using a single machine with the same number of cores.

Docker

Docker has stood the test of time and has many benefits:

• Easy to deploy by just downloading a compose file.
• Trivially scales down to a 4-core laptop and way up to a 48-core server.
• Easily integrates with existing Docker based solutions like Watchdog or other services running in Docker.
• Backup and restore can be achieved via simple file management via bind mounted volumes.
• Just enough complexity to securely and soundly run the CHT without extra complexities of Kubernetes
• Natively supports one-click CHT upgrades in the admin GUI

Docker Compose deployment has the following services when running the CHT:

• A service for each CouchDB instance.
• A CHT API service.
• HAProxy Healthcheck service.
• HAProxy service.
• Upgrade Service service.
• A CHT Sentinel service.
• An nginx service to act as a reverse proxy and terminate TLS connections

Kubernetes (k8s)

Running CHT on k8s requires familiarity with the k8s platform and can be more complex to deploy.

k8s offers:

• Resilient network across either physical or VM nodes in the cluster. This allows strong distribution of the heavy CPU and RAM loads that CouchDB can incur under heavy use.
• Orchestration of multiple deployments ensuring easy hosting of multi-tenants. For example you may opt to have a user acceptance testing (UAT), staging, and production instances all in one cluster.
• Service discovery which enables routing of public requests to deployments within the cluster
• Integration with hypervisors such as VMware and ESX. This is possible due to CRD support in Kubernetes that allows third parties to create integrations
• Helm support and integration that makes it easy to deploy applications onto the cluster
• Highly efficient snapshot backups when used with a storage area network (SAN) or Amazon’s Elastic Block Storage (EBS).

The main components of a Kubernetes CHT deployment include one less service than a Compose-based deployment as ingress via nginx isn’t needed as it’s handled externally:

• A pod for each CouchDB instance.
• A CHT API pod.
• A CHT HAProxy Healthcheck pod.
• A CHT HAProxy pod.
• Upgrade Service pod.
• CHT-Sentinel pod.

Example deployments

Below are examples from real world large-scale CHT deployments which have been anonymized. User counts represent the number of active users within a 30 days interval.

CHT Deployment Example 1

This deployment hosts 47 production instances in a data center running Docker. It trades ease and simplicity of Docker Compose deployment for a bit of fragility: if a VM fails, there is no automatic failover, and a restore from backup is needed.

CHT Deployment Example 2

This deployment hosts 1 CHT production instance in a data center running K3s.

CHT Deployment Example 3

This deployment hosts 46 production instances on Amazon Elastic Kubernetes Service (EKS).

To give more specific examples of what’s running in the 46 instances, here’s details on two including their Apdex:

• One large instance has 2.4k active users, 8.9M documents in the Medic database in CouchDB and a mean replication Apdex of 100.0.
• Another medium instance has 232 active users, 279k documents in the Medic database in CouchDB and a mean replication Apdex of 100.