CKAD theory pt.3 - Multi Container Pods

This is part 3 of my personal notes I’ve written during preparation to CKAD.

Multi-Container Pods

• Understand Multi-Container Pods
• Patterns

Multi-Container Pods

Multi-container pods provide an opportunity to enhance containers with helper containers. Multi-container Pods runs in the same namespaces, so containers can share resources and communicate with each other.

Among the ways that containers can use to interact are:

• Shared Network - all listening ports are accessible to other containers in POD, even if they are not exposed outside of the POD
• Shared Storage Volumes - containers can interact with each other by reading and modifying files in a shared storage that is mounted to both containers
• Shared Process Namespace - with processes namespace sharing enabled, containers in the same pod can interact with and signal one another’s processes. Requires to explicitly enable shareProcessNamespace: true in the POD spec.

Patterns

CKAD concentrates on three main multi-container pod patterns:

• Sidecar container - adds functionality to main container in some way. For example, a container that syncs files from main container to GIT. By separating this functionality, it’s possible to evolve both containers independently.
• Ambassador container - act as a network proxy, by accepting network traffic, possibly modifying it and passing to main container.
• Adapter container - takes the output from main container and modifies it, representing in required format. Very useful for monitoring.

Sidecar example

LoadBalancer-> Service with IP of POD -> PORT 443 -> Container with NGINX, rm SSL-> Localhost:80 -> Apache web server

Ambassador example

In this example, ambassador acts as an entrypoint, balancing traffic between 2 web servers PODs

LoadBalancer-> Ambassador POD:80 -> Nginx server1 weight=3 - > Service Server1:80 -> Server1 POD
                                 -> Nginx server2            > Service server2:80 -> Server2 POD

Adapter example

In this example, adapter grabs the data from redis and prepare it for prometheus

Prometheus -> Service Redis Adapter:9121 -> Redis_Exporter Container -> Redis Container

Links

• blog.nillsf.com
• Designing Distributes Systems
• Sidecar Example