design doc: Per Service Filter Chains

[draychev]

With #1725 we implemented filter chain per outbound service.

This GitHub issue is for documenting the the OSM documentation around per-service filterchains.

• Document filterchoin per outbound service -- since this is not a standalone feature - we need to also as port of this issue find the best place for this. Not sure whether this belongs to a design doc or as part of on existing doc around SMI policy (Traffic Target? TCP Route?)

• Create a small demo with sample apps and SMI showing how this feature works

  • What are the SMI policies that affect this feature? What causes OSM to create these filter chains?
  • Give examples of Envoy configuration with and without these filterchains.

• List Common Issues

  • Are there known issues that may arise from this design?
  • What happens if pods on the cluster churn quickly?

• Create Troubleshooting Guide - what tools and techniques could be used to troubleshoot this? Can we use OSM CLI to fetch Envoy config and observe JSON then compare w/ actual Kubernetes context/services/endpoints?

• Automate Troubleshooting Guide in pkg/troubleshooter (create appropriate functions) - alternatively create a GitHub Issue with the stub of the function that could be eventually created within pkg/troubleshooter package to automatically troubleshoot this feature.

[shashankram]

If we document this, kindly drop this in a design doc instead of user-facing docs. User's don't care about knowing the internals of Envoy configurations which can get quite complex based on configurations.

[eduser25]

We'll look forward to move it into docs if needed
https://docs.google.com/document/d/1LD0Ntc3arh2EJNoB8wCCmtIK0oSAqI2dYx8bZZDNtZg/edit