Why extensibility

As with any good architecture, extensibility is very important because there is no one size fits all approach to technology that can adapt to every application. Extensibility is important in Istio as it provides options to users to build corner cases and extend Istio as per their individual needs. In the early days of Istio and Envoy, the projects took different approaches to build extensibility. Istio took the approach of building a generic out-of-process extension model called Mixer (https://istio.io/v1.6/docs/reference/config/policy-and-telemetry/mixer-overview/), whereas Envoy focused on in-proxy extensions (https://www.envoyproxy.io/docs/envoy/latest/extending/extending). Mixer is now deprecated; it was a plugin-based implementation used for building extensions (also called adaptors) for various infrastructure backends. Some examples of adapters are Bluemix, AWS, Prometheus, Datadog, and SolarWinds. These adapters allowed Istio to interface with various kinds...