I believe we can (and should) converge on an architecture that abstract the VM/bare-metal case away and give the Controller direct access to the containers for each action. Lots of proposals out there already go into that direction (see: Problem Analysis - Autonomous Container Scheduling v1, Clustered Singleton Invoker for HA on Mesos) and it is a natural fit for the orchestrator based deployments, which the whole community seems to move to.
The overall architecture revolves around a clear distinction between user-facing abstractions (implemented by the Controller as of today) and the execution system, which is responsible for loadbalancing and scaling. On the hot path, only HTTP is spoken directly into the containers (similar to Problem Analysis - Autonomous Container Scheduling v1 in a sense). The Controller orchestrates the full workflow of an activation (calling `/run`, getting the result, composing an activation record, writing that activation record). This eliminates both Kafka and the Invoker as we know it today from the critical path. Picking a container boils down to choosing a free (warm) container from a locally known list of containers for a specific action in the ContainerRouter.
The path to optimize gets a lot thinner which could result in easier trackable performance bottlenecks and less code to optimize for performance. In this architecture, all deployment cases look at the very same path when it comes to optimizing burst performance for instance.
Possible execution backend implementations
- Raw VM/Bare-metal based: The ContainerManager and ContainerManagerAgent work in conjunction to create containers. The ContainerManager will need to implement a scheduling algorithm to place the container intelligently.
- Kubernetes based: The ContainerManager talks to the Kubernetes API to create new Pods.
- Mesos based: The ContainerManager talks to the Mesos API to create new containers.
- Knative based: The whole execution layer above vanishes and is replaced by Knative serving, which implements all the routing, scaling etc. itself.