How SlimFaas Works (Architecture)

Under the hood, SlimFaas is an HTTP proxy that intercepts requests for your functions, jobs, or events. It handles scaling, routing, and state management.

1. Core Concepts

  1. SlimFaas Pod Runs as a Deployment or StatefulSet (commonly 3 replicas in production). Each pod has internal workers:

    • SlimWorker: Handles async call processing.
    • SlimDataSynchronizationWorker: Manages the embedded database cluster (Raft-based).
    • HistorySynchronizationWorker: Syncs request history and logs.
    • ReplicasSynchronizationWorker: Keeps track of your function pods’ replicas and statuses in Kubernetes.
    • ReplicasScaleWorker: If the SlimFaas pod is leader, it scales up/down your function pods.

  2. SlimData A built-in key-value store based on Raft, provided by .NET’s dotNext. This database is crucial for consistent state among SlimFaas pods.

  3. Annotations Add or remove SlimFaas annotations on your pods/Deployments to control scaling, concurrency, visibility, and timeouts.

  4. Public vs. Private Restricts who can access a function/job (any external caller vs. same-namespace or trusted pods).

2. Request Flow

Synchronous HTTP Calls

  1. Client → SlimFaas GET /function/<functionName>/...
  2. SlimFaas ensures the target function is scaled up and ready.
  3. SlimFaas → Function Wait for the function’s response.
  4. SlimFaas returns the function’s response to the client.

sync_http_call.PNG

Asynchronous HTTP Calls

  1. Client → SlimFaas GET /async-function/<functionName>/... (returns immediately with 202 Accepted).
  2. SlimFaas enqueues the request in SlimData.
  3. SlimWorker processes requests in the background, respecting concurrency limits.
  4. Function handles each request. SlimFaas logs outcomes in SlimData.

async_http_call.PNG

Publish/Subscribe (Events)

  1. Client → SlimFaas POST /publish-event/<eventName> with JSON payload.
  2. SlimFaas synchronously broadcasts the payload to each subscribed function’s replicas.
  3. Each replica processes the event and responds individually to SlimFaas.

publish_sync_call.png

3. Scaling Logic

  • Scale to 0 after a defined inactivity (SlimFaas/TimeoutSecondBeforeSetReplicasMin).
  • Scale from 0 to 1+ when a new request arrives or wake-function is called.
  • Optional: Use standard K8s Horizontal Pod Autoscalers or KEDA if you need more advanced scaling triggers.

4. Build & Technology Stack

SlimFaas is developed in .NET, chosen for its:

  • High performance in web APIs.
  • Excellent concurrency model.
  • Constant improvements in speed and memory usage.
  • Compact container images.

That’s the architecture in a nutshell! SlimFaas ensures your functions and jobs scale efficiently while remaining lightweight and easy to set up.