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SlimFaas: The Slimmest, Simplest & Autoscaling-First Function-as-a-Service

SlimFaas is a lightweight, plug-and-play Function-as-a-Service (FaaS) platform for Kubernetes (and Docker-Compose / Podman-Compose). It’s designed to be fast, simple, and extremely slim — with a very opinionated, autoscaling-first design:

• 0 → N wake-up from HTTP history & schedules,
• 0 → N wake-up from Kafka lag via the companion SlimFaas Kafka service,
• N → M scaling powered by PromQL,
• internal metrics store, debug endpoints, and scale-to-zero out of the box.

Looking for MCP integration? Check out SlimFaas MCP — the companion runtime that converts any OpenAPI definition into MCP-ready tools on the fly.

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Why Use SlimFaas?

🚀 Autoscaling that actually understands your traffic

• Scale-to-zero & wake-up

  • Scale down to 0 after inactivity with configurable timeouts.
  • Wake up from 0 → N based on real HTTP traffic and/or cron-like schedules.
  • Wake up from 0 → N based on Kafka topic activity, using SlimFaas Kafka to monitor consumer lag and call the SlimFaas wake-up API.
  • Control initial capacity with ReplicasAtStart to reduce cold-start impact.

• Two-phase scaling model

  • 0 → N: driven by HTTP history, schedules, and Kafka lag (SlimFaas Kafka) to bring functions online only when they’re needed.
  • N → M: driven by a built-in PromQL mini-evaluator on top of an internal metrics store.
  • Metrics-based autoscaling only runs when at least one pod exists — no reliance on non-existent metrics.

• PromQL-driven autoscaler

  • Express scaling rules with PromQL-style queries, for example:
    • sum(rate(http_server_requests_seconds_count{namespace="...",job="..."}[1m]))
    • max_over_time(slimfaas_function_queue_ready_items{function="my-func"}[30s])
    • histogram_quantile(0.95, sum by (le) ( rate(http_server_requests_seconds_bucket{...}[1m]) ))
  • Choose whether thresholds are per pod (AverageValue) or global (Value).
  • Configure scale-up/scale-down policies and stabilization windows inspired by HPA/KEDA.

• Integrated metrics scraping

  • SlimFaas scrapes only the Prometheus-style HTTP metrics endpoints of pods with prometheus.io/scrape: "true".
  • It stores only the metric keys that are requested in autoscaling triggers or debug queries.
  • A single designated node scrapes and persists metrics; all other nodes read from the same store.

• Debug-friendly

  • POST /debug/promql/eval – evaluate a PromQL expression against the internal store and see the scalar result.
  • GET /debug/store – inspect what metrics are being scraped, how many series exist, and retention size.
  • Designed so you can easily answer: “What does SlimFaas see when it decides to scale?”

• FinOps-minded

  • 30-minute metrics retention window for predictable memory usage.
  • Native scale-to-zero and schedules to keep non-critical workloads cold when they’re not needed.
  • Slim control-plane footprint to avoid burning resources in your autoscaling logic itself.

🧵 Synchronous and Asynchronous Functions

• Simple HTTP endpoints for both sync and async calls.
• Async mode:
  • Limit the number of concurrent requests per function.
  • Configure retry behaviors and backoff strategies.
  • Drive autoscaling decisions from queue metrics.

⏱ Jobs

• Run one-off, batch, and scheduled (cron) jobs via HTTP calls.
• Configure:
  • concurrency,
  • visibility (public/private),
  • retry behavior.

🔐 Private/Public Functions and Jobs

• Mark functions as public or private:
  • Private: only accessible from within the cluster or from trusted pods.
  • Public: fronted by Ingress / API Gateways as usual.

📣 Publish/Subscribe Internal Events

• Synchronously send events to every replica of selected functions.
• No additional event bus required — ideal for cluster-local fan-out, cache invalidation, configuration refresh, etc.

🧠 “Mind Changer” (Status & Wake-up API)

• Built-in REST APIs to:
  • monitor function and replica status,
  • wake functions up on demand,
  • integrate autoscaling state into your own tools/dashboards.

🔌 Plug and Play

• Deploy SlimFaas as a standard pod/StatefulSet with minimal configuration.
• Onboard existing workloads simply by adding annotations:
  • let SlimFaas manage their scaling without rewriting your applications.

⚡ Slim & Fast

• Written in .NET with:
  • focus on performance and low memory footprint,
  • AOT-friendly design,
  • minimal dependency surface.

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Ready to Get Started?

Check out:

• Get Started – Learn how to deploy SlimFaas on Kubernetes or Docker Compose.
• Autoscaling – Deep-dive into 0 → N / N → M autoscaling, PromQL triggers, metrics scraping, and debug endpoints.
• Kafka Connector – Use Kafka topic lag to wake functions from 0 → N and keep workers alive while messages are still flowing.
• Functions – See how to call functions synchronously or asynchronously.
• Events – Explore how to use internal synchronous publish/subscribe events.
• Jobs – Learn how to define and run one-off jobs.
• How It Works – Dive into SlimFaas’s architecture and design.
• Planet Saver – See how to start and monitor replicas from a JavaScript frontend.
• MCP – Discover how to convert any OpenAPI definition into MCP-ready tools on the fly.

We hope SlimFaas helps you simplify autoscaling, reduce costs, and keep your serverless workloads slim.

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Community & Governance

• CNCF Project SlimFaas is proud to be part of the Cloud Native Computing Foundation (CNCF) landscape.

  

• Community Meeting Join us through our Community Meeting Calendar

  • ICS file

• Slack Channel Join our channel on the CNCF Slack to connect with other SlimFaas users.

• Code of Conduct SlimFaas follows the CNCF Code of Conduct.

Enjoy SlimFaas!

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Adopters

List of organizations using this project in production or at stages of testing.

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Add your logo via a pull request:

• Logo must be at PNG format, 100 px width, and 100 px height.
• Add your logo to the documentation/adopters folder.