Spry with OpenFaaS: Deploying Dart Servers as Serverless Functions

Deploy your Spry Dart applications on OpenFaaS—the open‑source serverless platform—with custom Docker images, automatic scaling, and GitOps workflows.

OpenFaaS (Functions as a Service) is a popular open‑source serverless platform that runs on Kubernetes or Docker Swarm. With its support for custom containers, you can package Spry applications as OpenFaaS functions, combining Dart's performance with the flexibility of self‑hosted serverless.

In this tutorial, you’ll learn:

• Why run Dart on OpenFaaS? Open‑source, Kubernetes‑native, and portable
• How to create a custom Docker image for OpenFaaS with Dart
• Packaging Spry applications as OpenFaaS functions
• Deploying to Kubernetes with the OpenFaaS Operator
• Environment configuration, logging, and monitoring
• Auto‑scaling and GitOps workflows

All examples are based on a real Spry microservice and can be adapted for your own projects.

Why OpenFaaS for Spry?

• Open‑source freedom – No vendor lock‑in, run on any cloud or on‑premises
• Kubernetes‑native – Leverage Kubernetes for scaling, networking, and storage
• Custom runtime support – Package any language as a Docker container
• Portable – Deploy the same function across different clusters
• GitOps ready – Manage functions via Git with the OpenFaaS CLI
• Community‑driven – Active ecosystem with plugins and extensions

Prerequisites

• A Kubernetes cluster (minikube, kind, k3d, or cloud‑managed)
• OpenFaaS installed (arkade install openfaas)
• Docker installed (for building images)
• OpenFaaS CLI (faas‑cli)
• Basic familiarity with Kubernetes and Docker
• Spry application ready for containerization

Project Structure

spry‑openfaas‑example/
├── Dockerfile.openfaas          # Docker image for OpenFaaS function
├── spry‑function.yml            # OpenFaaS function definition
├── kubernetes/
│   ├── namespace.yaml           # OpenFaaS namespace
│   ├── ingress.yaml             # Ingress configuration
│   └── values.yaml              # Helm values (optional)
├── lib/
│   └── main.dart                # Spry application entry point
├── scripts/
│   └── deploy‑openfaas.sh       # Deployment automation
└── README.md

1. Docker Image for OpenFaaS

OpenFaaS expects a Docker image that listens on port 8080 and responds to HTTP requests. Create a Dockerfile that follows the OpenFaaS watchdog pattern:

# ──────────────────────────────────────────────────────────────────────────────
# Builder stage (AOT compilation)
# ──────────────────────────────────────────────────────────────────────────────
FROM dart:stable AS builder

WORKDIR /app

COPY pubspec.yaml pubspec.lock ./
RUN dart pub get --offline

COPY . .
RUN dart compile exe lib/main.dart -o /app/bin/spry‑server

# ──────────────────────────────────────────────────────────────────────────────
# Runtime stage (OpenFaaS‑compatible)
# ──────────────────────────────────────────────────────────────────────────────
FROM alpine:3.20

# Install runtime dependencies
RUN apk add --no‑cache libc6‑compat ca‑certificates tzdata

# Copy compiled binary
COPY --from=builder /app/bin/spry‑server /app/bin/

# Create non‑root user
RUN addgroup -S spry && adduser -S spry -G spry
USER spry

WORKDIR /app

# Health check
HEALTHCHECK --interval=30s --timeout=3s --start‑period=5s --retries=3 \
    CMD ["/app/bin/spry‑server", "--health‑check"] || exit 1

EXPOSE 8080

CMD ["/app/bin/spry‑server"]

2. OpenFaaS Function Definition

Create an OpenFaaS function definition file:

# spry‑function.yml
version: 1.0
provider:
  name: openfaas
  gateway: http://127.0.0.1:8080

functions:
  spry‑function:
    lang: dockerfile
    handler: .
    image: yourregistry/spry‑openfaas:latest
    environment:
      LOG_LEVEL: info
      DATABASE_URL: postgres://...
    secrets:
      - database‑password
    labels:
      com.openfaas.scale.min: "3"
      com.openfaas.scale.max: "10"
      com.openfaas.scale.factor: "20"
    annotations:
      prometheus.io.scrape: "true"
      prometheus.io.port: "8080"
    limits:
      memory: 256Mi
      cpu: 500m
    requests:
      memory: 128Mi
      cpu: 100m

3. Spry Application with OpenFaaS Support

Update your Spry application to be OpenFaaS‑compatible:

// lib/main.dart
import 'dart:io';
import 'package:spry/spry.dart';
import 'package:spry_health/spry_health.dart';

Future<void> main() async {
  final app = Application();

  // Health check endpoint (required by OpenFaaS)
  app.get('/health', (request) async {
    return Response.json({
      'status': 'healthy',
      'timestamp': DateTime.now().toIso8601String(),
      'runtime': 'Dart on OpenFaaS',
    });
  });

  // OpenFaaS watchdog expects requests at `/`
  app.post('/', (request) async {
    // Parse OpenFaaS request
    final body = await request.body.json();
    final method = body['method'] ?? 'GET';
    final path = body['path'] ?? '/';
    final headers = Map<String, String>.from(body['headers'] ?? {});
    final query = Map<String, String>.from(body['query'] ?? {});

    // Your business logic
    final result = await processRequest(method, path, headers, query);
    return Response.json(result);
  });

  // Your application routes
  app.get('/', (request) async {
    return Response.text('Spry on OpenFaaS 🚀');
  });

  final server = await app.listen(port: 8080);
  print('Server running on http://${server.address.host}:${server.port}');
}

4. Deployment with faas‑cli

Build and push the Docker image:

faas‑cli build -f spry‑function.yml
docker tag yourregistry/spry‑openfaas:latest
docker push yourregistry/spry‑openfaas:latest

Deploy to OpenFaaS:

# Log in to OpenFaaS gateway
export OPENFAAS_URL=http://127.0.0.1:8080
export OPENFAAS_PASSWORD=$(cat ~/openfaas‑password.txt)

faas‑cli login --password‑stdin < ~/openfaas‑password.txt

# Deploy the function
faas‑cli deploy -f spry‑function.yml

# List functions
faas‑cli list

# Invoke function
echo '{"method": "GET", "path": "/"}' | faas‑cli invoke spry‑function

5. Kubernetes Deployment (Alternative)

For production deployments, use the OpenFaaS Operator with Kubernetes manifests:

# kubernetes/function.yaml
apiVersion: openfaas.com/v1
kind: Function
metadata:
  name: spry‑function
  namespace: openfaas‑fn
spec:
  name: spry‑function
  image: yourregistry/spry‑openfaas:latest
  environment:
    LOG_LEVEL: info
    DATABASE_URL: postgres://...
  secrets:
    - database‑password
  labels:
    com.openfaas.scale.min: "3"
    com.openfaas.scale.max: "10"
  annotations:
    prometheus.io.scrape: "true"
    prometheus.io.port: "8080"
  limits:
    memory: 256Mi
    cpu: 500m
  requests:
    memory: 128Mi
    cpu: 100m

Apply with kubectl:

kubectl apply -f kubernetes/function.yaml

6. Monitoring and Observability

Prometheus metrics:

OpenFaaS automatically exposes metrics via Prometheus. Add custom metrics in your Spry application:

import 'package:spry_prometheus/spry_prometheus.dart';

final prometheus = Prometheus();
app.use(prometheus.middleware());
app.get('/metrics', (request) async {
  return Response.text(prometheus.export());
});

Logging with OpenFaaS logging provider:

import 'package:logger/logger.dart';

final logger = Logger(
  printer: JsonPrinter(),
);

app.use((request, next) async {
  final start = DateTime.now();
  final response = await next(request);
  final duration = DateTime.now().difference(start);
  logger.i({
    'function': 'spry‑function',
    'method': request.method,
    'path': request.path,
    'status': response.status,
    'duration_ms': duration.inMilliseconds,
  });
  return response;
});

7. Auto‑scaling

OpenFaaS provides automatic scaling based on metrics. Configure scaling behavior in the function definition:

labels:
  com.openfaas.scale.min: "3"
  com.openfaas.scale.max: "10"
  com.openfaas.scale.factor: "20"
  com.openfaas.scale.zero: "true"   # Scale to zero when idle

8. GitOps Workflow

Manage your OpenFaaS functions with GitOps using the git‑faas plugin or ArgoCD:

# Install git‑faas
arkade install git‑faas

# Deploy from Git repository
git‑faas deploy \
  --repo https://github.com/yourorg/spry‑openfaas‑example \
  --branch main \
  --path ./spry‑function.yml

9. Troubleshooting

Common Issues

• Cold starts: Use com.openfaas.scale.min to keep instances warm
• Timeout errors: Increase timeout in OpenFaaS gateway configuration
• Permission errors: Ensure Docker image is accessible and secrets exist
• Memory limits: Adjust memory requests/limits based on Dart VM usage
• Network connectivity: Check Kubernetes network policies and service mesh

Debugging Commands

# View function logs
faas‑cli logs spry‑function

# Describe function
faas‑cli describe spry‑function

# Invoke with verbose output
echo '{}' | faas‑cli invoke --verbose spry‑function

# Check function health
curl http://gateway.openfaas.svc.cluster.local:8080/health

# Scale function
faas‑cli scale --factor=5 spry‑function

Conclusion

OpenFaaS provides a flexible, open‑source platform for running Spry Dart applications in production. With custom Docker images and Kubernetes integration, you can deploy Spry applications as serverless functions with full control over the infrastructure.

This tutorial covered essential patterns—Docker images, OpenFaaS function definitions, deployment workflows, and monitoring—to get your Spry application running on OpenFaaS.

Next Steps:

1. Implement CI/CD – Automate builds and deployments with GitHub Actions
2. Add authentication – Integrate with OpenFaaS IAM or external auth providers
3. Event‑driven architecture – Connect to NATS, Kafka, or Redis streams
4. Multi‑cluster deployment – Deploy across clusters for high availability
5. Security hardening – Apply network policies, PodSecurity admission, and secrets management

All code from this tutorial is available in the spry‑openfaas‑example GitHub repository.

Happy serverless building! 🚀

This is a draft tutorial. Final content will include more detailed examples, troubleshooting guides, and best practices for production deployments.