Serverless Best Practices and Monitoring

Building serverless applications requires a different mindset from traditional application development. This lesson covers essential best practices for Azure serverless — from function design and cold start mitigation to security, cost optimisation, and comprehensive monitoring with Application Insights.

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Function Design Best Practices

Keep Functions Small and Focused

Each function should do one thing well:

• Single responsibility — one trigger, one purpose
• Short execution time — aim for seconds, not minutes
• Small deployment packages — fewer dependencies mean faster cold starts
• Clear naming — the function name should describe what it does

Good:  validateOrder, processPayment, sendConfirmation
Bad:   handleEverything, processData, myFunction

Design for Idempotency

Serverless functions may execute multiple times for the same event (at-least-once delivery). Every function must produce the same result regardless of how many times it runs:

// Idempotent — uses upsert, safe to run multiple times
app.storageQueue('processOrder', {
    handler: async (message, context) => {
        await cosmosContainer.items.upsert({
            id: message.orderId,    // Same ID = same document
            status: 'processed',
            processedAt: new Date().toISOString()
        });
    }
});

Externalise State

Functions are stateless. Never rely on in-memory state between invocations: