Cold starts in Azure Functions — what they are and how to mitigate

A cold start happens when Azure has to allocate a new function instance from scratch — typically because (a) no instance is running, or (b) load just spiked and a new instance was needed. The first invocation pays the platform setup cost.

What happens during a cold start

1. Azure allocates a worker VM           (~50-200 ms)
2. Mounts your function app             (~100-500 ms)
3. Starts the language host             (~100-300 ms)
4. JIT-compiles your assemblies          (~500-2000 ms, .NET)
5. Runs static initializers + DI setup   (~100-1000 ms, depends on app)
6. Runs your function code              (variable)

Total for .NET on Consumption: 1-5 seconds, dominated by JIT + DI setup.

How cold starts compare across languages

Runtime	Typical cold start
Node.js	500 ms – 2 s
Python	500 ms – 2 s
.NET 8+ isolated worker	1 – 3 s
.NET 8+ AOT (Native AOT)	200 – 800 ms
Java	3 – 10 s (worst)

When you DON'T pay cold start

• Subsequent invocations within ~10-20 min of the previous one — the instance stays warm
• All invocations on Premium plan (instances are pre-warmed)
• All invocations on App Service Plan (always running)

Mitigation strategies

1. Move to Premium or Flex Consumption

The most reliable fix. Premium pre-warms instances; Flex Consumption has much faster cold starts than legacy Consumption.

resource plan 'Microsoft.Web/serverfarms@2023-12-01' = {
  name: 'plan-functions'
  sku: { name: 'EP1', tier: 'ElasticPremium' }
  properties: { maximumElasticWorkerCount: 20 }
}

2. Use .NET 8+ Native AOT

.NET Native AOT removes the JIT step entirely. Cold start drops from 1-5 s to 200-800 ms.

<!-- .csproj -->
<PropertyGroup>
  <PublishAot>true</PublishAot>
  <InvariantGlobalization>true</InvariantGlobalization>
</PropertyGroup>

Limitations: no reflection-heavy libraries, code must be AOT-compatible.

3. Reduce app size and dependencies

• Each NuGet package adds startup time
• Use PublishTrimmed=true to strip unused code
• Move heavy initialization OUT of static constructors and DI registration

// ❌ Runs on cold start, even for unrelated functions
public class HeavyService {
    static HeavyService() {
        BuildExpensiveLookupTable();   // 800 ms
    }
}

// ✅ Lazy — only runs when the service is actually used
public class HeavyService {
    private static readonly Lazy<LookupTable> _table = new(BuildExpensiveLookupTable);
}

4. Health-check ping ("keep-warm")

Schedule a Timer function that hits your HTTP function every 5 minutes. The instance stays warm.

[Function("KeepWarm")]
public async Task KeepWarmAsync(
    [TimerTrigger("0 */5 * * * *")] TimerInfo timer,
    [HttpClientFactory] HttpClient http)
{
    await http.GetAsync("https://myapp.azurewebsites.net/api/health");
}

Cheap on Consumption (small executions), works well for low-traffic apps that occasionally get a critical user request.

5. Use AlwaysReady instances on Flex Consumption

Flex Consumption lets you set a minimum number of always-ready instances per function. Best of both worlds: scale to zero for cold paths, never cold-start for the hot path.

"functionAppConfig": {
  "scaleAndConcurrency": {
    "alwaysReady": [
      { "name": "PaymentHandler", "instanceCount": 1 }
    ]
  }
}

When cold start doesn't matter

• Webhook receivers (the caller doesn't notice a 2 s response)
• Background queue processors (a 2 s startup adds to ALL processing, but throughput is what matters)
• Scheduled jobs (no user waits)
• Internal automation

When cold start is unacceptable

• User-facing API where p95 latency must be < 500 ms
• Real-time chatbot responses
• Financial transaction APIs with hard SLAs

For these, you should NOT be on Consumption. Move to Premium or use Web API on App Service.

Common interview follow-up

"Our webhook is on Consumption, occasionally gets a 5 s response. Stripe is complaining. What's the cheapest fix?"

Tiered answer:

1. Cheapest: add a Timer keep-warm pinging the webhook every 5 min (~₹50/month extra)
2. Better: migrate to Flex Consumption (~5x reduction in cold start, similar cost)
3. Best: Premium EP1 with always-ready=1 (no cold start, but ~₹10,000/month)

Pick based on how often the cold start actually fires. If it's once a day at 3 AM, keep-warm; if it's during business hours, Flex or Premium.