Azure Functions Cold Start Mitigation

Cold starts occur when a function app must allocate infrastructure, load the runtime, and initialize your code before handling a request. Impact and mitigation options differ significantly by hosting plan.

Cold Start Behavior by Plan

Cold-start duration depends on the runtime, dependencies, package size, and initialization work. Measure the latency of the deployed app instead of relying on a fixed estimate.

Plan	Platform behavior	Primary mitigation
Consumption (Y1)	Scales to zero; cold starts are expected	Reduce dependencies and startup work, or move to another plan
Flex Consumption (FC1)	Improved scale-from-zero behavior	Configure always-ready instances per function or trigger group
Premium (EP1-EP3)	Keeps app-level always-ready instances and an HTTP prewarmed buffer	Configure the app's always-ready instance count
Dedicated	Host runs continuously when Always On is enabled	Enable Always On
Container Apps (Functions-on-ACA)	Scales to zero when minReplicas is 0	Set minReplicas to 1 or higher

Mitigation Strategies

Consumption Plan

Consumption has no built-in always-ready setting. Reduce the work required to specialize a new instance:

Strategy	How	Trade-off
Reduce package size	Trim unused dependencies; use tree-shaking/bundling	Development effort
Optimize startup code	Lazy-load heavy modules; defer non-critical connections	Code changes required

💡 Tip: A timer-based keep-alive isn't a cold-start guarantee and creates extra executions. If cold starts are a consistent problem, move to Flex Consumption or Premium. See hosting-plans.md for the comparison.

Flex Consumption Plan

Configure always-ready instances for a function group with the dedicated CLI command (do not hand-edit the functionAppConfig ARM array — indexing into it by position can silently overwrite other always-ready groups):

# Set 1 always-ready instance for the "http" function group
az functionapp scale config always-ready set \
  -g $RG -n $APP \
  --settings http=1

Bicep — Always-Ready Configuration

resource functionApp 'Microsoft.Web/sites@2024-04-01' = {
  name: appName
  location: location
  kind: 'functionapp,linux'
  properties: {
    serverFarmId: flexPlan.id
    functionAppConfig: {
      runtime: {
        name: 'node'