Azure Functions vs App Service vs Container Apps — decision matrix

Three popular Azure compute options for HTTP and background workloads. Same code might run on any of them; the right choice depends on scaling shape, runtime control, and price.

	Azure Functions (Consumption)	App Service (Web App)	Container Apps
Compute model	Event-driven serverless	VM-backed, long-running	Serverless containers (KEDA + Envoy)
Cold start	Yes (sub-second to seconds)	None (warm)	Yes, but scale-to-zero optional
Max execution time	10 min (default), 60 min (premium)	Unlimited	Unlimited
Bring your own container?	Custom handlers via container	Yes (Linux)	Yes (first-class)
Scale-to-zero	Yes	No (always >= 1 instance)	Yes
Best for	Bursty workloads, queue consumers, webhooks	Web sites, APIs with steady traffic	Microservices with custom containers, KEDA scalers
Pricing	Pay per execution + GB-s	Per hour of VM	Per vCPU-second + memory-second

When to pick Functions

• Workload is bursty — Sundays do 100 RPS, Tuesdays do 0.
• Event source (Service Bus, Storage Queue, Event Grid, Cosmos changefeed) — bindings are first-class.
• You do not want to think about scale tuning.

When to pick App Service

• Predictable, sustained traffic — a public marketing site, internal LOB app.
• You need built-in SSL/custom domains, deployment slots, easy "swap" rollouts.
• Workload includes long-lived processes (SignalR with many open connections) that do not match the Functions model.

When to pick Container Apps

• You already ship as a container.
• You want scale-to-zero plus KEDA-driven scaling on custom signals (RabbitMQ queue length, Kafka lag, anything KEDA supports).
• You need revisions (multiple versions live with traffic split — "blue/green out of the box").
• You want Dapr for service-to-service calls and pub/sub abstraction.

Sample matrix decisions

• Webhook receiver for Stripe — Functions. Bursty, short, exactly fits the event-driven model.
• Customer-facing API at 100 RPS sustained — App Service P1V3 with 2-3 instances. Functions cold start hurts at scale.
• Microservice that consumes from Service Bus and scales 1-50 based on queue depth — Container Apps. Scale-to-zero off-hours saves real money.
• Cron job nightly batch — Functions Timer trigger if under 10 min, otherwise Container Apps Job or App Service WebJob.

Common cost gotchas

• Functions Premium plan is no longer pay-per-execution — it is per VM-hour, with cold-start guarantees. Pick it only when you actually need the no-cold-start guarantee.
• App Service "Always On" prevents cold start but charges 24/7 — for low traffic, Functions Consumption is much cheaper.
• Container Apps minReplicas = 1 disables scale-to-zero. Set to 0 explicitly for cost saving.

Migration paths

• Functions can run as a container today — migrating to Container Apps is mostly a YAML change.
• App Service can run containers — the migration to Container Apps is incremental (start with a single service).
• Container Apps "Jobs" cover one-off and scheduled batch tasks; you do not need to keep Functions around just for cron.