ECS vs Lambda vs App Runner: which to use in 2025

Quick decision matrix

Workload	Best fit
HTTP API, always-on, predictable	ECS Fargate
Event-driven, spiky, scale-to-zero	Lambda
HTTP API, minimal ops overhead	App Runner
WebSocket / long-lived connections	ECS Fargate
Scheduled batch	Lambda + EventBridge

ECS Fargate

resource "aws_ecs_service" "api" {
  name = "payment-api"; cluster = aws_ecs_cluster.main.id
  task_definition = aws_ecs_task_definition.api.arn
  desired_count = 2; launch_type = "FARGATE"
  network_configuration {
    subnets = var.private_subnet_ids; assign_public_ip = false
    security_groups = [aws_security_group.api.id]
  }
}

Watch out: idle containers still cost money (billed per-second).

App Runner

resource "aws_apprunner_service" "api" {
  service_name = "user-api"
  source_configuration {
    image_repository {
      image_configuration { port = "8080" }
      image_identifier = "${aws_ecr_repository.api.repository_url}:latest"
      image_repository_type = "ECR"
    }
    auto_deployments_enabled = true
  }
}

Watch out: less networking control than ECS.

Cost comparison (1M req/month, 100ms avg, 512MB)

Lambda:       ~$0.41/month
App Runner:   ~$5-8/month
ECS Fargate:  ~$9.27/month (24/7)

Lambda wins at spiky/low volume. ECS wins at steady-state high volume.

My recommendation

• Small team, ops is a constraint: App Runner
• Need networking control or persistent connections: ECS Fargate
• Event-driven processing: Lambda — genuinely the right tool

All three are supported in Step2Dev.

👉 step2dev.com

What compute pattern has worked best for your team and why?