Do Any Major Apps Rely on AWS Lambda for Their Backend?

Yes, many companies successfully rely on Lambda; however, you need to be cautious about how you structure your code. Keep your business logic separate from AWS services to allow easier transitions between Lambda and ECS. That way, if your app grows and demands change, you’re not stuck with a difficult migration.

Absolutely, there are production applications handling millions of requests that run on AWS Lambda. Just be sure to consider potential issues like cold starts and the costs associated with high invocation rates. If your application's traffic is consistent rather than spikey, ECS might be more cost-effective in the long run.

There are certainly large companies that use Lambda extensively, like Capital One. But one major concern is cost. While Lambda can be efficient at lower scales, it might become pricey as your usage increases, particularly if you're dealing with a lot of requests. For steady workloads, you might find that using ECS is more economical.

When using Lambda, it’s crucial to design your application to separate core functionality from the Lambda abstraction. This way, when you decide to migrate to ECS, you won't have to rebuild everything from scratch. It's all about making your application as modular as possible.

Yes, there are definitely apps with millions of active users that utilize Lambda. However, the concurrency limit you mentioned isn't quite as flexible as it seems; it’s actually a regional limit shared across all your functions. But you can request an increase if needed. Common bottlenecks are usually downstream issues, such as rate-limited APIs or databases, rather than Lambda itself.