[gighi]

Both approaches are reasonable (and there's also a third one, ship your application in containers and replace containers instead of instances).

We update existing instances because in our test environment we deploy at every single new commit (we absolutely love that), and we have hundreds (or more) a day. At that pace, replacing instances would be more time consuming (again, for our specific use case) and less cost efficient.

Plus, updating existing instances is handled automatically by AWS Code Deploy, which provides a very good deploying pipeline that you can control using the aws cli tool.

There are other minor advantages but those are the two main ones.

[nzoschke]

That's an awesomely aggressive deployment rate and a great reason to do instance mutation.

Does something verify every commit in the testing environment too?

[gighi]

Yes, every commit gets pulled by jenkins which builds the whole thing, runs unit tests and then starts the deployment once the tests pass.