We have an asp.net application, it makes no use of async or parallel methods(except for some owin middleware).
I want to start using the async version of some methods using the async-await syntax.
I know that there is danger in mixing async and sync operations as that might cause a deadlocks.
It is not possible to rewrite the whole app in one go.
Where should I Start? is the safe way to do it to make the controllers actions async and work my way down? or the other way around?
Is there some clear warning sign(s) that I can watch for along the way like: "never have a sync method call an async method, but it's fine to have async call sync"
Where should I Start? is the safe way to do it to make the controllers actions async and work my way down? or the other way around?
I recommend starting at the "leaves" and creating "vertical partitions". That is, within a service or data access layer, identify I/O-bound operations. Then create asynchronous copies of those methods that use naturally-asynchronous APIs. Once you have those, then continue "up" your layers, creating asynchronous copies, until you can make a controller action asynchronous.
You'll end up with a fair amount of code duplication - both synchronous and asynchronous versions of the same method - but this is only temporary. Once all the other code only uses the asynchronous version of a method, the synchronous version can be removed.