Async await

in C#

Why should we care?

• Thundering herd
• Horses = users

What is our goal

• Do more with less
• Juggle more users

How? An analogy

• Imagine a 'wash and fold' laundry business
• When people drop off clothes, you have a choice:
• Sync vs async

Sync (blocking)

• A customer arrives with clothes
• You put the clothes in the machine
• You sit in front of the machine until they are done
• You take the clothes back to the front desk
• You can now serve the next customer

Computer time

Async (non-blocking)

• A customer arrives with clothes
• You take their phone number and give them a ticket
• You put the clothes in the machine
• You go back to the desk to help the next customer
• When the clothes are done, you call the first customer

Apply the analogy

• Laundromat = your application
• Customer = request
• Employee = thread
• Washing clothes = calling an external service
• We want to be able to give people tickets, not sit and wait

Talk Outline

• Key concepts
• Best practices and common gotchas
• 5 take aways

What is a task?

• Think of a task like a laundry ticket
• You give it to the customer immediately
• When the items are ready, we'll tell you
• "An abstraction over an activity that is ongoing and will eventually complete." Mads Torgersen

What is await?

• Like the front desk
• Place to come back when the task is done
• Where we can resume execution for that customer
• Await does not mean 'wait'

Waiting is relative

• The customer still has to wait
• But the employee does not
• They can 'juggle' many customers

Is it faster?

• The total washing time is still an hour
• The user has to wait 1 hour plus service time
• This is still faster than sync approach of a queue
• In sync, your wait includes the washing time of other people

Threads

• It's like the employee
• One person can handle hundreds of customers because they do things concurrently

Parallel vs concurrent

I/O vs CPU

Summary (so far)

• Task is like a ticket
• Await is like the front desk
• Thread is like an employee
• We seem fast by doing this concurrently

Best practices

When I/O, then async

• Any time we call an external service
• E.g. file system, API, DB
• Use async await

Don't block

• Avoid methods like .Result() or .WaitAll() or .Wait()
• These will force the thread to sit in front of the washing machine
• This means async code will spread

Be suspicious of Task.Run

• This moves the work to another thread
• Like hiring another person
• If it's I/O they are still blocked - they still sit and wait for machine to finish
• Might be okay for CPU bound work but there are better ways to do parallelization 

Avoid await inside a loop

Use list of tasks instead

Use .ConfigureAwait(false) for UI thread

• This allows ANY thread to resume the work, not the calling thread
• This is important for any applications with a UI
• Any laundromat employee can give your clothes back, not just the one who served you originally
• Dotnet core does not have this problem

5 take aways

• All I/O should be async
• Avoid blocking keywords
• Be suspicious of Task.Run
• Avoid await inside a loop
• Use .ConfigureAwait(false) for old UI work