Introduction to concurrency

Correct concurrent code is one that allows for a certain variability in the order of its execution while still being deterministic on the result. For this to be possible, different parts of the code need to have some level of independence, and some degree of orchestration may also be required. The best way to understand concurrency is by comparing sequential code with concurrent code. Let's start by looking at some non-concurrent code:

fun getProfile(id: Int) : Profile {
    val basicUserInfo = getUserInfo(id)
    val contactInfo = getContactInfo(id)

    return createProfile(basicUserInfo, contactInfo)
}

If I ask you what is going to be obtained first, the user information or the contact information – assuming no exceptions – you will probably agree with me that 100% of the time the user information will be retrieved first. And you will be correct. That is, first and foremost, because the contact information is not being requested until the contact information has already been retrieved:

Timeline of getProfile

And that's the beauty of sequential code: you can easily see the exact order of execution, and you will never have surprises on that front. But sequential code has two big issues:

• It may perform poorly compared to concurrent code
• It may not take advantage of the hardware that it's being executed on

Let's say, for example, that both getUserInfo and getContactInfo call a web service, and each service will constantly take around one second to return a response. That means that getProfile will take not less than two seconds to finish, always. And since it seems like getContactInfo doesn't depend on getUserInfo, the calls could be done concurrently, and by doing so it would be possible can halve the execution time of getProfile.

Let's imagine a concurrent implementation of getProfile:

suspend fun getProfile(id: Int) {
    val basicUserInfo = asyncGetUserInfo(id) 
    val contactInfo = asyncGetContactInfo(id)

    createProfile(basicUserInfo.await(), contactInfo.await())
}

In this updated version of the code, getProfile() is a suspending function – notice the suspend modifier in its definition – and the implementation of asyncGetUserInfo() and asyncGetContactInfo() are asynchronous – which will not be shown in the example code to keep things simple.

Because asyncGetUserInfo() and asyncGetContactInfo() are written to run in different threads, they are said to be concurrent. For now, let's think of it as if they are being executed at the same time – we will see later that it's not necessarily the case, but will do for now. This means that the execution of asyncGetContactInfo() will not depend on the completion of asyncGetUserInfo(), so the requests to the web services could be done around at the same time. And since we know that each service takes around one second to return a response, createProfile() will be called around one second after getProfile() is started, sooner than it could ever be in the sequential version of the code, where it will always take at least two seconds to be called. Let's take a look at how this may look:

Concurrent timeline for getProfile

But in this updated version of the code, we don't really know if the user information will be obtained before the contact information. Remember, we said that each of the web services takes around one second, and we also said that both requests will be started at around the same time.

This means that if asyncGetContactInfo is faster than asyncGetUserInfo, the contact information will be obtained first; but the user information could be obtained first if asyncGetUserInfo returns first; and since we are at it, it could also happen that both of them return the information at the same time. This means that our concurrent implementation of getProfile, while possibly performing twice as fast as the sequential one, has some variability in its execution.

That's the reason there are two await() calls when calling createProfile(). What this is doing is suspending the execution of getProfile() until both asyncGetUserInfo() and asyncGetContactInfo() have completed. Only when both of them have completed createProfile() will be executed. This guarantees that regardless of which of the concurrent call ends first, the result of getProfile() will be deterministic.

And that's where the tricky part of concurrency is. You need to guarantee that no matter the order in which the semi-independent parts of the code are completed, the result needs to be deterministic. For this example, what we did was suspend part of the code until all the moving parts completed, but as we will see later in the book, we can also orchestrate our concurrent code by having it communicate between coroutines.