Asynchronous APIs are a Step Backwards for Non-Blocking Code

async and await are must-have features of modern programming languages, yet they represent the industry’s doubling-down on a clumsy, error-prone, and hard-to-debug mechanism for managing non-blocking IO. Rather than managing non-blocking IO in their runtimes transparently, they instead chose to force them into the forefront of the developer’s mind like manual memory management.

Asynchronous APIs are often sold as a positive feature of newer frameworks and libraries yet usually means littering code with async/await or promises, waving goodbye to comprehensible stacktraces, and dividing the technology stack into two incompatible parts: the old blocking libraries and the new non-blocking ones.


Pausing execution while waiting for something to return a value is seen as blocking but is actually being synchronous. While non-blocking asynchronous code is encouraged and the industry increasingly discourages blocking synchronous code, many developers forget about the synchronous non-blocking approach. This might be because so few language runtimes support it, such as Erlang, Go, and Haskell’s GHC. The three big runtimes don’t support it: web browsers, the JVM, and .NET. They get more entrenched in the asynchronous non-blocking approach over time.

Asynchronous APIs have many problems, such as:

Asynchronous APIs hamper abstraction yet developers are enamoured with them. Here are some of the points that come up.

Asynchronous APIs force code to be architectually non-blocking and performant from the ground up.

Asynchronosity forces asynchronous APIs but it doesn’t encourage performant design. The transformation from synchronous to asynchronous is an entirely mechanistic transformation. If it wasn’t, async/await wouldn’t work.

Asynchronous APIs mandate a greater cognitive overhead for developers due to the aforementioned reasons. The time they spend making code asynchronous is time they could otherwise spend on architectural performance concerns.

Asynchronous code helps avoid the nightmare of threads, such as non-atomic operations, critical regions, and race conditions.

This is the problem of shared resources across concurrent tasks, which is orthogonal to asynchronosity and parallelism. Asynchronous tasks can race promises that manipulate shared state, which opens itself up to race conditions. Of course, this can also happen with synchronous APIs too.

Threads open up developers to the painful world of CPU caching and memory fences, it’s true. However, this still requires sharing of mutable resources, which has its own share of problems in non-threaded code. Developers are thankfully moving away from shared memory and towards messaging and queuing. Synchronous non-blocking tasks exist that don’t expose low-level threading details, such as Erlang’s processes or Haskell’s green threads.

Systems that have neither threads nor a lightweight threading solution like Erlang’s restrict themselves from a lot of fine-grained parallelism tricks, which exclude them from certain domains. For example, Node.js’s cluster module is restricted to coarse parallelism. Most languages with asynchronous APIs, such as Java, C#, and Python expose threads anyway, and usually force threading concerns onto reasonably complex codebases.

To summarise, synchronous APIs don’t mean direct thread manipulation, and most asynchronous tech stacks foist threading concerns onto developers anyway.

The alternative is OS threads, which perform badly.

Erlang processes, goroutines, and Haskell’s green threads are not OS threads, but they can utilise threads behind the scenes for parallelism.

Threads are too coarse-grained for concurrency, but are a great OS feature for implementing low-level parallelism. Apache and early Java attempts at concurrency made the mistake of using them for concurrency and paid the price by getting a reputation of not scaling, with the C10k problem many years ago helping to bring the problem to light.

Threads are an implementation detail of parallelism in modern technology stacks. To dwell on them when considering asynchronous and synchronous API design is mistaken.

Promises are monads. If we had proper do-notation in mainstream languages they’d be the cleanest way of managing non-blocking IO.

Even the language most synonymous with monads, Haskell, provides runtime support for non-blocking synchronous IO without forcing users through async monads. One of their fast HTTP servers, Warp, takes advantage of this.

async/await are essentially the do-notation specialised to the async “monad”, so they inherit all of their problems despite being a more general abstraction.

Asynchronisity is required for our event-driven system.

Interpreting events as many callbacks that can be triggered at any time sounds inherently asynchronous, but what about seeing it as an infinite stream of events that a single waiting task dispatches in a loop? Wait, isn’t that what epoll and kqueue fundamentally do, the foundations of asynchronous systems like Node.js?

Events are inherently concurrent but they don’t necessitate an asynchronous API. In fact, triggering callbacks ad hoc rules out many powerful tools of abstraction that are available with the streaming model.

The goal of projects like ReactiveX and Java’s Reactor is to turn ad hoc events into such organised streams. Of course, it’s a lot easier just to iterate over a stream with a synchronous API and wait for results as they come in. When non-blocking is supported directly by a language, this roughly performs in the same way yet doesn’t lead to the horribly mangled stacktraces and duplication of existing language constructs that occurs in reactive programming libraries.

For example, if Java had lightweight tasks, Reactor would be unnecessary as developers could use java.util.Stream even for results that blocked due to operations like network calls.

My application can’t afford lightweight processes or goroutines.

If your application can afford garbage collection and dynamic dispatch for polymorphism, it can probably afford a lightweight task runtime too.

Like garbage collection it’s ultimately trading off a bit of performance, in this case memory, to make programming less error-prone and more robust.

If you’re programming in Java or C# you’re almost certainly already working on a problem for which a lightweight task system has perfectly acceptable performance. This is even more true for JavaScript and Python.

If you’re working in C, C++, or Rust, then you possibly are working in a domain in which a lightweight task runtime is prohibitive.


Like manual memory management, explicitly asynchronous APIs should reside solely in the realm of systems and high-performance programming.

We don’t hesitate to reach for a garbage collector when developing high-level applications, so why is the industry so keen on manually managing non-blocking IO?

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