I know it sounds really basic, but it turns out there’s much more into it. First of all, a thread in Java is called a user thread. Essentially, what we do is that we just create an object of type thread, we parse in a piece of code. When we start such a thread here on line two, this thread will run somewhere in the background. The virtual machine will make sure that our current flow of execution can continue, but this separate thread actually runs somewhere. At this point in time, we have two separate execution paths running at the same time, concurrently.

So if you have many long-running IO tasks, they aren’t going to waste a kernel thread and have it sit around idle waiting on IO. This is similar to async libraries, but without the mental overhead. You should be able to just code synchronously and the JVM will take care of the rest. The improvements that Project Loom brings are exciting.

Project Loom provides ‘virtual’ threads as a first class concept within Java. There is plenty of good information in the 2020 blog post ‘State of Loom’ although details have changed in the last two years. Yes – all subsystems same as in production No – detailed simulations, but test doubles relied upon Useful for debugging No – distributed systems failures never fun to debug.

project loom

Those are technically very similar and address the same problem. However, there’s at least one small but interesting difference from a developer’s perspective. For coroutines, there are special keywords in the respective languages (in Clojure a macro for a “go block”, in Kotlin the “suspend” keyword). The virtual threads in Loom come without additional syntax.

Project Loom: Why Are Virtual Threads Not The Default?

Providing an orderly way to stop a service is not only good practice and good manners, but in our case, also very useful when implementing the Raft in-memory simulation. If you are doing the actual debugging, so you want to step over your code, you want to see, what are the variables? Because when your virtual thread runs, it’s a normal Java thread. It’s a normal platform thread because it uses carrier thread underneath.

project loom

Today Java is heavily used in backend web applications, serving concurrent requests from users and other applications. In traditional blocking I/O, a thread will block from continuing its execution while waiting https://globalcloudteam.com/ for data to be read or written. Due to the heaviness of threads, there is a limit to how many threads an application can have, and thus also a limit to how many concurrent connections the application can handle.

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Do you imagine that go programmers secretly wish they were writing Java syntax, in my experience this is very much not true. Go has an advantage in the niche cases where you can get away with value types. But that is a very rare use case, reminiscent of embedded programs. You almost always need heap allocations, especially for long running, large apps — and Java has the state of the art GC implementation on both throughput and low-latency front. On a 1 TB server machine guess which platform will have better throughput by far? Maybe a little disappointing for low level nuts and other languages like kotlin, but the right move IMO.

Blocking and waiting on one queue is easy, blocking and waiting on a set of queues waiting for any to get an element is a good deal trickier. Having that baked into the language and the default channel data-structures really does pay dividends over a library in a case like this. You can still use a fixed thread pool with a custom task scheduler if you like, but probably not exactly what you are after. Why should Clojure benefit more from Loom than other languages? I think it simplifies reactive/concurrent programming in any JVM language.

It voluntarily says that it no longer wishes to run because we asked that thread to sleep. Unparking or waking up means basically, that we would like ourselves to be woken up after a certain period of time. Before we put ourselves to sleep, we are scheduling an alarm clock. It will continue running our thread, it will continue running our continuation after a certain time passes by. In between calling the sleep function and actually being woken up, our virtual thread no longer consumes the CPU.

Java Concurrency: An Introduction To Project Loom

However, as far as Raft implementation was concerned, this did not really matter a lot. In this aspect, there’s no additional type-safety benefits from the wrapped representation. Are the concurrency constructs really equivalent in both approaches? However, if we go a bit deeper, that’s not always the case. Thanks to the nice syntax provided by our Loom.fork method , if we parsed these into sufficiently high-level abstract syntax trees, we would get the same result. What’s important to keep in mind is that the ZIO implementation came first, and the result might have been completely different had I started with Loom, and then translated to ZIO.

Structured concurrency aims to simplify multi-threaded and parallel programming. It treats multiple tasks running in different threads as a single unit of work, streamlining error handling and cancellation while improving reliability and observability. This helps to avoid issues like thread leaking and cancellation delays. Being an incubator feature, this might go through further changes during stabilization. Before looking more closely at Loom’s solution, it should be mentioned that a variety of approaches have been proposed for concurrency handling.

If we’re the leader, make sure the other nodes are up to date with us’. Certain parts of the system need some closer attention. For example, there are many potential failure modes for RPCs that must be considered; network failures, retries, timeouts, slowdowns etc; we can encode logic that accounts for a realistic model of this. It’s typical to test the consistency protocols of distributed systems via randomized failure testing.

project loom

For shared datastructures that see accesses from multiple threads, one could write unit tests which check that properties are maintained using the framework. Start by building a simulation of core Java primitives (concurrency/threads/locks/caches, filesystem access, RPC). Implement the ability to insert delays, errors in the results as necessary. One could implement a simulation of core I/O primitives like Socket, or a much higher level primitive like a gRPC unary RPC4. FoundationDB’s usage of this model required them to build their own programming language, Flow, which is transpiled to C++.


It is the goal of this project to add a public delimited continuation construct to the Java platform. Virtual threads help in achieving the same high scalability and throughput as the asynchronous APIs with the same hardware configuration, without adding the syntax complexity. To explore this idea the Loon team started out flying early prototypes through California’s central valley to see if the idea had promise. In 2013 the team found some friendly New Zealanders who were the first in the world to connect to the internet via a stratospheric balloon. The bulk of the Raft implementation can be found in RaftResource, and the bulk of the simulation in DefaultSimulation.

It’s great for the people who use Java, but there are tons of reasons why other people use other languages. I don’t know that I’d say Scala gets immutability right in that it still provides you equal access to the mutable collections , but I cede the point it’s way better than either Go or Java here. I readily admit Golang gets this wrong, just, -slightly- better than Java. I’m coming from an Erlang background, and that’s the main influence I’m looking at concurrency from; the JVM as a whole gives me a sad when it comes to helping me write correctly behaving code.

Let’s say that we have a two-lane road , and 10 cars want to use the road at the same time. Naturally, this is not possible, but think about how this situation is currently handled. Traffic lights allow a controlled number of cars onto the road and make the traffic use the road in an orderly fashion.

Thread Locals

A virtual thread is very lightweight, it’s cheap, and it’s a user thread. By lightweight, I mean you can really allocate millions of them without using too much memory. A carrier thread is the real one, it’s the kernel one that’s actually running your virtual threads. Of course, the bottom line is that you can run a lot of virtual threads sharing the same carrier thread. In some sense, it’s like an implementation of an actor system where we have millions of actors using a small pool of threads. All of this can be achieved using a so-called continuation.

It turns out that user threads are actually kernel threads these days. To prove that that’s the case, just check, for example, jstack utility that shows you the stack trace of Java Loom your JVM. Besides the actual stack, it actually shows quite a few interesting properties of your threads. For example, it shows you the thread ID and so-called native ID.

After looking through the code, I determined that I was not parallelizing calls to the two followers on one codepath. After making the improvement, after the same number of requests only 6m14s of simulated time (and 240ms of wall clock time!) had passed. This makes it very easy to understand performance characteristics with regards to changes made. As you build your distributed system, write your tests using the simulation framework.

This allows you to use the experimental and Preview API in Java which is disabled by default. Try hands-on activities in the OpenShift Sandbox Access Red Hat’s products and technologies without setup or configuration, and start developing quicker than ever before with our new, no-cost sandbox environments. Learning paths Customize your learning to align with your needs and make the most of your time by exploring our massive collection of paths and lessons. I stopped the experiment after about 35 minutes of continuous operation.

Establishing the memory visibility guarantees necessary for migrating continuations from one kernel thread to another is the responsibility of the fiber implementation. In the following example, we are submitting 10,000 tasks and waiting for all of them to complete. The code will create 10,000 virtual threads to complete these 10,000 tasks. So we can say that virtual threads also improve the code quality by adapting the traditional syntax while having the benefits of reactive programming. The first eight threads took a wallclock time of about two seconds to complete, the next eight took about four seconds, etc.

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