concurrency How fibers can exist in Java

If your program gets stuck instead, try to figure out with a debugger where there’s a restarting method and use that stack as a reference. Sometimes you want to use library methods that will end up calling your suspendable code, so they too must be marked suspendable and instrumented. So, suppose https://www.globalcloudteam.com/ method f is declared in interface I, and we’d like to make its implementation in class C suspendable. The compiler will not let us declare that we throw SuspendExecution because that will conflict with f’s declaration in I. As we said above, fibers are great as a replacement for callbacks.

We see Virtual Threads complementing reactive programming models in removing barriers of blocking I/O while processing infinite streams using Virtual Threads purely remains a challenge. ReactiveX is the right approach for concurrent scenarios in which declarative concurrency (such as scatter-gather) matters. The underlying Reactive Streams specification defines a protocol for demand, back pressure, and cancellation of data pipelines without limiting itself to non-blocking API or specific Thread usage. While implementing async/await is easier than full-blown continuations and fibers, that solution falls far too short of addressing the problem. In other words, it does not solve what’s known as the “colored function” problem.

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However, Quasar can gracefully handle these calls if they happen occasionally, so they can be allowed by passing the b argument to the Quasar Java agent, or by setting the allowBlocking property on the instrumentation Ant task. As one of the reasons for implementing continuations as an independent construct of fibers (whether or not they are exposed as a public API) is a clear separation of concerns. Continuations, therefore, are not thread-safe and none of their operations creates cross-thread happens-before relations.

Internally, a fiber is a continuation which is then scheduled in a scheduler. A continuation captures the instantaneous state of a computation, and allows it to be suspended and then resumed at a later time from the point where it was suspended. Quasar creates continuations by instrumenting (at the bytecode level) suspendable methods. For scheduling, Quasar uses ForkJoinPool, which is a very efficient, work-stealing, multi-threaded scheduler. These methods too block the kernel threads too and by default they are not allowed in fibers, causing Quasar instrumentation to fail.

Will your application benefit from Virtual Threads?

In particular, Loom offers a lighter alternative to threads along with new language constructs for managing them. When these features are production ready, it should not affect regular Java developers much, as these developers may be using libraries for concurrency use cases. But it can be a big deal in those rare scenarios where you are doing a lot of multi-threading without using libraries. Virtual threads could be a no-brainer replacement for all use cases where you use thread pools today. This will increase performance and scalability in most cases based on the benchmarks out there. Structured concurrency can help simplify the multi-threading or parallel processing use cases and make them less fragile and more maintainable.

• The kernel has no knowledge of fi­bers,
  and when a fiber enters the kernel, the thread it is executing on may
  block and the kernel will schedule an arbitrary thread on the
  processor, making it unavailable to run other fibers.
• If your actor extends BasicActor, there’s another form of the receive method that allows for selective receive.
• First, if we want to run h in a fiber, then it must be suspendable because it calls f which is suspendable.
• If it is about an actor that has died but has been unlinked or unwatched already, it just ignores the message.
• Running actors can migrate from one cluster node to another, while preserving their state.

If fibers are represented by Threads, then some changes would need to be made to such striped data structures. In any event, it is expected that the addition of fibers would necessitate adding an explicit API for accessing processor identity, java fiber whether precisely or approximately. It helped me think of virtual threads as tasks, that will eventually run on a real thread⟨™) (called carrier thread) AND that need the underlying native calls to do the heavy non-blocking lifting.

What is a Continuation?

Continuations are a very low-level primitive that will only be used by library authors to build higher-level constructs (just as java.util.Stream implementations leverage Spliterator). It is expected that classes making use of contiuations will have a private instance of the continuation class, or even, more likely, of a subclass of it, and that the continuation instance will not be directly exposed to consumers of the construct. In the literature, nested continuations that allow such behavior are sometimes call “delimited continuations with multiple named prompts”, but we’ll call them scoped continuations. The motivation for adding continuations to the Java platform is for the implementation of fibers, but continuations have some other interesting uses, and so it is a secondary goal of this project to provide continuations as a public API.

Not all classes will actually be instrumented – only those with suspendable methods (see below) – so simply give the task all of the class files in your program. In terms of basic capabilities, fibers must run an arbitrary piece of Java code, concurrently with other threads (lightweight or heavyweight), and allow the user to await their termination, namely, join them. Obviously, there must be mechanisms for suspending and resuming fibers, similar to LockSupport’s park/unpark. We would also want to obtain a fiber’s stack trace for monitoring/debugging as well as its state (suspended/running) etc..

What is the difference between a thread and a fiber?

The actor model does not only make concurrency easy; it also helps build fault-tolerant systems by compartmentalizing failure. Each actor is it’s own execution context – if it encounters an exception, only the actor is directly affected (like a thread, only actors are lightweight). Unlike regular functions/objects, where an exception has to be caught and handled immediately on the callstack, with actors we can completely separate code execution from error handling. An example for that are the synchronization primitives in the co.paralleluniverse.strands.concurrent package, which implement interfaces declared in java.util.concurrent, and we want to maintain compatibility. Also, no harm will come if we use these classes in regular threads.

What is even better – it automatically establishes optimal order of execution of all parts of code. This is the primary reason why I see no big value in fibers/coroutines. The only reason of their existence is ability to continue writing classic synchronous code.

Kilim: Continuations, Fibers, Actors and message passing for the JVM

Many applications make use of data stores, message brokers, and remote services. I/O-intensive applications are the primary ones that benefit from Virtual Threads if they were built to use blocking I/O facilities such as InputStream and synchronous HTTP, database, and message broker clients. Running such workloads on Virtual Threads helps reduce the memory footprint compared to Platform Threads and in certain situations, Virtual Threads can increase concurrency. An alternative solution to that of fibers to concurrency’s simplicity vs. performance issue is known as async/await, and has been adopted by C# and Node.js, and will likely be adopted by standard JavaScript.

The last behavior actor, the supervisor deserves a chapter of its own, as it’s at the core of the actor model’s error handling philosophy. To create an event source actor, simply construct an instance of the EventSourceActor class. Event handlers can be registered or unregistered with the actor, and events sent to the actor, through the behavior’s interface, the EventSource class.

Building Quasar

In particular, it is quite different from the existing mental constructs that Java developers have traditionally used. Also, JavaRX can’t match the theoretical performance achievable by managing virtual threads at the virtual machine layer. The solution is to introduce some kind of virtual threading, where the Java thread is abstracted from the underlying OS thread, and the JVM can more effectively manage the relationship between the two. That is what project Loom sets out to do, by introducing a new virtual thread class called a fiber.