Parasite

Safe, structured asynchronous tasks

Parasite provides asynchronous tasks, built upon Java threads or virtual threads. Asynchronous tasks form a supervisor hierarchy, where each task is "owned" by a supervising parent task, and cancelation of tasks cascades through the hierarchy. Capture checking is used to avoid thread leaks.

Features

• simple interface for creating and running tasks
• ideally suited for use on Java 20+
• asynchronous tasks form an intuitive hierarchy
• capture checking avoids thread leakage

Availability Plan

Parasite has not yet been published. The medium-term plan is to build Parasite with Fury and to publish it as a source build on Vent. This will enable ordinary users to write and build software which depends on Parasite.

Subsequently, Parasite will also be made available as a binary in the Maven Central repository. This will enable users of other build tools to use it.

For the overeager, curious and impatient, see building.

Getting Started

All Parasite terms and types are defined in the parasite package:

import parasite.*

A Parasite task, an instance of Async, is similar to a Scala or Java Future: it encapsulates a block of code which it starts executing immediately, and once evaluated, holds the result. There are, however, a few differences.

We can create one inside a supervise block with:

import contingency.errorHandlers.throwUnsafely

def slowTask(): Unit = ???

def run() = supervise:
  val async = Async:
    slowTask()

This creates a new Async[ResultType] where ResultType is the return type of someSlowTask().

This will create and start the new task in a thread forked from the current thread. Execution of someSlowTask() will proceed in a forked thread until it completes.

We can call async.await() in the current thread to wait until the forked thread finishes, and return the value which results from its execution.

Asynchronous tasks form a hierarchy, and a task spawned within the body of another task will use the latter's context to determine its owner, effectively making the former a child task. This has few implications for how the task runs, unless the parent task is canceled, in which case all descendants will also be canceled.

Async methods

An Async instance has several useful methods:

• await(), which blocks the current thread until the Async produces a value
• await(timeout), which takes a timeout, after which a TimeoutError will be thrown if no value has been produced
• map and flatMap, providing standard monadic operations on the Async
• cancel(), which will stop the task running

Platform or Virtual threading

From JDK 20 and above, threads may be either platform (corresponding to threads managed by the operating system) or virtual (managed by the JVM). Prior to JDK 20, all threads are platform threads. Parasite needs to know which type of thread to use, and this requires one of two imports:

• parasite.threading.virtual
• parasite.threading.platform

Note that choosing threading.virtual will result an a runtime error on JDKs older than JDK 20.

Cancelation

A task may be cancelled at any time, though cancellation is co-operative: it requires the body of the task to be written to expect possible cancellation. Within a task's body, the method acquiesce() may be called multiple times. For as long as the task is running normally, acquiesce() will do nothing. But if a task is cancelled, the task will stop immediately, without a value being produced. Any await() calls on the task will throw a CancelError.

However, this happens only when acquiesce() is called, so if no such calls are run as the task is executing, that task cannot be cancelled, and it must execute to completion.

Status

Parasite is classified as fledgling. For reference, Soundness projects are categorized into one of the following five stability levels:

• embryonic: for experimental or demonstrative purposes only, without any guarantees of longevity
• fledgling: of proven utility, seeking contributions, but liable to significant redesigns
• maturescent: major design decisions broady settled, seeking probatory adoption and refinement
• dependable: production-ready, subject to controlled ongoing maintenance and enhancement; tagged as version 1.0.0 or later
• adamantine: proven, reliable and production-ready, with no further breaking changes ever anticipated

Projects at any stability level, even embryonic projects, can still be used, as long as caution is taken to avoid a mismatch between the project's stability level and the required stability and maintainability of your own project.

Parasite is designed to be small. Its entire source code currently consists of 587 lines of code.

Building

Parasite will ultimately be built by Fury, when it is published. In the meantime, two possibilities are offered, however they are acknowledged to be fragile, inadequately tested, and unsuitable for anything more than experimentation. They are provided only for the necessity of providing some answer to the question, "how can I try Parasite?".

1. Copy the sources into your own project

   Read the fury file in the repository root to understand Parasite's build structure, dependencies and source location; the file format should be short and quite intuitive. Copy the sources into a source directory in your own project, then repeat (recursively) for each of the dependencies.

   The sources are compiled against the latest nightly release of Scala 3. There should be no problem to compile the project together with all of its dependencies in a single compilation.

2. Build with Wrath

   Wrath is a bootstrapping script for building Parasite and other projects in the absence of a fully-featured build tool. It is designed to read the fury file in the project directory, and produce a collection of JAR files which can be added to a classpath, by compiling the project and all of its dependencies, including the Scala compiler itself.

   Download the latest version of wrath, make it executable, and add it to your path, for example by copying it to /usr/local/bin/.

   Clone this repository inside an empty directory, so that the build can safely make clones of repositories it depends on as peers of parasite. Run wrath -F in the repository root. This will download and compile the latest version of Scala, as well as all of Parasite's dependencies.

   If the build was successful, the compiled JAR files can be found in the .wrath/dist directory.

Contributing

Contributors to Parasite are welcome and encouraged. New contributors may like to look for issues marked beginner.

We suggest that all contributors read the Contributing Guide to make the process of contributing to Parasite easier.

Please do not contact project maintainers privately with questions unless there is a good reason to keep them private. While it can be tempting to repsond to such questions, private answers cannot be shared with a wider audience, and it can result in duplication of effort.

Author

Parasite was designed and developed by Jon Pretty, and commercial support and training on all aspects of Scala 3 is available from Propensive OÜ.

Name

A tick indicates the completion of a task, while also being the name of a common human parasite, hence the name, and the allusion to the parasitic nature of threads.

In general, Soundness project names are always chosen with some rationale, however it is usually frivolous. Each name is chosen for more for its uniqueness and intrigue than its concision or catchiness, and there is no bias towards names with positive or "nice" meanings—since many of the libraries perform some quite unpleasant tasks.

Names should be English words, though many are obscure or archaic, and it should be noted how willingly English adopts foreign words. Names are generally of Greek or Latin origin, and have often arrived in English via a romance language.

Logo

The logo shows a tick symbol, indicative of a task (which has been completed).

License

Parasite is copyright © 2024 Jon Pretty & Propensive OÜ, and is made available under the Apache 2.0 License.