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https://github.com/propensive/parasite

Structured asynchronous task management in Scala
https://github.com/propensive/parasite

async asynchronous concurrency scala task threads virtual-threads

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Structured asynchronous task management in Scala

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# 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



## Getting Started



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

```scala

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:

```scala

import contingency.strategies.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 631 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](https://github.com/propensive/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`](https://github.com/propensive/wrath/releases/latest), 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](https://github.com/propensive/parasite/labels/beginner).



We suggest that all contributors read the [Contributing

Guide](/contributing.md) 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](https://propensive.com/).



## 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](/license.md).