Miou, a simple scheduler for OCaml 5
Miou is a small library that facilitates asynchronous and parallel programming in OCaml. It is a project of the Robur cooperative, aimed at developing system and network applications. This library only requires OCaml 5 and can be obtained via opam.
$ opam install miou
Miou offers three key features:
- a multi-domain runtime for executing asynchronous code.
- flexibility in defining interactions between the system and Miou.
- essential components for asynchronous and/or parallel programming.
Miou's role in your project
When developing an application that heavily interacts with the system to offer services like an HTTP server, embracing asynchronous programming is recommended. However, OCaml lacks built-in support for asynchronous programming, necessitating the use of a library such as Miou, which provides the required runtime.
Such a runtime emerges as a pivotal element of your application, orchestrating and executing tasks to ensure continued service availability despite varying workloads. Miou caters to diverse systems, ranging from unikernels to large servers with numerous cores or even small embedded devices.
While pivotal, this runtime represents the final frontier between your application's intended functionality and its current execution by interacting with the system. Hence, we advise users to defer selecting the scheduler, such as Miou, until the application's design phase is complete.
A multi-domain runtime
Since OCaml 5, it has been possible to execute tasks in parallel. Miou provides this capability by solving the inter-domain synchronization problems involved. Miou allocates multiple domains that are available to the user to manage in parallel, for example, clients.
We recommend referring to the OCaml manual to learn more about domains. Indeed, Miou manages domains itself because they can be a costly resource. As such, Miou handles their allocation, transfers your tasks to them, manages synchronization when you want to obtain the results of your tasks, and ultimately deallocates these domains properly.
Agnostic to the system
Miou only requires OCaml to operate. This choice stems from our ambition to integrate Miou as a scheduler for our unikernels, which are highly specialized systems. However, more generally and based on experience, we understand that interactions with the system are inherently complex and cannot be standardized through a common interface.
Therefore, we believe that the best person to determine how to interact with the system is you! Miou thus provides this capability so that you can leverage the full potential of your system.
However, Miou offers a small extension allowing interaction with your system
through the miou.unix library. While rudimentary, it is adequate for most
system and network applications.
Essential components for asynchronous/parallel programming
Finally, Miou provides essential elements for parallel and/or asynchronous programming. These components help address synchronization challenges inherent in parallel and/or asynchronous programming.
It is worth noting that these elements may seem somewhat rudimentary. However, we would like to caution the user that the topic of synchronization is a vast realm of solutions and research, and we do not claim to have omniscience over it. Therefore, we prefer to leave this space open for the user.
When not to use Miou
Although Miou is useful for many projects that need to do a lot of things simultaneously, there are also some use-cases where Miou is not a good fit: speeding up CPU-bound computations by running them in parallel on several domains. Miou is designed for IO-bound applications where each individual task spends most of its time waiting for IO. If the only thing your application does is run computations in parallel, you should use moonpool. That said, it is still possible to "mix & match" if you need to do both.