Parallelism is a crucial concept in the HTML Standard, specifically in the realm of web development. It plays a significant role in enabling modern web applications to perform efficiently and respond to user interactions promptly. The WhatWG (Web Hypertext Application Technology Working Group) has defined a common infrastructure and terminology that deals with parallelism in web standards. IHere we will delve into the key aspects of parallelism in the HTML Standard, exploring how it is defined, its practical applications, and how it aids in avoiding race conditions.
To begin with, parallelism, in the context of the HTML Standard, refers to the execution of multiple tasks simultaneously. This means that certain operations or steps are executed concurrently, in parallel, alongside other logical processes. Parallelism is a fundamental feature of modern web browsers and allows them to carry out various tasks simultaneously, enhancing user experience and improving overall performance.
However, it's important to note that the HTML Standard does not specify the exact mechanisms by which parallelism is achieved. Whether it's through time-sharing cooperative multitasking, fibers, threads, processes, or other methods, the standard focuses on the outcome – the ability to execute tasks in parallel.
Parallelism vs. Immediate Execution: In the HTML Standard, parallelism is distinguished from immediate execution. When an operation is marked to run in parallel, it means that it will be executed concurrently with other tasks in the event loop or standard logic. In contrast, an operation marked for immediate execution must interrupt the currently running task, execute itself, and then resume the previously running task.
Guidance for Leveraging Parallelism: For developers and specification writers, the HTML Standard provides guidance on how to write specifications that leverage parallelism effectively. This guidance is particularly useful when dealing with the event loop from other specifications. It helps ensure that parallel tasks are correctly coordinated and executed.
Avoiding Race Conditions with Parallel Queues: One of the challenges of parallelism is avoiding race conditions, particularly when multiple algorithms operate on the same data simultaneously. To address this issue, the HTML Standard introduces the concept of a parallel queue. A parallel queue is a queue of algorithm steps that must be executed in series, ensuring the correct order of operations.
Key Characteristics of a Parallel Queue:
- Algorithm Queue: A parallel queue has an algorithm queue, which is initially empty.
- Enqueueing Steps: To add steps to a parallel queue, developers should enqueue the algorithm steps to the queue's algorithm queue.
Starting a New Parallel Queue
Starting a new parallel queue involves the following steps:
- Create a new parallel queue, which is represented by the variable parallelQueue.
- Execute the following steps in parallel, which allows multiple tasks to be carried out concurrently.
- Continuously dequeue and execute steps from the algorithm queue within parallelQueue.
- Ensure that no exceptions are thrown during the execution of these steps in parallel.
Note: Implementations are not expected to run this as a continuously running loop. Standards aim to be comprehensible and may not optimize for battery life or performance.
Practical Application: Avoiding Race Conditions
To illustrate the practical use of parallel queues in avoiding race conditions, consider the following scenario:
Suppose there is a standard-defined nameList (a list) and a method to add a name to nameList, with the condition that it must reject if the name is already in nameList.
The following solution, without parallel queues, could lead to race conditions:
- Create a new promise p.
- Execute the following steps in parallel:
- If nameList contains the name, reject promise p with a TypeError and abort these steps.
- Perform potentially lengthy work.
- Append the name to nameList.
- Resolve promise p with undefined.
- Return promise p.
In this scenario, two simultaneous invocations could run, leading to a race condition. The name might not be in nameList during step 2.1, but it could be added before step 2.3 runs, resulting in the name being added to nameList twice.
Parallel queues provide a solution to this problem. By creating a new parallel queue (nameListQueue), the steps can be enqueued in the correct order, ensuring that the race condition is avoided. The steps are queued and executed one after the other, preventing conflicts in adding names to nameList.
Parallelism is a fundamental concept in the HTML Standard, allowing web browsers and web applications to execute multiple tasks simultaneously. The WhatWG's common infrastructure terminology provides guidance and tools, such as parallel queues, to effectively leverage parallelism and avoid race conditions. This ensures the efficient execution of algorithms and improves the overall performance and user experience in the modern web environment.
