Contemporary control concept
Concurrency control refers to the process of managing and coordinating concurrent operations during programming. In modern applications, there are often multiple tasks or operations that run simultaneously, such as concurrent network requests, file read and write, and database queries. If not controlled, these concurrent operations may lead to problems such as resource competition and data inconsistency. Therefore, concurrent control is crucial, which ensures program accuracy, reliability and performance optimization.
Challenges of concurrent operations
Concurrent operations bring some challenges, including resource competition, race conditions and deadlocks. Resource competition refers to multiple threads or processes reading and writing shared resources at the same time, which may lead to data inconsistency. Race conditions refer to the nondeterminism of the results that may result in execution interleaving between multiple processes or threads. Deadlock refers to multiple processes or threads that cannot continue to execute because they are waiting for resources they hold. These problems can cause application crashes, performance degradation and even data corruption. Appropriate concurrency control mechanisms can solve these problems and ensure the correctness and reliability of the program.
The role and advantages of Promise
Promise is a technology in JavaScript that handles asynchronous operations, which provides a more elegant way to handle concurrent tasks and serialized operations. Promise allows developers to express the dependencies between asynchronous operations more clearly through chain calls. It can encapsulate the results of asynchronous operations and provide an error handling mechanism similar to try-catch. The characteristics of Promise make it a powerful tool for implementing concurrent control, which can simplify code logic, improve readability and maintenance.
Implementation of queue scheduler
Implement concurrency control of Promise: The provided Promise array needs to be executed in a limited number of concurrency, and ensure that the results are output in the order in the array. The function needs to have the following functions:
1. Accept a numeric parameter limit to represent the maximum number of promises executed simultaneously.
2. Implement a queue to store the Promise tasks to be executed.
3. Implement an enqueue method to queue the Promise task and schedule according to concurrency restrictions.
4. Implement a _next method to execute the next task and continue to schedule the next task after the task is completed.
5. In the enqueue method, decide whether to perform the next task based on the concurrency limit and the status of the queue.
6. When executing tasks, use Promise methods (such as then, catch, and finally) to track the completion status of the task and perform necessary cleaning operations after the task is completed.
accomplish:
function promiseLimit(limit) {
let running = 0;
const queue = [];
const enqueue = (task) => {
(task);
_next();
};
const _next = () => {
if (running >= limit || === 0) {
return;
}
running++;
const task = ();
task()
.then(() => {
(`Task ${task} completed`);
})
.catch(() => {
(`Task ${task} failed`);
})
.finally(() => {
running--;
_next();
});
};
return { enqueue };
}test:
const task = [
() => new Promise((resolve) => setTimeout(() => { (1); resolve(); }, 100)),
() => new Promise((resolve) => setTimeout(() => { (2); resolve(); }, 50)),
() => new Promise((resolve) => setTimeout(() => { (3); resolve(); }, 200)),
() => new Promise((resolve) => setTimeout(() => { (4); resolve(); }, 150)),
];
const promise = promiseLimit(2);
((t) => (t));explain:
The function contains two core methods:
- The enqueue method is used to queue tasks and schedule execution.
- The _next method is used to execute the next task in the queue.
In the enqueue method, we add the task to the queue and immediately call the _next method for dispatch. If the number of promises currently being executed is less than the limit limit, the next task is taken from the queue to execute; otherwise, wait for the executed promise to complete before continuing to schedule the next one.
When executing a task, the promise returned by each task uses the finally method, ensuring that the execution of the promise count is reduced after the task is completed, and the _next method is scheduled again.
In the usage example, we create a task with 4 delayed outputs and queue them for execution by methods. The passed limit parameter is 2, indicating that up to 2 tasks are executed in parallel. The output results will be output in sequence according to the delay time of the task, and ensure that no more than 2 tasks are executed simultaneously.
Concurrent control solution
In addition to the promiseLimit function, there are other concurrency control solutions to choose from. One solution is to use Semaphore to manage concurrent access resources. Semaphore can limit the number of threads or processes that access a resource simultaneously, thereby avoiding resource competition. Another solution is to use third-party libraries, such as, which provide a wealth of concurrent control functions and tools to make the management of concurrent tasks more flexible and simple. In addition, new JavaScript features such as async/await also provide a more intuitive and concise way to manage asynchronous operations and control concurrency.
Practice and usage scenarios of concurrent control
Batch data request: When you need to get a large amount of data from the backend server, you can use Promise concurrency control to limit the number of requests sent simultaneously to avoid excessive load pressure on the server. For example, on a page that requires 100 user information, Promise concurrency control can be used to limit the number of concurrent requests per time, maintaining the server's response speed and performance stability.
Concurrent API calls: When interacting with an external API, it is sometimes necessary to limit the number of concurrent requests made simultaneously to avoid exceeding the API provider's limit or triggering the traffic control mechanism. For example, when calling an API, the number of concurrent requests is limited to 5, ensuring that the concurrency limit of the API provider is not exceeded while maintaining the application's high responsiveness performance.
Concurrent download: Use Promise concurrency control to increase download speed when multiple files or resources are required. The file download task can be executed concurrently and the number of downloads performed simultaneously is limited to avoid excessive network connections and bandwidth usage. This is especially useful for applications that require batch download of files, such as file synchronization tools or explorer.
Concurrent Operation Limitation: Sometimes in a specific context, concurrent operations need to be limited to a certain number to ensure the consistency and correctness of the data. For example, in a database transaction, it is necessary to limit the number of concurrent operations performed simultaneously to avoid race conditions and inconsistencies in the data. Through Promise concurrency control, it is possible to ensure that only a certain number of operations are performed simultaneously within a given time.
Summarize
When it comes to concurrent control, Promise is a powerful tool that is widely used in modern programming. This article introduces the relevant concepts of Promise concurrency control.
In summary, Promise concurrency control is a powerful and flexible tool that optimizes application performance and user experience. Depending on specific requirements and application scenarios, we can flexibly use Promise concurrency control to manage concurrent tasks and ensure the consistency and correctness of the data.
By mastering the practice of Promise concurrency control, developers can better handle concurrent tasks, provide efficient applications, and improve user satisfaction. Whether it is handling large amounts of data requests, interacting with external APIs, or performing concurrent downloads, Promise concurrency control is a powerful tool that deserves our in-depth understanding and application.
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