Is it possible for two separate tasks operating concurrently to access and modify the same array simultaneously?

Question

Is it possible for two separate tasks operating concurrently to access and modify the same array simultaneously?

Imagine you have an array being altered by one asynchronous process and then another asynchronous process reading from the same array. The modifications and reads happen synchronously. So, the big question here is: can the read process somehow access the array while it's being modified by the write process?

Based on my experience, it seems unlikely that the read process would be able to access the array while synchronous operations are happening. When a thread is busy with synchronous tasks, all resources it operates on (including the array) are blocked. Is this behavior a direct result of JavaScript being single-threaded, or is it explicitly specified in the language specifications?

javascript typescript asynchronous

Answer 1

Answer №1

Is it possible for the read process to access the array while it is being modified by the write process?

In JavaScript, since it is single-threaded, there is no way for the read process to access the data concurrently as the write process modifies it. The only option to achieve this in JS is by using shared resources between different scripts. For instance, on the web, you can utilize WebWorkers with SharedArrayBuffers or in Nodejs, certain databases allow for concurrent mutations.

Does this restriction stem from the fact that JavaScript is single-threaded, or is it explicitly stated in the specifications?

The specifications indeed define threads known as Agents:

8.3 Agents

An agent encompasses a collection of ECMAScript execution contexts, an execution context stack, a running execution context, a set of named job queues, an Agent Record, and an executing thread. Apart from the executing thread, all components of an agent are exclusively assigned to that agent.

This implies that all code runs on the same thread (job queue) and variables are only accessible within one thread (execution context). To share memory among different agents (threads), a SharedArrayBuffer must be used, requiring distinct Agents such as Webworkers.

Answer 2

Is it possible for the read process to access the array while it is being modified by the write process?

In JavaScript, since it is single-threaded, there is no way for the read process to access the data concurrently as the write process modifies it. The only option to achieve this in JS is by using shared resources between different scripts. For instance, on the web, you can utilize WebWorkers with SharedArrayBuffers or in Nodejs, certain databases allow for concurrent mutations.

Does this restriction stem from the fact that JavaScript is single-threaded, or is it explicitly stated in the specifications?

The specifications indeed define threads known as Agents:

8.3 Agents

An agent encompasses a collection of ECMAScript execution contexts, an execution context stack, a running execution context, a set of named job queues, an Agent Record, and an executing thread. Apart from the executing thread, all components of an agent are exclusively assigned to that agent.

This implies that all code runs on the same thread (job queue) and variables are only accessible within one thread (execution context). To share memory among different agents (threads), a SharedArrayBuffer must be used, requiring distinct Agents such as Webworkers.

Answer 3

Answer №2

To tackle this situation, I suggest creating 3 separate arrays.

 let kingArray = [];
 let arrayForProcess1 = [];
 let arrayForProcess2 = [];

 someProcessOneExecution().then(response => {
    if (kingArray.length === 0) {
       arrayForProcess1 = response;
    } else {
       doSomethingWithResponse(response);
    }
 });

 someProcessTwoExecution().then(response => {
    if (kingArray.length === 0) {
       arrayForProcess2 = response;
    } else {
       doSomethingWithResponse(response);
    }
 });

 function doSomethingWithResponse() {
    // perform an action here..
 }

While not a direct answer to your query, it's worth noting that the javascript event loop or call stack will handle these promises sequentially due to JS being single-threaded. As they resolve, either process1 or process2 will be executed. Monitoring them manually (as shown above) may not be the most efficient method.

Alternate Approach

In such cases, I would recommend chaining the promises together and awaiting their resolution individually before updating the respective arrays.

From my perspective (a personal opinion), attempting simultaneous updates on the same array may result in execution issues; reevaluating the strategy might be necessary in such scenarios.

Answer 4

To tackle this situation, I suggest creating 3 separate arrays.

 let kingArray = [];
 let arrayForProcess1 = [];
 let arrayForProcess2 = [];

 someProcessOneExecution().then(response => {
    if (kingArray.length === 0) {
       arrayForProcess1 = response;
    } else {
       doSomethingWithResponse(response);
    }
 });

 someProcessTwoExecution().then(response => {
    if (kingArray.length === 0) {
       arrayForProcess2 = response;
    } else {
       doSomethingWithResponse(response);
    }
 });

 function doSomethingWithResponse() {
    // perform an action here..
 }

While not a direct answer to your query, it's worth noting that the javascript event loop or call stack will handle these promises sequentially due to JS being single-threaded. As they resolve, either process1 or process2 will be executed. Monitoring them manually (as shown above) may not be the most efficient method.

Alternate Approach

In such cases, I would recommend chaining the promises together and awaiting their resolution individually before updating the respective arrays.

From my perspective (a personal opinion), attempting simultaneous updates on the same array may result in execution issues; reevaluating the strategy might be necessary in such scenarios.

Is it possible for two separate tasks operating concurrently to access and modify the same array simultaneously?

Answer №1

Answer №2

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