50 lines
18 KiB
HTML
50 lines
18 KiB
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<!--replace-end-7--><!--replace-end-4--><!--replace-end-1--></head><body><div class="ui fluid container universe"><!--replace-start-2--><!--replace-start-3--><!--replace-start-6--><div class="ui text container" id="zettel-container" style="position: relative"><div class="zettel-view"><article class="ui raised attached segment zettel-content"><div class="pandoc"><h1 id="title-h1">The Event Loop</h1><p>Node.js provides a single-threaded asynchronous architecture which is achieved via means of the Event Loop.</p><h2 id="multi-threaded-synchronous-architectures">Multi-threaded synchronous architectures</h2><p>In the context of backend, a thread as an instance of a request-response transaction.</p><p>For example a request is made from the client for a resource contained in a database. The back-end language is an intermediary between the client machine and the server. It receives the request and returns the resource as a response.</p><p>Many backend frameworks are synchronous but multithreaded. This means that a thread can only process one request-response cycle at a time. The thread cannot initiate a new cycle until it has finished with its current cycle.</p><p>If there was only one thread, this would be inefficient and unworkable. Therefore the framework will be multi-threaded: multiple request-response cycles can be executed at once by different threads.</p><p><img alt="sync-thread.svg" src="/static/sync-thread.svg" /></p><p>To accomodate the ability to increase the scale of synchronous applications you need to be able to spawn more threads commensurate to increased demand. This increases the resource consumption of the framework (more cores, more memory etc). Moreover it is possible to reach a point where all threads are active and no more can be spawned. In this case there will simply be delays in the return of data.</p><h2 id="node-as-a-single-threaded-asynchronous-architecture">Node as a single-threaded asynchronous architecture</h2><p>In contrast, Node only has a single thread but it works asynchronously, not synchronously. Thus it has a <strong>single-threaded asynchronous architecture</strong>. This means whilst there is only a single thread it can juggle responses by dispatching them asynchronously. When a request is made it sends it off and continues with its execution and handling new requests. Once these resolve, the data is returned to the main thread.</p><p><img alt="async.svg" src="/static/async.svg" /></p><h2 id="the-event-loop-1">The Event Loop</h2><p>Node implements its single-threaded asynchronous architecture through the Event Loop.</p><p>This is the mechanism by which Node keeps track of incoming requests and their fulfillment status: whether the data has been returned successfully or if there has been error.</p><p>Node is continually monitoring the Event Loop in the background.</p><p>A running Node application is a single running process. Like everything that happens within the OS, a process is managed by the <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: The kernel"><a href="The_kernel.html">kernel</a></span></span> that dispatches operations to the CPU in a clock cycle. A thread is a sequence of code that resides within the process and utilises its memory pool (the amount of memory assigned by the kernel to the Node process). The Event Loop runs on CPU ticks: a tick is a single run of the Event Loop.</p><h3 id="phases-of-the-event-loop">Phases of the Event Loop</h3><p>The Event Loop comprises six phases. The Event Loop starts at the moment Node begins to execute your <code>index.js</code> file or any other application entry point. These six phases create one cycle, or loop, equal to one <strong>tick</strong>. A Node.js process exits when there is no more pending work in the Event Loop, or when <code>process.exit()</code> is called manually. A program only runs for as long as there are tasks queued in the Event Loop, or present on the <span class="zettel-link-container cf"><span class="zettel-link" title="Zettel: The call-stack"><a href="Call_stack.html">call stack</a>
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