317 lines
7.9 KiB
HTML
317 lines
7.9 KiB
HTML
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<html>
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<head>
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<meta charset="utf-8" />
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<meta name="generator" content="pandoc" />
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<title>Why use promises?</title>
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<script>// Pandoc 2.9 adds attributes on both header and div. We remove the former (to
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dl dd {
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dl dt {
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ul {
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margin-top: 0;
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ul li {
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list-style: circle outside;
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ul ul {
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color: #333;
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pre {
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border-radius: 3px;
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code {
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div.figure {
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text-align: center;
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}
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img {
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background-color: #FFFFFF;
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padding: 2px;
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border-radius: 3px;
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border: 1px solid #CCCCCC;
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margin: 0 5px;
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}
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h1 {
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margin-top: 0;
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font-size: 35px;
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line-height: 40px;
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}
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h2 {
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border-bottom: 4px solid #f7f7f7;
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padding-top: 10px;
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padding-bottom: 2px;
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font-size: 145%;
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}
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h3 {
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}
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h4 {
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font-size: 105%;
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}
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h5, h6 {
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border-bottom: 1px solid #ccc;
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font-size: 105%;
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}
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a {
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}
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a:hover {
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code > span.er { color: #a61717; background-color: #e3d2d2; }
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</style>
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</head>
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<body>
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<h1 class="title toc-ignore">Why use promises?</h1>
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<p>The <code>promises</code> library is designed to make it possible for
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R users to integrate <em>asynchronous programming</em> techniques into
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their code. It is intended to improve the scalability of certain classes
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of Shiny applications, and can also be used completely separately from
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Shiny.</p>
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<p>Asynchronous programming means starting an operation without waiting
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around for its results. Contrast this with standard (synchronous) R
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programming, where the caller of a function must wait for the function
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to either return a result or throw an error.</p>
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<p>Asynchronous programming is a particularly important capability for
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languages that are both 1) single threaded, and 2) need to serve
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multiple users concurrently. R is single threaded, but until recent
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years has mostly been restricted to desktop use or batch execution. The
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rise of Shiny and other web frameworks for R has meant much greater
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demand for serving multiple users concurrently, and thus a greater need
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for asynchronous programming.</p>
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<div id="what-are-promises-good-for" class="section level2">
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<h2>What are promises good for?</h2>
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<p>Promises work well when you have a Shiny application with a
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particular operation, especially a calculation, that takes a long time
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(measured in seconds or even minutes). Without promises, such operations
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not only block the current user’s session from proceeding, but also
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block <em>all</em> other requests being made to the R process, no matter
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how trivial. Even loading a small CSS file, which should be nearly
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instantaneous, can be delayed by many seconds if another user’s Shiny
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session is busy crunching through hundreds of gigabytes of data or
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querying a particularly slow backend database.</p>
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<p>With promises, you can convert long-running operations to be
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asynchronous, which frees up the R process to do other work. For Shiny
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applications, this has the potential to greatly increase scalability for
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each R process (depending on how slow the operations are in the first
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place, and what resources they make use of).</p>
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<p><strong>DO use promises if you have a Shiny app with long-running
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operations, and want to serve multiple users
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simultaneously.</strong></p>
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</div>
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<div id="what-arent-promises-good-for" class="section level2">
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<h2>What <em>aren’t</em> promises good for?</h2>
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<p>While promises can make a huge difference in the scalability of a
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Shiny app, they make relatively little difference in the latency of a
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single session. That is to say, if a Shiny application is slow when only
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a single user is hitting it, converting it to use promises is unlikely
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to make it perform any faster (and in fact may slightly slow it down).
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Promises will just help prevent <em>other</em> sessions from being
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slowed down by one session’s computations.</p>
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<p>(The exception to this is if the Shiny app has several distinct long
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computations that don’t depend very much on each other—then you could
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use asynchronous programming to exploit a little parallelism. But I
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think this will be a less common use of async programming, as R already
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has other good tools designed specifically for data parallelism.)</p>
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<p><strong>DON’T use promises to improve the performance of Shiny apps
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for a single user.</strong></p>
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<div style="font-size: 20px; margin-top: 40px; text-align: right;">
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<p>Next: <a href="promises_02_intro.html">An informal intro to async
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programming</a></p>
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</div>
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</div>
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<!-- code folding -->
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<!-- dynamically load mathjax for compatibility with self-contained -->
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<script>
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var script = document.createElement("script");
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script.type = "text/javascript";
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script.src = "https://mathjax.rstudio.com/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML";
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document.getElementsByTagName("head")[0].appendChild(script);
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})();
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</script>
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</body>
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</html>
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