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</head>
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<body>
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<h1 class="title toc-ignore">An informal introduction to async
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programming</h1>
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<p>Hello, R and/or Shiny user! Let’s talk about async programming!</p>
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<p><strong>Async programming? Sounds complicated.</strong></p>
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<p>It is, very! You may want to grab some coffee.</p>
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<p><strong>Ugh. Tell me why I even need to know this?</strong></p>
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<p>Async programming is a major new addition to Shiny that can make
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certain classes of apps dramatically more responsive under load.</p>
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<p>Because R is single threaded (i.e. it can only do one thing at a
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time), a given Shiny app process can also only do one thing at a time:
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if it is fitting a linear model for one client, it can’t simultaneously
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serve up a CSV download for another client.</p>
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<p>For many Shiny apps, this isn’t a big problem; if no one processing
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step takes very long, then no client has to wait an undue amount of time
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before they start seeing results. But for apps that perform long-running
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operations — either expensive computations that take a while to
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complete, or waiting on slow network operations like database or web API
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queries — your users’ experience can suffer dramatically as traffic
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ramps up. Operations that normally are lightning quick, like downloading
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a small JavaScript file, can get stuck in traffic behind something
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slow.</p>
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<p><strong>Oh, OK—more responsiveness is always good. But you said
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this’ll only help for certain classes of Shiny apps?</strong></p>
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<p>It’s mostly helpful for apps that have a few specific operations that
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take a long time, rather than lots of little operations that are all a
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bit slow on their own and add up to one big slow mess. We’re looking for
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watermelons, not blueberries.</p>
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<p><strong>Watermelons… sure. So then, how does this all
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work?</strong></p>
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<p>It all starts with <em>async functions</em>. An async function is one
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that performs an operation that takes a long time, yet returns control
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to you immediately. Whereas a normal function like <code>read.csv</code>
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will not return until its work is done and it has the value you
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requested, an asynchronous <code>read.csv.async</code> function would
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kick off the CSV reading operation, but then return immediately, long
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before the real work has actually completed.</p>
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<div class="sourceCode" id="cb1"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(future)</span>
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<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a><span class="fu">plan</span>(multisession)</span>
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<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a>read.csv.async <span class="ot"><-</span> <span class="cf">function</span>(file, <span class="at">header =</span> <span class="cn">TRUE</span>, <span class="at">stringsAsFactors =</span> <span class="cn">FALSE</span>) {</span>
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<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a> <span class="fu">future_promise</span>({</span>
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<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a> <span class="fu">read.csv</span>(file, <span class="at">header =</span> header, <span class="at">stringsAsFactors =</span> stringsAsFactors)</span>
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<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a> })</span>
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<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div>
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<p>(Don’t worry about what this definition means for now. You’ll learn
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more about defining async functions in <a href="promises_04_futures.html">Launching tasks</a> and <a href="promises_05_future_promise.html">Advanced <code>future</code> and
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<code>promises</code> usage</a>.)</p>
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<p><strong>So instead of “read this CSV file” it’s more like “begin
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reading this CSV file”?</strong></p>
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<p>Yes! That’s what async functions do: they start things, and give you
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back a special object called a <em>promise</em>. If it doesn’t return a
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promise, it’s not an async function.</p>
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<p><strong>Oh, I’ve heard of promises in R! From <a href="http://adv-r.had.co.nz/Computing-on-the-language.html">the NSE
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chapter</a> in Hadley’s Advanced R book!</strong></p>
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<p>Ah… this is awkward, but no. I’m using the word “promise”, but I’m
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not referring to <em>that</em> kind of promise. For the purposes of
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async programming, try to forget that you’ve ever heard of that kind of
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promise, OK?</p>
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<p>I know it seems needlessly confusing, but the promises we’re talking
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about here are <del>shamelessly copied from</del> directly inspired by a
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central abstraction in modern JavaScript, and the JS folks named them
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“promises”.</p>
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<p><strong>Fine, whatever. So what are these promises?</strong></p>
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<p>Conceptually, they’re a stand-in for the <em>eventual result</em> of
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the operation. For example, in the case of our
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<code>read.csv.async</code> function, the promise is a stand-in for a
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data frame. At some point, the operation is going to finish, and a data
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frame is going to become available. The promise gives us a way to get at
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that value.</p>
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<p><strong>Let me guess: it’s an object that has
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<code>has_completed()</code> and <code>get_value()</code>
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methods?</strong></p>
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<p>Good guess, but no. Promises are <em>not</em> a way to directly
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inquire about the status of an operation, nor to directly retrieve the
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result value. That is probably the simplest and most obvious way to
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build an async framework, but in practice it’s very difficult to build
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deeply async programs with an API like that.</p>
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<p>Instead, a promise lets you <em>chain together operations</em> that
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should be performed whenever the operation completes. These operations
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might have side effects (like plotting, or writing to disk, or printing
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to the console) or they might transform the result values somehow.</p>
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<p><strong>Chain together operations? Using the <code>%>%</code>
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operator?</strong></p>
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<p>A lot like that! You can’t use the <code>%>%</code> operator
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itself, but we provide a promise-compatible version of it:
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<code>%...>%</code>. So whereas you might do this to a regular data
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frame:</p>
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<div class="sourceCode" id="cb2"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(dplyr)</span>
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<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="fu">read.csv</span>(<span class="st">"https://rstudio.github.io/promises/data.csv"</span>) <span class="sc">%>%</span></span>
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<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a> <span class="fu">filter</span>(state <span class="sc">==</span> <span class="st">"NY"</span>) <span class="sc">%>%</span></span>
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<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a> <span class="fu">View</span>()</span></code></pre></div>
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<p>The async version would look like:</p>
|
||
<div class="sourceCode" id="cb3"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="fu">library</span>(dplyr)</span>
|
||
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a><span class="fu">read.csv.async</span>(<span class="st">"https://rstudio.github.io/promises/data.csv"</span>) <span class="sc">%...>%</span></span>
|
||
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a> <span class="fu">filter</span>(state <span class="sc">==</span> <span class="st">"NY"</span>) <span class="sc">%...>%</span></span>
|
||
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a> <span class="fu">View</span>()</span></code></pre></div>
|
||
<p>The <code>%...>%</code> operator here is the secret sauce. It’s
|
||
called the <em>promise pipe</em>; the <code>...</code> stands for
|
||
promise, and <code>></code> mimics the standard pipe operator.</p>
|
||
<p><strong>What a strange looking operator. Does it work just like a
|
||
regular pipe?</strong></p>
|
||
<p>In many ways <code>%...>%</code> does work like a regular pipe: it
|
||
rewrites each stage’s function call to take the previous stage’s output
|
||
as the first argument. (All the <a href="https://CRAN.R-project.org/package=magrittr/vignettes/magrittr.html">standard
|
||
magrittr tricks</a> apply here: <code>.</code>, <code>{</code>,
|
||
parenthesized lambdas, etc.) But the differences, while subtle, are
|
||
profound.</p>
|
||
<p>The first and most important difference is that
|
||
<code>%...>%</code> <em>must</em> take a promise as input; that is,
|
||
the left-hand side of the operator must be an expression that yields a
|
||
promise. The <code>%...>%</code> will do the work of “extracting” the
|
||
result value from the promise, and passing that (unwrapped) result to
|
||
the function call on the right-hand side.</p>
|
||
<p>This last fact—that <code>%...>%</code> passes an unwrapped, plain
|
||
old, not-a-promise value to the right-hand side—is critically important.
|
||
It means we can use promise objects with non-promise-aware functions,
|
||
with <code>%...>%</code> serving as the bridge between asynchronous
|
||
and synchronous code.</p>
|
||
<p><strong>So the left-hand side of <code>%...>%</code> needs to be
|
||
one of these special promise objects, but the right-hand side can be
|
||
regular R base functions?</strong></p>
|
||
<p>Yes! R base functions, dplyr, ggplot2, or whatever.</p>
|
||
<p>However, that work often can’t be done in the present, since the
|
||
whole point of a promise is that it represents work that hasn’t
|
||
completed yet. So <code>%...>%</code> does the work of extracting and
|
||
piping not at the time that it’s called, but rather, sometime in the
|
||
future.</p>
|
||
<p><strong>You lost me.</strong></p>
|
||
<p>OK, let’s slow down and take this step by step. We’ll generate a
|
||
promise by calling an async function:</p>
|
||
<div class="sourceCode" id="cb4"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a>df_promise <span class="ot"><-</span> <span class="fu">read.csv.async</span>(<span class="st">"https://rstudio.github.io/promises/data.csv"</span>)</span></code></pre></div>
|
||
<p>Even if <code>data.csv</code> is many gigabytes,
|
||
<code>read.csv.async</code> returns immediately with a new promise. We
|
||
store it as <code>df_promise</code>. Eventually, when the CSV reading
|
||
operation successfully completes, the promise will contain a data frame,
|
||
but for now it’s just an empty placeholder.</p>
|
||
<p>One thing we definitely <em>can’t</em> do is treat
|
||
<code>df_promise</code> as if it’s simply a data frame:</p>
|
||
<div class="sourceCode" id="cb5"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a><span class="co"># Doesn't work!</span></span>
|
||
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a>dplyr<span class="sc">::</span><span class="fu">filter</span>(df_promise, state <span class="sc">==</span> <span class="st">"NY"</span>)</span></code></pre></div>
|
||
<p>Try this and you’ll get an error like
|
||
<code>no applicable method for 'filter_' applied to an object of class "promise"</code>.
|
||
And the pipe won’t help you either;
|
||
<code>df_promise %>% filter(state == "NY")</code> will give you the
|
||
same error.</p>
|
||
<p><strong>Right, that makes sense. <code>filter</code> is designed to
|
||
work on data frames, and <code>df_promise</code> isn’t a data
|
||
frame.</strong></p>
|
||
<p>Exactly. Now let’s try something that actually works:</p>
|
||
<div class="sourceCode" id="cb6"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a>df_promise <span class="sc">%...>%</span> <span class="fu">filter</span>(state <span class="sc">==</span> <span class="st">"NY"</span>)</span></code></pre></div>
|
||
<p>At the moment it’s called, this code won’t appear to do much of
|
||
anything, really. But whenever the <code>df_promise</code> operation
|
||
actually completes successfully, then the result of that operation—the
|
||
plain old data frame—will be passed to
|
||
<code>filter(., state = "NY")</code>.</p>
|
||
<p><strong>OK, so that’s good. I see what you mean about
|
||
<code>%...>%</code> letting you use non-promise functions with
|
||
promises. But the whole point of using the <code>filter</code> function
|
||
is to get a data frame back. If <code>filter</code> isn’t even going to
|
||
be called until some random time in the future, how do we get its value
|
||
back?</strong></p>
|
||
<p>I’ll tell you the answer, but it’s not going to be satisfying at
|
||
first.</p>
|
||
<p>When you use a regular <code>%>%</code>, the result you get back
|
||
is the return value from the right-hand side:</p>
|
||
<div class="sourceCode" id="cb7"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a>df_filtered <span class="ot"><-</span> df <span class="sc">%>%</span> <span class="fu">filter</span>(state <span class="sc">==</span> <span class="st">"NY"</span>)</span></code></pre></div>
|
||
<p>When you use <code>%...>%</code>, the result you get back is a
|
||
promise, whose <em>eventual</em> result will be the return value from
|
||
the right-hand side:</p>
|
||
<div class="sourceCode" id="cb8"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a>df_filtered_promise <span class="ot"><-</span> df_promise <span class="sc">%...>%</span> <span class="fu">filter</span>(state <span class="sc">==</span> <span class="st">"NY"</span>)</span></code></pre></div>
|
||
<p><strong>Wait, what? If I have a promise, I can do stuff to it using
|
||
<code>%...>%</code>, but then I just end up with another promise? Why
|
||
not just have <code>%...>%</code> return a regular value instead of a
|
||
promise?</strong></p>
|
||
<p>Remember, the whole point of a promise is that we don’t know its
|
||
value yet! So to write a function that uses a promise as input and
|
||
returns some non-promise value as output, you’d need to either be a time
|
||
traveler or an oracle.</p>
|
||
<p>To summarize, once you start working with a promise, any calculations
|
||
and actions that are “downstream” of that promise will need to become
|
||
promise-oriented. Generally, this means once you have a promise, you
|
||
need to use <code>%...>%</code> and keep using it until your pipeline
|
||
terminates.</p>
|
||
<p><strong>I guess that makes sense. Still, if the only thing you can do
|
||
with promises is make more promises, that limits their usefulness,
|
||
doesn’t it?</strong></p>
|
||
<p>It’s a different way of thinking about things, to be sure, but it
|
||
turns out there’s not much limit in usefulness—especially in the context
|
||
of a Shiny app.</p>
|
||
<p>First, you can use promises with Shiny outputs. If you’re using an
|
||
async-compatible version of Shiny (version >=1.1), all of the
|
||
built-in <code>renderXXX</code> functions can deal with either regular
|
||
values or promises. An example of the latter:</p>
|
||
<div class="sourceCode" id="cb9"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a>output<span class="sc">$</span>table <span class="ot"><-</span> <span class="fu">renderTable</span>({</span>
|
||
<span id="cb9-2"><a href="#cb9-2" aria-hidden="true" tabindex="-1"></a> <span class="fu">read.csv.async</span>(<span class="st">"https://rstudio.github.io/promises/data.csv"</span>) <span class="sc">%...>%</span></span>
|
||
<span id="cb9-3"><a href="#cb9-3" aria-hidden="true" tabindex="-1"></a> <span class="fu">filter</span>(state <span class="sc">==</span> <span class="st">"NY"</span>)</span>
|
||
<span id="cb9-4"><a href="#cb9-4" aria-hidden="true" tabindex="-1"></a>})</span></code></pre></div>
|
||
<p>When <code>output$table</code> executes the <code>renderTable</code>
|
||
code block, it will notice that the result is a promise, and wait for it
|
||
to complete before continuing with the table rendering. While it’s
|
||
waiting, the R process can move on to do other things.</p>
|
||
<p>Second, you can use promises with reactive expressions. Reactive
|
||
expressions treat promises about the same as they treat other values,
|
||
actually. But this works perfectly fine:</p>
|
||
<div class="sourceCode" id="cb10"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a><span class="co"># A reactive expression that returns a promise</span></span>
|
||
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a>filtered_df <span class="ot"><-</span> <span class="fu">reactive</span>({</span>
|
||
<span id="cb10-3"><a href="#cb10-3" aria-hidden="true" tabindex="-1"></a> <span class="fu">read.csv.async</span>(<span class="st">"https://rstudio.github.io/promises/data.csv"</span>) <span class="sc">%...>%</span></span>
|
||
<span id="cb10-4"><a href="#cb10-4" aria-hidden="true" tabindex="-1"></a> <span class="fu">filter</span>(state <span class="sc">==</span> <span class="st">"NY"</span>) <span class="sc">%...>%</span></span>
|
||
<span id="cb10-5"><a href="#cb10-5" aria-hidden="true" tabindex="-1"></a> <span class="fu">arrange</span>(median_income)</span>
|
||
<span id="cb10-6"><a href="#cb10-6" aria-hidden="true" tabindex="-1"></a>})</span>
|
||
<span id="cb10-7"><a href="#cb10-7" aria-hidden="true" tabindex="-1"></a></span>
|
||
<span id="cb10-8"><a href="#cb10-8" aria-hidden="true" tabindex="-1"></a><span class="co"># A reactive expression that reads the previous</span></span>
|
||
<span id="cb10-9"><a href="#cb10-9" aria-hidden="true" tabindex="-1"></a><span class="co"># (promise-returning) reactive, and returns a</span></span>
|
||
<span id="cb10-10"><a href="#cb10-10" aria-hidden="true" tabindex="-1"></a><span class="co"># new promise</span></span>
|
||
<span id="cb10-11"><a href="#cb10-11" aria-hidden="true" tabindex="-1"></a>top_n_by_income <span class="ot"><-</span> <span class="fu">reactive</span>({</span>
|
||
<span id="cb10-12"><a href="#cb10-12" aria-hidden="true" tabindex="-1"></a> <span class="fu">filtered_df</span>() <span class="sc">%...>%</span></span>
|
||
<span id="cb10-13"><a href="#cb10-13" aria-hidden="true" tabindex="-1"></a> <span class="fu">head</span>(input<span class="sc">$</span>n)</span>
|
||
<span id="cb10-14"><a href="#cb10-14" aria-hidden="true" tabindex="-1"></a>})</span>
|
||
<span id="cb10-15"><a href="#cb10-15" aria-hidden="true" tabindex="-1"></a></span>
|
||
<span id="cb10-16"><a href="#cb10-16" aria-hidden="true" tabindex="-1"></a>output<span class="sc">$</span>table <span class="ot"><-</span> <span class="fu">renderTable</span>({</span>
|
||
<span id="cb10-17"><a href="#cb10-17" aria-hidden="true" tabindex="-1"></a> <span class="fu">top_n_by_income</span>()</span>
|
||
<span id="cb10-18"><a href="#cb10-18" aria-hidden="true" tabindex="-1"></a>})</span></code></pre></div>
|
||
<p>Third, you can use promises in reactive observers. Use them to
|
||
perform asynchronous tasks in response to reactivity.</p>
|
||
<div class="sourceCode" id="cb11"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a><span class="fu">observeEvent</span>(input<span class="sc">$</span>save, {</span>
|
||
<span id="cb11-2"><a href="#cb11-2" aria-hidden="true" tabindex="-1"></a> <span class="fu">filtered_df</span>() <span class="sc">%...>%</span></span>
|
||
<span id="cb11-3"><a href="#cb11-3" aria-hidden="true" tabindex="-1"></a> <span class="fu">write.csv</span>(<span class="st">"ny_data.csv"</span>)</span>
|
||
<span id="cb11-4"><a href="#cb11-4" aria-hidden="true" tabindex="-1"></a>})</span></code></pre></div>
|
||
<p><strong>Alright, I think I see what you mean. You can’t escape from
|
||
promise-land, but there’s no need to, because Shiny knows what to do
|
||
with them.</strong></p>
|
||
<p>Yes, that’s basically right. You just need to keep track of which
|
||
functions and reactive expressions return promises instead of regular
|
||
values, and be sure to interact with them using <code>%...>%</code>
|
||
or other promise-aware operators and functions.</p>
|
||
<p><strong>Wait, there are other promise-aware operators and
|
||
functions?</strong></p>
|
||
<p>Yes. The <code>%...>%</code> is the one you’ll most commonly use,
|
||
but there is a variant <code>%...T>%</code>, which we call the
|
||
<em>promise tee</em> operator (it’s analogous to the magrittr
|
||
<code>%T>%</code> operator). The <code>%...T>%</code> operator
|
||
mostly acts like <code>%...>%</code>, but instead of returning a
|
||
promise for the result value, it returns the original value instead.
|
||
Meaning <code>p %...T>% cat("\n")</code> won’t return a promise for
|
||
the return value of <code>cat()</code> (which is always
|
||
<code>NULL</code>) but instead the value of <code>p</code>. This is
|
||
useful for logging, or other “side effecty” operations.</p>
|
||
<p>There’s also <code>%...!%</code>, and its tee version,
|
||
<code>%...T!%</code>, which are used for error handling. I won’t confuse
|
||
you with more about that now, but you can read more <a href="promises_03_overview.html#error-handling">here</a>.</p>
|
||
<p>The <code>promises</code> package is where all of these operators
|
||
live, and it also comes with some additional functions for working with
|
||
promises.</p>
|
||
<p>So far, the only actual async function we’ve talked about has been
|
||
<code>read.csv.async</code>, which doesn’t actually exist. To learn
|
||
where actual async functions come from, read <a href="promises_04_futures.html">this guide to the <code>future</code>
|
||
package</a>.</p>
|
||
<p>There are the lower-level functions <code>then</code>,
|
||
<code>catch</code>, and <code>finally</code>, which are the non-pipe,
|
||
non-operator equivalents of the promise operators we’ve been discussing.
|
||
See <a href="promises_03_overview.html#accessing-results-with-then">reference</a>.</p>
|
||
<p>And finally, there are <code>promise_all</code>,
|
||
<code>promise_race</code>, and <code>promise_lapply</code>, used to
|
||
combine multiple promises into a single promise. Learn more about them
|
||
<a href="../reference/promise_all.html">here</a>.</p>
|
||
<p><strong>OK, looks like I have a lot of stuff to read up on. And I’ll
|
||
probably have to reread this conversation a few times before it fully
|
||
sinks in.</strong></p>
|
||
<p>Sorry. I told you it was complicated. If you make it through the rest
|
||
of the guide, you’ll be 95% of the way there.</p>
|
||
<div style="font-size: 20px; margin-top: 40px; text-align: right;">
|
||
<p>Next: <a href="promises_03_overview.html">Working with
|
||
promises</a></p>
|
||
</div>
|
||
|
||
|
||
|
||
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|
||
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||
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