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<ol class="chapter"><li class="chapter-item expanded "><a href="introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="examples.html"><strong aria-hidden="true">2.</strong> Examples</a></li><li class="chapter-item expanded affix "><li class="part-title">User guide</li><li class="chapter-item expanded "><a href="concepts.html"><strong aria-hidden="true">3.</strong> Core concepts</a></li><li class="chapter-item expanded "><a href="async_procs.html"><strong aria-hidden="true">4.</strong> async functions</a></li><li class="chapter-item expanded "><a href="error_handling.html"><strong aria-hidden="true">5.</strong> Errors and exceptions</a></li><li class="chapter-item expanded "><a href="threads.html"><strong aria-hidden="true">6.</strong> Threads</a></li><li class="chapter-item expanded "><a href="tips.html"><strong aria-hidden="true">7.</strong> Tips, tricks and best practices</a></li><li class="chapter-item expanded "><a href="porting.html" class="active"><strong aria-hidden="true">8.</strong> Porting code to chronos</a></li><li class="chapter-item expanded "><a href="http_server_middleware.html"><strong aria-hidden="true">9.</strong> HTTP server middleware</a></li><li class="chapter-item expanded affix "><li class="part-title">Developer guide</li><li class="chapter-item expanded "><a href="book.html"><strong aria-hidden="true">10.</strong> Updating this book</a></li></ol>
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<h1 id="porting-code-to-chronos-v4"><a class="header" href="#porting-code-to-chronos-v4">Porting code to <code>chronos</code> v4</a></h1>
<ul>
<li><a href="#chronos-v3">Chronos v3</a></li>
<li><a href="#asyncdispatch"><code>asyncdispatch</code></a></li>
<li><a href="#supporting-multiple-backends">Supporting multiple backends</a></li>
</ul>
<p>Thanks to its macro support, Nim allows <code>async</code>/<code>await</code> to be implemented in
libraries with only minimal support from the language - as such, multiple
<code>async</code> libraries exist, including <code>chronos</code> and <code>asyncdispatch</code>, and more may
come to be developed in the futures.</p>
<h2 id="chronos-v3"><a class="header" href="#chronos-v3">Chronos v3</a></h2>
<p>Chronos v4 introduces new features for IPv6, exception effects, a stand-alone
<code>Future</code> type as well as several other changes - when upgrading from chronos v3,
here are several things to consider:</p>
<ul>
<li>Exception handling is now strict by default - see the <a href="./error_handling.html">error handling</a>
chapter for how to deal with <code>raises</code> effects</li>
<li><code>AsyncEventBus</code> was removed - use <code>AsyncEventQueue</code> instead</li>
<li><code>Future.value</code> and <code>Future.error</code> panic when accessed in the wrong state</li>
<li><code>Future.read</code> and <code>Future.readError</code> raise <code>FutureError</code> instead of
<code>ValueError</code> when accessed in the wrong state</li>
</ul>
<h2 id="asyncdispatch"><a class="header" href="#asyncdispatch"><code>asyncdispatch</code></a></h2>
<p>Code written for <code>asyncdispatch</code> and <code>chronos</code> looks similar but there are
several differences to be aware of:</p>
<ul>
<li><code>chronos</code> has its own dispatch loop - you can typically not mix <code>chronos</code> and
<code>asyncdispatch</code> in the same thread</li>
<li><code>import chronos</code> instead of <code>import asyncdispatch</code></li>
<li>cleanup is important - make sure to use <code>closeWait</code> to release any resources
you're using or file descriptor and other leaks will ensue</li>
<li>cancellation support means that <code>CancelledError</code> may be raised from most
<code>{.async.}</code> functions</li>
<li>Calling <code>yield</code> directly in tasks is not supported - instead, use <code>awaitne</code>.</li>
<li><code>asyncSpawn</code> is used instead of <code>asyncCheck</code> - note that exceptions raised
in tasks that are <code>asyncSpawn</code>:ed cause panic</li>
</ul>
<h2 id="supporting-multiple-backends"><a class="header" href="#supporting-multiple-backends">Supporting multiple backends</a></h2>
<p>Libraries built on top of <code>async</code>/<code>await</code> may wish to support multiple async
backends - the best way to do so is to create separate modules for each backend
that may be imported side-by-side - see <a href="https://github.com/status-im/nim-metrics/blob/master/metrics/">nim-metrics</a>
for an example.</p>
<p>An alternative way is to select backend using a global compile flag - this
method makes it diffucult to compose applications that use both backends as may
happen with transitive dependencies, but may be appropriate in some cases -
libraries choosing this path should call the flag <code>asyncBackend</code>, allowing
applications to choose the backend with <code>-d:asyncBackend=<backend_name></code>.</p>
<p>Known <code>async</code> backends include:</p>
<ul>
<li><code>chronos</code> - this library (<code>-d:asyncBackend=chronos</code>)</li>
<li><code>asyncdispatch</code> the standard library <code>asyncdispatch</code> <a href="https://nim-lang.org/docs/asyncdispatch.html">module</a> (<code>-d:asyncBackend=asyncdispatch</code>)</li>
<li><code>none</code> - <code>-d:asyncBackend=none</code> - disable <code>async</code> support completely</li>
</ul>
<p><code>none</code> can be used when a library supports both a synchronous and
asynchronous API, to disable the latter.</p>
<footer id="open-on-gh">Found a bug? <a href="https://github.com/status-im/nim-chronos//edit/master/docs/src/porting.md">Edit this page on GitHub.</a></footer>
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