signalk-server
Version:
An implementation of a [Signal K](http://signalk.org) server for boats.
358 lines (314 loc) • 23.5 kB
HTML
<html lang="en" class="sidebar-visible no-js light">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>Processing Data - Signal K Server Documentation</title>
<!-- Custom HTML head -->
<meta name="description" content="A Guide for users and developers.">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="theme-color" content="#ffffff" />
<link rel="shortcut icon" href="../../favicon.png">
<link rel="stylesheet" href="../../css/variables.css">
<link rel="stylesheet" href="../../css/general.css">
<link rel="stylesheet" href="../../css/chrome.css">
<link rel="stylesheet" href="../../css/print.css" media="print">
<!-- Fonts -->
<link rel="stylesheet" href="../../FontAwesome/css/font-awesome.css">
<link rel="stylesheet" href="../../fonts/fonts.css">
<!-- Highlight.js Stylesheets -->
<link rel="stylesheet" href="../../highlight.css">
<link rel="stylesheet" href="../../tomorrow-night.css">
<link rel="stylesheet" href="../../ayu-highlight.css">
<!-- Custom theme stylesheets -->
</head>
<body>
<div id="body-container">
<!-- Provide site root to javascript -->
<script>
var path_to_root = "../../";
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
</script>
<!-- Work around some values being stored in localStorage wrapped in quotes -->
<script>
try {
var theme = localStorage.getItem('mdbook-theme');
var sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') && theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
</script>
<!-- Set the theme before any content is loaded, prevents flash -->
<script>
var theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = default_theme; }
var html = document.querySelector('html');
html.classList.remove('no-js')
html.classList.remove('light')
html.classList.add(theme);
html.classList.add('js');
</script>
<input type="checkbox" id="sidebar-toggle-anchor" class="hidden">
<!-- Hide / unhide sidebar before it is displayed -->
<script>
var html = document.querySelector('html');
var sidebar = null;
var sidebar_toggle = document.getElementById("sidebar-toggle-anchor");
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
} else {
sidebar = 'hidden';
}
sidebar_toggle.checked = sidebar === 'visible';
html.classList.remove('sidebar-visible');
html.classList.add("sidebar-" + sidebar);
</script>
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<div class="sidebar-scrollbox">
<a href="/" style="width:155px;height:30px;background-image:url(/signal-k-logo-image-text.svg);display:inline-block;background-repeat:no-repeat;background-position:center center;background-size:150px auto;"></a>
<ol class="chapter"><li class="chapter-item expanded affix "><a href="../../index.html">Introduction</a></li><li class="chapter-item expanded affix "><li class="part-title">Getting Started</li><li class="chapter-item expanded "><a href="../../installation/install.html"><strong aria-hidden="true">1.</strong> Installation</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../installation/raspberry_pi_installation.html"><strong aria-hidden="true">1.1.</strong> Installing on Raspberry Pi</a></li><li class="chapter-item expanded "><a href="../../installation/updating.html"><strong aria-hidden="true">1.2.</strong> Updating your installation</a></li><li class="chapter-item expanded "><a href="../../installation/command_line.html"><strong aria-hidden="true">1.3.</strong> Runtime environment & options</a></li></ol></li><li class="chapter-item expanded "><a href="../../security.html"><strong aria-hidden="true">2.</strong> Security</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../setup/generating_tokens.html"><strong aria-hidden="true">2.1.</strong> Generating tokens</a></li></ol></li><li class="chapter-item expanded "><li class="part-title">Setup</li><li class="chapter-item expanded "><a href="../../setup/configuration.html"><strong aria-hidden="true">3.</strong> Configuration</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../setup/seatalk/seatalk.html"><strong aria-hidden="true">3.1.</strong> Seatalk Connections</a></li></ol></li><li class="chapter-item expanded "><li class="part-title">Feature How Tos</li><li class="chapter-item expanded "><a href="../../features/anchoralarm/anchoralarm.html"><strong aria-hidden="true">4.</strong> Anchor Alarm</a></li><li class="chapter-item expanded "><a href="../../features/navdataserver/navdataserver.html"><strong aria-hidden="true">5.</strong> NMEA0183 Server</a></li><li class="chapter-item expanded "><a href="../../features/datalogging/datalogging.html"><strong aria-hidden="true">6.</strong> Data Logging</a></li><li class="chapter-item expanded affix "><li class="part-title">Support</li><li class="chapter-item expanded "><a href="../../support/help.html"><strong aria-hidden="true">7.</strong> Help & Support</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../support/faq.html"><strong aria-hidden="true">7.1.</strong> FAQs</a></li></ol></li><li class="chapter-item expanded "><a href="../../support/sponsor.html"><strong aria-hidden="true">8.</strong> Sponsor</a></li><li class="chapter-item expanded affix "><li class="part-title">Develop</li><li class="chapter-item expanded "><a href="../../develop/developer_notes.html"><strong aria-hidden="true">9.</strong> Notes for Developers</a></li><li class="chapter-item expanded "><a href="../../whats_new.html"><strong aria-hidden="true">10.</strong> What's New in V2</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../breaking_changes.html"><strong aria-hidden="true">10.1.</strong> Changes & Deprecations</a></li></ol></li><li class="chapter-item expanded "><a href="../../develop/webapps.html"><strong aria-hidden="true">11.</strong> WebApps</a></li><li class="chapter-item expanded "><a href="../../develop/plugins/server_plugin.html"><strong aria-hidden="true">12.</strong> Plugins</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../develop/plugins/deltas.html" class="active"><strong aria-hidden="true">12.1.</strong> Processing Data</a></li><li class="chapter-item expanded "><a href="../../develop/plugins/server_plugin_api.html"><strong aria-hidden="true">12.2.</strong> Server API</a></li><li class="chapter-item expanded "><a href="../../develop/plugins/resource_provider_plugins.html"><strong aria-hidden="true">12.3.</strong> Resource Providers</a></li><li class="chapter-item expanded "><a href="../../develop/rest-api/course_calculations.html"><strong aria-hidden="true">12.4.</strong> Course Providers</a></li><li class="chapter-item expanded "><a href="../../develop/plugins/autopilot_provider_plugins.html"><strong aria-hidden="true">12.5.</strong> Autopilot Providers</a></li></ol></li><li class="chapter-item expanded "><a href="../../develop/plugins/publishing.html"><strong aria-hidden="true">13.</strong> Publishing to the AppStore</a></li><li class="chapter-item expanded "><a href="../../develop/rest-api/open_api.html"><strong aria-hidden="true">14.</strong> REST APIs</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../develop/rest-api/course_api.html"><strong aria-hidden="true">14.1.</strong> Course API</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../develop/rest-api/course_calculations.html"><strong aria-hidden="true">14.1.1.</strong> Course Calculations</a></li></ol></li><li class="chapter-item expanded "><a href="../../develop/rest-api/resources_api.html"><strong aria-hidden="true">14.2.</strong> Resources API</a></li><li class="chapter-item expanded "><a href="../../develop/rest-api/notifications_api.html"><strong aria-hidden="true">14.3.</strong> Notifications API</a></li><li class="chapter-item expanded "><a href="../../develop/rest-api/autopilot_api.html"><strong aria-hidden="true">14.4.</strong> Autopilot API</a></li><li class="chapter-item expanded "><a href="../../develop/rest-api/anchor_api.html"><strong aria-hidden="true">14.5.</strong> Anchor API</a></li></ol></li><li class="chapter-item expanded "><a href="../../develop/contributing.html"><strong aria-hidden="true">15.</strong> Contribute</a></li></ol>
</div>
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
</nav>
<!-- Track and set sidebar scroll position -->
<script>
var sidebarScrollbox = document.querySelector('#sidebar .sidebar-scrollbox');
sidebarScrollbox.addEventListener('click', function(e) {
if (e.target.tagName === 'A') {
sessionStorage.setItem('sidebar-scroll', sidebarScrollbox.scrollTop);
}
}, { passive: true });
var sidebarScrollTop = sessionStorage.getItem('sidebar-scroll');
sessionStorage.removeItem('sidebar-scroll');
if (sidebarScrollTop) {
// preserve sidebar scroll position when navigating via links within sidebar
sidebarScrollbox.scrollTop = sidebarScrollTop;
} else {
// scroll sidebar to current active section when navigating via "next/previous chapter" buttons
var activeSection = document.querySelector('#sidebar .active');
if (activeSection) {
activeSection.scrollIntoView({ block: 'center' });
}
}
</script>
<div id="page-wrapper" class="page-wrapper">
<div class="page">
<div id="menu-bar-hover-placeholder"></div>
<div id="menu-bar" class="menu-bar sticky">
<div class="left-buttons">
<label id="sidebar-toggle" class="icon-button" for="sidebar-toggle-anchor" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
<i class="fa fa-bars"></i>
</label>
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
<i class="fa fa-paint-brush"></i>
</button>
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
<li role="none"><button role="menuitem" class="theme" id="light">Light</button></li>
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
</ul>
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
<i class="fa fa-search"></i>
</button>
</div>
<h1 class="menu-title">Signal K Server Documentation</h1>
<div class="right-buttons">
<a href="../../print.html" title="Print this book" aria-label="Print this book">
<i id="print-button" class="fa fa-print"></i>
</a>
<a href="https://github.com/SignalK/signalk-server/tree/master/docs/src" title="Suggest an edit" aria-label="Suggest an edit">
<i id="git-edit-button" class="fa fa-edit"></i>
</a>
</div>
</div>
<div id="search-wrapper" class="hidden">
<form id="searchbar-outer" class="searchbar-outer">
<input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
</form>
<div id="searchresults-outer" class="searchresults-outer hidden">
<div id="searchresults-header" class="searchresults-header"></div>
<ul id="searchresults">
</ul>
</div>
</div>
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
<script>
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
</script>
<div id="content" class="content">
<main>
<h1 id="processing-data-from-the-server"><a class="header" href="#processing-data-from-the-server">Processing data from the server</a></h1>
<p>A plugin will generally want to:</p>
<ol>
<li>Subscribe to data published by the server <em>(i.e. received from a NMEA 2000 bus, etc)</em></li>
<li>Emit data.</li>
</ol>
<p>In both cases the plugin will use <em>deltas</em> which the server uses to signal changes in the Signal K full data model. Delta messages contain the new value associated with a path (not the amount of change from the previous value.)_</p>
<p><em>See the <a href="http://signalk.org/specification/1.7.0/doc/data_model.html#delta-format">Signal K Delta Specification</a> for details.</em></p>
<p>Using the server API, plugins can either:</p>
<ol>
<li>Get the current value of a path in the full model or</li>
<li>Subscribe to a path and access a stream of <em>deltas</em> that updates every time the value is updated.</li>
</ol>
<p>By specifying a context <em>e.g. 'vessels.self'</em> you can limit the number of delta messages received to those of host vesseel.
To receive all deltas you can specify <code>*</code> as the context.</p>
<p>You can also limit the deltas received by the path you supply.
If you supply a specific path <em>e.g. navigation.position</em>, only updates in the value will be received.
Since paths are hierarchical, paths can contain wildcards _e.g.<em>navigation.*</em> which will deliver deltas containing updates to all paths under <code>navigation</code>.</p>
<p>The data received is formatted as per the following example:</p>
<pre><code class="language-javascript"> {
path: 'navigation.position',
value: { longitude: 24.7366117, latitude: 59.72493 },
context: 'vessel.self',
source: {
label: 'n2k-sample-data',
type: 'NMEA2000',
pgn: 129039,
src: '43'
},
$source: 'n2k-sample-data.43',
timestamp: '2014-08-15T19:00:02.392Z'
}
</code></pre>
<h2 id="reading-the-current-path-value"><a class="header" href="#reading-the-current-path-value">Reading the current path value</a></h2>
<p>The server API provides the following methods for retrieving values from the full data model.</p>
<ul>
<li><code>getSelfPath(path)</code> returns the value of the supplied <code>path</code> in the <code>vessels.self</code> context.</li>
</ul>
<pre><code class="language-javascript">const value = app.getSelfPath('uuid');
app.debug(value); // Should output something like urn:mrn:signalk:uuid:a9d2c3b1-611b-4b00-8628-0b89d014ed60
</code></pre>
<ul>
<li><code>getPath(path)</code> returns the value of the path (including the context) starting from the <em>root</em> of the full data model.</li>
</ul>
<pre><code class="language-javascript">const baseStations = app.getPath('shore.basestations');
</code></pre>
<h2 id="subscribing-to-deltas"><a class="header" href="#subscribing-to-deltas">Subscribing to Deltas</a></h2>
<p>A can subscribe to a stream of updates (deltas) by creating the subscription.</p>
<p>Subcriptions are generally manged in the plugin <code>start()</code> and <code>stop()</code> methods to ensure the subscribtions are <em>unsubscribed</em> prior to the plugin stopping to ensure all resources are freed.</p>
<p>The following example illustrates the pattern using the <code>subscriptionmanager</code> API method.</p>
<pre><code class="language-javascript">let unsubscribes = [];
plugin.start = (options, restartPlugin) => {
app.debug('Plugin started');
let localSubscription = {
context: '*', // Get data for all contexts
subscribe: [{
path: '*', // Get all paths
period: 5000 // Every 5000ms
}]
};
app.subscriptionmanager.subscribe(
localSubscription,
unsubscribes,
subscriptionError => {
app.error('Error:' + subscriptionError);
},
delta => {
delta.updates.forEach(u => {
app.debug(u);
});
}
);
};
plugin.stop = () => {
unsubscribes.forEach(f => f());
unsubscribes = [];
};
</code></pre>
<p>In the <code>start()</code> method create a subscription definition <code>localSubscription</code> which is then passed to <code>app.subscriptionmanager.subscribe()</code> as the first argument, we also pass the <code>unsubscribes</code> array in the second argument.</p>
<p>The third argument is a function that will be called when there's an error.</p>
<p>The final argument is a function that will be called every time an update is received.</p>
<p>In the <code>stop()</code> method each subcription in the <code>unsubscribes</code> array is <em>unsubscribed</em> and the resources released.</p>
<h2 id="sending-deltas"><a class="header" href="#sending-deltas">Sending Deltas</a></h2>
<p>A SignalK plugin can not only read deltas, but can also send them. This is done using the <code>handleMessage()</code> API method and supplying:</p>
<ol>
<li>The plugin id</li>
<li>A formatted delta update message</li>
<li>The Signal K version ['v1' or 'v2'] <em>(if omitted the default is 'v1')</em>. See <a href="../openapi/index.html">REST APIs</a> for details.</li>
</ol>
<p><em>Example:</em></p>
<pre><code class="language-javascript">app.handleMessage(
plugin.id,
{
updates: [{
values: [{
path: 'environment.outside.temperature',
value: -253
}]
}]
},
'v1'
);
</code></pre>
<h2 id="sending-nmea-2000-data-from-a-plugin"><a class="header" href="#sending-nmea-2000-data-from-a-plugin">Sending NMEA 2000 data from a plugin</a></h2>
<p>A SignalK plugin can not only emit deltas, but can also send data such as NMEA 2000 data.</p>
<p>This is done using the <code>emit()</code> API and specifying the provider as well as the formatted data to send.</p>
<p><em>Example: Send NMEA using Actisense serial format:</em></p>
<pre><code class="language-javascript"> app.emit(
'nmea2000out',
'2017-04-15T14:57:58.468Z,0,262384,0,0,14,01,0e,00,88,b6,02,00,00,00,00,00,a2,08,00');
</code></pre>
<p><em>Example: Send NMEA using Canboat JSON format:</em></p>
<pre><code class="language-javascript"> app.emit('nmea2000JsonOut', {
pgn: 130306,
'Wind Speed': speed,
'Wind Angle': angle < 0 ? angle + Math.PI*2 : angle,
'Reference': "Apparent"
});
</code></pre>
<h3 id="sending-a-message-on-nmea2000-startup"><a class="header" href="#sending-a-message-on-nmea2000-startup">Sending a message on NMEA2000 startup</a></h3>
<p>If you need to send an NMEA2000 message out at startup, <em>e.g get current state from a device</em> you will need to wait until the provider is ready before sending your message.</p>
<p><em>Example: Send NMEA after the provider is ready:</em></p>
<pre><code class="language-javascript"> app.on('nmea2000OutAvailable', () => {
app.emit(
'nmea2000out',
'2017-04-15T14:57:58.468Z,2,6,126720,%s,%s,4,a3,99,01,00');
});
</code></pre>
</main>
<nav class="nav-wrapper" aria-label="Page navigation">
<!-- Mobile navigation buttons -->
<a rel="prev" href="../../develop/plugins/server_plugin.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next" href="../../develop/plugins/server_plugin_api.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
<div style="clear: both"></div>
</nav>
</div>
</div>
<nav class="nav-wide-wrapper" aria-label="Page navigation">
<a rel="prev" href="../../develop/plugins/server_plugin.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next" href="../../develop/plugins/server_plugin_api.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
</nav>
</div>
<script>
window.playground_copyable = true;
</script>
<script src="../../elasticlunr.min.js"></script>
<script src="../../mark.min.js"></script>
<script src="../../searcher.js"></script>
<script src="../../clipboard.min.js"></script>
<script src="../../highlight.js"></script>
<script src="../../book.js"></script>
<!-- Custom JS scripts -->
</div>
</body>
</html>