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<article class="post" itemscope itemtype="http://schema.org/BlogPosting">
<header class="post-header">
<h1 class="post-title" itemprop="name headline">Using Eclipse Hono's Command&Control with Eclipse Ditto</h1>
<p class="post-meta">Published by <img src="https://www.gravatar.com/avatar/ce9011e32b5ea7c8fe00893ae278cb0a?s=135" alt="A photo of Dominik Guggemos" style="width:50px;border-radius:50%;display:inline-block;margin-right:5px;" /><span itemprop="author" itemscope itemtype="http://schema.org/Person"><span itemprop="name"><a href="https://github.com/dguggemos" target="_blank">Dominik Guggemos</a> </span></span> on <time datetime="2018-12-05T00:00:00+00:00" itemprop="datePublished">Dec 5, 2018</time> - Tags:
<a href="tag_blog.html">blog</a>
</p>
</header>
<div class="post-content" itemprop="articleBody">
<p>With <a href="release_notes_080.html">version 0.8.0</a> Eclipse Ditto can now interact with Eclipse Hono using the
“Command & Control” feature. It is possible to send a
Thing (or Feature) message at the Ditto Message API, which is then forwarded to Hono as a
command message. Hono routes the message to the device, which in turn can send a response to the command including a
status, telling if the command was successfully processed or not.
This response is routed back via Hono to the Ditto Message API.</p>
<p>In this example we connect the <a href="https://ditto.eclipseprojects.io/">Ditto sandbox</a> and the
<a href="https://www.eclipse.org/hono/sandbox/">Hono sandbox</a> to send a message (3) to a simulated device via the
Ditto Messages API. The device receives the command from the Hono HTTP Adapter and responds with a message (4)
that is routed back to the caller at the Ditto Message API (5). For the sake of simplicity we use simple curl
commands both for the Ditto and Hono HTTP APIs.</p>
<p>The following steps are covered in this example:</p>
<ol>
<li>Setup a connection between Eclipse Ditto and Hono sandboxes</li>
<li>Signal availability of the device</li>
<li>Send a Ditto message</li>
<li>Device receives command and sends command response</li>
<li>Caller receives response at Ditto Message API</li>
</ol>
<p><img src="images/blog/2018-12-14-command-and-control.png" alt="Command and Control" /></p>
<h3 id="prerequisites-a-ditto-digital-twin-and-a-hono-device">Prerequisites: A Ditto digital twin and a Hono device</h3>
<p>The creation of a Hono device and Ditto digital twin has already been covered in the blog post
<a href="2018-05-02-connecting-ditto-hono.html">Connecting Eclipse Ditto to Eclipse Hono</a>. For brevity we will just list the
required commands to create a twin/device here. For a detailed explanation of the steps please refer to the previous
post.</p>
<h4 id="create-hono-device">Create Hono device</h4>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c"># setup a tenant</span>
<span class="nv">$ </span>curl <span class="nt">-X</span> POST <span class="nt">-i</span> <span class="nt">-H</span> <span class="s1">'Content-Type: application/json'</span> <span class="nt">-d</span> <span class="s1">'{"tenant-id": "org.eclipse.ditto"}'</span> https://hono.eclipseprojects.io:28080/tenant
<span class="c"># create a device</span>
<span class="nv">$ </span>curl <span class="nt">-X</span> POST <span class="nt">-i</span> <span class="nt">-H</span> <span class="s1">'Content-Type: application/json'</span> <span class="nt">-d</span> <span class="s1">'{"device-id": "org.eclipse.ditto:teapot"}'</span> https://hono.eclipseprojects.io:28080/registration/org.eclipse.ditto
<span class="c"># add device credentials</span>
<span class="nv">$ PWD_HASH</span><span class="o">=</span><span class="si">$(</span><span class="nb">echo</span> <span class="nt">-n</span> <span class="s1">'teapot'</span> | openssl dgst <span class="nt">-binary</span> <span class="nt">-sha512</span> | <span class="nb">base64</span> <span class="nt">-w</span> 0<span class="si">)</span>
<span class="nv">$ </span>curl <span class="nt">-X</span> POST <span class="nt">-i</span> <span class="nt">-H</span> <span class="s1">'Content-Type: application/json'</span> <span class="nt">-d</span> <span class="s1">'{
"device-id": "org.eclipse.ditto:teapot",
"type": "hashed-password",
"auth-id": "teapot",
"secrets": [{
"hash-function" : "sha-512",
"pwd-hash": "'</span><span class="nv">$PWD_HASH</span><span class="s1">'"
}]
}'</span> https://hono.eclipseprojects.io:28080/credentials/org.eclipse.ditto
</code></pre></div></div>
<h4 id="create-ditto-policy-for-digital-twin">Create Ditto policy for digital twin</h4>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c"># create policy in Ditto</span>
<span class="nv">$ </span>curl <span class="nt">-X</span> PUT <span class="nt">-i</span> <span class="nt">-u</span> demo5:demo <span class="nt">-H</span> <span class="s1">'Content-Type: application/json'</span> <span class="nt">-d</span> <span class="s1">'{
"policyId": "org.eclipse.ditto:teapot-policy",
"entries": {
"DEMO": {
"subjects": {
"nginx:demo5": {
"type": "basic auth user and user used in connection'</span>s authorizationContext<span class="s2">"
}
},
"</span>resources<span class="s2">": {
"</span>thing:/<span class="s2">": {
"</span>grant<span class="s2">": ["</span>READ<span class="s2">", "</span>WRITE<span class="s2">"],
"</span>revoke<span class="s2">": []
},
"</span>policy:/<span class="s2">": {
"</span>grant<span class="s2">": ["</span>READ<span class="s2">", "</span>WRITE<span class="s2">"],
"</span>revoke<span class="s2">": []
},
"</span>message:/<span class="s2">": {
"</span>grant<span class="s2">": ["</span>READ<span class="s2">", "</span>WRITE<span class="s2">"],
"</span>revoke<span class="s2">": []
}
}
}
}
}' https://ditto.eclipseprojects.io/api/2/policies/org.eclipse.ditto:teapot-policy
</span></code></pre></div></div>
<h4 id="create-ditto-digital-twin">Create Ditto digital twin</h4>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c"># create thing in Ditto</span>
<span class="nv">$ </span>curl <span class="nt">-X</span> PUT <span class="nt">-i</span> <span class="nt">-u</span> demo5:demo <span class="nt">-H</span> <span class="s1">'Content-Type: application/json'</span> <span class="nt">-d</span> <span class="s1">'{
"policyId": "org.eclipse.ditto:teapot-policy",
"features": {
"water": {
"properties": {
"temperature": 20
}
}
}
}'</span> https://ditto.eclipseprojects.io/api/2/things/org.eclipse.ditto:teapot
</code></pre></div></div>
<h3 id="setup-a-connection-for-command--control">Setup a connection for Command & Control</h3>
<p>In order to forward Ditto Messages to the device as a Hono command we first need to setup and configure a
<a href="connectivity-manage-connections.html">connection</a> between Eclipse Ditto and Eclipse Hono that is prepared for
<em>Command & Control</em> messages. According to the
<a href="https://www.eclipse.org/hono/">Hono documentation</a>
the connection must contain a target with the address <code class="language-plaintext highlighter-rouge">control/<tenant-id>/<device-id></code>
and a source with the address <code class="language-plaintext highlighter-rouge">control/<tenant-id>/<reply-identifier></code>. The <code class="language-plaintext highlighter-rouge">reply-identifier</code> can be
chosen arbitrarily, but must be set as the <code class="language-plaintext highlighter-rouge">reply-to</code> header of a command exactly as defined in the connection:</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code>curl <span class="nt">-X</span> POST <span class="nt">-i</span> <span class="nt">-u</span> devops:devopsPw1! <span class="se">\</span>
<span class="nt">-H</span> <span class="s1">'Content-Type: application/json'</span> <span class="se">\</span>
<span class="nt">-d</span> <span class="s1">'{
"targetActorSelection": "/system/sharding/connection",
"headers": {
"aggregate": false
},
"piggybackCommand": {
"type": "connectivity.commands:createConnection",
"connection": {
"id": "command-and-control-connection",
"connectionType": "amqp-10",
"connectionStatus": "open",
"uri": "amqp://consumer%40HONO:verysecret@hono.eclipse.org:15672",
"failoverEnabled": true,
"sources": [{
"addresses": [
"control/org.eclipse.ditto/replies"
],
"authorizationContext": [
"nginx:demo5"
],
"headerMapping": {
"correlation-id": "{{ header:correlation-id }}",
"status": "{{ header:status }}",
"content-type": "{{ header:content-type }}"
}
}
],
"targets": [{
"address": "control/org.eclipse.ditto/{{ thing:name }}",
"authorizationContext": [
"nginx:demo5"
],
"topics":["_/_/things/live/messages"],
"headerMapping": {
"message-id": "{{ header:correlation-id }}",
"subject": "{{ topic:subject }}",
"content-type": "application/vnd.eclipse.ditto+json",
"reply-to": "control/org.eclipse.ditto/replies"
}
}
]
}
}
}'</span> <span class="se">\</span>
https://ditto.eclipseprojects.io/devops/piggyback/connectivity?timeout<span class="o">=</span>10s
</code></pre></div></div>
<p>As described in the
<a href="https://www.eclipse.org/hono/">Hono API description</a>
a command message has three mandatory properties: <code class="language-plaintext highlighter-rouge">correlation-id</code>, <code class="language-plaintext highlighter-rouge">subject</code> and <code class="language-plaintext highlighter-rouge">reply-to</code>, these are defined in the
target header mapping of the connection.
The source header mapping defines a mapping for <code class="language-plaintext highlighter-rouge">correlation-id</code> and <code class="language-plaintext highlighter-rouge">status</code> to internal headers, they are required
to properly map the Hono command response to a Ditto message response.</p>
<h3 id="signal-availability-of-device">Signal availability of device</h3>
<p>As we are using the Hono HTTP Adapter to connect our device, send telemetry and receive commands, the designated way
is therefor to signal readiness to receive a command by specifying the <code class="language-plaintext highlighter-rouge">hono-ttd</code> parameter on an arbitrary event (for
detailed description please consult the
[Hono HTTP Adapter] (https://www.eclipse.org/hono/)
guide).</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code>curl <span class="nt">-X</span> POST <span class="nt">-i</span> <span class="nt">-u</span> teapot@org.eclipse.ditto:teapot <span class="nt">-H</span> <span class="s1">'hono-ttd: 60'</span> <span class="nt">-H</span> <span class="s1">'Content-Type: application/json'</span> <span class="se">\</span>
<span class="nt">-d</span> <span class="s1">'{
"topic": "org.eclipse.ditto/teapot/things/twin/commands/modify",
"path": "/features/water/properties/temperature",
"value": 23
}'</span> <span class="se">\</span>
https://hono.eclipseprojects.io:8080/telemetry
</code></pre></div></div>
<p>The request is now open to receive a command for 60 seconds before it is terminated.</p>
<h3 id="send-a-ditto-message">Send a Ditto message</h3>
<p>Now we can use the <a href="protocol-specification-things-messages.html#using-the-messages-api">Ditto Messages API</a> to send a
message to the device waiting for a command:</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code>curl <span class="nt">-i</span> <span class="nt">-X</span> POST <span class="s1">'https://ditto.eclipseprojects.io/api/2/things/org.eclipse.ditto:teapot/inbox/messages/brew?timeout=60'</span> <span class="se">\</span>
<span class="nt">-u</span> demo5:demo <span class="se">\</span>
<span class="nt">-H</span> <span class="s1">'x-correlation-id: command-and-control'</span> <span class="se">\</span>
<span class="nt">-d</span> <span class="s1">'{"targetTemperature":85}'</span>
</code></pre></div></div>
<h3 id="device-receives-the-command">Device receives the command</h3>
<p>The message is forwarded to Hono as configured in the connection and finally terminates the pending request we opened
before with a status code of <code class="language-plaintext highlighter-rouge">200 OK</code>:</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>HTTP/1.1 200 OK
hono-command: brew
hono-cmd-req-id: 013command-and-controlreplies
Content-Type: application/octet-stream
Content-Length: 17
Connection: Keep-Alive
</code></pre></div></div>
<p>Hono adds two headers besides the standard HTTP headers: <code class="language-plaintext highlighter-rouge">hono-command</code> and <code class="language-plaintext highlighter-rouge">hono-cmd-req-id</code>.
<code class="language-plaintext highlighter-rouge">hono-command</code> contains the subject of the message and <code class="language-plaintext highlighter-rouge">hono-cmd-req-id</code> identifies the messages and is used to
correlate the request and the response we are now going to send.</p>
<h3 id="device-sends-a-command-response">Device sends a command response</h3>
<p>We use the header value of <code class="language-plaintext highlighter-rouge">hono-cmd-req-id</code> to construct the response address:
<code class="language-plaintext highlighter-rouge">https://hono.eclipseprojects.io:8080/control/res/013command-and-controlreplies</code></p>
<p>Another curl command completes the roundtrip with a response from the simulated device:</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code>curl <span class="nt">-i</span> <span class="nt">-X</span> POST <span class="nt">-u</span> teapot@org.eclipse.ditto:teapot <span class="se">\</span>
<span class="nt">-H</span> <span class="s1">'Content-Type: application/json'</span> <span class="se">\</span>
<span class="nt">-H</span> <span class="s1">'hono-cmd-status: 200'</span> <span class="se">\</span>
<span class="nt">-d</span> <span class="s1">'{
"topic": "org.eclipse.ditto/teapot/things/live/messages/brew",
"headers": {
"content-type": "application/json",
"correlation-id": "command-and-control"
},
"path": "/inbox/messages/brew",
"value": { "eta": 56},
"status": 200
}'</span> <span class="se">\</span>
https://hono.eclipseprojects.io:8080/control/res/013command-and-controlreplies
</code></pre></div></div>
<h3 id="message-response-is-received-at-ditto-message-api">Message response is received at Ditto Message API</h3>
<p>And finally we receive the command response at the Ditto Message API where we sent the original message:</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>HTTP/1.1 200 OK
correlation-id: command-and-control
message-id: command-and-control
status: 200
Content-Type: application/json
Content-Length: 10
{"eta":56}
</code></pre></div></div>
<h3 id="alternative-receive-command-and-send-response-via-mqtt">Alternative: Receive command and send response via MQTT</h3>
<p>Alternatively we can also receive the command by subscribing to the MQTT topic <code class="language-plaintext highlighter-rouge">control/+/+/req/#</code> at the
<a href="https://www.eclipse.org/hono/">Hono MQTT Adapter</a>:</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nv">$ </span>mosquitto_sub <span class="nt">-d</span> <span class="nt">-h</span> hono.eclipse.org <span class="nt">-p</span> 8883 <span class="nt">-v</span> <span class="nt">-u</span> <span class="s1">'teapot@org.eclipse.ditto'</span> <span class="nt">-P</span> teapot <span class="nt">-t</span> <span class="s1">'control/+/+/req/#'</span>
</code></pre></div></div>
<p>And also publish the command response on the MQTT topic <code class="language-plaintext highlighter-rouge">control///res/013command-and-controlreplies/200</code>:</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code>mosquitto_pub <span class="nt">-d</span> <span class="nt">-h</span> hono.eclipse.org <span class="nt">-p</span> 8883 <span class="nt">-u</span> <span class="s1">'teapot@org.eclipse.ditto'</span> <span class="nt">-P</span> teapot <span class="se">\</span>
<span class="nt">-t</span> control///res/013command-and-controlreplies/200 <span class="se">\</span>
<span class="nt">-m</span> <span class="s1">'{
"topic": "org.eclipse.ditto/teapot/things/live/messages/brew",
"headers": {
"content-type": "application/json",
"correlation-id": "command-and-control"
},
"path": "/inbox/messages/brew",
"value": {
"eta": 58
},
"status": 200
}'</span>
</code></pre></div></div>
<p>If you have any wishes, improvements, are missing something
or just want to get in touch with us, you can use one of
our <a href="feedback.html">feedback channels</a>.</p>
<figure><img class="docimage" src="images/ditto.svg" alt="Ditto" style="max-width: 500px" /></figure>
<p>–<br />
The Eclipse Ditto team</p>
</div>
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