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| <div class="post-header"> |
| <h1 class="post-title-main">Things - Messages protocol specification</h1> |
| </div> |
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| <div class="post-content"> |
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| |
| <p>Messages within the Ditto Protocol allow sending, receiving and responding to |
| Messages. They contain an arbitrary <em>payload</em>, so you can choose what content |
| fits your solution best. If you want to learn more about the basic concepts of the Messages |
| functionality, please have a look at the <a href="basic-messages.html">Messages page</a>.</p> |
| |
| <div class="alert alert-success" role="alert"><i class="fa fa-check-square-o"></i> <b>Tip:</b> If you only need to send Messages, but don’t need to receive |
| or respond, you could also use the <a href="httpapi-messages.html">HTTP Messages API</a></div> |
| |
| <h2 id="messages-protocol">Messages protocol</h2> |
| |
| <p>The Messages protocol is part of the <a href="protocol-specification.html">Ditto Protocol</a> and therefore |
| conforms to its specification. This section describes how the protocol envelope can be filled |
| for sending Messages. If you want to jump right into using the API, head over to the |
| next section that describes how to <a href="#using-the-messages-api">use the messages API</a>.</p> |
| |
| <p>There are three protocol parameters that have special meaning for Messages:</p> |
| <ul> |
| <li><code class="highlighter-rouge">topic</code> : <em>{namespace}</em>/<em>{thingId}</em>/things/live/messages/<em>{messageSubject}</em></li> |
| <li><code class="highlighter-rouge">path</code> : <em>{addressedPartOfThing}</em>/<em>{mailbox}</em>/messages/<em>{messageSubject}</em></li> |
| <li><code class="highlighter-rouge">headers</code> : The headers for Messages must include <em>content-type</em></li> |
| </ul> |
| |
| <p><br /></p> |
| |
| <p>The <code class="highlighter-rouge">topic</code> definition for Messages needs the <em>namespace</em> and <em>thingId</em> |
| of the Thing you’re sending Messages to. The <em>messageSubject</em> describes the Message |
| and must conform to the <em>path</em> as described in <a href="https://tools.ietf.org/html/rfc3986">RFC-3986</a>. |
| Examples for valid topics are:</p> |
| <ul> |
| <li><code class="highlighter-rouge">org.eclipse.ditto/smartcoffee/things/live/messages/ask/question</code></li> |
| <li><code class="highlighter-rouge">com.example/smarthome/things/live/messages/turnoff</code></li> |
| </ul> |
| |
| <p><br /></p> |
| |
| <p>The <code class="highlighter-rouge">path</code> also contains the <em>messageSubject</em> that describes the Message. |
| <em>mailbox</em> can be <em>inbox</em> (Message is sent to a Thing) or <em>outbox</em> (Message is |
| sent from a Thing). The <em>addressedPartOfThing</em> tells which part of the Thing |
| is addressed. <code class="highlighter-rouge">/features/water-tank</code> would address the water-tank Feature of |
| the Thing, while `` would address the whole Thing. Valid paths are e.g.:</p> |
| <ul> |
| <li><code class="highlighter-rouge">/inbox/messages/ask/question</code></li> |
| <li><code class="highlighter-rouge">/features/lights/inbox/messages/turnoff</code></li> |
| <li><code class="highlighter-rouge">/features/smokedetector/outbox/messages/smokedetected</code></li> |
| </ul> |
| |
| <p><br /></p> |
| |
| <p>In the <code class="highlighter-rouge">headers</code> of the envelope the Messages API requires:</p> |
| <ul> |
| <li><code class="highlighter-rouge">content-type</code> : The type of the payload you are sending, e.g. <em>text/plain</em></li> |
| </ul> |
| |
| <h2 id="using-the-messages-api">Using the Messages API</h2> |
| |
| <p>The following parts contain examples that will show you how to leverage the Messages API. |
| In the examples we will use some kind of smart coffee machine with the id <em>smartcoffee</em>.</p> |
| |
| <div class="alert alert-info" role="alert"><i class="fa fa-info-circle"></i> <b>Note:</b> We encourage you to play around with the examples. You can use |
| the WebSocket binding to do so. Make sure the Thing you are sending Messages to is existing and has |
| the correct access rights.</div> |
| |
| <h3 id="sending-a-message-to-a-thing">Sending a Message to a Thing</h3> |
| |
| <p>When sending a Message to a Thing, we send it to the inbox of the receiving entity.<br /> |
| What follows is a simple example Message that asks our Thing <em>smartcoffee</em> how it is feeling today:</p> |
| |
| <div class="language-json highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="p">{</span><span class="w"> |
| </span><span class="s2">"topic"</span><span class="p">:</span><span class="w"> </span><span class="s2">"org.eclipse.ditto/smartcoffee/things/live/messages/ask"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"headers"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"content-type"</span><span class="p">:</span><span class="w"> </span><span class="s2">"text/plain"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"correlation-id"</span><span class="p">:</span><span class="w"> </span><span class="s2">"a-unique-string-for-this-message"</span><span class="w"> |
| </span><span class="p">},</span><span class="w"> |
| </span><span class="s2">"path"</span><span class="p">:</span><span class="w"> </span><span class="s2">"/inbox/messages/ask"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"value"</span><span class="p">:</span><span class="w"> </span><span class="s2">"Hey, how are you?"</span><span class="w"> |
| </span><span class="p">}</span><span class="w"> |
| </span></code></pre></div></div> |
| |
| <p>Notice that our <code class="highlighter-rouge">topic</code> adheres to the <a href="protocol-specification-topic.html">Ditto Protocol topic definition</a> |
| with <em>messages</em> as the criterion, <em>live</em> as channel, and the message-subject as action.</p> |
| |
| <p>We encourage you to always send a <code class="highlighter-rouge">correlation-id</code> with your Messages. |
| This is especially important, since the WebSocket Ditto Protocol binding |
| sends messages in a fire-and-forget manner. Ditto wouldn’t know who to |
| respond to if there was no <code class="highlighter-rouge">correlation-id</code> set in the Message.</p> |
| |
| <div class="alert alert-success" role="alert"><i class="fa fa-check-square-o"></i> <b>Tip:</b> If you want to receive the response to |
| a Message, make sure to always send a <code class="highlighter-rouge">correlation-id</code> with it.</div> |
| |
| <p>The response we would get from our coffee machine could look something like this:</p> |
| |
| <div class="language-json highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="p">{</span><span class="w"> |
| </span><span class="s2">"topic"</span><span class="p">:</span><span class="w"> </span><span class="s2">"org.eclipse.ditto/smartcoffee/things/live/messages/ask"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"headers"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"correlation-id"</span><span class="p">:</span><span class="w"> </span><span class="s2">"a-unique-string-for-this-message"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"auth-subjects"</span><span class="p">:</span><span class="w"> </span><span class="p">[</span><span class="s2">"ditto"</span><span class="p">,</span><span class="w"> </span><span class="s2">"nginx:ditto"</span><span class="p">],</span><span class="w"> |
| </span><span class="s2">"content-type"</span><span class="p">:</span><span class="w"> </span><span class="s2">"text/plain"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"version"</span><span class="p">:</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> |
| </span><span class="p">},</span><span class="w"> |
| </span><span class="s2">"path"</span><span class="p">:</span><span class="w"> </span><span class="s2">"/outbox/messages/ask"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"value"</span><span class="p">:</span><span class="w"> </span><span class="s2">"I do not know, since i am only a coffee machine."</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"status"</span><span class="p">:</span><span class="w"> </span><span class="mi">418</span><span class="w"> |
| </span><span class="p">}</span><span class="w"> |
| </span></code></pre></div></div> |
| |
| <p>The answer of the coffee machine has the same <code class="highlighter-rouge">topic</code> and <code class="highlighter-rouge">correlation-id</code> |
| as the original message. As we can see, the response does not only contain a |
| <code class="highlighter-rouge">value</code> but also the <code class="highlighter-rouge">status</code> of the response |
| which is based on the <a href="protocol-specification.html#status">HTTP status codes</a>. |
| Notice, that the <code class="highlighter-rouge">path</code> of the Message has changed from <em>inbox</em> to <em>outbox</em>, |
| which means the Message was sent <em>from</em> the Thing. |
| Ditto automatically added some headers that we can ignore for now.</p> |
| |
| <h3 id="receiving-a-message">Receiving a message</h3> |
| |
| <p>To be able to show how to receive Messages, we need to use one of the provided Ditto Protocol |
| bindings. We will use the WebSocket binding for now. With it, it is amazingly easy to |
| receive Messages sent <em>to</em> or <em>from</em> Things. You only need to fulfill these <em>three</em> simple requirements:</p> |
| |
| <ol> |
| <li>Having an open connection to the Ditto WebSocket</li> |
| <li>Having sent the <a href="httpapi-protocol-bindings-websocket.html#request-events">WebSocket binding specific message</a> |
| <code class="highlighter-rouge">START-SEND-MESSAGES</code> to the WebSocket to be able to retrieve Messages</li> |
| <li>You are allowed (<a href="basic-auth.html">authorized</a>) to receive Messages</li> |
| </ol> |
| |
| <p>If we have a user <em>ditto</em> that has <code class="highlighter-rouge">READ</code> permission on <em>smartcoffee</em>, we could receive Messages |
| for it using a local Ditto instance using simple JavaScript:</p> |
| |
| <div class="language-javascript highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1">// connect to the WebSocket</span> |
| <span class="kd">var</span> <span class="nx">websocket</span> <span class="o">=</span> <span class="k">new</span> <span class="nx">WebSocket</span><span class="p">(</span><span class="s1">'ws://ditto:ditto@localhost:8080/ws/1'</span><span class="p">);</span> |
| <span class="nx">websocket</span><span class="p">.</span><span class="nx">onmessage</span><span class="p">(</span><span class="kd">function</span><span class="p">(</span><span class="nx">message</span><span class="p">)</span> <span class="p">{</span> |
| <span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="s1">'received message data: '</span> <span class="o">+</span> <span class="nx">message</span><span class="p">.</span><span class="nx">data</span><span class="p">);</span> |
| <span class="p">});</span> |
| <span class="nx">websocket</span><span class="p">.</span><span class="nx">onopen</span><span class="p">(</span><span class="kd">function</span><span class="p">(</span><span class="nx">ws</span><span class="p">)</span> <span class="p">{</span> |
| <span class="c1">// register for receiving messages</span> |
| <span class="nx">ws</span><span class="p">.</span><span class="nx">send</span><span class="p">(</span><span class="s1">'START-SEND-MESSAGES'</span><span class="p">);</span> |
| <span class="p">});</span> |
| </code></pre></div></div> |
| |
| <p>If we would send the Message described in <a href="#sending-a-message-to-a-thing">Sending a Message to a Thing</a> |
| to the WebSocket, our JavaScript receiver would receive the following data:</p> |
| |
| <div class="language-json highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="p">{</span><span class="w"> |
| </span><span class="s2">"topic"</span><span class="p">:</span><span class="w"> </span><span class="s2">"org.eclipse.ditto/smartcoffee/things/live/messages/ask"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"headers"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"correlation-id"</span><span class="p">:</span><span class="w"> </span><span class="s2">"demo-6qaal9l"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"auth-subjects"</span><span class="p">:</span><span class="w"> </span><span class="p">[</span><span class="s2">"ditto"</span><span class="p">,</span><span class="w"> </span><span class="s2">"nginx:ditto"</span><span class="p">],</span><span class="w"> |
| </span><span class="s2">"content-type"</span><span class="p">:</span><span class="w"> </span><span class="s2">"text/plain"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"version"</span><span class="p">:</span><span class="w"> </span><span class="mi">1</span><span class="w"> |
| </span><span class="p">},</span><span class="w"> |
| </span><span class="s2">"path"</span><span class="p">:</span><span class="w"> </span><span class="s2">"/inbox/messages/ask"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"value"</span><span class="p">:</span><span class="w"> </span><span class="s2">"Hey, how are you?"</span><span class="w"> |
| </span><span class="p">}</span><span class="w"> |
| </span></code></pre></div></div> |
| |
| <p>The content is almost the same, except for automatically added headers that we can ignore. |
| In the next part you can learn how to respond to this Message.</p> |
| |
| <h3 id="responding-to-a-message">Responding to a Message</h3> |
| |
| <p>After <a href="#sending-a-message-to-a-thing">sending a Message to a Thing</a> and |
| <a href="#receiving-a-message">receiving the Message</a>, we are able to respond to the |
| Message we received. To do this, we can re-use the relevant Message contents |
| and change the type from incoming to outgoing. Here is a simple JavaScript |
| function that shows how you could respond to a given Message. It takes |
| the original Message, the response payload and status code and returns the |
| response Message you can send using a Ditto Protocol binding.</p> |
| |
| <div class="language-javascript highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nx">createTextResponse</span> <span class="o">=</span> <span class="kd">function</span><span class="p">(</span><span class="nx">originalMessage</span><span class="p">,</span> <span class="nx">payload</span><span class="p">,</span> <span class="nx">statusCode</span><span class="p">)</span> <span class="p">{</span> |
| <span class="kd">var</span> <span class="nx">topic</span> <span class="o">=</span> <span class="nx">originalMessage</span><span class="p">.</span><span class="nx">topic</span><span class="p">;</span> |
| <span class="kd">var</span> <span class="nx">correlationId</span> <span class="o">=</span> <span class="nx">originalMessage</span><span class="p">.</span><span class="nx">headers</span><span class="p">[</span><span class="s2">"correlation-id"</span><span class="p">];</span> |
| <span class="kd">var</span> <span class="nx">outboxPath</span> <span class="o">=</span> <span class="nx">originalMessage</span><span class="p">.</span><span class="nx">path</span><span class="p">.</span><span class="nx">replace</span><span class="p">(</span><span class="s2">"inbox"</span><span class="p">,</span> <span class="s2">"outbox"</span><span class="p">);</span> |
| |
| <span class="k">return</span> <span class="p">{</span> |
| <span class="s2">"topic"</span><span class="p">:</span> <span class="nx">topic</span><span class="p">,</span> |
| <span class="s2">"headers"</span><span class="p">:</span> <span class="p">{</span> |
| <span class="s2">"correlation-id"</span><span class="p">:</span> <span class="nx">correlationId</span><span class="p">,</span> |
| <span class="s2">"content-type"</span><span class="p">:</span> <span class="s2">"text/plain"</span> |
| <span class="p">},</span> |
| <span class="s2">"path"</span><span class="p">:</span> <span class="nx">outboxPath</span><span class="p">,</span> |
| <span class="s2">"status"</span><span class="p">:</span> <span class="nx">statusCode</span><span class="p">,</span> |
| <span class="s2">"value"</span><span class="p">:</span> <span class="nx">payload</span> |
| <span class="p">};</span> |
| <span class="p">};</span> |
| </code></pre></div></div> |
| |
| <p>With this method you could create a simple text response, and send it |
| using e.g. the WebSocket binding. Since the original Message and the response |
| Message have the same <code class="highlighter-rouge">correlation-id</code>, the issuer would receive your response:</p> |
| |
| <div class="language-json highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="p">{</span><span class="w"> |
| </span><span class="s2">"topic"</span><span class="p">:</span><span class="w"> </span><span class="s2">"org.eclipse.ditto/smartcoffee/things/live/messages/ask"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"headers"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"correlation-id"</span><span class="p">:</span><span class="w"> </span><span class="s2">"demo-6qaal9l"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"auth-subjects"</span><span class="p">:</span><span class="w"> </span><span class="p">[</span><span class="s2">"ditto"</span><span class="p">,</span><span class="w"> </span><span class="s2">"nginx:ditto"</span><span class="p">],</span><span class="w"> |
| </span><span class="s2">"content-type"</span><span class="p">:</span><span class="w"> </span><span class="s2">"text/plain"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"version"</span><span class="p">:</span><span class="w"> </span><span class="mi">1</span><span class="w"> |
| </span><span class="p">},</span><span class="w"> |
| </span><span class="s2">"path"</span><span class="p">:</span><span class="w"> </span><span class="s2">"/outbox/messages/ask"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"status"</span><span class="p">:</span><span class="w"> </span><span class="mi">418</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"value"</span><span class="p">:</span><span class="w"> </span><span class="s2">"I don't know since i am only a coffee machine"</span><span class="w"> |
| </span><span class="p">}</span><span class="w"> |
| </span></code></pre></div></div> |
| |
| <h3 id="talking-to-features">Talking to Features</h3> |
| |
| <p>When sending Messages to or from Features, almost everything stays the same as with |
| Things. The <code class="highlighter-rouge">path</code> in the JSON and an additional <code class="highlighter-rouge">feature-id</code> header are the only parts to change. |
| A Message to a Feature could therefore have the following JSON:</p> |
| |
| <div class="language-json highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="p">{</span><span class="w"> |
| </span><span class="s2">"topic"</span><span class="p">:</span><span class="w"> </span><span class="s2">"org.eclipse.ditto/smartcoffee/things/live/messages/heatUp"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"headers"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"content-type"</span><span class="p">:</span><span class="w"> </span><span class="s2">"text/plain"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"correlation-id"</span><span class="p">:</span><span class="w"> </span><span class="s2">"a-unique-string-for-this-message"</span><span class="w"> |
| </span><span class="p">},</span><span class="w"> |
| </span><span class="s2">"path"</span><span class="p">:</span><span class="w"> </span><span class="s2">"/features/water-tank/inbox/messages/heatUp"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"value"</span><span class="p">:</span><span class="w"> </span><span class="s2">"47"</span><span class="w"> |
| </span><span class="p">}</span><span class="w"> |
| </span></code></pre></div></div> |
| |
| <h3 id="sending-and-handling-claim-messages">Sending and handling Claim Messages</h3> |
| |
| <p>Claim Messages are handled like standard Messages with the difference that the message subject is <em>claim</em> and they |
| can only be sent to a Thing. As the purpose of claiming is to gain access to a Thing, you do not require <code class="highlighter-rouge">WRITE</code> |
| permission to send a Claim Message to a Thing. This however means for a receiver that incoming Claim Messages have to be |
| carefully verified before granting access to a Thing.</p> |
| |
| <p>A Claim Message to gain access to our smart coffee machine might look like this:</p> |
| |
| <div class="language-json highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="p">{</span><span class="w"> |
| </span><span class="s2">"topic"</span><span class="p">:</span><span class="w"> </span><span class="s2">"org.eclipse.ditto/smartcoffee/things/live/messages/claim"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"headers"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"content-type"</span><span class="p">:</span><span class="w"> </span><span class="s2">"text/plain"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"correlation-id"</span><span class="p">:</span><span class="w"> </span><span class="s2">"a-unique-string-for-this-claim-message"</span><span class="w"> |
| </span><span class="p">},</span><span class="w"> |
| </span><span class="s2">"path"</span><span class="p">:</span><span class="w"> </span><span class="s2">"/inbox/messages/claim"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"value"</span><span class="p">:</span><span class="w"> </span><span class="s2">"some-claiming-secret"</span><span class="w"> |
| </span><span class="p">}</span><span class="w"> |
| </span></code></pre></div></div> |
| |
| <p>After verifying the Message, in particular the correctness of the payload, the smart coffee machine could grant access |
| to the issuer by setting an additional permission and respond with a <em>status</em> of <code class="highlighter-rouge">200</code> or <code class="highlighter-rouge">204</code> (as you can see the |
| <code class="highlighter-rouge">path</code> changed to <em>outbox</em> and the <code class="highlighter-rouge">direction</code> is now <em>from</em>, same as above for the Thing Message):</p> |
| |
| <div class="language-json highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="p">{</span><span class="w"> |
| </span><span class="s2">"topic"</span><span class="p">:</span><span class="w"> </span><span class="s2">"org.eclipse.ditto/smartcoffee/things/live/messages/claim"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"headers"</span><span class="p">:</span><span class="w"> </span><span class="p">{</span><span class="w"> |
| </span><span class="s2">"content-type"</span><span class="p">:</span><span class="w"> </span><span class="s2">"text/plain"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"correlation-id"</span><span class="p">:</span><span class="w"> </span><span class="s2">"a-unique-string-for-this-claim-message"</span><span class="w"> |
| </span><span class="p">},</span><span class="w"> |
| </span><span class="s2">"path"</span><span class="p">:</span><span class="w"> </span><span class="s2">"/outbox/messages/claim"</span><span class="p">,</span><span class="w"> |
| </span><span class="s2">"status"</span><span class="p">:</span><span class="w"> </span><span class="mi">204</span><span class="w"> |
| </span><span class="p">}</span><span class="w"> |
| </span></code></pre></div></div> |
| |
| <p>In case the Claim Message does not contain the required information the smart coffee machine can reject the |
| claim request by <em>NOT</em> granting access and responding with a status different from <code class="highlighter-rouge">200</code> or <code class="highlighter-rouge">204</code>.</p> |
| |
| |
| <div class="tags"> |
| |
| <b>Tags: </b> |
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