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<a href="#"><span></span>→ Policies examples</a>
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<li><a href="protocol-examples-policies-retrievepolicyentry.html">Retrieve a single entry</a></li>
<li><a href="protocol-examples-policies-modifysubjects.html">Modify subjects</a></li>
<li><a href="protocol-examples-policies-retrievesubjects.html">Retrieve subjects</a></li>
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<li><a href="protocol-examples-policies-modifysubject.html">Modify a single subject</a></li>
<li><a href="protocol-examples-policies-retrievesubject.html">Retrieve a single subject</a></li>
<li><a href="protocol-examples-policies-modifyresources.html">Modify resources</a></li>
<li><a href="protocol-examples-policies-retrieveresources.html">Retrieve resources</a></li>
<li><a href="protocol-examples-policies-createresource.html">Create a single resource</a></li>
<li><a href="protocol-examples-policies-deleteresource.html">Delete a single resource</a></li>
<li><a href="protocol-examples-policies-modifyresource.html">Modify a single resource</a></li>
<li><a href="protocol-examples-policies-retrieveresource.html">Retrieve a single resource</a></li>
<li><a href="protocol-examples-policies-errorresponses.html">Error responses</a></li>
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<li><a href="protocol-examples-search.html">→ Search examples</a></li>
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<p>Messages <em>do not affect</em> the state of a digital twin or an actual device.
Therefore Ditto does not handle messages like <a href="basic-signals-command.html">commands</a>: there are no responses which are
produced by Ditto and no events which are emitted for messages.</p>
<div class="alert alert-info" role="alert"><i class="fa fa-info-circle"></i> <b>Note:</b> Ditto has no knowledge of the payload of messages but merely routes messages between
connected devices.</div>
<p>Messages provide the possibility to send something <strong>to</strong> or <strong>from</strong> an actual device using an arbitrary subject/topic.
They contain a custom payload with a custom content-type, so you can choose what content best
fits your solution.</p>
<p>Expressed differently, Messages</p>
<ul>
<li><strong>to</strong> devices are operations which should trigger an action on a device (e.g. with a subject <code class="highlighter-rouge">turnOff</code>),</li>
<li><strong>from</strong> devices are events/alarms which are emitted by devices (e.g. with a subject <code class="highlighter-rouge">smokeDetected</code>).</li>
</ul>
<figure><img class="docimage" src="images/pages/basic/ditto-messages.png" alt="Ditto Messages" style="max-width: 600px" /><figcaption>How Ditto acts as router for Messages</figcaption></figure>
<h2 id="characteristics-of-messages">Characteristics of Messages</h2>
<p>Eclipse Ditto is not a message broker and does not want to offer features a message broker does.</p>
<p>It can be seen as a message router which:</p>
<ul>
<li>accepts messages via 2 APIs (<a href="httpapi-messages.html">HTTP</a> and
<a href="protocol-specification-things-messages.html">Ditto Protocol</a>, e.g. via <a href="httpapi-protocol-bindings-websocket.html">WebSocket binding</a>)</li>
<li>checks for <strong>currently connected</strong> interested parties whether they may receive a specific Message
(performs <a href="basic-auth.html#authorization">authorization checks</a>)</li>
<li>
<p>routes the Message and reply Messages in between connected clients</p>
<div class="alert alert-danger" role="alert"><i class="fa fa-exclamation-circle"></i> <b>Warning:</b> Ditto offers no message retention. If a device isn’t connected when a Message should
be routed, it will never receive the Message.</div>
<div class="alert alert-danger" role="alert"><i class="fa fa-exclamation-circle"></i> <b>Warning:</b> Ditto makes no statement about Message QoS. Messages are routed <strong>at most once</strong>.</div>
<div class="alert alert-danger" role="alert"><i class="fa fa-exclamation-circle"></i> <b>Warning:</b> Ditto does deliver messages only in “fan out” style,
if the same credentials are connected twice, both connections will receive Messages if the credential is authorized
to read a Message.</div>
</li>
</ul>
<h2 id="elements">Elements</h2>
<p>Ditto messages always have to have at least this elements:</p>
<ul>
<li><strong>Direction</strong>: <em>to</em> / <em>from</em>,</li>
<li><strong>Thing ID</strong>: the ID of the <code class="highlighter-rouge">Thing</code> (actual device) which should receive/send the message and</li>
<li><strong>Subject</strong>: the custom subject/topic.</li>
</ul>
<p>Additionally they can contain more information:</p>
<ul>
<li><strong>Feature ID</strong>: if a message should be addressed to the <a href="basic-feature.html">Feature</a> of a Thing, this specifies
its ID.</li>
<li><strong>content-type</strong>: defines which content-type the optional payload has.</li>
<li><strong>correlation-id</strong>: Ditto can route message responses back to the issuer of a message. Therefore a correlation-id has
to be present in the message.</li>
</ul>
<h2 id="payload">Payload</h2>
<p>A message can optionally contain a payload. As Ditto does neither have to understand the message nor its payload, the
content-type and serialization is arbitrary.</p>
<h2 id="apis">APIs</h2>
<p>Messages can be sent via</p>
<ul>
<li>the <a href="httpapi-protocol-bindings-websocket.html">WebSocket API</a> as <a href="protocol-overview.html">Ditto Protocol</a> messages,</li>
<li>the <a href="httpapi-overview.html">HTTP API</a> either as “fire and forget” messages or, when expecting a response, in a
blocking way at the <a href="http-api-doc.html#/Messages">Messages HTTP API endpoint</a></li>
</ul>
<p>Messages can, however, be received only via the <a href="httpapi-protocol-bindings-websocket.html">WebSocket API</a> as
<a href="protocol-overview.html">Ditto Protocol</a> messages.</p>
<h2 id="receiving-messages">Receiving Messages</h2>
<p>To be able to receive Messages for a Thing, you need to have <code class="highlighter-rouge">READ</code> access on that Thing.
When a Message is sent to or from a Thing, <strong>every</strong> connected WebSocket with the correct
access rights will receive the Message. If there is more than one response, only the
first one will be routed back to the initial issuer of a Message.</p>
<div class="alert alert-info" role="alert"><i class="fa fa-info-circle"></i> <b>Note:</b> Currently, Messages can only be received using the
Ditto Protocol WebSocket binding</div>
<h2 id="sending-messages">Sending Messages</h2>
<p>If you want to send a Message to or from a Thing, you need <code class="highlighter-rouge">WRITE</code> permissions on that Thing.
Every WebSocket that is able to receive Messages for the Thing (<code class="highlighter-rouge">READ</code> permission), will receive your message.</p>
<h2 id="responding-to-messages">Responding to Messages</h2>
<p>Since WebSocket messages are stateless there is no <em>direct</em> response to a Message.<br />
For Ditto to be able to route the response of a Message back to the issuer, the
correlation-ids need to match. E.g. when the sender uses correlation-id <code class="highlighter-rouge">random-aa98s</code>,
any receiver can reply by using the same correlation-id <code class="highlighter-rouge">random-aa98s</code>.</p>
<div class="alert alert-info" role="alert"><i class="fa fa-info-circle"></i> <b>Note:</b> Currently, you can only respond to Messages using the
Ditto Protocol WebSocket binding</div>
<h2 id="permissions">Permissions</h2>
<h3 id="api-version-1">API version 1</h3>
<p>Permissions in API version 1 are simple for the Message API. If you want to receive Messages of a Thing,
you need <code class="highlighter-rouge">READ</code> access on the Thing. To be able to send Messages to or from a Thing
you need to have <code class="highlighter-rouge">WRITE</code> permissions.</p>
<p>There is one sole exception, which are <a href="#claim-messages">Claim Messages</a>. You do
not need access rights for sending them.</p>
<h3 id="api-version-2">API version 2</h3>
<p>Permissions in API version 2 can be more fine grained. In order to be able to receive all Messages of a Thing,
you need <code class="highlighter-rouge">READ</code> permission for the <code class="highlighter-rouge">message:/</code> resource in the used <a href="basic-policy.html#message">Policy</a>.<br />
There can however be specified that you may only receive specific Messages (with a defined subject), also
expressed via <a href="basic-policy.html#message">Policy entry</a>.<br />
The same applies for being able to send Messages, here a <code class="highlighter-rouge">WRITE</code> permission is required either globally for
all messages via the <code class="highlighter-rouge">message:/</code> resource or only for specific ones.</p>
<p>There is one sole exception, which are <a href="#claim-messages">Claim Messages</a>. You do
not need access rights for sending them.</p>
<h2 id="claim-messages">Claim Messages</h2>
<p>A Claim Message is used to gain access to a Thing. For this purpose a Claim Messages has two specifics:</p>
<ul>
<li>the predefined message subject is always <em>claim</em></li>
<li>you do not require <code class="highlighter-rouge">WRITE</code> permission to send a Claim Message</li>
</ul>
<p>Apart from that the Claim Message is handled like a standard Message. It is forwarded to all Ditto Protocol bindings
that registered for Claim Messages of the specific Thing. The decision whether to grant access (by setting permissions)
is completely up to the receiver of the Claim Message e.g. after verifying the payload of the Message.</p>
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