96 lines
3.2 KiB
ReStructuredText
96 lines
3.2 KiB
ReStructuredText
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Introduction
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============
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.. contents::
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Terminology
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-----------
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MQTT is a server-client protocol in which all clients *must* connect to a server
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in order to exchange information which have to go through the server.
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Network connection
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~~~~~~~~~~~~~~~~~~
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The standard defines network connection as the following.
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::
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A construct provided by the underlying transport protocol that is being
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used by MQTT.
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* It connects the Client to the Server.
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* It provides the means to send an ordered, lossless, stream of bytes in
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both directions.
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I believe that this definition was mainly provided as to not limit the network
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connection, specifically the network protocol, to TCP as it is possibly the most
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common network protocol used when implementing MQTT. You can emulate the bullet
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points under non-TCP network protocols e.g., UDP and implement MQTT on top of
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it.
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Session
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~~~~~~~
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::
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A stateful interaction between a Client and a Server. Some Sessions last
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only as long as the Network Connection, others can span multiple
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consecutive Network Connections between a Client and a Server.
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This allows for the possibility of attempting re-connection after it has been
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lost to preserve the same session.
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Application message
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~~~~~~~~~~~~~~~~~~~
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The information sent between clients through servers is called the application
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message which is carried by the MQTT protocol across the network. It contains
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payload data, a Quality of Service (QoS), a collection of properties and a topic
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name.
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Subscription
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^^^^^^^^^^^^
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A subscription comprises a topic filter and a maximum QoS. A subscription is
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associated with a single session. A session can contain more than one
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subscription. Each subscription within a session has a different topic filter.
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Shared subscription can be associated with more than one session. I assume this
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is used for the case of multiple servers? I don't see the need to have multiple
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sessions to the same server but it is allowed by the standard.
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Wildcard subscription is a subscription with a topic filter containing one or
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more wildcard characters. This allows the subscription to match more than one
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topic name. Basically ``*`` in regular expression.
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write UML diagram about user <-- session 0..* -- 1..1 subscription
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Topic name
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^^^^^^^^^^
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The label attached to an application message which is matched against the
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subscriptions known to the server. This is up to the application to use it
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efficiently. In the case of media synchronization, would you rather use do
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central topics e.g., ``/media/filename`` and users push to that topic or would
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you do per-user topics ``/renken/media/filename`` and only ``renken`` can
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publish to that topic. Similar ideas to that come to mind I guess.
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Topic filter
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^^^^^^^^^^^^
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An expression contained in a subscription to indicate an interest in one or more
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topics. A topic filter can include wildcard characters.
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Client
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~~~~~~
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As discussed before, in MQTT, the client can only communicate with a server and
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not with other clients. Note that the client is not limited to a single server.
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It also not limited to a fixed number of application messages published or
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requested.
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The server acts as an intermediary between clients which publish application
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messages and clients which have made subscriptions.
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.. uml:: client.uml
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