DDS learning notes of opendds

DDS:data distribution service Data distribution services

    DDS Information publishing middleware is a kind of portable 、 Middleware technology that can provide real-time information transmission .DDS Middleware is a software layer , Slave operating system 、 The application is abstracted from the details of network transmission and underlying data format . The same concept and api Available to different programming languages , Make the application in different operating systems 、 Exchange information between codification language and processing architecture . The underlying details include the data transmission format 、 Find out 、 Connect 、 Reliability and 、 agreement 、Qos Policies are managed by middleware .

    DDS Adopt release / Subscription architecture , Data centric , Provide a wealth of Qos Service quality strategy .

     The data centric core is DDS Know the data in the whole integrated network and the sharing of control data . Using traditional message centric middleware , The problem of sending messages must be considered . Using data centric middleware , Just think about how to share data , When to share data , Data values can be shared between .DDS Directly realize controlled for users 、 Managed 、 Secure data sharing , Instead of managing all the responsible logic in the application code . Among the numerous communications ,DDS Is the only data centric standard , For the Internet of things .

    DDS Offer based on Qos Controlled shared data . This should be achieved by publishing and subscribing to topics , Topics are identified by their topic names . Subscribers can specify time and content filters , In this way, you can obtain a subset of the published data of the topic . Different DDS Fields are completely non overlapping . stay DDS There is no sharing between domains .

   DDS Global data space

       DDS All locally stored data is called global data space . For applications , The global data space looks like passing through api To access memory . When you use it , Just like using local storage . in fact ,DDS Send a message to update the corresponding stored value of the remote node . such , When use , Like local storage .

         To make a long story short ,DDS Make the application provide a virtual global data pool concept when data access is required , In fact, there is no global device that stores all data . Each application only stores the data it needs locally , When any application in the whole network needs them , Provided to the corresponding application . All data distribution work is done by DDS take the trouble for . Global data space is a virtual concept , Is actually a locally stored collection . Every application , Can be written in almost any language , Run on any system , You can read the data displayed in the best data format on the local memory . The global data space can be used in embedded systems 、 Data sharing between mobile and Cloud Applications , Use any transmission mode , Regardless of language or system , And the delay is very low .

DDS Release / Subscription model

        DDS Data centric Publishing - The subscription model creates a virtual shared global data space for all distributed nodes . In this model, distributed nodes are distributed in the network

Transmit data by publishing or subscribing on , Nodes can be publishers or subscribers , Or both . Data objects in the network are identified by topics , Distributed nodes publish or subscribe to interesting topic information in the global data space . Each node has no master-slave relationship in logic , Points are equal to each other , The communication mode can be point-to-point , Point to many , Many to many , stay Qos Establish connection under the control of policy , Automatically discover and configure network parameters .

        DDS The specification has two layers ： Namely DLRL layer （ Data local reconstruction ） and DCPS layer （ Data centric publishing and subscription ）.DCPS Layer is DDS Core and foundation of , It provides basic communication services ;DLRL Layers will DCPS The services provided by the layer are abstracted , stay DLRL The layer establishes a mapping relationship with the underlying services .

 

DDS The basic structure is the domain domain, It is uniquely identified by the domain number .domain Bind applications together to communicate , Only communicating entities in the same domain can communicate , There is no logical relationship between entities in different domains .

        DDS All the members in the are entity,DDS Any two entity All communications must be in the same domain Interact within .domain Internal domainparticipant It's the entry point of the service , whatever DDS Applications need to get domain participant, And then through domain participant Get other services , Such as publisher,subscriber,topic etc. .

        domain Participant As an entry point for data distribution services , Contains several publishers , Subscribers and registration topics , Responsible for creating , Delete and manage these entities .

        publisher Role as publisher , At least one DataWriter, And responsible for creating , Delete and manage datawriter. Again ,subcri As a subscriber , With at least one datareader relation , And be responsible for publishing data , The data publisher calls datawriter Of write Function to publish data , But the data will not be sent out immediately , The actual message is generated through publisher and Qos Integrated control .datareader Responsible for subscribing to data , The subscription mode can be asynchronous （listener）, Synchronous mode and non blocking mode .

        topic yes datawriter and datareader The subject agreed upon when communicating with each other , Every datawriter/datareader Must be bound to a topic , Communicating with each other datawriter and datareader The subject data type must be the same between ,Qos Must match . in addition ,topic There must be a definite data type .

 
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