The Data Distribution Service Tutorial
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This presentation introduces the key concepts behind DDS along with a small library that encapsulates the common programming idioms.
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The Data Distribution Service Tutorial
- 1. Angelo Corsaro, PhD Chief Technology Officer angelo.corsaro@prismtech.com Data Distribution Service The Tutorial
- 2. DDS is a standard technology for ubiquitous, interoperable, secure, platform independent, and real-time data sharing across network connected devices DDS in131 Characters
- 4. Standard
- 5. Adoption
- 6. CopyrightPrismTech,2015 John Deere’s machinery uses DDS for internal communication as well as for vehicle-to-vehicle coordination Telemetry Data is constantly sent to a Cloud for preventive maintenance In this use case VORTEX enables fog computing as well as cloud computing John Deere Autonomous FarmingSmart farming
- 7. smart cities
- 8. Smart Lightbulbs 96Kbytes Memory Connected Medical Devices
- 9. CopyrightPrismTech,2015 Smart grids and power generation
- 10. European air traffic control DDS is the standard recommended by Eurocontrol/Eurocae for Pan-European flight data sharing
- 12. CopyrightPrismTech,2015 DDS provides a Distributed Data Space abstraction where applications can autonomously and asynchronously read and write data enjoying spatial and temporal decoupling Its built-in dynamic discovery isolates applications from network topology and connectivity details DDS’ Data Space is completely decentralised High Level Abstraction DDS Global Data Space ... Data Writer Data Writer Data Writer Data Reader Data Reader Data Reader Data Reader Data Writer TopicA QoS TopicB QoS TopicC QoS TopicD QoS
- 13. Conceptual Model DDS Global Data Space ... Data Writer Data Writer Data Writer Data Reader Data Reader Data Reader Data Reader Data Writer TopicA QoS TopicB QoS TopicC QoS TopicD QoS
- 14. Conceptual Model Actual Implementation Data Writer Data Writer Data Writer Data Reader Data Reader Data Reader Data Writer TopicA QoS TopicB QoS TopicC QoS TopicD QoS TopicD QoS TopicD QoS TopicA QoS DDS Global Data Space ... Data Writer Data Writer Data Writer Data Reader Data Reader Data Reader Data Reader Data Writer TopicA QoS TopicB QoS TopicC QoS TopicD QoS
- 15. The communication between the DataWriter and matching DataReaders can be peer-‐to-‐ peer exploiting UDP/IP (Unicast and Multicast)or TCP/IP Data Writer Data Writer Data Writer Data Reader Data Reader Data Reader Data Writer TopicA QoS TopicB QoS TopicC QoS TopicD QoS TopicD QoS TopicD QoS TopicA QoS The communication between the DataWriter and matching DataReaders can be “brokered” but still exploiting UDP/IP (Unicast and Multicast)or TCP/IP
- 16. CopyrightPrismTech,2014 DDS is independent from the - Programming language, - Operating System - HW architecture Platform Independent
- 17. Decomposing DDS
- 19. Copyright2013,PrismTech–AllRightsReserved. Domain • DDS data lives within a domain • A domain is identified with a non negative integer, such as 1, 3, 31 • The number 0 identifies the default domain • A domain represent an impassable communication plane DDS Domain
- 20. Copyright2013,PrismTech–AllRightsReserved. Partitions • Partitions are the mechanism provided by DDS to organise information within a domain • Access to partitions is controlled through QoS Policies • Partitions are defined as strings: • “system:telemetry” • “system:log” • “data:row-‐2:col-‐3” • Partitions addressed by name or regular expressions: • ”system:telemetry” • “data:row-‐2:col-‐*” Partitions
- 22. Copyright2013,PrismTech–AllRightsReserved. Topic • A Topic defines a domain-wide information’s class • A Topic is defined by means of a (name, type, qos) tuple, where • name: identifies the topic within the domain • type: is the programming language type associated with the topic.Types are extensible and evolvable • qos: is a collection of policies that express the non- functional properties of this topic, e.g. reliability, persistence, etc. Topic Type Name QoS
- 23. Copyright2013,PrismTech–AllRightsReserved. Topic and Instances • As explained in the previous slide a topic defines a class/type of information • Topics can be defined as Singleton or can have multiple Instances • Topic Instances are identified by means of the topic key • A Topic Key is identified by a tuple of attributes -- like in databases • Remarks: • A Singleton topic has a single domain-wide instance • A “regular” Topic can have as many instances as the number of different key values, e.g., if the key is an 8-bit character then the topic can have 256 different instances
- 24. Example
- 25. Copyright2013,PrismTech–AllRightsReserved. Active Floor • Assume we are building an active floor • This active floor is made by a matrix of pressure sensors used to detects position, and indirectly movement • This information is then used by the application that uses the active floor for positioning or entertainment Cell: (i,j)
- 26. Copyright2013,PrismTech–AllRightsReserved. Active Floor • The generic active cell can be modeled with a topic that has an instance for each value of (i,j). The topic type can be defined as: • Each cell is now distinguishable and associated with a topic instance Cell: (i,j) struct TCell { short row; short column; float pressure; // in kPa }; #pragma keylist TCell row column
- 27. Copyright2013,PrismTech–AllRightsReserved. Active Floor • How can we know when something is on the cell? • The detection can be based on the difference between the atmospheric pressure, say P0, and the pressure sensed by the cell • We can model this as a Singleton Topic ReferencePressure defined by the type: Cell: (i,j) struct TReferencePressure { float pressure; // in kPa float precision; }; #pragma keylist TReferencePressure
- 28. Copyright2013,PrismTech–AllRightsReserved. Active Floor • Each sensor has associated a topic instance identified by the (row,column) coordinate -- the instance key • Each instance produces a stream of pressure values that in DDS terms are called samples 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time struct TCell { short row; short column; float pressure; // in kPa }; #pragma keylist Cell row column
- 29. Going 3D
- 30. Copyright2013,PrismTech–AllRightsReserved. Active Floors on a Building • Let’s assume now that we have a building that uses active floors to detect presence and movement • How can we organize the out data model? 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time
- 31. Copyright2013,PrismTech–AllRightsReserved. Active Floors on a Building • The first thing to do is to introduce the third dimension to our cell: struct TCell { short row; short column; short floor; float pressure; // in kPa }; #pragma keylist TCell row column floor 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time
- 32. Copyright2013,PrismTech–AllRightsReserved. Active Floors on a Building • As we move from a single floor to a building we need to add some more structure to our data • We can now use: • A Domain for each Building • A Partition for each Floor • A Partition for reference value, i.e. Partition • A Partition for the configuration information, e.g. how many floors, how many rows/cols per floor 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time
- 33. Copyright2013,PrismTech–AllRightsReserved. Active Floors on a Building • Thus the resulting structure is: • Floor Partitions: • “building:f-‐1” • “building:f-‐2” • ... • ReferenceValues Partition: • “building:refvals” • Configuration Partition: • “building:config” 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time
- 35. Copyright2013,PrismTech–AllRightsReserved. DataWriter • A DataWriter (DW) is a strongly typed entity used to produce samples for one or more instances of a Topic, with a given QoS • Conceptually, the DataWriter QoS should be the same as the Topic QoS or more stringent • However, DDS does enforce a specific relationship between the Topic and DataWriter QoS DW Type Topic QoS
- 36. Copyright2013,PrismTech–AllRightsReserved. DataWriter • The DataWriter controls the life-cycle of Topic Instances and allows to: • Define a new topic instance • Write samples for a topic instance • Dispose the topic instance DW Type Topic QoS
- 38. Copyright2013,PrismTech–AllRightsReserved. DataReader • A DataReader (DR) is a strongly typed entity used to access and/or consume samples for a Topic, with a given QoS • Conceptually, the DataReader QoS should be the same as the Topic QoS or less stringent • However, DDS does enforce a specific relationship between the Topic and DataReader QoS DR Type Topic QoS
- 39. Copyright2013,PrismTech–AllRightsReserved. DataReader • Depending on its QoS a DataReader may provide access to: • last sample • last n samples • all samples produced since the DataReader was created 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time
- 40. Copyright2013,PrismTech–AllRightsReserved. DataReader • Depending on its QoS a DataReader may provide access to: • last sample • last n samples • all samples produced since the DataReader was created n=3 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time
- 41. Copyright2013,PrismTech–AllRightsReserved. DataReader • Depending on its QoS a DataReader may provide access to: • last sample • last n samples • all samples produced since the DataReader was created n=3 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time
- 42. Copyright2013,PrismTech–AllRightsReserved. DataReader • Depending on its QoS a DataReader may provide access to: • last sample • last n samples • all samples produced since the DataReader was created n=3 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time
- 43. Copyright2013,PrismTech–AllRightsReserved. DataReader • Depending on its QoS a DataReader may provide access to: • last sample • last n samples • all samples produced since the DataReader was created n=3 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time
- 44. Copyright2013,PrismTech–AllRightsReserved. DataReader • Depending on its QoS a DataReader may provide access to: • last sample • last n samples • all samples produced since the DataReader was created 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time n=3
- 45. Copyright2013,PrismTech–AllRightsReserved. DataReader • Depending on its QoS a DataReader may provide access to: • last sample • last n samples • all samples produced since the DataReader was created 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time n=3
- 46. Copyright2013,PrismTech–AllRightsReserved. DataReader • Depending on its QoS a DataReader may provide access to: • last sample • last n samples • all samples produced since the DataReader was created 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time n=3
- 47. Copyright2013,PrismTech–AllRightsReserved. DataReader • Samples are stored in the DataReader Cache • Samples can be read or taken from the cache • Samples taken are evicted from the cache • Samples read remain in the cache and are simply market as read • The cache content can be selected based on content or state. More on this later... 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time
- 49. CopyrightPrismTech,2015 Anatomy of a DDS Application
- 50. Copyright2013,PrismTech–AllRightsReserved. DomainParticipant • The DomainParticipant is the programming entity that gives access to a DDS domain • A DomainParticipant is created as follows: // ISO C++ DDS API int domain_id = 18; auto dp = DomainParticipant(domain_id); // Java 5 DDS API int domain_id = 18; DomainParticipantFactory dpf = DomainParticipantFactory.getInstance(env) DomainParticipant dp = dpf.createParticipant(domainId);
- 51. Copyright2013,PrismTech–AllRightsReserved. Topic • Given a DomainParticipant we can define (or discover) Topics within a domain.This can be done as follows: • As this declaration does not explicitly provide QoS for the Topic, the default QoS will be used // ISO C++ DDS API auto topic = Topic<TCell>(dp, “Cell”); // Java 5 DDS API Topic<TCell> topic = dp.createTopic(“Cell”, TCell.class);
- 52. Copyright2013,PrismTech–AllRightsReserved. Publisher/Subscriber • Publisher/Subscriber, through the Partitions they are associated with, define the scope of a write/read operation • Partitions association is done through the Partition QoS Policy • This association can be defined as a list of string as well as a list of regular expressions
- 53. Copyright2013,PrismTech–AllRightsReserved. Publisher/Subscriber • Definition of a Publisher/Subscriber in the default partition: • Definition of a Publisher/Subscriber with Partition settings // ISO C++ DDS API auto pub = Publisher(dp); // ISO C++ DDS API auto pub_qos = dp.default_publisher_qos() << Partition(”af:telemetry”); auto pub = Publisher(dp, pub_qos); // ISO C++ DDS API auto sub = Subscriber(dp); // ISO C++ DDS API auto sub_qos = dp.default_subscriber_qos() << Partition(”af:telemetry”); auto sub = Subscriber(dp, sub_qos);
- 54. Copyright2013,PrismTech–AllRightsReserved. Publisher/Subscriber • Definition of a Publisher/Subscriber in the default partition: • Definition of a Publisher/Subscriber with Partition settings // Java 5 DDS API Publisher pub = dp.createPublisher(); // Java 5 DDS API PublisherQos pubQoS = dp.getDefaultPublisherQos() .with(pf.Partition(”af:telemetry”)); Publisher pub = dp.createPublisher(pubQoS); // Java 5 DDS API Subscriber sub = dp.createSubscriber(); // Java 5 DDS API SubscriberQos subQoS = dp.getDefaultSubscriberQos() .with(pf.Partition(”af:telemetry”)); Subscriber sub = dp.createSubscriber(subQoS);
- 55. Copyright2013,PrismTech–AllRightsReserved. DataWriter • A DataWriter with default QoS can be declared as follows: // ISO C++ DDS API auto dw = DataWriter<TCell>(pub, topic); // Write the cell c(1,1) using `writer` TCell c11 = {1, 1, 15}; dw.write(c11); // Write the cell c(1,2) using the `operator <<` TCell c12 = {1, 2, 5}; dw << c12; // Java 5 DDS API DataWriter<TCell> dw = pub.createDataWriter<TCell>(topic); TCell c11 = new TCell(1, 2, 15); dw.write(c11);
- 56. Copyright2013,PrismTech–AllRightsReserved. DataReader • A DataReader with default QoS can be declared as follows: // ISO C++ DDS API auto dr = DataReader<TCell>(sub, topic); // Read Samples auto samples = dr.read(); // Do something with it std::for_each(samples.begin(), samples.end(), do_something); // Java 5 DDS API DataReader<TCell> dr = sub.createDataReader<TCell>(topic);
- 57. Data Selection
- 58. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Reading Samples • Samples can be read from the Data Reader History Cache • The action of reading a sample is non-destructive. Samples are not removed from the cache DataReader Cache DataReader ... DataReader Cache DataReader ... read
- 59. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Taking Samples • Samples can be taken from the Data Reader History Cache • The action of taking a sample is destructive. Samples are removed from the cache DataReader Cache DataReader ... DataReader Cache DataReader ... take
- 60. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Sample Selectors • DDS provides some very flexible mechanisms for selecting the samples to be read/take: • Content • Status • These mechanisms are composable
- 62. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Filters and Queries • DDS Filters allow to control what gets into a DataReader cache • DDS Queries allow to control what gets out of a DataReader cache • Filters are defined by means of ContentFilteredTopics • Queries operate in conjunction with read operations • Filters and Queries are expressed as SQL where clauses DataReader Cache DataReader ... ... ... ... Filter Query Application
- 63. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Filters // == ISO C++ DDS API // Create a Topic auto topic = Topic<ShapeType>(dp, “Circle”); // Define filter expression and parameters auto filter = Filter(“x < 100 AND y < 200”); // Define content filtered topic auto cftopic = ContentFilteredTopic<ShapeType>(“CFCircle”, topic, filter) // Create a DataReader for the content-filtered Topic auto dr = DataReader<ShapeType>(sub,cftopic) struct ShapeType { string color; long x; long y; long shapesize; }; #pragma keylist ShapeType color
- 64. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Filters // == Java 5DDS API == final PolicyFactory pf = runtime.policyFactory(); final DataReaderQos drqos = sub.getDefaultDataReaderQos() .withPolicy ( pf.ContentFilter() .withFilter( new JavaScriptFilter<ShapeType>("data.x > data.y")) ); final DataReader<ShapeType> dr = sub.createDataReader(shape, drqos); struct ShapeType { string color; long x; long y; long shapesize; }; #pragma keylist ShapeType color
- 65. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Query // == ISO C++ DDS API == // Define filter expression and parameters auto dr = DataReader<ShapeType>(sub, topic) val query = Query(dr, “x < 100 AND y < 200”); dr.select() .content(query) .read(); struct ShapeType { string color; long x; long y; long shapesize; }; #pragma keylist ShapeType color
- 66. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Query // == Java 5 DDS API == Filter<ShapeType> filter = new JavaScriptFilter<ShapeType>("data.x > data.y")) dr.select() .content(filter) .read(); struct ShapeType { string color; long x; long y; long shapesize; }; #pragma keylist ShapeType color
- 67. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Instances • DDS provides a very efficient way of reading data belonging to a specific Topic Instance • Obviously, one could use queries to match the key’s value, but this is not as efficient as the special purpose instance selector // == ISO C++ DDS API == auto handle = dr.lookup_instance(ShapeType(“RED”, 0, 0, 0)); auto data = dr.select() .instance(handle) .read();
- 69. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Sample, Instance, andView State • The samples included in the DataReader cache have associated some meta-information which, among other things, describes the status of the sample and its associated stream/instance • The Sample State (READ, NOT_READ) allows to distinguish between new samples and samples that have already been read • The View State (NEW, NOT_NEW) allows to distinguish a new instance from an existing one • The Instance State (ALIVE, NOT_ALIVE_DISPOSED, NOT_ALIVE_NO_WRITERS) allows to track the life-cycle transitions of the instance to which a sample belongs
- 70. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. State Selector in Action // == ISO C++ DDS API == // Read only new samples auto data = dr.read() // Read any samples from live instances auto data = dr.select() .state(DataState::any_data()) .read();
- 71. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. State Selector in Action // == Java 5 DDS API == // Read only new samples Iterator<Sample<ShapeType>> data = dr.read() // Read any samples from live instances Iterator<Sample<ShapeType>> data = dr.select() .dataState(sub.createDataState().withAnySampleState()) .read();
- 72. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. // == ISO C++ DDS API == auto data = dr.select() .content(query) .state(data_state) .instance(handle) .read(); Putting all Together • Selectors can be composed in a flexible and expressive manner
- 73. Application / DDS Interaction Models
- 74. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Interaction Models Polling •The application proactively polls for data availability as well as special events, such as a deadline being missed, etc. Notice that all DDS API calls, exclusion made for wait operations, are non-blocking Synchronous Notification •The application synchronously waits for some conditions to be verified, e.g., data availability, instance lifecycle change, etc. Asynchronous Notification •The application registers the interest to be asynchronously notified when specific condition are satisfied, e.g. data available, a publication matched, etc.
- 75. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Synchronous Notifications • DDS provides a mechanism known as WaitSet to synchronously wait for a condition • Condition can predicate on: • communication statuses • data availability • data availability with specific content • user-triggered conditions
- 76. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. WaitSet // == Java 5 DDS API == // Create the waitset WaitSet ws = runtime.createWaitSet(); Subscriber.DataState ds = sub.createDataState(); // Create the condition Condition c = dr.createReadCondition( ds.withAnyViewState() .with(InstanceState.ALIVE) .with(SampleState.NOT_READ)); // Attach the condition ws.attachCondition(c); // Wait for the condition ws.wait();
- 77. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Asynchronous Notifications • DDS provides a mechanism known as Listeners for asynchronous notification of a given condition • Listener interest can predicate on: • communication statuses • data availability
- 78. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Listener Declaration // == ISO C++ DDS API == class ShapeListener : public dds::sub::NoOpDataReaderListener<ShapeType> { public: ShapeListener() {} virtual void on_data_available(dds::sub::DataReader<ShapeType>& dr) { auto samples = dr.read(); std::for_each(samples.begin(), samples.end(), [](const dds::sub::Sample<ShapeType>& sample) { if (sample.info().valid()) // Check if sample contains valid data std::cout << sample.data() << std::endl; }); } virtual void on_liveliness_changed(dds::sub::DataReader<ShapeType>& the_reader, const dds::core::status::LivelinessChangedStatus& status) { std::cout << ">> Liveliness Changed! " << std::endl; } };
- 79. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Listener Registration // == ISO C++ DDS API == auto l = new ShapeListener(); // Create a “nothing” status mask StatusMask mask = StatusMask::none(); // Add the statuses we are interested in. mask << StatusMask::data_available() << StatusMask::liveliness_changed() << StatusMask::liveliness_lost(); // Register the listener with the associated mask dr.listener(l, mask);
- 81. CopyrightPrismTech,2015 For data to flow from a DataWriter (DW) to one or many DataReader (DR) a few conditions have to apply: The DR and DW domain participants have to be in the same domain The partition expression of the DR’s Subscriber and the DW’s Publisher should match (in terms of regular expression match) The QoS Policies offered by the DW should exceed or match those requested by the DR Quality of Service Domain Participant DURABILITY OWENERSHIP DEADLINE LATENCY BUDGET LIVELINESS RELIABILITY DEST. ORDER Publisher DataWriter PARTITION DataReader Subscriber Domain Participant offered QoS Topic writes reads Domain Id joins joins produces-in consumes-from RxO QoS Policies requested QoS
- 82. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. QoS DSL • The ISO C++ and Java 5 APIs provide DSL for dealing with QoS Policies configuration • The DSL uses language specific idioms, such as fluid interfaces, as well as specific features of the languages • Policies as well as Entity QoS are immutable — this allows for better safety and object sharing • Policies are treated as algebraic data types and the DSL provide constructors of each of the cases • A QoS Provider can now be used to retrieve QoS settings from external sources, e.g. a file, an HTTP server, DDS durability
- 83. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. C++ QoS Policy DSL // == ISO C++ DDS API == DataWriterQos dwqos = pub.default_datawriter_qos() << History.KeepLast(10) << Durability.Transient();
- 84. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Data Delivery
- 85. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Reliability QoS Policy The Reliability Policy controls the level of guarantee offered by the DDS in delivering data to subscribers •Reliable. In steady-state, and with no data writer crashes, guarantees that all samples in the DataWriter history will eventually be delivered to all the DataReader •Best Effort. Indicates that it is acceptable not to retry propagation of samples QoS Policy Applicability RxO Modifiable RELIABILITY T, DR, DW Y N
- 89. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. History QoS Policy The DataWriter HISTORY QoS Policy controls the amount of data that can be made available to late joining DataReaders under TRANSIENT_LOCAL Durability The DataReader HISTORY QoS Policy controls how many samples will be kept on the reader cache •Keep Last. DDS will keep the most recent “depth” samples of each instance of data identified by its key •Keep All. The DDS keep all the samples of each instance of data identified by its key -- up to reaching some configurable resource limits QoS Policy Applicability RxO Modifiable HISTORY T, DR, DW N N 0 1 2 3 0 1 2 3 4 Pressure time Pressure time Pressure time KeepLast(3) KeepLast(1) KeepAll
- 90. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Durability QoS Policy The DURABILITY QoS controls the data availability w.r.t. late joiners, specifically the DDS provides the following variants: • Volatile. No need to keep data instances for late joining data readers • Transient Local. Data instance availability for late joining data reader is tied to the data writer availability • Transient. Data instance availability outlives the data writer • Persistent. Data instance availability outlives system restarts QoS Policy Applicability RxO Modifiable DURABILITY T, DR, DW Y N
- 91. Copyright2013,PrismTech–AllRightsReserved. Data Writer Data Reader TopicA QoS Volatile Durability • No Time Decoupling • Readers get only data produced after they joined the Global Data Space 1
- 92. Copyright2013,PrismTech–AllRightsReserved. Volatile Durability • No Time Decoupling • Readers get only data produced after they joined the Global Data Space Data Writer Data Reader TopicA QoS Data Reader 1 Late Joiner
- 93. Copyright2013,PrismTech–AllRightsReserved. Volatile Durability • No Time Decoupling • Readers get only data produced after they joined the Global Data Space Data Writer Data Reader TopicA QoS Data Reader 1 Late Joiner 22
- 94. Copyright2013,PrismTech–AllRightsReserved. Data Writer Data Reader TopicA QoS Transient Local Durability • Some Time Decoupling • Data availability is tied to the availability of the data writer and the history settings 11
- 95. Copyright2013,PrismTech–AllRightsReserved. Transient Local Durability • Some Time Decoupling • Data availability is tied to the availability of the data writer and the history settings Data Writer Data Reader TopicA QoS Data Reader 1 Late Joiner 11
- 96. Copyright2013,PrismTech–AllRightsReserved. Data Writer Data Reader TopicA QoS Data Reader 2 Transient-Local Durability • Some Time Decoupling • Data availability is tied to the availability of the data writer and the history settings 1 2 1 12
- 97. Copyright2013,PrismTech–AllRightsReserved. Data Writer Data Reader TopicA QoS Transient Durability • Time Decoupling • Data availability is tied to the availability of the durability service 1 1
- 98. Copyright2013,PrismTech–AllRightsReserved. Data Writer Data Reader TopicA QoS Data Reader 1 Transient Durability • Time Decoupling • Data availability is tied to the availability of the durability service 1 Late Joiner 1
- 99. Copyright2013,PrismTech–AllRightsReserved. Data Writer Data Reader TopicA QoS Data Reader 2 Transient Durability • Time Decoupling • Data availability is tied to the availability of the durability service 1 2 1 1 2
- 100. Copyright2013,PrismTech–AllRightsReserved. Data Reader TopicA QoS Data Reader Transient Durability • Time Decoupling • Data availability is tied to the availability of the durability service 1 1 2 2 12
- 101. Copyright2013,PrismTech–AllRightsReserved. Transient Durability • Time Decoupling • Data availability is tied to the availability of the durability service 1 Late Joiner 1 Data Reader TopicA QoS Data Reader Data Reader 2 2 12 12
- 102. Idioms
- 103. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Soft State • In distributed systems you often need to model soft-state -- a state that is periodically updated • Examples are the reading of a sensor (e.g.Temperature Sensor), the position of a vehicle, etc. • The QoS combination to model Soft-State is the following: Reliability => BestEffort Durability => Volatile History => KeepLast(n) [with n = 1 in most of the cases] Deadline => updatePeriod LatencyBudget => updatePeriod/3 [rule of thumb] DestinationOrder => SourceTimestamp [if multiple writers per instance]
- 104. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Hard State • In distributed systems you often need to model hard-state -- a state that is sporadically updated and that often has temporal persistence requirements • Examples are system configuration, a price estimate, etc. • The QoS combination to model Hard-State is the following: Reliability => Reliable Durability => Transient | Persistent History => KeepLast(n) [with n = 1 in most of the cases] DestinationOrder => SourceTimestamp [if multiple writers per instance]
- 105. Copyright2013,PrismTech–AllRightsReserved.Copyright2013,PrismTech–AllRightsReserved. Events • In distributed systems you often need to model events -- the occurrence of something noteworthy for our system • Examples are a collision alert, the temperature beyond a given threshold, etc. • The QoS combination to model Events is the following: Reliability => Reliable Durability => any [depends on system requirements] History => KeepAll [on both DataWriter and DataReader!] DestinationOrder => SourceTimestamp
- 106. Idiomatic DDS
- 107. CopyrightPrismTech,2015 State Writer template <typename T> class StateWriter { public: StateWriter(const std::string& name, int retain = 1, bool persistent = false, int domainId = 0); void write(const T& data); };
- 108. CopyrightPrismTech,2015 State Reader template <typename T> class StateReader { public: StateReader(const std::string& name, bool persistent = false, int domainId = 0); Maybe<T> get(const T& key); dds::sub::LoanedSamples<T> get(); std::function<void(StateReader<T>&)> on_change() const; void on_change(std::function<void(StateReader<T>)> f); };
- 109. CopyrightPrismTech,2015 StateVector Reader template <typename T> class StateVectorReader { public: StateVectorReader(const std::string& name, int retain, bool persistent = false, int domainId = 0); Maybe<dds::sub::LoanedSamples<T>> get(const T& key); dds::sub::LoanedSamples<T> get();
- 110. CopyrightPrismTech,2015 StateStream template <typename T> class StateStream { public: StateStream(const std::string& name, std::function<void(const T&)> fun, int retain = 1, bool persistent = false, int domainId = 0); : ~StateStream(); };
- 111. CopyrightPrismTech,2015 Queue template <typename T> class Queue { public: Queue(const std::string name, int retain = 0, bool persistent = false, int domainId = 0) void enqueue(const T& data); T peek(const dds::core::Duration timeout = dds::core::Duration::infinite()); T dequeue(const dds::core::Duration timeout = dds::core::Duration::infinite()); void ack(const T& data); };
- 112. CopyrightPrismTech,2015 Enqueue template <typename T> class Enqueue { public: Enqueue(const std::string name, int retain = 0, bool persistent = false, int domainId = 0); void enqueue(const T& data); };
- 113. Live Coding lab