DDS::DurabilityQosPolicy Struct Reference
[DURABILITY]

This QoS policy specifies whether or not RTI Data Distribution Service will store and deliver previously published data samples to new DDS::DataReader entities that join the network later. More...

#include <managed_infrastructure.h>

List of all members.

Static Public Member Functions
static System::String^	get_durability_qos_policy_name ()
	Stringified human-readable name for DDS::DurabilityQosPolicy.
Public Attributes
DurabilityQosPolicyKind	kind
	The kind of durability.
Properties
System::Boolean	direct_communication [get, set]
	<<eXtension>> Indicates whether or not a TRANSIENT or PERSISTENT DDS::DataReader should receive samples directly from a TRANSIENT or PERSISTENT DDS::DataWriter

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Detailed Description

This QoS policy specifies whether or not RTI Data Distribution Service will store and deliver previously published data samples to new DDS::DataReader entities that join the network later.

Entity:

DDS::Topic, DDS::DataReader, DDS::DataWriter

Status:

DDS::StatusKind::OFFERED_INCOMPATIBLE_QOS_STATUS, DDS::StatusKind::REQUESTED_INCOMPATIBLE_QOS_STATUS

Properties:

RxO = YES
Changeable = UNTIL ENABLE

See also:

DURABILITY_SERVICE

Usage

It is possible for a DDS::DataWriter to start publishing data before all (or any) DDS::DataReader entities have joined the network.

Moreover, a DDS::DataReader that joins the network after some data has been written could potentially be interested in accessing the most current values of the data, as well as potentially some history.

This policy makes it possible for a late-joining DDS::DataReader to obtain previously published samples.

By helping to ensure that DataReaders get all data that was sent by DataWriters, regardless of when it was sent, using this QoS policy can increase system tolerance to failure conditions.

Note that although related, this does not strictly control what data RTI Data Distribution Service will maintain internally. That is, RTI Data Distribution Service may choose to maintain some data for its own purposes (e.g., flow control) and yet not make it available to late-joining readers if the DURABILITY policy is set to DDS::DurabilityQosPolicyKind::VOLATILE_DURABILITY_QOS.

Transient and Persistent Durability

For the purpose of implementing the DURABILITY QoS kind TRANSIENT or PERSISTENT, RTI Data Distribution Service behaves as if for each Topic that has DDS::DurabilityQosPolicy::kind of DDS::DurabilityQosPolicyKind::TRANSIENT_DURABILITY_QOS or DDS::DurabilityQosPolicyKind::PERSISTENT_DURABILITY_QOS there is a corresponding "built-in" DDS::DataReader and DDS::DataWriter configured with the same DURABILITY kind. In other words, it is as if somewhere in the system, independent of the original DDS::DataWriter, there is a built-in durable DDS::DataReader subscribing to that Topic and a built-in durable DataWriter re-publishing it as needed for the new subscribers that join the system. This functionality is provided by the RTI Persistence Service.

The Persistence Service can configure itself based on the QoS of your application's DDS::DataWriter and DDS::DataReader entities. For each transient or persistent DDS::Topic, the built-in fictitious Persistence Service DDS::DataReader and DDS::DataWriter have their QoS configured from the QoS of your application's DDS::DataWriter and DDS::DataReader entities that communicate on that DDS::Topic.

For a given DDS::Topic, the usual request/offered semantics apply to the matching between any DDS::DataWriter in the domain that writes the DDS::Topic and the built-in transient/persistent DDS::DataReader for that DDS::Topic; similarly for the built-in transient/persistent DDS::DataWriter for a DDS::Topic and any DDS::DataReader for the DDS::Topic. As a consequence, a DDS::DataWriter that has an incompatible QoS will not send its data to the RTI Persistence Service, and a DDS::DataReader that has an incompatible QoS will not get data from it.

Incompatibilities between local DDS::DataReader and DDS::DataWriter entities and the corresponding fictitious built-in transient/persistent entities cause the DDS::StatusKind::REQUESTED_INCOMPATIBLE_QOS_STATUS and DDS::StatusKind::OFFERED_INCOMPATIBLE_QOS_STATUS to change and the corresponding Listener invocations and/or signaling of DDS::Condition objects as they would with your application's own entities.

The value of DDS::DurabilityServiceQosPolicy::service_cleanup_delay controls when RTI Persistence Service is able to remove all information regarding a data instances.

Information on a data instance is maintained until the following conditions are met:

1. The instance has been explicitly disposed (instance_state = NOT_ALIVE_DISPOSED),

and

2. While in the NOT_ALIVE_DISPOSED state, the system detects that there are no more 'live' DDS::DataWriter entities writing the instance. That is, all existing writers either unregister the instance (call unregister) or lose their liveliness,

and

3. A time interval longer that DDS::DurabilityServiceQosPolicy::service_cleanup_delay has elapsed since the moment RTI Data Distribution Service detected that the previous two conditions were met.

The utility of DDS::DurabilityServiceQosPolicy::service_cleanup_delay is apparent in the situation where an application disposes an instance and it crashes before it has a chance to complete additional tasks related to the disposition. Upon restart, the application may ask for initial data to regain its state and the delay introduced by the service_cleanup_delay will allow the restarted application to receive the information on the disposed instance and complete the interrupted tasks.

Compatibility

The value offered is considered compatible with the value requested if and only if the inequality offered kind >= requested kind evaluates to 'TRUE'. For the purposes of this inequality, the values of DURABILITY kind are considered ordered such that DDS::DurabilityQosPolicyKind::VOLATILE_DURABILITY_QOS < DDS::DurabilityQosPolicyKind::TRANSIENT_LOCAL_DURABILITY_QOS < DDS::DurabilityQosPolicyKind::TRANSIENT_DURABILITY_QOS < DDS::DurabilityQosPolicyKind::PERSISTENT_DURABILITY_QOS.

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Member Data Documentation

DurabilityQosPolicyKind DDS::DurabilityQosPolicy::kind

The kind of durability.

[default] DDS::DurabilityQosPolicyKind::VOLATILE_DURABILITY_QOS

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Property Documentation

System:: Boolean DDS::DurabilityQosPolicy::direct_communication [get, set]

<<eXtension>> Indicates whether or not a TRANSIENT or PERSISTENT DDS::DataReader should receive samples directly from a TRANSIENT or PERSISTENT DDS::DataWriter

When direct_communication is set to true, a TRANSIENT or PERSISTENT DDS::DataReader will receive samples from both the original DDS::DataWriter configured with TRANSIENT or PERSISTENT durability and the DDS::DataWriter created by the persistence service. This peer-to-peer communication pattern provides low latency between end-points.

If the same sample is received from the original DDS::DataWriter and the persistence service, the middleware will discard the duplicate.

When direct_communication is set to false, a TRANSIENT or PERSISTENT DDS::DataReader will only receive samples from the DDS::DataWriter created by the persistence service. This brokered communication pattern provides a way to guarantee eventual consistency.

[default] true