QoS Policies

Quality of Service (QoS) policies. More...

Modules
	DISCOVERY
	<<eXtension>> Specifies the attributes required to discover participants in the domain.
	DEADLINE
	Expresses the maximum duration (deadline) within which an instance is expected to be updated.
	OWNERSHIP
	Specifies whether it is allowed for multiple DDS_DataWriter (s) to write the same instance of the data and if so, how these modifications should be arbitrated.
	OWNERSHIP_STRENGTH
	Specifies the value of the strength used to arbitrate among multiple DDS_DataWriter objects that attempt to modify the same instance of a data type (identified by DDS_Topic + key).
	LIVELINESS
	Determines the mechanism and parameters used by the application to determine whether a DDS_Entity is alive.
	RELIABILITY
	Indicates the level of reliability offered/requested by RTI Connext Micro. Only best-effort is supported.
	HISTORY
	Specifies the behavior of RTI Connext Micro in the case where the value of an instance changes (one or more times) before it can be successfully communicated to one or more existing subscribers.
	DURABILITY
	<<eXtension>> Specifies the Durability properties used by a DDS_DataWriter and/or DDS_DataReader.
	RESOURCE_LIMITS
	Specifies the resources that RTI Connext Micro can consume in order to meet the requested QoS.
	ENTITY_FACTORY
	A QoS policy for all DDS_Entity types that can act as factories for one or more other DDS_Entity types.
	Extended Qos Support
	<<eXtension>> Types and defines used in extended QoS policies.
	SYSTEM_RESOURCE_LIMITS
	<<eXtension>> Specifies the system-specific resources that RTI Connext Micro can use.
	WIRE_PROTOCOL
	<<eXtension>> Specifies the wire protocol related attributes for the DDS_DomainParticipant.
	DATA_READER_PROTOCOL
	<<eXtension>> Specifies the DataReader-specific protocol QoS.
	DATA_WRITER_PROTOCOL
	<<eXtension>> Specifies the DataWriter-specific protocol QoS.
	TRANSPORT
	<<eXtension>> <<cert>> Specify transport settings
	DOMAIN_PARTICIPANT_RESOURCE_LIMITS
	<<eXtension>> Specifies the DomainParticipant-specific resources that RTI Connext Micro can use.
	ENTITY_NAME
	<<eXtension>> Name of the entity.

Enumerations
enum	DDS_QosPolicyId_t {   DDS_INVALID_QOS_POLICY_ID ,   DDS_DEADLINE_QOS_POLICY_ID ,   DDS_OWNERSHIP_QOS_POLICY_ID,   DDS_OWNERSHIPSTRENGTH_QOS_POLICY_ID,   DDS_LIVELINESS_QOS_POLICY_ID ,   DDS_RELIABILITY_QOS_POLICY_ID ,   DDS_HISTORY_QOS_POLICY_ID ,   DDS_ENTITYFACTORY_QOS_POLICY_ID }
	Type to identify QosPolicies. More...

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Detailed Description

Quality of Service (QoS) policies.

Data Distribution Service (DDS) relies on the use of QoS. A QoS is a set of characteristics that controls some aspect of the behavior of DDS. A QoS is comprised of individual QoS policies (objects conceptually deriving from an abstract QosPolicy class).

RTI Connext Micro supports a subset of these QoS.

"DDS QoS policies"

The QosPolicy provides the basic mechanism for an application to specify quality of service parameters. It has an attribute name that is used to uniquely identify each QosPolicy.

QosPolicy implementation is comprised of a name, an ID, and a type. The type of a QosPolicy value may be atomic, such as an integer or float, or compound (a structure). Compound types are used whenever multiple parameters must be set coherently to define a consistent value for a QosPolicy.

QoS (i.e., a list of QosPolicy objects) may be associated with all DDS_Entity objects in the system such as DDS_Topic, DDS_DataWriter, DDS_DataReader, DDS_Publisher, DDS_Subscriber, and DDS_DomainParticipant.

Specifying QoS on entities

QosPolicies can be set programmatically when an DDS_Entity is created, or modified with the DDS_Entity's set_qos (abstract) method .

To customize a DDS_Entity's QoS before creating the entity, the correct pattern is:

• First, initialize a QoS object with the appropriate INITIALIZER constructor.
• Call the relevant get_<entity>_default_qos() method.
• Modify the QoS values as desired.
• Finally, create the entity.

Each QosPolicy is treated independently from the others. This approach has the advantage of being very extensible. However, there may be cases where several policies are in conflict. Consistency checking is performed each time the policies are modified via the set_qos (abstract) operation, or when the DDS_Entity is created.

When a policy is changed after being set to a given value, it is not required that the new value be applied instantaneously; RTI Connext Micro is allowed to apply it after a transition phase. In addition, some QosPolicy have immutable semantics, meaning that they can only be specified either at DDS_Entity creation time or else prior to calling the DDS_Entity_enable operation on the entity.

Each DDS_Entity can be configured with a list of QosPolicy objects. However, not all QosPolicies are supported by each DDS_Entity. For instance, a DDS_DomainParticipant supports a different set of QosPolicies than a DDS_Topic or a DDS_Publisher.

QoS compatibility

In several cases, for communications to occur properly (or efficiently), a QosPolicy on the publisher side must be compatible with a corresponding policy on the subscriber side. For example, if a DDS_Subscriber requests to receive data reliably while the corresponding DDS_Publisher defines a best-effort policy, communication will not happen as requested.

To address this issue and maintain the desirable decoupling of publication and subscription as much as possible, the QosPolicy specification follows the subscriber-requested, publisher-offered pattern.

In this pattern, the subscriber side can specify a "requested" value for a particular QosPolicy. The publisher side specifes an "offered" value for that QosPolicy. RTI Connext Micro will then determine whether the value requested by the subscriber side is compatible with what is offered by the publisher side. If the two policies are compatible, then communication will be established. If the two policies are not compatible, RTI Connext Micro will not establish communications between the two DDS_Entity objects and will record this fact by means of the DDS_OFFERED_INCOMPATIBLE_QOS_STATUS on the publisher end and DDS_REQUESTED_INCOMPATIBLE_QOS_STATUS on the subscriber end. The application can detect this fact by means of a DDS_Listener.

The following properties are defined on a QosPolicy.

• RxO property
  The QosPolicy objects that need to be set in a compatible manner between the publisher and subscriber end are indicated by the setting of the RxO property:
  • RxO = YES indicates that the policy can be set both at the publishing and subscribing ends and the values must be set in a compatible manner. In this case the compatible values are explicitly defined.
  • RxO = NO indicates that the policy can be set both at the publishing and subscribing ends but the two settings are independent. That is, all combinations of values are compatible.
  • RxO = N/A indicates that the policy can only be specified at either the publishing or the subscribing end, but not at both ends. So compatibility does not apply.
• Changeable property
  Determines whether a QosPolicy can be changed.
  NO – policy can only be specified at DDS_Entity creation time.
  UNTIL ENABLE – policy can only be changed before the DDS_Entity is enabled.
  YES – policy can be changed at any time.

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Enumeration Type Documentation

enum DDS_QosPolicyId_t

Type to identify QosPolicies.

Note that the value of these constants disagree with the values of the corresponding parameter IDs in the RTPS protocol. This conflict is unavoidable since these values are given in the DDS specification, which is not tied to RTPS.

Enumerator:

DDS_INVALID_QOS_POLICY_ID	Identifier for an invalid QoS policy.
DDS_DEADLINE_QOS_POLICY_ID	Identifier for DDS_DeadlineQosPolicy.
DDS_OWNERSHIP_QOS_POLICY_ID	Identifier for DDS_OwnershipQosPolicy.
DDS_OWNERSHIPSTRENGTH_QOS_POLICY_ID	Identifier for DDS_OwnershipStrengthQosPolicy.
DDS_LIVELINESS_QOS_POLICY_ID	Identifier for DDS_LivelinessQosPolicy.
DDS_RELIABILITY_QOS_POLICY_ID	Identifier for DDS_ReliabilityQosPolicy.
DDS_HISTORY_QOS_POLICY_ID	Identifier for DDS_HistoryQosPolicy.
DDS_ENTITYFACTORY_QOS_POLICY_ID	Identifier for DDS_EntityFactoryQosPolicy.