RTI Connext DDS Core Libraries Qo S Reference Guide
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RTI® Connext™ DDS—Comprehensive Summary of QoS Policies
Chg. = Changeable
A = Always
F = DomainParticipantFactory
P = Publisher
W = DataWriter
Y = Yes
RxO = Request/Offered Semantics
BC = Before Creation
D = DomainParticipant
S = Subscriber
R = DataReader
Blank = No
Ext. = Extension to DDS standard
BE = Before Enable
T = Topic
Inst. = Behavior is applied per-instance
QoS Policy
Description
Chg.
RxO
Inst.
Entities
Ext.
AsynchronousPublisher
Configures mechanism that sends user data in a separate middleware thread
BC
P
Y
Availability
Configures Collaborative DataWriters and Durable Subscriptions
A
R,W
Y
Batch
Configures mechanism for collecting multiple data samples to be sent in a single network packet
BE
W
Y
Database
Configures threads & resource limits that control internal Connext DDS data storage
BC
D
Y
DataReaderProtocol,
DataWriterProtocol
Configures DDS on-the-wire protocol (RTPS), along with WireProtocol QoS for Reliable Topics
BC
R,W
Y
DataReaderResourceLimits,
DataWriterResourceLimits
Configures how Readers/Writers allocate and use physical memory for Connext DDS specific internal
resources
BC
R,W
Y
Deadline
Maximum duration within which an instance is expected to be updated.
A
Y
Y
T,R,W
DestinationOrder
Controls how RTI deals with data from multiple DataWriters for the same instance; "by reception
timestamp" or "by source timestamp"
BE
Y
Y
T,R,W
Discovery, DiscoveryConfig
Configures which DomainParticipants to contact, how to automatically discover & connect with them
BC
D
Y
DomainParticipantResourceLimits
Configures how DomainParticipants allocate and use physical memory for internal resources
BC
D
Y
Durability
Specifies if Connext DDS will store and deliver previously published data to new/late-joining Readers
BE
Y
T,R,W
DurabilityService
Configures external Persistence Service for Writers with PERSISTENT or TRANSIENT Durability
BE
T,W
EntityFactory
Controls Entity behavior as a factory for other entities (i.e., if child entities are created enabled)
A
F,D,P,S
EntityName
Assigns a name to an Entity, pairs DomainParticipant with a Durable Subscription
BE
D,P,S,
R,W
Y
Event
Configures the internal thread in a DomainParticipant that handles timed events
BC
D
Y
ExclusiveArea
Configures multi-thread concurrency and deadlock prevention capabilities
BC
P,S
Y
History
Controls how much data to store and how stored data is managed for the DataWriter/Reader within
specified resource limits (see ResourceLimits)
BE
Y
T,R,W
LatencyBudget
Suggests to the middleware how much time is allowed to deliver data
A
Y
T,R,W
Lifespan
Specifies how long Connext DDS considers data sent by a user application to be valid
A
T,W
Liveliness
Controls mechanism that allows DataReaders to detect when matching DataWriters become
disconnected/dead
BE
Y
T,R,W
Logging
Configures the Logging feature.
A
F
Y
MultiChannel
Configures DataWriter to send data using different multicast groups (addresses) based on data value
BC
W
Y
Ownership
Specifies if a DataReader can receive new samples for an instance of data from multiple DataWriters at the
same time
BE
Y
Y
T,R,W
OwnershipStrength
Specifies strength used to arbitrate among multiple DataWriters of same instance
A
Y
W
Partition
Adds string IDs for finer-grained matching between DataWriters and DataReaders of same Topic
A
P,S
Presentation
Controls how Connext DDS presents data received by an application to DataReaders
BE
Y
Y
P,S
Profile
Configures how the DomainParticipantFactory loads XML documents containing QoS profiles
BC
F
Y
Property
Specifies name/value pairs used to configure parameters not exposed via formal QoS policies
A
D,R,W
Y
PublishMode
Specifies whether data samples will be sent in the application thread or a middleware thread
BC
W
Y
ReaderDataLifecycle
Controls how a DataReader manages the lifecycle of the data that it has received
A
R
ReceiverPool
Configures threads used to receive data from network transports (for example, UDP) and deliver data to
the application
BC
D
Y
Reliability
Indicates whether samples lost by the network should be repaired by the middleware
BE
Y
T,R,W
ResourceLimits
Limits amount of data cached by the middleware
BE
T,R,W
SystemResourceLimits
Configures process-level resources, independent of particular entities
BC
F
Y
Service
Specifies the service for the entity. Only for use by RTI infrastructure services, not user applications
BE
R,W
Y
TimeBasedFilter
Sets minimum time period before new data for an instance is provided to a DataReader
BE
Y
R
TransportBuiltin
Specifies which built-in transports are used
BC
D
Y
TransportMulticast
Sets multicast address and port number on which DataReader will receive data
BC
R
Y
TransportMulticastMapping
Specifies automatic mapping between list of topic expressions and multicast addresses used by
DataReaders to receive data
BC
D
Y
TransportPriority
On OS/transport combinations that understand priority, specifies priority on per-DataReader/Writer basis
BC
T,R,W
TransportSelection
Selects which transports a DataReader/Writer may use to send or receive its data
BC
R,W
Y
TransportUnicast
Specifies a subset of transports and a port number that can be used by an Entity to receive data
BC
D,R,W
Y
TypeConsistencyEnforcement
Sets rule to determine if a type used to publish given topic is consistent with that used to subscribe to it
BE
R
TypeSupport
Attaches application-specific values to a DataWriter/DataReader that are passed to serialization /
deserialization routine of associated data type. Controls if padding bytes are set to 0 during serialization.
A
R,W
Y
UserData, GroupData, TopicData
Attaches arbitrary application data (a buffer of bytes) to discovery meta-data
A
R,W;
P,S; T
WireProtocol
Configures properties for the DDS wire protocol (RTPS)
BC
D
Y
WriterDataLifecycle
Controls how a DataWriter handles the lifecycle of the instances that it is registered to manage
A
W
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QoS Policies per Entity Type
DomainParticipant
UserData
EntityFactory
WireProtocol
TransportBuiltin
TransportUnicast
Discovery
DomainParticipantResourceLimits
Event
ReceiverPool
Database
DiscoveryConfig
Property
EntityName
TypeSupport
TransportMulticastMapping
Topic
TopicData
Durability
DurabilityService
Deadline
LatencyBudget
Liveliness
Reliability
DestinationOrder
History
ResourceLimits
TransportPriority
Lifespan
Ownership
Publisher
Presentation
Partition
GroupData
EntityFactory
AsynchronousPublisher
ExclusiveArea
Subscriber
Presentation
Partition
GroupData
EntityFactory
ExclusiveArea
DataWriter
Durability
DurabilityService
Deadline
LatencyBudget
Liveliness
Reliability
DestinationOrder
History
ResourceLimits
TransportPriority
Lifespan
UserData
Ownership
OwnershipStrength
WriterDataLifecycle
DataWriterResourceLimits
DataWriterProtocol
TransportSelection
TransportUnicast
PublishMode
Property
Service
Batch
MultiChannel
Availability
EntityName
TypeSupport
DataReader
Durability
Deadline
LatencyBudget
Liveliness
Reliability
DestinationOrder
History
ResourceLimits
TransportPriority
UserData
Ownership
TimeBasedFilter
ReaderDataLifecycle
TypeConsistencyEnforcement
DataReaderResourceLimits
DataReaderProtocol
TransportSelection
TransportUnicast
TransportMulticast
Property
Service
Availability
EntityName
TypeSupport
DomainParticipantFactory
EntityFactory
SystemResourceLimits
Profile
Logging
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QoS Policy Highlights
1. AsynchronousPublisher Configures the mechanism that sends user data in an external middleware thread.
Must be used to send "large" data reliably. "Large" means data that cannot be sent as a single packet by a transport. For
example, to send data larger than 63K reliably using UDP/IP, you must configure Connext DDS to send the data using
asynchronous publishers.
Can reduce the amount of time spent in the user thread to send data.
For small samples, can reduce bandwidth consumption. Using an asynchronous publisher provides aggregation of samples
(collecting many RTPS data samples into 1 network packet) since samples written for the same topic to the same destination are
coalesced within the same UDP packet, which reduces bandwidth. For bandwidth reduction, see also Batch QoS. However, for
aggregation the preferred mechanism is Batch.
Often used with:
PublishMode—activates asynchronous publishing, a prerequisite for the use of this policy
LatencyBudget—controls the scheduling of asynchronously published data onto the network across Data Writers
2. Availability Configures Collaborative DataWriters and Durable Subscriptions.
For a Collaborative DataWriter, specifies the group of DataWriters expected to collaboratively provide data and the timeouts
that control when to allow data to be available that may skip samples.
For a Durable Subscription, configures a set of Durable Subscriptions for a DataWriter.
Supported in release 4.5e, 4.6a and higher.
3. Batch Configures mechanism for collecting multiple user data samples to be sent in a single network packet.
This allows you to take advantage of the efficiency of sending larger packets and thus increase effective throughput.
Can dramatically increase the effective throughput for small data packets. Usually, throughput for small packets (data < 2048
bytes) is limited by CPU capacity, not by network bandwidth. Batching many smaller data packets so they are sent in 1 large
packet increases network utilization and thus throughput in terms of user data packets per second.
For Reliable topics, batching reduces the Reliable (RTPS) traffic because the Reliable entity is the batch, not the sample.
4. Database Configures threads and resource limits that control the internal Connext DDS database.
Essentially a list of lists, RTI uses an internal "database" to store information about entities created locally as well as remote
entities of other participants found during the discovery process. This QoS configures how RTI manages its database including
how often it cleans up, priority of the database thread, and limits on resources that may be allocated by the database.
You may be interested in modifying the shutdown_timeout and shutdown_cleanup_period parameters to decrease the time it
takes to delete a DomainParticipant when your application is shutting down.
5/6. DataReaderProtocol, DataWriterProtocol Configures DDS on-the-wire protocol (RTPS).
Connext DDS uses a standard protocol for packet (user- and meta-data) exchange between applications. These QoS policies
control configurable portions of the protocol, including configuration of the reliable data delivery mechanism of the protocol on
a per DataWriter/DataReader basis.
These parameters control timing and timeouts and give you the ability to tradeoff between speed of data-loss detection and
repair, versus the network and CPU bandwidth used to maintain reliability.
It is important to tune the reliability protocol (on a per DataWriter and DataReader basis) to meet the requirements of the end-
user application so that data can be sent between DataWriters and DataReaders in an efficient and optimal manner in the
presence of data loss. You can also use these QoS policies to control how Connext DDS responds to "slow" reliable DataReaders
or ones that disconnect or are otherwise lost.
See the Reliability QoS policy for more on configuring per-DataReader/DataWriter reliability. The History and ResourceLimits
QoS also play an important role in the reliable protocol.
7/8. DataReaderResourceLimits, DataWriterResourceLimits Configures how DataReaders/DataWriters allocate and use physical
memory for internal resources.
DataReaders must allocate internal structures to handle the maximum number of DataWriters that may connect to it, whether
or not a DataReader handles data fragmentation and how many data fragments that it may handle, how many simultaneous,
outstanding loans of internal memory holding data samples can be provided to user code, as well as others.
DataWriters must allocate internal structures to handle the simultaneously blocking of threads trying to call write() on the same
DataWriter, for the storage used by batched DataWriters, and for filters for content-based filtering DataReaders.
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Most of these internal structures start at an initial size and by default will grow as needed by dynamically allocating additional
memory. You may set fixed, maximum sizes for these internal structures if you want to bind the amount of memory that can be
used by a DataReader/DataWriter. Setting the initial size to the maximum size will prevent Connext DDS from dynamically
allocating any memory after the DataReader/DataWriter is created.
9. Deadline For DataReaders: specifies the maximum expected elapsed time between arriving data samples.
For DataWriters: specifies a commitment to publish samples with no greater than this elapsed time between
them.
Can be used during system integration to ensure that applications have been coded to meet design specifications.
Can be used at run time to detect when systems are performing outside of design specifications. Receiving applications can take
appropriate actions to prevent total system failure when data is not received in time. For topics on which data is not expected
to be periodic, the deadline period should be set to an infinite value.
Loss of Deadline results in loss of ownership for topics with Exclusive Ownership.
10. DestinationOrder Controls how Connext DDS deals with data sent by multiple DataWriters for the same topic.
When multiple DataWriters send data for the same Topic, the order in which data from different DataWriters is received by the
applications of different DataReaders may be different. So different DataReaders may not receive the same "last" value when
DataWriters stop sending data.
If set to "by reception timestamp":
Data will be delivered by a DataReader in the order in which it was received (which may lead to inconsistent final values).
If set to "by source timestamp":
Data will be delivered by a DataReader in the order in which it was sent. If data arrives on the network with a source timestamp
earlier than the source timestamp of the last data delivered, the new data will be dropped. This ordering therefore works best
when system clocks are relatively synchronized among writing machines.
Not all data sent by multiple DataWriters may be delivered to a DataReader, and not all DataReaders will see the same data sent
by DataWriters. However, all DataReaders will see the same "final" data when DataWriters stop sending data.
Can be used to create systems that have "eventual consistency." Thus intermediate states across multiple applications may be
inconsistent, but when DataWriters stop sending changes to the same topic, all applications will end up having the same state.
11. Discovery Configures mechanism used to automatically discover and connect with new remote DomainParticipants.
This QoS specifies the identifiers used by the DomainParticipant to discover other DomainParticipants with which to
communicate. This is done via an initial peers list. The middleware will periodically send network packets to these locations,
announcing itself to any remote applications that may be present, and will listen for announcements from those applications.
This QoS policy also controls the extent of and the transports used for discovery.
Often used with: DiscoveryConfig – to tune timeliness and reliability related parameters for the discovery process.
12. DiscoveryConfig Configures timeliness and reliability settings for discovery.
This QoS policy controls how often discovery data is sent as well as the reliability settings used for discovery topics. The amount
of network traffic required by the discovery process can vary widely, based on how your application has configured the
middleware's network addressing (e.g., unicast vs. multicast, multicast TTL, etc.), the system’s size, whether all applications are
started at the same time or at staggered times, and other factors. Your application can use this policy to make tradeoffs
between discovery completion time and network bandwidth utilization. You can also introduce random back-off periods into
the discovery process to decrease the probability of network contention when many applications start simultaneously.
Often used with: Discovery – to define the discovery peers.
13. DomainParticipantResourceLimits Configures how DomainParticipants allocate and use physical memory for internal
resources, including maximum sizes of various properties.
This QoS policy sets size limits on variable-length parameters used by the participant and its contained entities. It also controls
the initial and maximum sizes of data structures used by the Domain Participant to store information about locally created and
remotely discovered entities (such as DataWriters/DataReaders), as well as parameters used by the internal database to size the
hash tables it uses.
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14. Durability Specifies whether Connext DDS will store and deliver previously published data to new/late-joining
DataReaders.
Durability controls whether or not new DataReaders get data which was written by DataWriters previously. The Durability can
vary from not at all (the default) to persistent (stored to disk). This QoS policy helps insulate system from startup dependencies
and can increase system tolerance to failure conditions.
Often used with:
DurabilityService – to configure the external service parameters.
History – to specify how much historical data is to be kept.
15. DurabilityService Configures external Persistence Service used for DataWriters with PERSISTENCE/TRANSIENT Durability.
When a DataWriter's Durability QoS is set to PERSISTENT_DURABILITY or TRANSIENT_DURABILITY, an external service, RTI
Persistence Service, is used to store and possibly forward the data sent by the DataWriter to DataReaders that are created after
the data was initially sent. This QoS policy configures certain parameters of RTI Persistence Service when it operates on behalf
of the DataWriter, such as how much data to store.
Often used with: Durability – to specify the level of durability for the data.
16. EntityFactory Controls an Entity’s behavior as a ‘factory’ for other entities, such as whether child entities are created in
the enabled state.
This QoS policy is useful to synchronize the initialization of Entities. For example, when a DataReader is created in an enabled
state, its existence is immediately propagated for discovery and the DataReader's listener is called as soon as data is received.
The initialization process for an application may extend beyond the creation of the DataReader, so you may not want for the
DataReader to start to receive or process any data until the initialization process is complete. By creating readers in a disabled
state, you can make sure that no data is received until the rest of the application initialization is complete.
17. EntityName Assigns a name to an Entity & associates a DomainParticipant with a Durable Subscription.
This QoS policy assigns a name to a DomainParticipant, Publisher, Subscriber, DataWriter, or DataReader; this name will be sent
as part of during discovery. The name will also be visible in RTI tools (except for Publishers and Subscribers) to help you visualize
and debug your system. It also assigns a role name, which specifies the Durable Subscription to which the DataReader belongs.
Often used with: Availability – if using Durable Subscriptions.
18. Event Configures the internal thread in a DomainParticipant that handles timed events.
In a DomainParticipant, the Event Thread is dedicated to handling all timed events, including checking for timeouts and
deadlines, and executing internal and user-defined timeout or exception handling routines/callbacks. An example of a timed
event is the reliable heartbeat rate. If this rate is very high, you should use a higher than normal event thread priority.
This QoS policy allows you to configure thread properties such as priority level and stack size. You can also configure the
maximum number of events that can be posted to the event thread. By default, a DomainParticipant will dynamically allocate
memory as needed for events posted to the event thread. However, by setting a maximum value or setting the initial and
maximum value to be the same, you can either bind the amount of memory allocated for the event thread or prevent a
DomainParticipant from dynamically allocating memory for the event thread after initialization.
19. ExclusiveArea Configures multi-thread concurrency and deadlock prevention capabilities.
An exclusive area (EA) is an abstraction of a multi-thread-safe region. Each entity is protected by one and only one EA, although
a single exclusive area may be shared by multiple entities. Conceptually, an EA is a mutex or monitor with additional deadlock
protection features.
Note: one only gets into exclusive area violations because entity creation/modifications are done in callbacks. If these
operations are one in user threads, exclusive violations cannot occur. Creating shared EA's is generally not a good practice since
it decreases concurrency.
20. GroupData Attaches arbitrary application data to discovery meta-data at the publisher/subscriber level.
See: UserData.
21. History Controls how much data to store and how stored data is managed for a DataWriter or DataReader.
Controls how Connext DDS manages data sent by a DataWriter or received for a DataReader. Two settings are available:
KEEP_ALL or KEEP_LAST with a value (depth). KEEP_ALL does not imply that Connext DDS will store infinite data. How much data
can actually be stored (and thus memory allocation) is controlled by the ResourceLimits QoS.
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When using the KEEP_LAST setting, the value of depth (number of data samples to keep) applies on a per instance-basis (unique
key value) for Topics that are keyed. For example, for a Topic that provides data about the positions of aircraft and using the
aircraft ID as a key, a KEEP_LAST setting of 1 will tell Connext DDS to keep the last value for each unique aircraft ID.
The major use of history is to help tune the reliability between DataWriters and DataReaders. You must use KEEP_ALL History
for both the DataWriter and DataReader if you want strict-reliability (the DataReader must receive all of the data sent by the
DataWriter). Using KEEP_LAST on either side, Connext DDS will only guarantee the reliability of the last N (depth) values sent.
Often used with:
Reliability – using the KEEP_LAST kind for History depth, the level of reliability can be tuned.
Durability – for the DataWriter side to specify how much data need to be stored and sent to new DataReaders who request
previously published data using the non-VOLATILE Durability settings.
See also: ResourceLimits
22. LatencyBudget Suggests how much time is allowed to deliver data.
This is an optional QoS; its implementation and usage may be determined by the implementer.
RTI uses this QoS policy to help schedule data to be sent by an asynchronous thread with a Flow Controller. Data with low
latency requirements can be sent ahead of data that is not latency sensitive when using this QoS policy and the asynchronous
publishing mode for a DataWriter.
The default value is 0, which implies you want to send with minimum latency.
If using this QoS policy, then all DataWriters whose data is sent by the same Flow Controller must set the value for this QoS
policy accordingly. The data of any DataWriter that uses the default value for this QoS policy will always be sent ahead of
DataWriters with non-zero values for this QoS policy.
Only used with:
PublishMode – specifically when using DDS_ASYNCHRONOUS_PUBLISH_MODE_QoS and a flow controller.
DDSFlowController – specifically the DDS_EDF_FLOW_CONTROLLER_SCHED_POLICY for DDS_FlowControllerProperty_t.
23. Lifespan Specifies how long Connext DDS should consider data sent by a user application to be valid.
Connext DDS will timestamp all data sent and received. When a finite Lifespan is specified for a DataWriter or DataReader,
Connext DDS will check to see how long the data has been stored in the DataWriter’s send queue or the DataReader’s receive
queue and remove any data that has exceeded its Lifespan duration.
Connext DDS will always compare times from the same local clock when checking Lifespan. Thus, the Lifespan for data in the
DataReader uses the time as stamped by the local clock when the data was received as the start time for the Lifespan
calculation.
You can use this QoS policy to limit how much data is stored by Connext DDS. Even if configured for KEEP_ALL History Qos, data
may be dropped by Connext DDS due to the use of the Lifespan QoS.
May interact with the Reliability QoS. Due to expired Lifespan, data in the DataWriter that has not yet been acknowledged as
have been received in a reliable connection may be dropped and thus never repaired if the initial sample was lost.
Data in the DataReader that has not yet been accessed by the user code may also be removed due to an expired Lifespan.
You can use this QoS policy to ensure applications don’t receive or act on data, commands, or messages that are too old and
have "expired."
24. Liveliness Configures the mechanism that allows DataReaders to detect when matching DataWriters become
disconnected or dead.
In a connectionless communications model, the only way a DataReader can know if it is able to receive data from a DataWriter
is if it actually receives a message from the DataWriter. This message can either be real data sent by user code, or a message
that indicates that the DataWriter is indeed alive and that all elements of the communications link between the DataWriter and
DataReader applications is working (network cards, switches, routers, etc.).
By using the Liveliness QoS, the user configures how the DataWriter application maintains liveliness with the DataReader
application and how fast the DataReader application is able to detect when the DataWriter is unable to send data to the
DataReader (because the sending application has died or perhaps the link between the two applications has broken).
You can use this to ensure that important messages can be received if they are sent. For example, if a command message like
“Emergency Stop” is never sent unless the situation is encountered, Liveliness can be used to periodically send a message to test
the end-to-end connectivity of the system so that when an “Emergency Stop” message is sent, it will likely be received by all
subscribers.
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You can use it at run time to detect when systems are performing outside of design specifications. Receiving applications can
take appropriate actions in response to disconnected DataWriters.
25. Logging Configures the logging feature.
Allows users to configure various aspects associated with logging via XML profile files: verbosity, category, print format, and
output file.
26. MultiChannel Configures DataWriter to send data using different multicast groups (addresses) based on data value.
Data published by a DataWriter can be divided into different “channels” within the same topic. This is useful when the total
volume is data is large—perhaps even larger than the capacity of a single network link—and the number of DataReaders is large,
but each DataReader is interested in only a subset of the available data. A “channel” is defined by a filter expression and a set of
multicast addresses (groups). When data is sent, it is passed through the filter expressions for all defined channels to see if it will
be sent on a particular channel. The filter expressions are applied on the value of the data (contents of the data structure). It is
possible for the same data value to be sent on multiple channels. When sent on a channel, Connext DDS will send the data on all
of the multicast addresses defined for the channel.
A DataWriter using the MultiChannel QoS policy will ignore the DataReader’s Transport Unicast and Transport Multicast QoS
settings. It will only send the data on the multicast addresses set for its channels. DataReaders are expected to subscribe to
Content-Filtered Topics and will automatically subscribe to data using the configured multicast addresses of a multi-channel
DataWriter for the channels whose filter intersects the Content-Filtered Topic of the DataReader.
Usually, the data of different instances within a topic are grouped together into different channels. Since there are usually more
objects than multicast addresses that can be practically used, a single multicast address assigned to a single channel will usually
carry the data for more than one instance. So using multi-channel DataWriters will greatly reduce, but probably not eliminate,
the undesired delivery and discarding of unwanted data by the subscribing machine.
Users must analyze the data usage across all applications/machines in their system and map out the data topology to create an
optimal mapping (channel filters) of objects/instances/independent substreams to minimize network bandwidth usage as well
as the CPU usage for dropping unwanted data.
Besides using Content-Filtered Topics to configure the substream of data desired, there is no additional configuration that is
needed for DataReaders.
See also: ContentFilteredTopics in the User’s Manual (not a QoS policy)
27/28. Ownership, OwnershipStrength Specifies if a DataReader can receive new samples for an instance of data from
multiple DataWriters at the same time.
By default, DataReaders for a given topic can receive data from any matching DataWriter for the same topic—this is the
"shared" setting for the Ownership QoS policy. You can also configure an application to use "exclusive" Ownership for a topic so
that DataReaders only receive data from one DataWriter at a time. Ownership applies on a per key value (instance) basis for
Topics that are keyed. Thus, with exclusive Ownership, DataReaders will only receive data from a single DataWriter for a
particular instance (unique value for the key) of a Topic. This implies that a DataReader may receive data from multiple
DataWriters as long as the DataWriters are sending data for different instances.
The value of Ownership must be the same for a DataWriter to be connected to a DataReader. Either both sides must be shared
or both sides must be exclusive. Mismatched DataWriter and DataWriter pairs are not connected and will never exchange data.
The OwnershipStrength QoS policy is used to determine which DataWriter is allowed to send data (or updates for instances for
keyed Topics) to DataReaders when Ownership is exclusive and there are multiple DataWriters all sending data for the same
instance. The DataWriter with the highest value for the OwnershipStrength QoS policy will be considered the owner of the
instance of the Topic and whose data is delivered to DataReaders. Data for the instance sent by all other DataWriters with lower
OwnershipStrength will be dropped by Connext DDS when received at the subscribing application.
An arbitrary but deterministic method is used to select a single DataWriter among multiple DataWriters with the same highest
OwnershipStrength, so all DataReaders will receive the data from the same DataWriter even when the OwnershipStrength is the
same value.
When the DataWriter with the highest Ownership strength loses its liveliness (as controlled by the Liveliness QoS policy) or
misses a deadline (as controlled by the Deadline QoS policy) or whose application quits, dies, or otherwise disconnects, Connext
DDS will change ownership of the Topic instance to the DataWriter with the highest OwnershipStrength from the remaining
DataWriters.
This QoS policy can help you build systems that have redundant elements to safeguard against component or application
failures. When systems have active and hot standby components, the Ownership QoS policy can be used to ensure that data
from standby applications are only delivered in the case of the failure of the primary.
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Ownership QoS policy can also be used to create data channels or Topics designed to be taken over by external applications for
testing or maintenance.
Often used with:
Liveliness – to detect when DataWriters are dead and change ownership.
Deadline – to detect when DataWriters have not sent data in time so that lower strength data is accepted.
29. Partition Adds additional string identifiers for matching DataWriters and DataReaders for the same Topic.
Specified for Publishers and Subscribers, the Partition QoS policy can be used to add additional identifiers in the form of strings
that Connext DDS will used to match DataWriters and DataReaders. Normally, DataWriters are connected (matched) to
DataReaders of the same Topic (assuming that their QoS settings are mutually compatible). However, by using the Partition QoS
policy, additional criteria is used to decide if a DataWriter’s data is allowed to be sent to a DataReader. Referred to as partitions,
one or more strings can be added to the DataWriter’s Publisher or DataReader’s Subscriber parent. When the Partition QoS
policy is used, then a DataWriter is only connected (matched) to a DataReader for the same Topic only if their Publisher and
Subscribers have a common partition (intersecting partitions).
Direct comparison of partition strings of the Publisher and Subscriber is the nominal way to determine if there is a common
partition. However, one can also use strings that contain wildcards (actually regular expressions as defined by the POSIX
fnmatch API) as partitions. In that case, as long as one of the wildcard partitions matches one of the concrete partition strings
on the other side, then the DataWriter and DataReader are connected.
The set of partitions of a Publisher or Subscriber can be dynamically changed. This can be used to quickly control which
DataWriters and which DataReaders are allowed to connect. This facility is useful for creating temporary separation groups
among entities that would otherwise be connected to and exchange data each other. Therefore, if sometimes you want an
application (or individual DataWriter or DataReader) to be connected to another application (or DataReader or DataWriter,
respectively) and at other times you don’t, then you can use the Partition QoS policy to dynamically configure the connection
topology without stopping/starting or destroying/re-creating applications or Entities.
30. Presentation Controls how Connext DDS presents data received by an application to the DataReaders of the data
Usually DataReaders receive data in the order it was sent by a DataWriter. (When using the Reliability QoS policy, samples
which arrive out of order will be buffered until all previous samples arrive.) Once in order, data is presented to the DataReader
immediately.
Sometimes you may want a set of data for the same topic to be presented to the receiving DataReader only after ALL of the
elements of the set have been received, but not before. You may also want the data to be presented in a different order than it
was received. Specifically, for keyed data, you may want Connext DDS to present the data grouped by instance (same key value).
Thus this QoS policy allows you to specify different scopes of presentation, within a topic, across instances of a topic, and across
different Topics of a Publisher (although this last option is not currently supported by RTI). It also controls whether or not a set
of changes within the scope is delivered at the same time or can be delivered as soon as each element is received.
See also: DestinationOrder—also impacts the order in which samples will be delivered by the middleware to the application.
31. Profile Configures how XML documents containing QoS profiles are loaded by the DomainParticipantFactory.
QoS values for Entities can be configured in QoS profiles defined in XML documents. The XML documents that define QoS
profiles can be passed to Connext DDS as a string in XML format or loaded through XML files found on a file system.
The Profile QoS policy for the DomainParticipantFactory configures Connext DDS to load XML documents via 3 independent
mechanisms that may be used together or individually disabled.
Environment variable – the value of the environment variable NDDS_QOS_PROFILES can be used to set the locations of files on
the file system that contain XML documents to be loaded automatically by Connext DDS. You can also set a XML-formatted
string as a value for the environment variable. You can disable the use of the environment variable using the Profile QoS policy.
Default XML files – there are default locations where Connext DDS will look for XML files to load QoS profiles. These include the
current working directory from where an application is started and a file in the distribution directory for Connext DDS. You may
disable any or all of these default locations using the Profile QoS policy.
The locations of files as well as strings containing XML-formatted QoS profiles can be specified directly via the Profile QoS policy.
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32. Property Specifies name/value pairs used to configure parameters not exposed via formal QoS policies
The Property QoS policy allows name/value pairs (in the form of strings) to be attached to the QoS of different entities. These
strings are often used to add configuration parameters to an entity without having to change formal QoS policies. Behaviors
configured by means of this policy are often not known a priori (for example, dynamically loaded network transport plug-ins) or
are provided on a provisional basis. You may also add your own name/value pairs to the Property QoS policy of an Entity and
direct Connext DDS to propagate this information with the Entity’s discovery information so it can be accessed in other
applications. This allows you to add meta-information about an Entity for application-specific use.
Used to configure the property of builtin-transports before DomainParticipant creation
Used to specify dynamic loading of extension transports (such as RTI Secure WAN Transport)
Used to specify multiple instances of the built-in transports
Allows full pluggable transport configuration for non-C/C++ languages (Java, C#, etc.)
Used to select a clock
Configures Durable Writer History and Durable Reader State for high reliability in the face of component or system failures (see
the User’s Manual’s chapter on Mechanisms for Achieving Information Durability and Persistence)
Allows you to attach additional information to entities—such as for identification/authentication/authorization
Often used with:
TransportBuiltin, TransportSelection, TransportUnicast, TransportMulticast—configures aspects of built-in and
dynamically loaded transports.
DomainParticipantResourceLimits—configures maximum size of discoverable properties.
Availability, Durability, DurabilityService—also relevant to highly reliable and available systems.
See also: UserData, GroupData, TopicData—these QoS policies also attach discoverable meta-data to entities.
33. PublishMode Specifies the mechanism that sends user data in an external middleware thread
By default, data is sent in the context of the user thread that calls write()—this is typically the lowest-latency approach.
However, there are times when you may want an internal middleware thread to send the data asynchronously instead. Thus
when the user thread calls write(), the data is only serialized and copied to an internal buffer. A thread owned by the Publisher
is responsible for sending the data when it is scheduled. This QoS policy configures this behavior. It may be SYNCHRONOUS
(default) or ASYNCHRONOUS. You can also use this QoS policy to select a Flow Controller to "shape" the flow of data onto the
network.
"Shaping" a data flow usually means limiting the maximum data rates that Connext DDS will use to send data for a DataWriter.
The flow controller will buffer data sent by the DataWriter faster than the maximum rate configured for the Flow Controller,
and then only send the excess data when the send rate drops below the maximum rate.
The Asynchronous publish mode must also be used to send large data with RELIABLE reliability. Large data is any data that
exceeds the maximum message (packet) size that a transport plugin is configured to send—between 9 and 64 KB in the case of
UDP transports, depending on the configuration.
Note that a single thread will be used to send data for all of the asynchronous DataWriter’s of a Publisher. If you want to have
different threads to send data asynchronously, you must use multiple Publishers.
Often used with:
LatencyBudget—controls the scheduling of asynchronously published data onto the network across DataWriters.
Property—configures maximum size of available transport plug-ins.
AsynchronousPublisher—lower-level settings pertaining to asynchronous publication, like thread priority and options.
See also: Flow Controllers (must be configured via the API, see the User’s Manual)
34. ReaderDataLifecycle Controls how a DataReader manages the lifecycle of the data that it has received.
When a DataReader receives data, it caches that data for your application. Your application may either take the data from the
cache or leave it there. This QoS policy controls when the middleware will automatically remove the stored data for a particular
instance of the Topic from the cache (such that the data is no longer accessible by the application) when it detects that there
are no more DataWriters alive for a specific instance or when a DataWriter has disposed the instance. For Topics without keys,
this applies when there are no DataWriters alive for the Topic, or if the Topic has been disposed by a DataWriter.
You can use this QoS policy to specify a delay in which the data removal will occur when the middleware detects one of the two
states described earlier. By default, the delays are specified to be infinite, which configures Connext DDS not to remove any
data from the DataReader’s cache.
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35. ReceiverPool Configures threads used to receive and process data from transports (for example, UDP sockets).
Configures the properties of the “receive” threads that the middleware uses to receive and process data from its installed
transports. This includes setting thread properties, such as priority level and stack size for these threads, as well as the size of
the buffer used to store packets received from a transport. This buffer size limits the largest single packet of data that a
DomainParticipant will accept from a transport. For many applications, the value 65,536 (64 KB) is a good choice, as this is the
largest packet that can be sent/received via UDP.
Note that Connext DDS does not currently share threads among the different transport plugins. Instead, each transport plugin is
automatically allocated a single thread by default; depending on the transport plugin, it may request a separate thread for each
receive port.
Often used with:
Property—configures the maximum message size of individual transports
TransportUnicast, TransportMulticast— RTI uses a separate receive thread per port per transport plug-in by default. To
force Connext DDS to use a separate thread to process the data for a DataReader, set a unique port in the
DataReader’s Transport Unicast or Transport Multicast QoS policy.
36. Reliability Enables reliability protocol for a DataWriter/DataReader connection.
This QoS policy turns on the RTPS reliability protocol between those DataWriters and DataReaders that set this QoS policy to the
RELIABLE value. When the reliability protocol is used, Connext DDS will attempt to repair samples that were not successfully
received by reliable DataReaders.
By itself, the Reliability policy does not set the level of reliability for a connection between a DataWriter and DataReader.
Instead, the level of reliability is controlled in conjunction with other policies, such as History and ResourceLimits, to determine
which data remains relevant and therefore eligible for repair. For example, as a tradeoff for less memory, CPU, and network
usage, you can choose a reduced level of reliability where only the last N values are guaranteed to be delivered reliably to
DataReaders (N is configurable). With the reduced level of reliability, there are no guarantees that data sent before the last N
are received; only the last N data packets are monitored and repaired if necessary.
A connection between a DataWriter and DataReader may be configured for BEST_EFFORT Reliability, which means Connext DDS
will not use any resources to monitor or guarantee that the data sent by a DataWriter is received by a DataReader. For some use
cases, such as the periodic update of sensor values to a GUI displaying values to a person, "best effort" delivery is often good
enough. It is the fastest, most efficient, and least resource-intensive (CPU and network bandwidth) method of getting the
newest/latest value for a topic from DataWriters to DataReaders. However, there is no guarantee that the data sent will be
received. It may be lost due to a variety of factors, including data loss by the physical transport such as wireless RF or even
Ethernet.
For some data streams (topics), you need to be assured that all data from the DataWriter is received reliably by the
DataReaders. Connext DDS will ensure data was received and repair any lost data by resending a copy as many times as needed
until the DataReader receives it. This level of reliability is considered “strictly reliable.” For strict reliability, use the KEEP_ALL
setting for the History QoS policy for both the DataWriter and DataReader along with the RELIABLE setting for the Reliability
QoS policy.
RELIABLE DataWriters may send data to both BEST_EFFORT and RELIABLE DataReaders, but RELIABLE DataReaders can only
receive data from RELIABLE DataWriters.
The RTPS reliability protocol is highly configurable using the DataWriterProtocol and DataReaderProtocol QoS policy. Most users
will need to configure the reliability protocol to meet their real-time requirements since the default configuration is not
designed for speedy packet-loss detection and repair. In the examples directory of the Connext DDS installation, you should find
an example QoS configuration for the reliability protocol that is better suited for many applications.
To maintain reliability in the face of component or system failures, you can use RTI Persistence Service to store sent data on
other nodes or even to disk elsewhere on the network using the Durability and DurabilityService QoS policies.
Often used with:
History—governs how many previously written samples remain relevant; set history kind to KEEP_ALL for TCP-like reliability
over connectionless transports like UDP
Lifespan—governs how long previously written samples remain relevant; data with an expired lifespan will not be repaired even
if lost
ResourceLimits—governs the total sizes of DataWriter sample caches used to maintain commitments if a finite reliability send
window has not been set (see DataWriterProtocol)
Availability, Durability, DurabilityService—configure additional data sources to maintain reliable delivery in the face of partial or
total system failure
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DataWriterProtocol—a variety of parameters for controlling the RTPS reliability protocol for a reliable DataWriter, including the
size of the reliability “send window” and the rate at which heartbeats are sent. This also controls whether reliability will be
maintained only by means of negative acknowledgments, or by a combination of positive and negative acknowledgments (the
default).
DataReaderProtocol—includes corresponding parameters for controlling the reliability protocol for the reliable DataReader.
Among other things, this controls whether reliability will be maintained only by means of negative acknowledgments, or by a
combination of positive and negative acknowledgments (the default).
37. ResourceLimits Controls amount of physical memory is allocated for middleware entities; if dynamic allocations are
allowed and how they occur; and memory usage among different instance values for keyed topics.
Configures the amount of memory a DataWriter or DataReader may allocate to store data in a local cache (also referred to as
send or receive queues, respectively). The max_samples parameter in this policy also has a role in throttling the send rate of
reliable DataWriters, although starting in version 4.5, using the “send window” properties of the DataWriterProtocol QoS policy
is the recommended way to configure this behavior.
This QoS policy can limit how much system memory can be allocated by the middleware. For embedded real-time systems and
safety-critical systems, predetermination of maximum memory usage is often required. In addition, dynamic memory allocation
may introduce non-deterministic latencies in time-critical paths. By setting the initial and maximum settings to the same values
for different parameters of this QoS policy, you can be assured that an entity will not dynamically allocate more memory after
its initialization phase.
Often used with:
History—configures the window of relevant previously written samples, which must fit within the limits set by the
ResourceLimits policy.
Reliability – the physical data storage limits of the DataWriter and DataReader caches can affect the performance of the
protocol for reliable connections.
See also: DataWriterProtocol—recommended for taking over the role of reliability throttling from the ResourceLimits policy as
of version 4.5.
38. Service Specifies the type of service for the entity.
This policy is intended for use by RTI infrastructure services and should not be modified by user applications.
39. SystemResourceLimits Configures DomainParticipant-independent resources.
Within a single process (or address space for some supported real-time operating systems), applications may create and use
multiple DomainParticipants. This QoS policy governs resources at the process level, across DomainParticipants.
This QoS policy places an effective limit on the number of DomainParticipants that can be created in a single process. You may
need to modify this QoS policy if you need to create more than 10 DomainParticipants in a process.
40. TimeBasedFilter Sets a minimum time period before new data for an instance is provided to a DataReader, excess data
sent faster than the period set are not sent or otherwise discarded.
DataWriters may send data faster than needed by a DataReader. For example, a DataReader of sensor data that is displayed to a
human in a GUI application often does not need data updates faster than a human can reasonably perceive changes in data
values. The period between updates is often measured in tenths (0.1) of a second up to several seconds. However, a sensor-
information DataWriter may have DataReaders that are processing the sensor information to control parts of the system and
thus need new data updates with periods of hundredths (0.01) or thousandths (0.001) of a second. This policy allows you to
optimize resource usage (CPU and possibly network bandwidth) by only delivering the required amount of data to different
DataReaders and filtering out samples that arrive faster than a rate you specify (in terms of a period of time between data
arrival).
You can use this QoS policy reduce the amount of data received by a DataReader in order to avoid overwhelming a potentially
slow consumer. You can also use it to reduce the amount of data placed on the network by a DataWriter, to avoid
overwhelming network resources.
Different DataReaders for the same Topic are allowed to set their own time-based filters, so that data published faster than the
period set by a DataReader will be dropped by the middleware and not delivered to the DataReader—without perturbing other
DataReaders.
This time-based data-filtering does not depend on any DataWriter settings. However, it also does not force the DataWriter to
send at a specific rate. How fast a DataWriter updates new data is entirely under the control of the user code.
See also: ContentFilteredTopics (Not a QoS policy but a feature in which you can create DataReaders that receive data that are
filtered based on the value of the data in addition to time. See the User’s Manual.)
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41. TopicData Attaches arbitrary application data to discovery meta-data at the topic level.
See: UserData.
42. TransportBuiltin Specifies which built-in transports are used.
Three transport plug-ins are built into the core Connext DDS libraries: UDPv4, shared memory, and UDPv6. (This is true for most
supported target platforms. However, on certain embedded platforms, shared memory and/or UDPv6 may not be supported.
Please consult the Platform Notes for more information.) This QoS policy controls which of these built-in transport plug-ins are
to be used by a DomainParticipant.
By default, the UDPv4 and (on platforms that support it) shared memory transports are enabled. Most applications will not need
to change this default.
You may want to disable the shared memory transport if you do not want applications to use shared memory to communicate
when running on the same node. This may be the case in a lab environment, for example, where some users kill their
applications ungracefully, leaving shared memory regions orphaned and unable to be cleaned up by other users of the same
machines. Be aware that the availability of the shared memory transport should be set in the same way for all applications
running on the same machine in the same domain. Otherwise, discovery will not complete.
See also: Property—Used to install external (that is, not built in) transports.
43. TransportMulticast Specifies the multicast address on which a DataReader is to receive its data.
By default, DataWriters will send individually addressed packets to each DataReader that subscribes to the topic of the
DataWriter; this is known as unicast delivery. One copy of the data is sent for each DataReader. This configuration is the
simplest to configure and the easiest to manage. However, the network bandwidth used by a DataWriter increases linearly with
the number of DataReaders. Multicast addressing (on UDP/IP transports) allows a DataWriter to send a single network packet
that will be received by many DataReaders. Thus the network bandwidth usage will be nearly constant independent of the
number of DataReaders.
Coordinating the multicast address specified by DataReaders can help optimize network bandwidth usage in systems where
there are multiple DataReaders for the same Topic. Note that you can also specify a port number as well as a subset of the
available transports with which to receive the multicast data. These settings—multicast address, port number, and transports—
do not need to be set uniformly for all DataReaders of the same Topic. A DataWriter will automatically detect which
DataReaders can be reached at the same network location and which other DataReaders need to be addressed separately.
See also:
TransportBuiltin—Identifies the available built-in transports
Property—Used to install external (that is, not built in) transports
44. TransportMulticastMapping Specifies the automatic mapping between a list of topic expressions and multicast addresses
that can be used by a DataReader to receive data for a specific topic.
This QoS policy provides an easy way to configure and assign multicast addresses to DataReaders based on the names of their
associated Topics. You can specify an automatic mapping between a list of topic expressions and a multicast address that can be
used by the DataReader to receive the data. To have a DataReader use this QoS policy to select its multicast address, its own
TransportMulticast QoS Policy must be set accordingly.
45. TransportPriority Tells Connext DDS that the data being sent has a different "priority" than other data. Provides a
priority value to the underlying transport protocol, for those that can use it.
Some transport protocols have a concept of user-settable “priorities” that may be used by operating system network stacks and
switching hardware in between the publisher and the subscriber. For such transports and on supported OSs, Connext DDS will
pass this value to the transport for its use. For other transports and other OSs, the middleware will ignore this value. For more
information about whether Connext DDS can preserve this value for your transports and OSs, please consult the Platform Notes.
46. TransportSelection Selects which transports a DataWriter or DataReader may use to send or receive its data.
This QoS policy is used in the advanced case where multiple transports have been installed and only a subset of them are to be
used with a particular DataReader or DataWriter. For example, suppose that transports for UDP/IP, shared memory, Infiniband,
and VME transports are available, and a DataReader should only receive data over Infiniband.
See also:
TransportBuiltin—Identifies the available built-in transports
Property—Used to install external (that is, not built in) transports
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47. TransportUnicast Specifies a subset of transports and a port number that can be used by an Entity to receive data.
Connext DDS may send data to a variety of Entities, not just DataReaders. For example, reliable DataWriters may receive
ACK/NACK packets from reliable DataReaders. During discovery, each Entity announces to remote applications a list of (up to 4)
unicast addresses to which the remote application should send data (either user data packets or reliable protocol meta-data
such as ACK/NACKs and heartbeats).
By default, the list of addresses is populated automatically with values obtained from the enabled transport plug-ins that are
allowed to be used by the Entity (see the TransportBuiltin and TransportSelection QoS policies). The associated ports are
automatically determined. This default behavior is appropriate for most applications.
Use this QoS policy to manually select a subset of the available receive addresses for an Entity or to specify a non-default
receive port.
See also:
TransportBuiltin—Specifies the available built-in transports at the DomainParticipant level
Property—Used to install additional external transports at the DomainParticipant level
TransportSelection—Specifies a subset of the available built-in and external transports available for a particular DataWriter or
DataReader
48. TypeConsistencyEnforcement Defines rules that determine whether the type used to publish a given topic is consistent
with that used to subscribe to it. Also
This policy specifies a type consistency kind, either DISALLOW_TYPE_COERCION or DDS_ALLOW_TYPE_COERCION (default).
When Connext DDS is introspecting the built-in topic data declaration of a remote DataReader in order to determine whether it
can match with a local DataWriter, if it observes that no TypeConsistencyEnforcementQosPolicy value is provided, it assumes a
kind of DISALLOW_TYPE_COERCION.
49. TypeSupport Attaches application-specific values to a DataWriter/DataReader, which are passed to the
serialization/deserialization routine of the associated data type. Also controls if padding bytes are set to 0
during serialization.
You can modify the rtiddsgen-generated code so that the de/serialization routines act differently depending on the information
passed in via the object pointer.
RTI generally recommends that you treat generated source files as compiler outputs (analogous to object files) and that you do
not modify them. RTI cannot support user changes to generated source files. Furthermore, such changes would make upgrading
to newer versions of Connext DDS more difficult, as this generated code is considered to be a part of the middleware
implementation and consequently does change from version to version. The plugin_data field in this QoS policy should be
considered a back door, only to be used after careful design consideration, testing, and consultation with your RTI
representative.
50. UserData [GroupData, TopicData] Attaches arbitrary application data (a buffer of bytes) to discovery meta-data.
These QoS policies attach extra data that you want propagated at discovery time. This extra data is interpreted by the end user
application and is not used by the middleware itself.
UserData attaches discoverable meta-data at the DataWriter/DataReader level.
GroupData attaches discoverable meta-data at the Publisher/Subscriber level.
TopicData attaches discoverable meta-data at the Topic level.
Use cases for these policies include application-to-application identification, authentication, authorization, and encryption
purposes. For example, applications can use these policies to send security certificates to each other for RSA-type security.
See also:
DomainParticipantResourceLimits – Sets the maximum length of the extra data that can be attached (default 256 bytes).
Property—an alternative means of attaching discoverable meta-data to entities.
51. WireProtocol Configures the DDS on-the-wire protocol (RTPS).
Configures some global RTPS properties. The DataWriterProtocol and DataReaderProtocol QoS policies configure RTPS and
reliability properties on a per DataWriter or DataReader basis.
52. WriterDataLifecycle Controls how a DataWriter handles the lifecycle of the instances that it is registered to manage.
The behavior controlled by this QoS policy applies on a per instance (key) basis for keyed Topics, so that when a DataWriter
unregisters an instance, Connext DDS can automatically also dispose that instance. This is the default behavior.
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In cases where the ownership of a Topic is exclusive (see the Ownership QoS policy), DataWriters may want to relinquish
ownership of a particular instance of the Topic to allow other DataWriters to send updates for the value of that instance
regardless of how the OwnershipStrength QoS policy is set. In that case, you may only want a DataWriter to unregister an
instance without disposing the instance. Disposing an instance is a statement that an instance no longer exists. User applications
may be coded to trigger on the disposal of instances, thus the ability to unregister without disposing may be useful to properly
maintain the semantic of disposal.
