Dds Knowledge Patch

Data Distribution Service (DDS)

Use this skill when implementing or debugging DDS data access, QoS matching, discovery, reliable RTPS delivery, product configuration, interoperability, or ROS 2 RMW behavior.

Reference index

Compatibility-critical behavior

Separate discovery from endpoint matching

A remote participant can be visible while its readers and writers remain unmatched.

Check the layers in order:

1. Confirm SPDP participant visibility and lease health.
2. Confirm SEDP endpoint announcements arrive.
3. Inspect advertised locators for bidirectional reachability.
4. Compare topic and type identity.
5. Compare partitions.
6. Compare every requested/offered QoS policy.

Multicast discovery success does not prove that advertised unicast user-data locators are reachable.

Apply requested-versus-offered QoS directionally

Compatibility is not an equality test:

Incompatible endpoints may still discover one another. Use requested- and offered-incompatible-QoS status to distinguish discovery from matching.

Treat configuration as release-specific

Configuration schemas, accepted elements, defaults, file-loading rules, and feature support can vary by deployed release.

For repeatable diagnosis, record:

• the implementation and release;
• every loaded configuration file;
• selected profile or participant configuration;
• inherited profile bases;
• environment-selected configuration;
• programmatic overrides;
• active transports and advertised interfaces.

An unknown or misplaced configuration element may prevent startup rather than being ignored.

Preserve a common network transport

Shared memory, data sharing, loaning, and zero-copy are local optimizations with eligibility and support constraints. They do not replace a mutually supported network path for remote or cross-implementation communication.

Data access quick reference

Choose read or take deliberately

read returns matching samples, leaves them in the reader cache, and marks them read. take removes returned samples.

Selection masks combine three independent dimensions:

• sample state: READ or NOT_READ;
• view state: NEW or NOT_NEW;
• instance state: ALIVE, NOT_ALIVE_DISPOSED, or NOT_ALIVE_NO_WRITERS.

Always inspect SampleInfo.valid_data. A lifecycle notification can provide instance metadata without a new valid value.

Do not treat status as an event queue

Statuses expose cumulative counts and changes since the relevant status was last accessed. Reading a status, taking data, or invoking a listener can reset the corresponding change or status flag.

Do not assume one queued callback per occurrence. Listener callbacks normally run on implementation-controlled threads, so keep them short and hand work to application-owned execution without blocking or taking application locks.

Treat topic QoS as a creation-time template

Copying topic QoS into a reader or writer does not keep the endpoint linked to later topic-QoS changes. Check each policy's mutability before updating an enabled entity; recreate the entity when an immutable policy must change.

Delivery and presentation quick reference

Bound durability and history expectations

TRANSIENT_LOCAL history lives in the originating writer and disappears with that writer. TRANSIENT and PERSISTENT depend on durability-service support that differs among implementations.

KEEP_ALL is still bounded by resource limits. With reliable delivery, a slow reader can cause writes to block or make historical delivery fail.

Keep coherent access in scope

PRESENTATION separately chooses INSTANCE, TOPIC, or GROUP access scope and requests coherent or ordered access.

Coherent-change brackets can expose a compatible set of writes together, but they do not provide database isolation or rollback. Group scope may also depend on implementation support.

Use partitions for matching, not security

PARTITION is configured on publishers and subscribers. Its default is one empty string, and matching requires the two string sequences to intersect.

For portable matching, use a literal on at least one side instead of expecting two wildcard expressions to be intersected as regular expressions.

Partitions are mutable after enablement and may add or remove matches. They are discovery-visible metadata, not an authorization boundary.

RTPS quick reference

Interpret wire identity correctly

An RTPS GUID combines a participant GuidPrefix with an entity-local EntityId. Predefined entity IDs identify built-in discovery endpoints.

A Locator carries a transport kind, port, and address. INFO_SRC, INFO_DST, and INFO_TS alter source, destination, or timestamp context for following submessages.

Diagnose reliable repair per writer

Ordering is based on writer GUID plus monotonically increasing writer-local sequence number, not a global sequence or wall-clock time.

• HEARTBEAT advertises the available sequence range.
• ACKNACK reports the next expected sequence and a missing-number bitmap.
• GAP says changes are filtered, removed, or otherwise unavailable.
• HEARTBEAT_FRAG and NACK_FRAG repair fragmented DATA_FRAG samples.

Inspect SPDP before SEDP

SPDP creates and refreshes remote-participant proxy state from participant announcements. Lease expiry or explicit disposal removes that state and its dependent matches.

SPDP does not carry the complete user-endpoint catalog. SEDP reliably announces publications and subscriptions, including identity, association, topic/type data, locators, and relevant QoS.

Implementation configuration quick reference

Cyclone DDS

CYCLONEDDS_URI selects XML configuration resources. A Domain/@Id="any" block is reusable across domains but does not choose the participant's domain ID.

On multi-homed hosts, containers, VPNs, or asymmetric networks, explicitly select and prioritize a reachable interface when automatic selection advertises the wrong address. If multicast is disabled, configure reachable unicast peers and preserve bidirectional reachability of discovered locators.

Check domain/participant-index port mapping, participant-index search limits, port conflicts, and firewalls. Verify manual-liveliness support before relying on it. Writer linger can allow final reliable acknowledgements, but orderly entity deletion is still required.

Fast DDS

Choose simple discovery, static EDP, Discovery Server, or manual assertion with their distinct operational assumptions. Static EDP requires consistent endpoint definitions and IDs. Discovery Server clients need explicitly configured, optionally redundant servers.

FASTDDS_DEFAULT_PROFILES_FILE selects a default XML profile file, while supported releases can use SKIP_DEFAULT_XML to suppress default loading. Profile names are local identifiers, not topic or partition names.

Synchronous publication performs transport work on the writer's thread. Asynchronous publication queues it for background work and can apply flow control. Neither mode changes DDS reliability or history semantics.

Connext

Keep modern C++ and classic C++ examples with their matching API mapping and installed release. Select XML QoS by library/profile identity and account for base-profile inheritance, defaults, loaded files, and programmatic overrides.

Keep an interoperable UDP path enabled when shared memory or transport plugins are present.

Cross-implementation checklist

Align all of the following:

• IDL type name and keyed fields;
• extensibility and type-consistency rules;
• XCDR representation;
• topic name;
• partition expressions;
• requested/offered QoS;
• reachable locators and common transports.

Generated source APIs may differ even when the wire contract is compatible.

ROS 2 RMW quick reference

SYSTEM_DEFAULT delegates policy selection to the RMW or DDS implementation. BEST_AVAILABLE, where supported, derives values from discovered endpoints. Use explicit profiles when behavior must be stable across RMW implementations.

Loaned messages, zero-copy or shared-memory delivery, content filtering, and unique network flows are optional capabilities. Query support, handle an unsupported result, and do not assume that switching RMW implementations preserves an implementation-specific optimized path.