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DDS in 5 minutes

DDS is a publish-subscribe data-distribution standard. Four nouns:

Domain

A logical isolation boundary, identified by a small integer (typically 0). Two participants on different domains never see each other, even on the same network.

Domain 0:  ┌─────────┐    ┌─────────┐
           │ Robot A │ ↔  │ Robot B │
           └─────────┘    └─────────┘

Domain 7:  ┌─────────┐    ┌─────────┐
           │ Sim env │ ↔  │ Replay  │
           └─────────┘    └─────────┘

Topic

A typed named channel. Topics carry a single IDL type; subscribers match on (topic_name, type_name) and the QoS contract.

Topic "Telemetry" / Type "Robot::Telemetry"
   ┌──────────┐                          ┌──────────┐
   │ Writer A │ ──── samples ────▶       │ Reader X │
   └──────────┘                          └──────────┘

Publisher / DataWriter

Publisher is a factory + scope; DataWriter is the per-topic endpoint that calls write(sample). One publisher can hold many data-writers across different topics.

Subscriber / DataReader

Symmetric to Publisher: Subscriber is the factory, DataReader is the per-topic endpoint that calls take() to pull arrived samples.

QoS — the contract

Publishers offer QoS, subscribers request QoS. Two endpoints match only if the offered ⊇ requested.

The most-used policies:

Policy Default Meaning
Reliability BestEffort Reliable retries until ack; BestEffort ships once
Durability Volatile TransientLocal lets late-joining readers replay
History KeepLast(1) KeepAll retains until cache cap; KeepLast(N) evicts oldest
Deadline infinite Period within which a sample must arrive
Liveliness infinite Heartbeat interval to assert "writer is alive"
Ownership Shared Exclusive → only highest-strength writer wins per instance

Full reference: 03 Configuration → QoS.

Discovery

Endpoints find each other automatically via:

  1. SPDP — Simple Participant Discovery Protocol on UDP multicast (default 239.255.0.1:7400 + 250×domain_id).
  2. SEDP — Simple Endpoint Discovery Protocol over reliable built-in topics, after participants have found each other.

This is why your hello-world publisher and subscriber find each other without any explicit address configuration.

Wire format

ZeroDDS speaks DDSI-RTPS 2.5 on the wire, byte-identical to Cyclone DDS, FastDDS, RTI Connext, and OpenSplice. Sample payloads are encoded in CDR / XCDR2 per OMG-XTypes 1.3.

Next

→ First publisher / subscriber — let's see this in code.