Durability QoS Policy

The Durability policy controls whether samples are available to late-joining readers.

Values

Value	Description	Persistence
volatile()	Samples only for connected readers	None
transient_local()	Writer caches samples for late joiners	In-memory
persistent()	Samples persisted to disk	Disk

Volatile (Default)

Samples are only delivered to readers that are connected when the sample is written.

use hdds::{Participant, QoS, TransportMode};

let participant = Participant::builder("streaming_app")
    .domain_id(0)
    .with_transport(TransportMode::UdpMulticast)
    .build()?;

let writer = participant
    .topic::<VideoFrame>("video/stream")?
    .writer()
    .qos(QoS::best_effort().volatile())
    .build()?;

Use cases:

• Real-time sensor data
• Video/audio streaming
• Data where history doesn't matter

Transient Local

Writers cache samples in memory. Late-joining readers receive cached samples.

use hdds::{Participant, QoS, TransportMode};

let participant = Participant::builder("config_publisher")
    .domain_id(0)
    .with_transport(TransportMode::UdpMulticast)
    .build()?;

let writer = participant
    .topic::<Configuration>("system/config")?
    .writer()
    .qos(QoS::reliable().keep_last(100).transient_local())
    .build()?;

// Write some samples
writer.write(&config1)?;
writer.write(&config2)?;

// Later, a new reader joins and receives config1, config2

Characteristics:

• Samples persist as long as writer exists
• Cache size controlled by History policy
• Memory usage = history depth × sample size
• Lost when writer terminates

Use cases:

• Configuration/state topics
• Last-known-good values
• Initialization data

Persistent

Samples are persisted to disk. Survives application restarts.

use hdds::{Participant, QoS, TransportMode};

let participant = Participant::builder("event_logger")
    .domain_id(0)
    .with_transport(TransportMode::UdpMulticast)
    .build()?;

// Persistent event log survives restarts
let writer = participant
    .topic::<AuditEvent>("audit/events")?
    .writer()
    .qos(QoS::reliable().keep_all().persistent())
    .build()?;

Characteristics:

• Samples survive writer/application restart
• Higher write latency (disk I/O)
• Disk usage = configured limits

Use cases:

• Event logs
• Audit trails
• Mission-critical state

Compatibility Rules

Durability has an ordered compatibility: writer must offer at least what reader requests.

Writer	Reader	Match?
persistent()	persistent()	✅ Yes
persistent()	transient_local()	✅ Yes
persistent()	volatile()	✅ Yes
transient_local()	transient_local()	✅ Yes
transient_local()	volatile()	✅ Yes
transient_local()	persistent()	❌ No
volatile()	volatile()	✅ Yes
volatile()	transient_local()	❌ No
volatile()	persistent()	❌ No

Rule: Writer.durability >= Reader.durability

Late Joiner Behavior

Memory Considerations

Transient Local Memory Usage

Memory = history_depth × avg_sample_size × max_instances

Example:

• History depth: 100
• Sample size: 1 KB
• Instances: 10
• Memory: 1 MB

Examples

Configuration Topic

use hdds::{Participant, QoS, TransportMode};

let participant = Participant::builder("config_server")
    .domain_id(0)
    .with_transport(TransportMode::UdpMulticast)
    .build()?;

// Writer publishes configuration, late joiners get latest
let writer = participant
    .topic::<AppConfig>("system/config")?
    .writer()
    .qos(QoS::reliable().keep_last(1).transient_local())
    .build()?;

// Reader expects to receive current config on join
let reader = participant
    .topic::<AppConfig>("system/config")?
    .reader()
    .qos(QoS::reliable().transient_local())
    .build()?;

Event Log

use hdds::{Participant, QoS, TransportMode};

let participant = Participant::builder("audit_logger")
    .domain_id(0)
    .with_transport(TransportMode::UdpMulticast)
    .build()?;

// Persistent event log survives restarts
let writer = participant
    .topic::<AuditEvent>("audit/events")?
    .writer()
    .qos(QoS::reliable().keep_all().persistent())
    .build()?;

Volatile Streaming

use hdds::{Participant, QoS, TransportMode};

let participant = Participant::builder("video_streamer")
    .domain_id(0)
    .with_transport(TransportMode::UdpMulticast)
    .build()?;

// Real-time video stream, no caching needed
let writer = participant
    .topic::<VideoFrame>("video/stream")?
    .writer()
    .qos(QoS::best_effort().keep_last(1).volatile())
    .build()?;

Interaction with History

Durability and History work together:

Durability	History	Cached Samples
volatile()	Any	0 (no cache)
transient_local()	keep_last(N)	Last N samples
transient_local()	keep_all()	All samples (bounded by ResourceLimits)
persistent()	keep_last(N)	Last N per instance on disk
persistent()	keep_all()	All samples on disk

Next Steps

• History - Sample buffering configuration
• Reliability - Delivery guarantees