QoS Property

Quality of Service options (Level, Interval, Timeout) for acknowledged delivery.

Syntax

property QoS: TsgcWSQoS_Options read FQoS write SetQoS;

Default Value

—

Remarks

Three delivery guarantees are supported, equivalent to the MQTT model:

Level 0 — "At most once": messages sent from the server are delivered best-effort over TCP with no retry and no acknowledgement.
Level 1 — "At least once": outgoing messages carry an Id; the server keeps them in a pending list and resends them if the client does not acknowledge within Timeout seconds. Duplicates are detected by Id.
Level 2 — "Exactly once": the server performs a two-step PubRec/PubRel handshake before releasing the message to subscribers. Use this for operations where duplicate or lost messages are not acceptable.

On Level 2 the PubRel that releases a message is accepted only from the connection that published it, and any Level 2 message still waiting for its PubRel is discarded when that connection closes. One client cannot release, or replay, a message published by another.

Interval controls how often the internal QoS timer scans pending messages; Timeout is the number of seconds without acknowledgement after which a message is resent to the client.

Example

oProtocol.QoS.Level := qosLevel1;
oProtocol.QoS.Interval := 60;
oProtocol.QoS.Timeout := 300;

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