Headers

Message headers are used in a variety of JetStream contexts, such de-duplication, auto-purging of messages, metadata from republished messages, and more.

Publish

Headers that can be set by a client when a message being published.

Name	Description	Example	Version
Nats-Msg-Id	Client-defined unique identifier for a message that will be used by the server apply de-duplication within the configured Duplicate Window.	9f01ccf0-8c34-4789-8688-231a2538a98b	2.2.0
Nats-Expected-Stream	Used to assert the published message is received by some expected stream.	my-stream	2.2.0
Nats-Expected-Last-Msg-Id	Used to apply optimistic concurrency control at the stream-level. The value is the last expected Nats-Msg-Id and the server will reject a publish if the current ID does not match.	9f01ccf0-8c34-4789-8688-231a2538a98b	2.2.0
Nats-Expected-Last-Sequence	Used to apply optimistic concurrency control at the stream-level. The value is the last expected sequence and the server will reject a publish if the current sequence does not match.	328	2.2.0
Nats-Expected-Last-Subject-Sequence	Used to apply optimistic concurrency control at the subject-level. The value is the last expected sequence and the server will reject a publish if the current sequence does not match for the message's subject.	38	2.3.1
Nats-Rollup	Used to apply a purge of all prior messages in the stream or at the subject-level.	all for stream, sub for subject	2.6.2

RePublish

Headers set messages that are republished.

Name	Description	Example	Version
Nats-Stream	Name of the stream the message was republished from.	Nats-Stream: my-stream	2.8.3
Nats-Subject	The original subject of the message.	events.mouse_clicked	2.8.3
Nats-Sequence	The original sequence of the message.	193	2.8.3
Nats-Last-Sequence	The last sequence of the message having the same subject, otherwise zero if this is the first message for the subject.	190	2.8.3
Nats-Time-Stamp	The original timestamp of the message.	2023-08-23T19:53:05.762416Z	2.10.0

Sources

Headers that are implicitly added to messages sourced from other streams.

Name	Description	Example	Version
Nats-Stream-Source	Specifies the origin stream name, the subject and the sequence number plus the subject filter and destination transform of the message being sourced.	my-stream	2.2.0

Headers-only

Headers added to messages when the consumer is configured to be "headers only" omitting the body.

Name	Description	Example	Version
Nats-Msg-Size	Indicates the message size in bytes.	1024	2.6.2

Mirror

Headers used for internal flow-control messages for a mirror.

Name	Description	Example	Version
Nats-Last-Consumer			2.2.1
Nats-Last-Stream			2.2.1
Nats-Consumer-Stalled			2.4.0
Nats-Response-Type			2.6.4