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Streams are 'message stores', each stream defines how messages are stored and what the limits (duration, size, interest) of the retention are. Streams consume normal NATS subjects, any message published on those subjects will be captured in the defined storage system. You can do a normal publish to the subject for unacknowledged delivery, though it's better to use the JetStream publish calls instead as the JetStream server will reply with an acknowledgement that it was successfully stored.
In the diagram above we show the concept of storing all ORDERS.* in the Stream even though there are many types of order related messages. We'll show how you can selectively consume subsets of messages later. Relatively speaking the Stream is the most resource consuming component so being able to combine related data in this manner is important to consider.
Streams can consume many subjects. Here we have ORDERS.* but we could also consume SHIPPING.state into the same Stream should that make sense (not shown here).
Streams support various retention policies which define when messages in the stream can be automatically deleted, such as when stream limits are hit (like max count, size or age) - if the discard policy is set to 'discard old' - or also more novel options that apply on top of the limits such as 'interest' (automatically deleted after all consumers have received acknowledgement of the delivery of the message to client applications) and 'working queue' (where a message is automatically deleted from the stream when the consumer receives acknowledgement of its consumption from the client application).
Streams support deduplication using a Nats-Msg-Id header and a sliding window within which to track duplicate messages. See the Message Deduplication section.
For examples on how to configure streams with your preferred NATS client, see NATS by Example.
Below are the set of stream configuration options that can be defined. The Version column indicates the version of the server the option was introduced. The Editable column indicates the option can be edited after the stream created. See client-specific examples here.
Name
Names cannot contain whitespace, ., *, >, path separators (forward or backwards slash), and non-printable characters.
2.2.0
No
Storage
The type of storage backend, File and Memory
2.2.0
No
Subjects
A list of subjects to consume, supports wildcards
2.2.0
No
Replicas
How many replicas to keep for each message in a clustered JetStream, maximum 5
2.2.0
Yes
MaxAge
Maximum age of any message in the Stream, expressed in nanoseconds.
2.2.0
Yes
MaxBytes
How many bytes the Stream may contain. Adheres to Discard Policy, removing oldest or refusing new messages if the Stream exceeds this size
2.2.0
Yes
MaxMsgs
How many messages may be in a Stream. Adheres to Discard Policy, removing oldest or refusing new messages if the Stream exceeds this number of messages
2.2.0
Yes
MaxMsgSize
The largest message that will be accepted by the Stream
2.2.0
Yes
MaxConsumers
How many Consumers can be defined for a given Stream, -1 for unlimited
2.2.0
No
NoAck
Disables acknowledging messages that are received by the Stream
2.2.0
Yes
Retention
How message retention is considered, LimitsPolicy (default), InterestPolicy or WorkQueuePolicy
2.2.0
No
Discard
When a Stream reaches it's limits either, DiscardNew refuses new messages while DiscardOld (default) deletes old messages
2.2.0
Yes
Duplicates
The window within which to track duplicate messages, expressed in nanoseconds.
2.2.0
Yes
Placement
Used to declare where the stream should be placed via tags and/or an explicit cluster name.
2.2.0
Yes
Sources
If defined, declares one or more stream this stream will source messages from. See sources.
2.2.0
Yes
MaxMsgsPerSubject
How many messages be in the stream per subject.
2.3.0
Yes
Description
A verbose description of the stream.
2.3.3
Yes
Sealed
Sealed streams do not allow messages to be deleted via limits or API, sealed streams can not be unsealed via configuration update. Can only be set on already created streams via the Update API.
2.6.2
Yes (once)
DenyDelete
Restricts the ability to delete messages from a stream via the API.
2.6.2
No
DenyPurge
Restricts the ability to purge messages from a stream via the API.
2.6.2
No
AllowRollup
Allows the use of the Nats-Rollup header to replace all contents of a stream, or subject in a stream, with a single new message.
2.6.2
Yes
RePublish
If set, messages stored to the stream will be immediately republished to the configured subject.
2.8.3
No (if defined)
AllowDirect
If true, and the stream has more than one replica, each replica will respond to direct get requests for individual messages, not only the leader.
2.9.0
Yes
MirrorDirect
If true, and the stream is a mirror, the mirror will participate in a serving direct get requests for individual messages from origin stream.
2.9.0
Yes
