Each Consumer, or related group of Consumers, of a Stream will need a Consumer defined. It's ok to define thousands of these pointing at the same Stream.
Consumers can either be
push based where JetStream will deliver the messages as fast as possible to a subject of your choice or
pull based to have control by asking the server for messages. The rate of message delivery in both cases is subject to
ReplayInstant Consumer will receive all messages as fast as possible while a
ReplayOriginal Consumer will receive messages at the rate they were received, which is great for replaying production traffic in staging.
In the orders example above we have 3 Consumers. The first two select a subset of the messages from the Stream by specifying a specific subject like
ORDERS.processed. The Stream consumes
ORDERS.* and this allows you to receive just what you need. The final Consumer receives all messages in a
Consumers track their progress, they know what messages were delivered, acknowledged, etc., and will redeliver messages they sent that were not acknowledged. When first created, the Consumer has to know what message to send as the first one. You can configure either a specific message in the set (
StreamSeq), specific time (
StartTime), all (
DeliverAll) or last (
DeliverLast). This is the starting point and from there, they all behave the same - delivering all of the following messages with optional Acknowledgement.
Acknowledgements default to
AckExplicit - the only supported mode for pull-based Consumers - meaning every message requires a distinct acknowledgement. But for push-based Consumers, you can set
AckNone that does not require any acknowledgement, or
AckAll which quite interestingly allows you to acknowledge a specific message, like message
100, which will also acknowledge messages
AckAll mode can be a great performance boost.
Some messages may cause your applications to crash and cause a never ending loop forever poisoning your system. The
MaxDeliver setting allow you to set a upper bound to how many times a message may be delivered.
To assist with creating monitoring applications, one can set a
SampleFrequency which is a percentage of messages for which the system should sample and create events. These events will include delivery counts and ack waits.
When defining Consumers the items below make up the entire configuration of the Consumer:
How messages should be acknowledged,
How long to allow messages to remain un-acknowledged before attempting redelivery
The initial starting mode of the consumer,
The subject to deliver observed messages. Useful to setup an alternate subject for a regular NatsSubcriber can listen on that subject. Not allowed for pull subscriptions.
The name of the Consumer, which the server will track, allowing resuming consumption where left off.
When consuming from a Stream with many subjects, or wildcards, select only a specific incoming subjects, supports wildcards
Maximum amount times a specific message will be delivered. Use this to avoid poison pills crashing all your services forever
When first consuming messages from the Stream start at this particular message in the set
How messages are sent
What percentage of acknowledgements should be samples for observability, 0-100
When first consuming messages from the Stream start with messages on or after this time
The rate of message delivery in bits per second
The maximum number of messages without acknowledgement that can be outstanding, once this limit is reached message delivery will be suspended