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 (while adhering to the rate limit policy) to a subject of your choice or pull to have control by asking the server for messages. The rate of message delivery in both cases is subject to ReplayPolicy. A 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 push fashion.
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 1 through 99. The 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 an 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. If an ack is required but is not received within the AckWait window, the message will be redelivered.
IMPORTANT
The server may consider an ack arriving out of the window. If a first process fails to ack within the window it's entirely possible, for instance in queue situation, that the message has been redelivered to another consumer. Since this will technically restart the window, the ack from the first consumer will be considered.
This is the default policy. It means that each individual message must be acknowledged. It is the only allowed option for pull consumers.
You do not have to ack any messages, the server will assume ack on delivery.
If you receive a series of messages, you only have to ack the last one you received. All the previous messages received are automatically acknowledged.
Ack Wait is the time in nanoseconds that the server will wait for an ack for any individual message. If an ack is not received in time, the message will be redelivered.
When a consumer is first created, it can specify where in the stream it wants to start receiving messages. This is the DeliverPolicy and it's options are as follows:
All is the default policy. The consumer will start receiving from the earliest available message.
When first consuming messages, the consumer will start receiving messages with the last message added to the stream, so the very last message in the stream when the server realizes the consumer is ready.
When first consuming messages, the consumer will only start receiving messages that were created after the consumer was created.
When first consuming messages, start at this particular message in the set. The consumer is required to specify OptStartSeq, the sequence number to start on. It will receive the closest available message moving forward in the sequence should the message specified have been removed based on the stream limit policy.
When first consuming messages, start with messages on or after this time. The consumer is required to specify OptStartTime, the time in the stream to start at. It will receive the closest available message on or after that time.
The subject to deliver observed messages. Not allowed for pull subscriptions. Deliver subject is required for queue subscribing as it configures a subject that all the queue consumers should listen on.
The name of the Consumer, which the server will track, allowing resuming consumption where left off. By default, a consumer is ephemeral. To make the consumer durable, set the name.
When consuming from a stream with a wildcard subject, this allows you to select a subset of the full wildcard subject to receive messages from.
Flow control is another way for the consumer to manage back pressure. Instead of relying on the rate limit, it relies on the pending limits of max messages and/or max bytes. If the server sends the number of messages or bytes without receiving an ack, it will send a status message letting you know it has reached this limit. Once flow control is tripped, the server will not start sending messages again until the client tells the server, even if all messages have been acknowledged. The message status header will have a code of 100 and have an address in the message reply to field. The reply to address is where (the subject) to publish an empty message to. The status message may have a description like "FlowControl Request"
If the idle heartbeat period is set, the server will send a status message with to the client when the period has elapsed but it has not received any new messages. This lets the client know that it's still there, but just isn't receiving messages. The message status header will have a code of 100. Unlike FlowControl, it will have no reply to address. It may have a description like "Idle Heartbeat"
The maximum number of messages without an acknowledgement that can be outstanding, once this limit is reached message delivery will be suspended. It cannot be used with AckNone ack policy.
The maximum number of times a specific message will be delivered. Applies to any message that is re-sent due to ack policy.
Used to throttle the delivery of messages to the consumer, in bits per second.
The replay policy applies when the DeliverPolicy is DeliverAll, DeliverByStartSequence or DeliverByStartTime since those deliver policies begin reading the stream at a position other than the end. If the policy is ReplayOriginal, the messages in the stream will be pushed to the client at the same rate that they were originally received, simulating the original timing of messages. If the policy is ReplayInstant (the default), the messages will be pushed to the client as fast as possible while adhering to the Ack Policy, Max Ack Pending and the client's ability to consume those messages.
Sets the percentage of acknowledgements that should be sampled for observability, 0-100 This value is a string and for example allows both 30 and 30% as valid values.