Create a NATS JetStream durable consumer and reliably process messages with acknowledgment

Steps

1. Ensure JetStream is enabled on the NATS server (--jetstream flag or jetstream: enabled in config); create a stream that captures your subject(s) by calling js.AddStream with Name, Subjects (e.g., ['orders.>']), Storage (file or memory), and retention policy
2. Create a durable consumer on the stream using js.AddConsumer or the consumer API with Durable set to a stable name; set DeliverPolicy (all, last, new, or by start time/sequence), AckPolicy to explicit, and MaxDeliver to limit redelivery attempts
3. Subscribe using the durable consumer name via js.Subscribe or js.PullSubscribe; for pull consumers call Fetch(n) in a loop to receive batches of messages — pull consumers give the application full control over consumption rate
4. Acknowledge each message explicitly (msg.Ack()) after successful processing; use msg.Nak() to request immediate redelivery, or msg.NakWithDelay(duration) to back off before redelivery; use msg.Term() to permanently discard a message without redelivery
5. Set the MaxDeliver limit on the consumer and configure a dead-letter stream: route terminated or MaxDeliver-exceeded messages to a separate stream using an advisory subject filter or a consumer with a separate subject transform
6. Monitor consumer lag (NumPending) and redelivery rate via the NATS monitoring endpoint or nats consumer info <stream> <consumer> CLI command; high NumPending with low throughput indicates a stalled consumer