Learning Center
Delivery and store-and-forward
Store-and-forward delivery holds a message until a path to the recipient appears. Learn how outboxes, acknowledgments, retries, deduplication and delay-tolerant networking work, and what a delivery receipt confirms.
Basics
How does store-and-forward messaging work?Accept, store, wait for a path, hand over: the cycle behind email, brokers and meshes.→How does delay-tolerant networking (DTN) work?Bundles, contacts, convergence layers and custody: DTN from the specifications.→What is an outbox?Write first, send later: the durable queue that keeps unsent messages safe.→What is an acknowledgment (ACK)?How a receiver's confirmation drives retries, and why it is not the end of the story.→How does opportunistic networking move data?Store, carry, forward, and the rules for who gets a copy at each meeting.→What is data muling?Carrying data physically between devices that are never in range at the same time.→
Reliability
At-most-once, at-least-once, and exactly-once deliveryWhat each guarantee can lose or repeat, and why exactly-once is end to end.→What is idempotency, and why does offline delivery need it?Making a repeated request safe, so a retry never does the work twice.→What is message deduplication?Recognising copies of a message you have already handled, and dropping them.→What is exponential backoff?Waiting longer between each retry, with a cap and some randomness.→What is a message TTL?Hop limits and expiry times, and why every message needs one.→Delivered, accepted, committed: what does "delivered" mean?Why reaching a device, being accepted and being saved are three different results.→What is a dead-letter queue?Where failed messages go so they can be inspected instead of lost.→What is a delivery receipt?What "delivered" confirms in email and chat, and what it leaves unproven.→How do large files move across a mesh?Chunks, per-chunk acknowledgments, resume and the final whole-file check.→
Relays and long waits
What is a relay server?The middle server that forwards what it cannot always read.→How long can a message wait to be delivered?Expiry limits in push services, email, MQTT and DTN, and how to set your own.→Why do push notifications fail offline?Why push depends on the internet and a vendor server, and what to use instead.→