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@ -28,6 +28,7 @@
* [Does adding a “max\_age” to a “channel” for NATS streaming server connected to a SQL store, retroactively delete messages?](faq.md#does-adding-a-max_age-to-a-channel-for-nats-streaming-server-connected-to-a-sql-store-retroactively-delete-messages)
* [What is the upgrade path from NATS 1.x to 2.x?](faq.md#what-is-the-upgrade-path-from-nats-1-x-to-2-x)
* [How many clients can connect simultaneously?](faq.md#how-many-clients-can-connect-simultaneously)
* [Does JetStream replace NATS Streaming?](faq.md#does-jetstream-replace-nats-streaming)
## General
@ -130,14 +131,13 @@ No. As of `nats-server` v0.8.0, there is no hard limit on the maximum number of
### Does NATS guarantee message delivery?
NATS is offered as two components: the core server \(referred to simply as "NATS" or "NATS Server"\) and [NATS Streaming](nats-streaming-concepts/intro.md), which is a data streaming service that sits atop NATS core like a client.
* **NATS** implements what is commonly referred to as "at-most-once" delivery. This means that messages are guaranteed to arrive intact, in order from a given publisher, but not across different publishers. NATS does everything required to remain on and provide a dial-tone. However, if a subscriber is problematic or goes offline it will not receive messages, as the basic NATS platform is a simple pub-sub transport system that offers only TCP reliability.
* [**NATS Streaming**](nats-streaming-concepts/intro.md) offers _at-least-once_ delivery guarantees by implementing publish and delivery acknowledgements, and persisting messages to memory or a secondary store until messages have been successfully delivered, or until resource limits or other administrator-defined limits have been reached.
NATS Core Server, simply referred to as NATS Server, offers "at-most-once" delivery. This means messages are guaranteed to arrive intact, in order from a given publisher, but not across different publishers. NATS does everything required to remain available and provide a dial-tone. However, if a subscriber is problematic or goes offline it will not receive messages, as the basic NATS platform is a simple pub-sub transport system that offers only TCP reliability.
As of NATS Server 2.2, NATS JetStream offers persistence as "at-least-once" delivery. See the [JetStream](jetstream/jetstream.md) documentation for detailed information.
### Does NATS support replay/redelivery of historical data?
Yes, historical data may be persisted to memory or secondary storage and replayed using [NATS Streaming](nats-streaming-concepts/intro.md), an event streaming service based on \(and compatible with\) NATS.
NATS [JetStream](jetstream/jetstream.md) offers message store and replay by time or sequence.
### How do I gracefully shut down an asynchronous subscriber?
@ -165,3 +165,7 @@ If you are using TLS you'll want to be sure the hardware can handle the CPU load
All said, each server can be tuned to handle a large number of clients, and given the flexibility and scalability of NATS with clusters, superclusters, and leaf nodes one can build a NATS deployment supporting many millions of connections.
### Does JetStream replace NATS Streaming?
As of NATS Server 2.2, NATS [JetStream](jetstream/jetstream.md) is the recommended option for persistence and message guarantees. See [NATS Streaming](https://github.com/nats-io/nats-streaming-server) README for the deprecation notice.