v2.12.0

## Changelog

Refer to the [2.12 Upgrade Guide](https://docs.nats.io/release-notes/whats_new/whats_new_212) for backwards compatibility notes with 2.11.x.

### Go Version

- 1.25.1 (#7273)

### Added

General

- Exponential backoff on route/gateway reconnection attempts (#7042, #7048)
- Added `allow_insecure_cipher_suites` configuration option to `tls` block, as insecure ciphers are now disabled by default (#7144)
- Added `X25519MLKEM768` option to the TLS curve preferences (#7280)
- No responders errors from the server now include the original subject in the `Nats-Subject` header (#5250)
- Added `exact_match` field to the filtering of various monitoring endpoints for server name, host or cluster matching (#7260)

JetStream

- Atomic batch publishes (#6966, #7057, #7060, #7085, #7090, #7091, #7119, #7152, #7175, #7274, #7294, #7313, #7314, #7311, #7316, #7321, #7322, #7327, #7330)
  - Stage-and-commit style batches for JetStream message publishes with normal consistency checks
  - ADR: https://github.com/nats-io/nats-architecture-and-design/blob/main/adr/ADR-50.md
- Counter streams (#6973, #6988, #7081, #7118)
  - Counters are incremented or decremented by publishing to the stream with `Nats-Incr` headers, with full aggregate consistency over sources and mirrors
  - ADR: https://github.com/nats-io/nats-architecture-and-design/blob/main/adr/ADR-49.md
- Prioritised mode for consumer priority groups (#7113)
  - Allows for low-latency switching between clients based on the priority set
  - ADR: https://github.com/nats-io/nats-architecture-and-design/blob/main/adr/ADR-42.md
- Support for trusted proxies (#7153)
  - Allows enforcing that connections arrive via a NATS protocol-aware proxy
  - ADR: https://github.com/nats-io/nats-architecture-and-design/blob/main/adr/ADR-55.md
- Added the ability to promote mirrors to normal streams by removing the mirror configuration (#7171)
  - This allows a mirror to take over operation if the original stream is lost, for example in a disaster recovery scenario
  - This operation cannot be undone and also requires configuring the stream subjects to continue operation
- Added delayed message scheduling (#7170, #7245, #7319)
  - Implements single schedule messages using the `Nats-Schedule-TTL` message header
  - ADR: https://github.com/nats-io/nats-architecture-and-design/blob/main/adr/ADR-51.md
- Added support for offline assets (#7158)
  - Streams and consumers from a higher API version or using unsupported features (i.e. after a downgrade) will be marked as offline to prevent corruption
  - ADR: https://github.com/nats-io/nats-architecture-and-design/blob/main/adr/ADR-44.md 
- Added `partition(n)` for deterministic hash-based partitioning and `random(n)` for non-deterministic random partitioning to subject transforms (#6950)
- Added the ability to specify the required API level in JetStream API requests with the `Nats-Required-Api-Level` request header (#7157)
- Added the ability for streams to opt into async writes (#7315, #7323)
- Add `server_metadata` block to server config, allowing arbitrary key-pairs to be set on a per-server basis, similar to stream/consumer metadata (#6935)
- Support for empty votes, which improves how the Raft election logic handles nodes that have lost their stable storage and attempt to rejoin the cluster regardless (#7038)
- Added `"leader_since"` to the stream and consumer info responses (#7189)

Leafnodes

- Added the ability to isolate leafnode subject interest propagation with the hub-side `isolate_leafnode_interest` configuration option, suppressing east-west traffic and reducing subscription overheads when there are many leafnodes (#7238, #7243, #7277)
- Added `disabled` option for leafnode remotes, with support for reloading from the config at runtime (#7054)

### Changed

General

- WebSocket and MQTT clients no longer use TCP keepalives (#7329)

JetStream

- The JetStream API level has been increased to `2` (#6969)
- The API surface is now in strict mode by default, erroring if unknown fields are provided in the request body (#7049)
- Replicated streams will now default to async flush as long as `sync: always` is not configured (#7018, #7163)
- The default value for `max_buffered_msgs` has been increased by 10x to 100,000 messages (#6633)

### Improved

General

- When logging closed connections, the remote is now logged if known, making it easier to identify where route/gateway/leafnode connections have dropped (#7077)
- Client-specific log lines now include the account and user if known (#7079)

JetStream

- Raft nodes will no longer respond success to catchup messages and will ignore responses if not leader, fixing some potential stream desync scenarios (#6944)
- Replicated streams can now be created even if some of the replica nodes are offline (#7075)
- The storage type is now logged when logging resource limits exceeded (#7076)
- JetStream API requests are always handled from the worker pool, improving the semantics of the API request queue and logging when requests take too long (#7125)
- Filestore in-memory caches now use weak pointers and can respond to garbage collector (GC) pressure, which should help to avoid a number of possible out-of-memory scenarios (#7180)
- Filestore buffer reuse has been fixed (#7295)
- Raft will no longer send any append entries in a known non-leader state (#7297)
- Improved recovery of filestore blocks with unexpected sequence ordering (#7303, #7304, #7305)
- Filestore fixes to improve write correctness, particularly when combined with async mode (#7318, #7331)

Leafnodes

- Leafnode connections without auth no longer unexpectedly connect to the global account (#7116)

### Fixed

JetStream

- Improved validation for stream/consumer configs (#7134)
- An error will now be correctly surfaced when the `Nats-Expected-Last-Subject-Sequence-Subject` header is supplied but the accompanying `Nats-Expected-Last-Subject-Sequence` header is not (#7196)

OCSP

- URL encoding of OCSP requests should now be RFC4648-compliant (#7184)

### Complete Changes
 
https://github.com/nats-io/nats-server/compare/v2.11.0...v2.12.0