* Federation sender should now use considerably less CPU cycles and RAM when sending events into large rooms
* The roomserver now uses considerably less CPU cycles by not calculating event IDs so often
* Experimental support for [MSC2836](https://github.com/matrix-org/matrix-doc/pull/2836) (threading) has been merged
* Dendrite will no longer hold federation HTTP connections open unnecessarily, which should help to reduce ambient CPU/RAM usage and hold fewer long-term file descriptors
### Fixes
* A bug in the latest event updater has been fixed, which should prevent the roomserver from losing forward extremities in some rare cases
* A panic has been fixed when federation is disabled (contributed by [kraem](https://github.com/kraem))
* The response format of the `/joined_members` endpoint has been fixed (contributed by [alexkursell](https://github.com/alexkursell))
* Federation can now be disabled with the `global.disable_federation` configuration option
### Fixes
* The `"since"` parameter is now checked more thoroughly in the sync API, which led to a bug that could cause forgotten rooms to reappear (contributed by [kaniini](https://github.com/kaniini))
* The polylith now proxies signing key requests through the federation sender correctly
* The code for checking if remote servers are allowed to see events now no longer wastes CPU time retrieving irrelevant state events
* Memory optimisation by reference passing, significantly reducing the number of allocations and duplication in memory
* A hook API has been added for experimental MSCs, with an early implementation of MSC2836
* The last seen timestamp and IP address are now updated automatically when calling `/sync`
* The last seen timestamp and IP address are now reported in `/_matrix/client/r0/devices` (contributed by [alexkursell](https://github.com/alexkursell))
* An optional configuration option `sync_api.real_ip_header` has been added for specifying which HTTP header contains the real client IP address (for if Dendrite is running behind a reverse HTTP proxy)
* Partial implementation of `/_matrix/client/r0/admin/whois` (contributed by [DavidSpenler](https://github.com/DavidSpenler))
### Fixes
* A concurrency bug has been fixed in the federation API that could cause Dendrite to crash
* The error when registering a username with invalid characters has been corrected (contributed by [bodqhrohro](https://github.com/bodqhrohro))
* Forward extremities are now calculated using only references from other extremities, rather than including outliers, which should fix cases where state can become corrupted ([#1556](https://github.com/matrix-org/dendrite/pull/1556))
* Old state events will no longer be processed by the sync API as new, which should fix some cases where clients incorrectly believe they have joined or left rooms ([#1548](https://github.com/matrix-org/dendrite/pull/1548))
* More SQLite database locking issues have been resolved in the latest events updater ([#1554](https://github.com/matrix-org/dendrite/pull/1554))
* Internal HTTP API calls are now made using H2C (HTTP/2) in polylith mode, mitigating some potential head-of-line blocking issues ([#1541](https://github.com/matrix-org/dendrite/pull/1541))
* Roomserver output events no longer incorrectly flag state rewrites ([#1557](https://github.com/matrix-org/dendrite/pull/1557))
* Notification levels are now parsed correctly in power level events ([gomatrixserverlib#228](https://github.com/matrix-org/gomatrixserverlib/pull/228), contributed by [Pestdoktor](https://github.com/Pestdoktor))
* Invalid UTF-8 is now correctly rejected when making federation requests ([gomatrixserverlib#229](https://github.com/matrix-org/gomatrixserverlib/pull/229), contributed by [Pestdoktor](https://github.com/Pestdoktor))
* This release makes breaking changes for polylith deployments, since they now use the multi-personality binary rather than separate binary files
* Users of polylith deployments should revise their setups to use the new binary - see the Features section below
* This release also makes breaking changes for Docker deployments, as are now publishing images to Docker Hub in separate repositories for monolith and polylith
* New repositories are as follows: [matrixdotorg/dendrite-monolith](https://hub.docker.com/repository/docker/matrixdotorg/dendrite-monolith) and [matrixdotorg/dendrite-polylith](https://hub.docker.com/repository/docker/matrixdotorg/dendrite-polylith)
* The new `latest` tag will be updated with the latest release, and new versioned tags, e.g. `v0.2.0`, will preserve specific release versions
* [Sample Compose configs](https://github.com/matrix-org/dendrite/tree/master/build/docker) have been updated - if you are running a Docker deployment, please review the changes
* Images for the client API proxy and federation API proxy are no longer provided as they are unsupported - please use [nginx](docs/nginx/) (or another reverse proxy) instead
### Features
* Dendrite polylith deployments now use a special multi-personality binary, rather than separate binaries
* This is cleaner, builds faster and simplifies deployment
* The first command line argument states the component to run, e.g. `./dendrite-polylith-multi roomserver`
* Database migrations are now run at startup
* Invalid UTF-8 in requests is now rejected (contributed by [Pestdoktor](https://github.com/Pestdoktor))
* Fully read markers are now implemented in the client API (contributed by [Lesterpig](https://github.com/Lesterpig))
* Missing auth events are now retrieved from other servers in the room, rather than just the event origin
*`m.room.create` events are now validated properly when processing a `/send_join` response
* The roomserver now implements `KindOld` for handling historic events without them becoming forward extremity candidates, i.e. for backfilled or missing events
### Fixes
* State resolution v2 performance has been improved dramatically when dealing with large state sets
* The roomserver no longer processes outlier events if they are already known
* A SQLite locking issue in the previous events updater has been fixed
* The client API `/state` endpoint now correctly returns state after the leave event, if the user has left the room
* The client API `/createRoom` endpoint now sends cumulative state to the roomserver for the initial room events
* The federation API `/send` endpoint now correctly requests the entire room state from the roomserver when needed
* Some internal HTTP API paths have been fixed in the user API (contributed by [S7evinK](https://github.com/S7evinK))
* A race condition in the rate limiting code resulting in concurrent map writes has been fixed
* Each component now correctly starts a consumer/producer connection in monolith mode (when using Kafka)
* State resolution is no longer run for single trusted state snapshots that have been verified before
* A crash when rolling back the transaction in the latest events updater has been fixed
* Typing events are now ignored when the sender domain does not match the origin server
* Duplicate redaction entries no longer result in database errors
* Recursion has been removed from the code path for retrieving missing events
*`QueryMissingAuthPrevEvents` now returns events that have no associated state as if they are missing
* Signing key fetchers no longer ignore keys for the local domain, if retrieving a key that is not known in the local config
* Federation timeouts have been adjusted so we don't give up on remote requests so quickly
*`create-account` no longer relies on the device database (contributed by [ThatNerdyPikachu](https://github.com/ThatNerdyPikachu))
### Known issues
* Old events can incorrectly appear in `/sync` as if they are new when retrieving missing events from federated servers, causing them to appear at the bottom of the timeline in clients