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---
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title: Room Versions
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type: docs
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weight: 60
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---
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Rooms are central to how Matrix operates, and have strict rules for what
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is allowed to be contained within them. Rooms can also have various
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algorithms that handle different tasks, such as what to do when two or
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more events collide in the underlying DAG. To allow rooms to be improved
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upon through new algorithms or rules, "room versions" are employed to
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manage a set of expectations for each room. New room versions are
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assigned as needed.
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There is no implicit ordering or hierarchy to room versions, and their
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principles are immutable once placed in the specification. Although
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there is a recommended set of versions, some rooms may benefit from
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features introduced by other versions. Rooms move between different
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versions by "upgrading" to the desired version. Due to versions not
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being ordered or hierarchical, this means a room can "upgrade" from
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version 2 to version 1, if it is so desired.
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## Feature matrix
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Some functionality is only available in specific room versions, such
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as knocking. The table below shows which versions support which features
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from a client's perspective. Server implementations are still welcome
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to reference the following table, however the detailed per-version
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specifications are more likely to be of interest.
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<!--
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Dev note: When the room version columns get overwhelming, merge versions
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1 through 6 as "1 ... 6" or similar given they don't add any features.
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Alternatively, consider flipping the column/row organization to be features
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up top and versions on the left.
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-->
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| Feature \ Version | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
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|-------------------|---|---|---|---|---|---|---|---|---|
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| **Knocking** | ❌ | ❌ | ❌ | ❌ | ❌ | ❌ | ✔ | ✔ | ✔ |
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| **Restricted join rules** | ❌ | ❌ | ❌ | ❌ | ❌ | ❌ | ❌ | ✔ | ✔ |
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## Complete list of room versions
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Room versions are divided into two distinct groups: stable and unstable.
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Stable room versions may be used by rooms safely. Unstable room versions
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are everything else which is either not listed in the specification or
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flagged as unstable for some other reason. Versions can switch between
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stable and unstable periodically for a variety of reasons, including
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discovered security vulnerabilities and age.
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Clients should not ask room administrators to upgrade their rooms if the
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room is running a stable version. Servers SHOULD use **room version 6** as
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the default room version when creating new rooms.
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The available room versions are:
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- [Version 1](/rooms/v1) - **Stable**. The current version of most
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rooms.
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- [Version 2](/rooms/v2) - **Stable**. Implements State Resolution
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Version 2.
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- [Version 3](/rooms/v3) - **Stable**. Introduces events whose IDs
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are the event's hash.
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- [Version 4](/rooms/v4) - **Stable**. Builds on v3 by using
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URL-safe base64 for event IDs.
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- [Version 5](/rooms/v5) - **Stable**. Introduces enforcement of
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signing key validity periods.
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- [Version 6](/rooms/v6) - **Stable**. Alters several
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authorization rules for events.
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- [Version 7](/rooms/v7) - **Stable**. Introduces knocking.
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- [Version 8](/rooms/v8) - **Stable**. Adds a join rule to allow members
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of another room to join without invite.
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- [Version 9](/rooms/v9) - **Stable**. Builds on v8 to fix issues when
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redacting some membership events.
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## Room version grammar
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Room versions are used to change properties of rooms that may not be
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compatible with other servers. For example, changing the rules for event
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authorization would cause older servers to potentially end up in a
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split-brain situation due to not understanding the new rules.
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A room version is defined as a string of characters which MUST NOT
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exceed 32 codepoints in length. Room versions MUST NOT be empty and
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SHOULD contain only the characters `a-z`, `0-9`, `.`, and `-`.
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Room versions are not intended to be parsed and should be treated as
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opaque identifiers. Room versions consisting only of the characters
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`0-9` and `.` are reserved for future versions of the Matrix protocol.
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The complete grammar for a legal room version is:
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room_version = 1*room_version_char
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room_version_char = DIGIT
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/ %x61-7A ; a-z
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/ "-" / "."
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Examples of valid room versions are:
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- `1` (would be reserved by the Matrix protocol)
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- `1.2` (would be reserved by the Matrix protocol)
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- `1.2-beta`
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- `com.example.version`
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