matrix-spec/content/rooms/v1.md

title	type	weight
Room Version 1	docs	10

This room version is the first ever version for rooms, and contains the building blocks for other room versions.

Client considerations

Clients which implement the redaction algorithm locally should refer to the redactions section below.

Redactions

{{% rver-fragment name="v1-redactions" %}}

Server implementation components

{{% boxes/warning %}} The information contained in this section is strictly for server implementors. Applications which use the Client-Server API are generally unaffected by the intricacies contained here. The section above regarding client considerations is the resource that Client-Server API use cases should reference. {{% /boxes/warning %}}

The algorithms defined here should only apply to version 1 rooms. Other algorithms may be used by other room versions, and as such servers should be aware of which version room they are dealing with prior to executing a given algorithm.

{{% boxes/warning %}} Although there are many rooms using room version 1, it is known to have undesirable effects. Servers implementing support for room version 1 should be aware that restrictions should be generally relaxed and that inconsistencies may occur. {{% /boxes/warning %}}

State resolution

{{% boxes/warning %}} Room version 1 is known to have bugs that can cause the state of rooms to reset to older versions of the room's state. For example this could mean that users who had joined the room may be removed from the room, admins and moderators could lose their power level, and users who have been banned from the room may be able to rejoin. Other state events such as the the room's name or topic could also reset to a previous version.

This is fixed in the state resolution algorithm introduced in room version 2. {{% /boxes/warning %}}

The room state S′(E) after an event E is defined in terms of the room state S(E) before E, and depends on whether E is a state event or a message event:

• If E is a message event, then S′(E) = S(E).
• If E is a state event, then S′(E) is S(E), except that its entry corresponding to E's event_type and state_key is replaced by E's event_id.

The room state S(E) before E is the resolution of the set of states {S′(E′), S′(E″), …} consisting of the states after each of E's prev_events {E′, E″, …}.

The resolution of a set of states is defined as follows. The resolved state is built up in a number of passes; here we use R to refer to the results of the resolution so far.

• Start by setting R to the union of the states to be resolved, excluding any conflicting events.
• First we resolve conflicts between m.room.power_levels events. If there is no conflict, this step is skipped, otherwise:
  • Assemble all the m.room.power_levels events from the states to be resolved into a list.
  • Sort the list by ascending depth then descending sha1(event_id).
  • Add the first event in the list to R.
  • For each subsequent event in the list, check that the event would be allowed by the authorization rules for a room in state R. If the event would be allowed, then update R with the event and continue with the next event in the list. If it would not be allowed, stop and continue below with m.room.join_rules events.
• Repeat the above process for conflicts between m.room.join_rules events.
• Repeat the above process for conflicts between m.room.member events.
• No other events affect the authorization rules, so for all other conflicts, just pick the event with the highest depth and lowest sha1(event_id) that passes authentication in R and add it to R.

A conflict occurs between states where those states have different event_ids for the same (event_type, state_key). The events thus affected are said to be conflicting events.

Authorization rules

{{% rver-fragment name="v1-auth-rules" %}}

Event format

Events in version 1 rooms have the following structure:

{{% definition path="api/server-server/definitions/pdu" %}}

Canonical JSON

{{% rver-fragment name="v1-canonical-json" %}}