matrix-spec-proposals/content/rooms/v1.md

---
title: Room Version 1
type: docs
weight: 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](#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 %}}

### Redactions

[See above](#redactions).

### Event format

Events in version 1 rooms have the following structure:

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

### Authorization rules

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

### 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_event`s {*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.

### Canonical JSON

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