OVERVIEW

This specification outlines how to achieve the “channel moderation” feature in Mattermost. Channel moderation provides system admins—and in future channel admins—with toggles to enable and disable various channel-level features including:

Making a channel “read-only” (or the inverse, enabling posting) for members and guests
Allowing or disallowing members and guest from adding and removing other members
Allowing or disallowing members and guests from using channel mentions

GOALS

Describe the backend architecture and required changes.
Describe the webapp changes.
Describe the mobile app changes.

SCOPE

In:

exposing channel moderation in the system console in the channel details view
updates to the UI (webapp and mobile) to apply the create_post permission
updates to the UI (webapp and mobile) to apply the new use_channel_mentions permission

Out:

CLI changes
chat-facing administration of channel moderation settings
plugin changes

BACKGROUND READING

TERMINOLOGY

“channel moderation”: although this term is used throughout this document it may not end up being the customer-facing terminology as some find it confusing compared to an alternative that describes that permissions are being overridden.

“higher-scoped scheme”: the system scheme or team scheme from which the channel derives all of its permissions in the absence of a channel scheme. This is a newly necessary term/concept because previously there was no ongoing inheritance-style relationship between schemes.

“channel mentions”: the at-mentions “@all”, “@here”, and “@channel”.

“channel scheme”: a Schemes record with a Type value of channel. It overrides the permissions for given channels much like a team scheme overrides the permissions for a given team.

SPECIFICATIONS

High-level Architecture

The channel moderation feature leverages the existing permissions system to create a channel scheme for each channel for which moderation is enabled. The channel scheme causes permissions to be overridden and configurable for the channel.

More technically, a record is created in the Schemes table with a Scope value channel. Three new Roles records are also created and their Id values used to set the new Schemes record’s DefaultChannelAdminRole, DefaultChannelUserRole, and DefaultChannelGuestRoles. The new Roles records initially copy their Permissions value from the higher-scoped scheme’s associated Roles records.

Guest and member Roles records Permissions values are then updated as desired to change the permissions of the specific channel associated to the channel scheme.

If moderation is disabled for a channel, the channel’s associated Schemes record is deleted and the channel members and guests automatically revert to using the permissions as configured on the higher-scoped scheme.

Relationship between a channel scheme and its higher-scoped scheme

Per the permissions system design, a channel scheme completely overrides all channel-scoped permissions on the associated channel(s). This means that if there are permissions that are not exposed by channel moderation, admins will expect those permissions to be configured as per the higher-scoped scheme—lest permissions be overridden behind the scenes on the channel scheme without the knowledge of admins.

For example, say the higher-scoped scheme removes the “Archive Channels” permission (technically the delete_public_channel and delete_private_channel permissions). That permission is not configurable on the channel scheme given the current UI, so the system admin would not expect that permission to remain present for all channel that have moderation enabled, in spite of the fact that the permissions architecture would leave it present on the channel scheme by default. So we must have code that removes that permission from the channel scheme for all affected channels.

Since there is no “inheritance” as such between schemes, all channel-scoped permissions that are not modified by the channel moderation UI are updated on the channel scheme upon each change to the higher-scoped scheme.

Channel-scoped permissions are the only type of permissions that can be used by channel schemes, thus they’re the only permissions modifiable by channel moderation settings, and the only permissions that must be updated per changes to the higher-scoped scheme.

Question for dev: Instead of keeping the non-channel-moderated channel-scoped permissions synchronized between the higher-scoped scheme and the channel schemes could we change the core way the permissions system works to use the channel scheme for a set of permissions and the higher-scoped scheme for the rest?

The following actions trigger synchronization of permissions from high-scoped schemes to channel schemes:

Updates to all channel schemes:

add a channel-scoped permission to the system scheme
remove a channel-scoped permission from the system scheme

Updates to all of the channel schemes for a given team:

add a channel-scoped permission to a team scheme (if it has an associated team)
remove a channel-scoped permissions from a team scheme (if it has an associated team)
add a team to a team scheme
remove a team from a team scheme
delete a team scheme
create a team scheme (if it has an associated team)

Question for Platform team: Is this synchronization compatible with the plan for the future custom roles?

Permissions

New permission:

A new permission named use_channel_mentions is created, without which users are not able to use channel mentions.

Guest role permissions modified by channel moderation:

edit_post
delete_post
add_reaction/remove_reaction
create_post
use_channel_mentions

Member role permissions modified by channel moderation:

manage_public_channel_members/manage_private_channel_members
edit_post
edit_others_posts
delete_post
delete_others_posts
add_reaction/remove_reaction
create_post
use_channel_mentions

Guest, member, and channel admin role permissions that are kept synchronized with the higher-scoped scheme:

manage_public_channel_properties
manage_private_channel_properties
delete_public_channel
delete_private_channel
remove_others_reactions
upload_file
create_post_public
create_post_ephemeral
manage_channel_roles
read_channel

Question for PM: Do we need to expose create_post and use_channel_mentions in the system and team schemes UI?

Schema

No schema changes.

REST API

When “enable channel moderation” (see design for exact wording) is toggled on, a POST request is made to the existing API endpoint /api/v4/schemes to create the channel scheme.

The new Roles records that are created will be updated to match the Permissions value from the higher-scoped scheme (as described in the high-level architecture above).

When “enable channel moderation” is toggled off, a DELETE request is made to /api/v4/schemes/:scheme_id.
Update the create post flow (for example in mattermost-server/app/notifications.go) to restrict access to channel mention notifications to those with the use_channel_mentions permission.

CLI

Out of scope.

Configuration

None needed unless channel moderation is experimental.

Question for PM: will channel moderation be experimental?

Webapp only

TBD. Designs for the channel moderation UI in the system console is pending.

Mobile and Webapp

Preventing posting based on permissions (rather than it currently being done with the “experimental read-only town square” feature) must be implemented throughout the UI.
Preventing access to channel mentions must be controlled by the new use_channel_mentions permission.

Performance

If the synchronization of permissions (as described above) occur synchronously then those actions will incur a performance degradation.

If the actions occur asynchronously—say in a background job—then the actions will not incur a performance degradation, but at the cost of added complexity.

Plugins