mirror of
https://mau.dev/maunium/synapse.git
synced 2024-12-15 13:53:50 +01:00
113 lines
4.9 KiB
Markdown
113 lines
4.9 KiB
Markdown
# Room DAG concepts
|
|
|
|
## Edges
|
|
|
|
The word "edge" comes from graph theory lingo. An edge is just a connection
|
|
between two events. In Synapse, we connect events by specifying their
|
|
`prev_events`. A subsequent event points back at a previous event.
|
|
|
|
```
|
|
A (oldest) <---- B <---- C (most recent)
|
|
```
|
|
|
|
|
|
## Depth and stream ordering
|
|
|
|
Events are normally sorted by `(topological_ordering, stream_ordering)` where
|
|
`topological_ordering` is just `depth`. In other words, we first sort by `depth`
|
|
and then tie-break based on `stream_ordering`. `depth` is incremented as new
|
|
messages are added to the DAG. Normally, `stream_ordering` is an auto
|
|
incrementing integer, but backfilled events start with `stream_ordering=-1` and decrement.
|
|
|
|
---
|
|
|
|
- `/sync` returns things in the order they arrive at the server (`stream_ordering`).
|
|
- `/messages` (and `/backfill` in the federation API) return them in the order determined by the event graph `(topological_ordering, stream_ordering)`.
|
|
|
|
The general idea is that, if you're following a room in real-time (i.e.
|
|
`/sync`), you probably want to see the messages as they arrive at your server,
|
|
rather than skipping any that arrived late; whereas if you're looking at a
|
|
historical section of timeline (i.e. `/messages`), you want to see the best
|
|
representation of the state of the room as others were seeing it at the time.
|
|
|
|
## Outliers
|
|
|
|
We mark an event as an `outlier` when we haven't figured out the state for the
|
|
room at that point in the DAG yet. They are "floating" events that we haven't
|
|
yet correlated to the DAG.
|
|
|
|
Outliers typically arise when we fetch the auth chain or state for a given
|
|
event. When that happens, we just grab the events in the state/auth chain,
|
|
without calculating the state at those events, or backfilling their
|
|
`prev_events`. Since we don't have the state at any events fetched in that
|
|
way, we mark them as outliers.
|
|
|
|
So, typically, we won't have the `prev_events` of an `outlier` in the database,
|
|
(though it's entirely possible that we *might* have them for some other
|
|
reason). Other things that make outliers different from regular events:
|
|
|
|
* We don't have state for them, so there should be no entry in
|
|
`event_to_state_groups` for an outlier. (In practice this isn't always
|
|
the case, though I'm not sure why: see https://github.com/matrix-org/synapse/issues/12201).
|
|
|
|
* We don't record entries for them in the `event_edges`,
|
|
`event_forward_extremeties` or `event_backward_extremities` tables.
|
|
|
|
Since outliers are not tied into the DAG, they do not normally form part of the
|
|
timeline sent down to clients via `/sync` or `/messages`; however there is an
|
|
exception:
|
|
|
|
### Out-of-band membership events
|
|
|
|
A special case of outlier events are some membership events for federated rooms
|
|
that we aren't full members of. For example:
|
|
|
|
* invites received over federation, before we join the room
|
|
* *rejections* for said invites
|
|
* knock events for rooms that we would like to join but have not yet joined.
|
|
|
|
In all the above cases, we don't have the state for the room, which is why they
|
|
are treated as outliers. They are a bit special though, in that they are
|
|
proactively sent to clients via `/sync`.
|
|
|
|
## Forward extremity
|
|
|
|
Most-recent-in-time events in the DAG which are not referenced by any other
|
|
events' `prev_events` yet. (In this definition, outliers, rejected events, and
|
|
soft-failed events don't count.)
|
|
|
|
The forward extremities of a room (or at least, a subset of them, if there are
|
|
more than ten) are used as the `prev_events` when the next event is sent.
|
|
|
|
The "current state" of a room (ie: the state which would be used if we
|
|
generated a new event) is, therefore, the resolution of the room states
|
|
at each of the forward extremities.
|
|
|
|
## Backward extremity
|
|
|
|
The current marker of where we have backfilled up to and will generally be the
|
|
`prev_events` of the oldest-in-time events we have in the DAG. This gives a starting point when
|
|
backfilling history.
|
|
|
|
Note that, unlike forward extremities, we typically don't have any backward
|
|
extremity events themselves in the database - or, if we do, they will be "outliers" (see
|
|
above). Either way, we don't expect to have the room state at a backward extremity.
|
|
|
|
When we persist a non-outlier event, if it was previously a backward extremity,
|
|
we clear it as a backward extremity and set all of its `prev_events` as the new
|
|
backward extremities if they aren't already persisted as non-outliers. This
|
|
therefore keeps the backward extremities up-to-date.
|
|
|
|
## State groups
|
|
|
|
For every non-outlier event we need to know the state at that event. Instead of
|
|
storing the full state for each event in the DB (i.e. a `event_id -> state`
|
|
mapping), which is *very* space inefficient when state doesn't change, we
|
|
instead assign each different set of state a "state group" and then have
|
|
mappings of `event_id -> state_group` and `state_group -> state`.
|
|
|
|
|
|
### Stage group edges
|
|
|
|
TODO: `state_group_edges` is a further optimization...
|
|
notes from @Azrenbeth, https://pastebin.com/seUGVGeT
|