mirror of
https://github.com/element-hq/synapse.git
synced 2024-11-29 07:28:55 +03:00
80 lines
3 KiB
Markdown
80 lines
3 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.
|
||
|
|
|
||
|
|
|
||
|
|
## Forward extremity
|
||
|
|
|
||
|
|
Most-recent-in-time events in the DAG which are not referenced by any other events' `prev_events` yet.
|
||
|
|
|
||
|
|
The forward extremities of a room are used as the `prev_events` when the next event is sent.
|
||
|
|
|
||
|
|
|
||
|
|
## Backwards extremity
|
||
|
|
|
||
|
|
The current marker of where we have backfilled up to and will generally be the
|
||
|
|
oldest-in-time events we know of in the DAG.
|
||
|
|
|
||
|
|
This is an event where we haven't fetched all of the `prev_events` for.
|
||
|
|
|
||
|
|
Once we have fetched all of its `prev_events`, it's unmarked as a backwards
|
||
|
|
extremity (although we may have formed new backwards extremities from the prev
|
||
|
|
events during the backfilling process).
|
||
|
|
|
||
|
|
|
||
|
|
## 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.
|
||
|
|
|
||
|
|
We won't *necessarily* have the `prev_events` of an `outlier` in the database,
|
||
|
|
but it's entirely possible that we *might*. The status of whether we have all of
|
||
|
|
the `prev_events` is marked as a [backwards extremity](#backwards-extremity).
|
||
|
|
|
||
|
|
For example, when we fetch the event auth chain or state for a given event, we
|
||
|
|
mark all of those claimed auth events as outliers because we haven't done the
|
||
|
|
state calculation ourself.
|
||
|
|
|
||
|
|
|
||
|
|
## 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
|