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Refactor EventContext (#12689)

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Refactor how the `EventContext` class works, with the intention of reducing the amount of state we fetch from the DB during event processing.

The idea here is to get rid of the cached `current_state_ids` and `prev_state_ids` that live in the `EventContext`, and instead defer straight to the database (and its caching). 

One change that may have a noticeable effect is that we now no longer prefill the `get_current_state_ids` cache on a state change. However, that query is relatively light, since its just a case of reading a table from the DB (unlike fetching state at an event which is more heavyweight). For deployments with workers this cache isn't even used.


Part of #12684
This commit is contained in:
Erik Johnston 2022-05-10 20:43:13 +01:00 • committed by GitHub
parent c997bfb926
commit c72d26c1e1
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13 changed files with 70 additions and 200 deletions
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177
synapse/events/snapshot.py
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@ -17,11 +17,8 @@ import attr
from frozendict import frozendict
from typing_extensions import Literal
from twisted.internet.defer import Deferred
from synapse.appservice import ApplicationService
from synapse.events import EventBase
from synapse.logging.context import make_deferred_yieldable, run_in_background
from synapse.types import JsonDict, StateMap
if TYPE_CHECKING:
@ -61,6 +58,9 @@ class EventContext:
If ``state_group`` is None (ie, the event is an outlier),
``state_group_before_event`` will always also be ``None``.
state_delta_due_to_event: If `state_group` and `state_group_before_event` are not None
then this is the delta of the state between the two groups.
prev_group: If it is known, ``state_group``'s prev_group. Note that this being
None does not necessarily mean that ``state_group`` does not have
a prev_group!
@ -79,73 +79,47 @@ class EventContext:
app_service: If this event is being sent by a (local) application service, that
app service.
_current_state_ids: The room state map, including this event - ie, the state
in ``state_group``.
(type, state_key) -> event_id
For an outlier, this is {}
Note that this is a private attribute: it should be accessed via
``get_current_state_ids``. _AsyncEventContext impl calculates this
on-demand: it will be None until that happens.
_prev_state_ids: The room state map, excluding this event - ie, the state
in ``state_group_before_event``. For a non-state
event, this will be the same as _current_state_events.
Note that it is a completely different thing to prev_group!
(type, state_key) -> event_id
For an outlier, this is {}
As with _current_state_ids, this is a private attribute. It should be
accessed via get_prev_state_ids.
partial_state: if True, we may be storing this event with a temporary,
incomplete state.
"""
_storage: "Storage"
rejected: Union[Literal[False], str] = False
_state_group: Optional[int] = None
state_group_before_event: Optional[int] = None
_state_delta_due_to_event: Optional[StateMap[str]] = None
prev_group: Optional[int] = None
delta_ids: Optional[StateMap[str]] = None
app_service: Optional[ApplicationService] = None
_current_state_ids: Optional[StateMap[str]] = None
_prev_state_ids: Optional[StateMap[str]] = None
partial_state: bool = False
@staticmethod
def with_state(
storage: "Storage",
state_group: Optional[int],
state_group_before_event: Optional[int],
current_state_ids: Optional[StateMap[str]],
prev_state_ids: Optional[StateMap[str]],
state_delta_due_to_event: Optional[StateMap[str]],
partial_state: bool,
prev_group: Optional[int] = None,
delta_ids: Optional[StateMap[str]] = None,
) -> "EventContext":
return EventContext(
current_state_ids=current_state_ids,
prev_state_ids=prev_state_ids,
storage=storage,
state_group=state_group,
state_group_before_event=state_group_before_event,
state_delta_due_to_event=state_delta_due_to_event,
prev_group=prev_group,
delta_ids=delta_ids,
partial_state=partial_state,
)
@staticmethod
def for_outlier() -> "EventContext":
def for_outlier(
storage: "Storage",
) -> "EventContext":
"""Return an EventContext instance suitable for persisting an outlier event"""
return EventContext(
current_state_ids={},
prev_state_ids={},
)
return EventContext(storage=storage)
async def serialize(self, event: EventBase, store: "DataStore") -> JsonDict:
"""Converts self to a type that can be serialized as JSON, and then
@ -158,24 +132,14 @@ class EventContext:
The serialized event.
"""
# We don't serialize the full state dicts, instead they get pulled out
# of the DB on the other side. However, the other side can't figure out
# the prev_state_ids, so if we're a state event we include the event
# id that we replaced in the state.
if event.is_state():
prev_state_ids = await self.get_prev_state_ids()
prev_state_id = prev_state_ids.get((event.type, event.state_key))
else:
prev_state_id = None
return {
"prev_state_id": prev_state_id,
"event_type": event.type,
"event_state_key": event.get_state_key(),
"state_group": self._state_group,
"state_group_before_event": self.state_group_before_event,
"rejected": self.rejected,
"prev_group": self.prev_group,
"state_delta_due_to_event": _encode_state_dict(
self._state_delta_due_to_event
),
"delta_ids": _encode_state_dict(self.delta_ids),
"app_service_id": self.app_service.id if self.app_service else None,
"partial_state": self.partial_state,
@ -193,16 +157,16 @@ class EventContext:
Returns:
The event context.
"""
context = _AsyncEventContextImpl(
context = EventContext(
# We use the state_group and prev_state_id stuff to pull the
# current_state_ids out of the DB and construct prev_state_ids.
storage=storage,
prev_state_id=input["prev_state_id"],
event_type=input["event_type"],
event_state_key=input["event_state_key"],
state_group=input["state_group"],
state_group_before_event=input["state_group_before_event"],
prev_group=input["prev_group"],
state_delta_due_to_event=_decode_state_dict(
input["state_delta_due_to_event"]
),
delta_ids=_decode_state_dict(input["delta_ids"]),
rejected=input["rejected"],
partial_state=input.get("partial_state", False),
@ -250,8 +214,15 @@ class EventContext:
if self.rejected:
raise RuntimeError("Attempt to access state_ids of rejected event")
await self._ensure_fetched()
return self._current_state_ids
assert self._state_delta_due_to_event is not None
prev_state_ids = await self.get_prev_state_ids()
if self._state_delta_due_to_event:
prev_state_ids = dict(prev_state_ids)
prev_state_ids.update(self._state_delta_due_to_event)
return prev_state_ids
async def get_prev_state_ids(self) -> StateMap[str]:
"""
@ -266,94 +237,10 @@ class EventContext:
Maps a (type, state_key) to the event ID of the state event matching
this tuple.
"""
await self._ensure_fetched()
# There *should* be previous state IDs now.
assert self._prev_state_ids is not None
return self._prev_state_ids
def get_cached_current_state_ids(self) -> Optional[StateMap[str]]:
"""Gets the current state IDs if we have them already cached.
It is an error to access this for a rejected event, since rejected state should
not make it into the room state. This method will raise an exception if
``rejected`` is set.
Returns:
Returns None if we haven't cached the state or if state_group is None
(which happens when the associated event is an outlier).
Otherwise, returns the the current state IDs.
"""
if self.rejected:
raise RuntimeError("Attempt to access state_ids of rejected event")
return self._current_state_ids
async def _ensure_fetched(self) -> None:
return None
@attr.s(slots=True)
class _AsyncEventContextImpl(EventContext):
"""
An implementation of EventContext which fetches _current_state_ids and
_prev_state_ids from the database on demand.
Attributes:
_storage
_fetching_state_deferred: Resolves when *_state_ids have been calculated.
None if we haven't started calculating yet
_event_type: The type of the event the context is associated with.
_event_state_key: The state_key of the event the context is associated with.
_prev_state_id: If the event associated with the context is a state event,
then `_prev_state_id` is the event_id of the state that was replaced.
"""
# This needs to have a default as we're inheriting
_storage: "Storage" = attr.ib(default=None)
_prev_state_id: Optional[str] = attr.ib(default=None)
_event_type: str = attr.ib(default=None)
_event_state_key: Optional[str] = attr.ib(default=None)
_fetching_state_deferred: Optional["Deferred[None]"] = attr.ib(default=None)
async def _ensure_fetched(self) -> None:
if not self._fetching_state_deferred:
self._fetching_state_deferred = run_in_background(self._fill_out_state)
await make_deferred_yieldable(self._fetching_state_deferred)
async def _fill_out_state(self) -> None:
"""Called to populate the _current_state_ids and _prev_state_ids
attributes by loading from the database.
"""
if self.state_group is None:
# No state group means the event is an outlier. Usually the state_ids dicts are also
# pre-set to empty dicts, but they get reset when the context is serialized, so set
# them to empty dicts again here.
self._current_state_ids = {}
self._prev_state_ids = {}
return
current_state_ids = await self._storage.state.get_state_ids_for_group(
self.state_group
assert self.state_group_before_event is not None
return await self._storage.state.get_state_ids_for_group(
self.state_group_before_event
)
# Set this separately so mypy knows current_state_ids is not None.
self._current_state_ids = current_state_ids
if self._event_state_key is not None:
self._prev_state_ids = dict(current_state_ids)
key = (self._event_type, self._event_state_key)
if self._prev_state_id:
self._prev_state_ids[key] = self._prev_state_id
else:
self._prev_state_ids.pop(key, None)
else:
self._prev_state_ids = current_state_ids
def _encode_state_dict(