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spanconfig: introduce the spanconfig.KVSubscriber
KVSubscriber presents a consistent[^1] snapshot of a
spanconfig.StoreReader that's incrementally maintained with changes made
to the global span configurations state. The maintenance happens
transparently; callers can subscribe to learn about what key spans may
have seen a configuration change. After learning about a span update,
consulting the embedded StoreReader would retrieve an up-to-date[^2]
config for it.
When a callback is first installed, it's invoked with the [min,max) span
-- a shorthand to indicate that callers should consult the StoreReader
for all spans of interest. Subsequent updates are of the more
incremental kind. It's possible that the span updates received are
no-ops, i.e. consulting the StoreReader for the given span would
retrieve the last config observed for the span[^2].
type KVSubscriber interface {
StoreReader
OnSpanConfigUpdate(func(updated roachpb.Span))
}
Internally we maintain a rangefeed over the global store of span
configurations (system.span_configurations), applying updates from it
into an embedded spanconfig.Store. A read-only view of this data
structure (spanconfig.StoreReader) is exposed as part of the
KVSubscriber interface. Rangefeeds used as is don't offer any ordering
guarantees with respect to updates made over non-overlapping keys, which
is something we care about[^4]. For that reason we make use of a
rangefeed buffer, accumulating raw rangefeed updates and flushing them
out en-masse in timestamp order when the rangefeed frontier is
bumped[^5]. If the buffer overflows (as dictated by the memory limit the
KVSubscriber is instantiated with), the subscriber is wound down and an
appropriate error is returned to the caller.
When running into the errors above, it's safe for the caller to
re-subscribe to effectively re-establish the underlying rangefeeds. When
re-establishing a new rangefeed and populating a spanconfig.Store using
the contents of the initial scan[^6], we wish to preserve the existing
spanconfig.StoreReader. Discarding it would entail either blocking all
external readers until a new spanconfig.StoreReader was fully populated,
or presenting an inconsistent view of the spanconfig.Store that's
currently being populated. For new rangefeeds what we do then is route
all updates from the initial scan to a fresh spanconfig.Store, and once
the initial scan is done, swap at the source for the exported
spanconfig.StoreReader. During the initial scan, concurrent readers
would continue to observe the last spanconfig.StoreReader if any. After
the swap, it would observe the more up-to-date source instead. Future
incremental updates will also target the new source. When this source
swap occurs, we inform any handlers of the need to possibly refresh
their view of all configs.
This commit also wires up the KVSubscriber into KV stores, replacing the
use of the gossiped system config span (possible given the StoreReader
interface, only happens if a testing flag/env var is set).
[^1]: The contents of the StoreReader at t1 corresponds exactly to the
contents of the global span configuration state at t0 where
t0 <= t1. If the StoreReader is read from at t2 where t2 > t1,
it's guaranteed to observe a view of the global state at t >= t0.
[^2]: For the canonical KVSubscriber implementation, this is typically
the closed timestamp target duration.
[^3]: The canonical KVSubscriber implementation internally
re-establishes feeds when errors occur, possibly re-transmitting
earlier updates (usually through a lazy [min,max) span) despite
possibly not needing to. We could do a bit better and diff the two
data structures, emitting only targeted updates.
[^4]: For a given key k, it's config may be stored as part of a larger
span S (where S.start <= k < S.end). It's possible for S to get
deleted and replaced with sub-spans S1...SN in the same
transaction if the span is getting split. When applying these
updates, we need to make sure to process the deletion event for S
before processing S1...SN.
[^5]: In our example above deleting the config for S and adding configs
for S1...SN, we want to make sure that we apply the full set of
updates all at once -- lest we expose the intermediate state where
the config for S was deleted but the configs for S1...SN were not
yet applied.
[^6]: When tearing down the subscriber due to underlying errors, we
could also surface a checkpoint to use the next time the
subscriber is established. That way we can avoid the full initial
scan over the span configuration state and simply pick up where we
left off with our existing spanconfig.Store.
Release note: None- Loading branch information
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