Patrick McFadin created CASSANDRA-21661:
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Summary: Stop rebuilding a map of every table on each
DistributedSchema construction
Key: CASSANDRA-21661
URL: https://issues.apache.org/jira/browse/CASSANDRA-21661
Project: Apache Cassandra
Issue Type: Improvement
Components: Cluster/Schema
Reporter: Patrick McFadin
Assignee: Patrick McFadin
h2. What
DistributedSchema's constructor builds a flat
ImmutableMap<TableId,TableMetadata>
over every table in every keyspace, then validate() walks them again. It runs
twice per DDL -- once in AlterSchema, once via Transformer.build ->
withLastModified. Keyspaces already maintains that mapping internally and
already
exposes getTableOrViewNullable(id), so it is a rebuild of data the constructor
was
just handed.
h2. Change
Delete keyspacesToTableMap and the field it populated; point the single reader
at
keyspaces.getTableOrViewNullable(id). Net -11/+1 lines, no API change.
validate() is left alone deliberately: its allocation is per-keyspace, not
per-table (the Preconditions.checkArgument calls bind Guava's 3-Object overload,
not varargs). The after-numbers confirm it -- they do not move when the table
count grows 8x.
h2. Result
Per withLastModified call:
|| fixture || 400 tables || 3200 tables || growth ||
| before, 10 keyspaces | 27,288 B | 182,808 B | 6.7x |
| before, 1 keyspace | 22,812 B | 179,825 B | 7.9x |
| after, 10 keyspaces | 2,032 B | 2,032 B | 1.0x |
| after, 1 keyspace | 401 B | 376 B | 0.9x |
h2. One behaviour not preserved
ImmutableMap.Builder.build() threw on duplicate keys, implicitly asserting no
two
keyspaces share a TableId. Keyspaces' BTreeMap collapses such a duplicate
silently, so that check is gone. Not restored here: Keyspaces had already
collapsed it before DistributedSchema saw it, every other TCM lookup already
went
through the collapsed map, and a set-based check would reintroduce the
allocation
this removes. Right home is an O(1) guard in Keyspaces.withTablesViews. No
existing test covers it. Recorded as a decision, not a silent loss.
h2. Tests
New DistributedSchemaScalingTest: 2 allocation assertions, 5 correctness guards
including views reachable by their own id (the old map came from
tablesAndViews(),
so delegation must not narrow to base tables). Verified red without the fix.
Regression: schema 28 suites/137 tests, tcm 13/55, 0 failures. checkstyle clean.
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