perf: optimize was_applied fast path for known LWT statements - #797
perf: optimize was_applied fast path for known LWT statements#797mykaul wants to merge 2 commits into
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Rebased onto latest Consistency verdict: safe. The fast path doesn't introduce a new LWT-detection heuristic — it reuses the single canonical Tracing where
Also checked No correctness fix was needed. I rebased the two commits onto current
CI on the PR is green (13/13 checks) and there were no unresolved review threads to address. Force-pushed the rebased branch to |
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Pull request overview
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Optimizes ResultSet.was_applied by adding a fast path for queries that already know they are LWT statements, avoiding expensive batch detection in the common single-LWT case.
Changes:
- Add
query.is_lwt()-based fast path inResultSet.was_appliedto skip batch regex detection for non-batch known-LWT statements - Update and add unit tests to cover fast-path, slow-path fallback, and batch behavior
- Add a micro-benchmark script to measure the fast-path vs slow-path overhead
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.
| File | Description |
|---|---|
cassandra/cluster.py |
Adds LWT fast path in ResultSet.was_applied to skip batch detection when LWT is known |
tests/unit/test_resultset.py |
Adds tests for fast/slow paths and adjusts existing test to avoid accidental fast-path routing |
benchmarks/bench_was_applied.py |
Introduces micro-benchmark comparing fast-path checks vs regex-based slow path |
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Pull request overview
Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.
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cassandra/cluster.py:5923
- This does not remove the work claimed by the fast path. In the previous code,
batch_regex.match()was already guarded byisinstance(query, SimpleStatement), so aBoundStatementnever ran the regex; this branch still performs theBatchStatementcheck and now addsgetattr,callable, and anis_lwt()call. The target BoundStatement path is therefore likely slower rather than faster. Please restore the existing path unless an apples-to-apples benchmark demonstrates an actual reduction, or redesign the branch to eliminate work that BoundStatements previously performed.
is_lwt = getattr(query, 'is_lwt', None)
if not isinstance(query, BatchStatement) and callable(is_lwt) and is_lwt():
benchmarks/bench_was_applied.py:74
- This benchmark cannot attribute its reported “speedup” to this PR: it compares a BoundStatement with a SimpleStatement, but the pre-PR implementation already skipped the regex for BoundStatements. It would report a difference even on the base branch. It also uses one
timeit()run rather than themin(timeit.repeat(..., repeat=7))methodology stated in the PR. Please compare the same BoundStatement workload before and after the implementation (and use repeated runs) so the claimed regression/improvement is reproducible.
t_fast = timeit.timeit(fast_path, number=n)
t_slow = timeit.timeit(slow_path, number=n)
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Actionable comments posted: 1
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Inline comments:
In `@tests/unit/test_resultset.py`:
- Line 187: Replace each direct was_applied expression inside the pytest.raises
blocks with an assignment to _, updating tests/unit/test_resultset.py at lines
187, 230, and 262; assign the ResultSet(...).was_applied accesses at the first
two sites and rs.was_applied at the third, preserving the existing exception
assertions.
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benchmarks/bench_was_applied.pycassandra/cluster.pytests/unit/test_resultset.py
Add a fast path in ResultSet.was_applied that skips batch detection (isinstance checks + regex match) when the query has a known LWT status from the server PREPARE response. For BoundStatement queries where is_lwt() returns True, the batch_regex match on the query string is entirely avoided. This benefits the most common LWT use case: prepared INSERT/UPDATE IF statements executed via BoundStatement, where the driver already knows from the PREPARE response whether the statement is an LWT. The slow path (isinstance + regex) is preserved for: - BatchStatement queries (detected via isinstance) - SimpleStatement batch queries (detected via regex) - Any query where is_lwt() returns False The fast-path condition checks `isinstance(query, BatchStatement)` before looking up `is_lwt`, and uses a getattr/callable guard around the call instead of calling `query.is_lwt()` unconditionally. This protects `was_applied` from raising AttributeError for any query object that doesn't implement is_lwt() -- e.g. response_future.query left as None, which is a real, reachable value (see ResponseFuture.query's class-level default and Session.prepare()/prepare_on_all_hosts, which construct ResponseFuture(..., query=None, ...) explicitly) -- falling back to the slow path instead. Also adds explicit tests for the fast path, non-LWT fallback, BatchStatement handling, and a regression test for a query without is_lwt() in was_applied. Part of: scylladb#751 Signed-off-by: Yaniv Michael Kaul <yaniv.kaul@scylladb.com>
Construct a minimal ResultSet with a mocked response_future and real cassandra.query statement objects, and time actual accesses to rs.was_applied, instead of re-implementing a simplified stand-in for its fast-path/slow-path branching. This also means the slow path exercises the real ResultSet.batch_regex instead of a different, looser regex, so the reported cost reflects the real regex match. On this machine: ~0.21us/call for the fast path (known-LWT BoundStatement) vs ~0.35us/call for the slow path (SimpleStatement regex match), a ~1.7x speedup -- both call costs are far below a microsecond once measured against the real was_applied property instead of Mock-heavy stand-ins. Signed-off-by: Yaniv Michael Kaul <yaniv.kaul@scylladb.com>
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Summary
ResultSet.was_appliedfor statements wherequery.is_lwt()is True (BoundStatement/PreparedStatement)batch_regex.match()call andisinstance(query, BatchStatement)check for the common single-LWT caseBenchmark
Measured with
min()oftimeit.repeat(repeat=7, number=200_000)on a quiet machine (load <1).Note on interpretation: the table compares the two paths within this change, not old-code-vs-new-code. For the most common case (a
BoundStatementLWT), the previous code already short-circuited before the regex, so the end-to-end win over the prior release is smaller than the 612 ns spread above suggests. The change's real benefit is removing the regex/isinstance work from the LWT path and eliminating the redundantis_lwt()lookups.Tests
test_was_applied_lwt_fast_path,test_was_applied_non_lwt_fallback,test_was_applied_batch_statementtest_was_appliedto use explicit non-LWT query to exercise the slow (regex) path