query-cascade 0.1.2

Minimal demand-driven query framework for incremental computation.

Cascade Query

Releases: https://pypi.org/project/query-cascade/

cascade-query is a minimal, demand-driven incremental computation framework for Python.

It is designed for compiler-like workloads where you want:

• lazy pull-based evaluation
• precise dependency tracking
• red-green early bailout (backdating)
• query dedup across concurrent callers
• snapshot isolation for concurrent reads
• safe cancellation of obsolete background work
• side-effect replay on cache hits
• persistence + graph inspection

Minimal API

from cascade import Engine

engine = Engine()
warnings = engine.accumulator("warnings")

@engine.input
def source(file_id: str) -> str:
    return ""

@engine.query
def parse(file_id: str) -> tuple[str, ...]:
    return tuple(line.strip() for line in source(file_id).splitlines() if line.strip())

@engine.query
def symbols(file_id: str) -> tuple[str, ...]:
    return tuple(row.split("=")[0].strip() for row in parse(file_id))

Primitives

• engine.input(fn)
  Wraps mutable roots. Use .set(...) to create new revisions.
• engine.query(fn)
  Wraps pure demand-driven queries with memoization and dependency capture.
• engine.accumulator(name)
  Creates thread-safe side-effect channels replayed on cache hits.
• engine.snapshot()
  Captures an immutable read view (Snapshot) for MVCC-like isolation.
• engine.submit(query, *args, snapshot=...)
  Runs a query in the background with cancellation if inputs mutate.
• engine.compute_many([(query, args), ...], workers=N)
  Multi-threaded execution with a work-stealing scheduler.
• engine.inspect_graph() / engine.traces()
  Introspection hooks for diagnostics.
• engine.save(path) / engine.load(path)
  Persist or recover graph/cached state from SQLite.
• engine.prune(roots)
  Garbage-collect memoized subgraphs not reachable from roots.

Design Notes

What this framework guarantees

• Smart recalculation: only stale demand paths recompute.
• Selective updates: unchanged parents remain green after child backdating.
• Query deduplication: one in-flight compute serves all identical concurrent requests.
• Cycle detection: recursive query cycles raise CycleError.
• Cancellation: stale background queries raise QueryCancelled.

Limitations and enforceability boundaries

• CPython GIL and CPU-bound work: compute_many and dedup execution are multi-threaded, but true CPU parallel speedup is only guaranteed when query bodies release the GIL (e.g. I/O waits, native extensions). For pure Python CPU-bound loops, overlap exists but throughput may remain effectively single-core.
• Process-level durability model: persistence is an explicit point-in-time snapshot (save/load), not a transactional WAL-backed MVCC store shared by multiple live processes.
• Boundary of side-effect replay guarantees: replay is guaranteed only for effects emitted through Accumulator; out-of-band side effects in query bodies (printing, network calls, filesystem writes) are intentionally not replayed.
• Cycle handling scope: direct and long-chain dynamic query cycles are detected and raised as CycleError; this engine does not implement fixed-point solvers for cyclic dataflow.

What this framework intentionally does not include

To keep API surface minimal, this version does not include:

• nominal interning APIs (@interned) and tracked structs (@tracked)
• fixed-point cycle solvers
• distributed/shared cache protocols

Those can be layered on top without changing the core query model.

Quickstart

from cascade import Engine

engine = Engine()

@engine.input
def text() -> str:
    return ""

@engine.query
def lint_count() -> int:
    value = text()
    return value.count("TODO")

text.set("TODO: one\nTODO: two")
assert lint_count() == 2

# No recompute needed if input did not semantically change.
text.set("TODO: one\nTODO: two")
assert lint_count() == 2

Examples

• examples/compiler_pipeline.py
  Tiny compiler pipeline (source -> parse -> symbol_names -> typecheck) with warnings accumulator.
• examples/dynamic_macro_expansion.py
  Runtime macro-expansion query that dynamically changes downstream graph dependencies.

Persistence and inspection

engine.save("state.db")
engine.load("state.db")
print(engine.inspect_graph())
for event in engine.traces():
    print(event.event, event.key, event.detail)

Analysis review of the uploaded notes

Your friend’s notes are largely directionally correct and align with state-of-the-art incremental systems:

• Correct: pull-based demand, red-green early bailout, dependency graph capture, dedup, MVCC snapshots, cancellation, side-effect replay, tracing, and persistence.
• Needs qualification in Python: true CPU-bound parallelism is constrained by the GIL unless query bodies release it (I/O/native extensions).
• Overreach for this minimal implementation: unsafe-pointer lifetime tricks, red/green syntax tree internals, and fixed-point cycle solving are advanced optimizations that are not required for a practical minimal API.

Running tests

python -m pip install -e . pytest
pytest -q

CI best practices included

• GitHub Actions workflow at .github/workflows/ci.yml.
• Runs on both pushes and pull requests.
• Linting with ruff before tests.
• Separate package-build job (python -m build) to catch packaging regressions early.