Caudal.RateLimiting 0.1.0-preview.2

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Caudal (Spanish): the volume of water flowing through a channel per unit of time.

Caudal is a .NET library for building bounded, cancellable, observable async pipelines. It is aimed at the code most services end up writing by hand — a SemaphoreSlim to limit concurrency, Task.WhenAll to fan out, a Channel<T> to connect stages — and at the four problems that code keeps getting wrong:

1. Bounded concurrency — never more in-flight work than you asked for.
2. Real backpressure — a fast producer blocks instead of filling memory.
3. Correct cancellation and shutdown — no orphaned tasks, no swallowed exceptions.
4. Operational diagnostics — you can explain a slow pipeline without adding logs to your own code.

Status: pre-release. The full 0.1–0.3 API surface from the roadmap is implemented and tested (151 tests, all packages build warning-clean) and preview packages are on NuGet. The contracts in docs/SEMANTICS.md stay open to change until 1.0 — preview releases break deliberately when a contract needs fixing. The build order and exit criteria are in ROADMAP.md.

What it looks like

await source
    .ToFlow(capacity: 128)          // bounded buffer: producer waits when full
    .SelectAsync(
        ProcessAsync,
        concurrency: 8)             // at most 8 concurrent invocations, always explicit
    .ForEachAsync(
        SaveAsync,
        cancellationToken);         // one token stops producer, workers, and sink

Pipelines are built on IAsyncEnumerable<T> at the edges and Channel<T> internally. Channel<T> is an implementation detail and is not part of the public API.

The operator that best shows why Caudal exists is LatestByKey, for real-time feeds where stale items should be replaced rather than queued:

priceUpdates
    .ToFlow(capacity: 1_024)
    .LatestByKey(x => x.Symbol, maximumKeys: 1_000)  // at most one pending item per key, bounded to 1,000 keys; newer replaces older
    .SelectAsync(CalculateIndicatorsAsync, concurrency: 8)
    .Batch(maximumSize: 100, maximumDelay: TimeSpan.FromMilliseconds(50))
    .ForEachAsync(UpdateDashboardAsync, ct);

Design principles

The full contract is in docs/SEMANTICS.md. The short version:

• No buffer is unbounded by default.
• Every operation accepts a CancellationToken.
• Concurrency is always explicit — there is no default parallelism.
• Ordering is never preserved by accident; you opt in with PreserveOrder.
• An exception cannot disappear silently. Error handling is a per-stage policy: Stop, Skip, or Capture (failures become FlowResult<T> values).
• Completing a pipeline means every internal task has finished or been cancelled.
• Telemetry never changes semantics.
• Each operator documents its behavior under saturation.

Time-based operators (Debounce, Throttle, Sample, IdleTimeout) depend on TimeProvider, so they are testable with a fake clock and no real delays. Resilience is an integration with Microsoft.Extensions.Resilience, not a reimplementation of Polly.

When not to use Caudal

Caudal is deliberately small: the goal is around ten operators with precise semantics, not a partial Rx. It is the wrong tool when:

• the work is small, sequential, CPU-bound loops — a plain loop wins;
• the collection is small and already materialized — Task.WhenAll is fine;
• you need concurrency but no backpressure, batching, or per-key semantics — Parallel.ForEachAsync is enough;
• you need a full reactive event system — use Rx.

The benchmark suite under benchmarks/Caudal.Benchmarks measures the cost of the abstraction against all of these; see docs/when-not-to-use.md for the full reasoning and docs/benchmarks.md for how to run the suite and read its results honestly.

Packages

Documentation

• docs/SEMANTICS.md — the behavioral contract: backpressure, errors, cancellation, completion, ordering.
• ROADMAP.md — phases, exit criteria, and what ships in each version.

License

MIT