scalo 2.10.3

# scalo

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> There's plenty of sage advice about running services in production at
> scale -- config cascades, structured logging, secret masking, Prometheus,
> OpenTelemetry, health probes, backpressure, graceful shutdown -- but almost
> none of it as code you can just install and use.
>
> This is that code.

scalo is an integrated, self-regulating runtime for hyperscale-grade
data-plane services. Config, logging and metrics come as one pre-wired
trinity -- global singletons you just use, no plumbing, no init dance.
Everything else leans on that same integration: the config cascade flows
straight into the CLI so `run`/`version`/`config-check` just work; the metrics
and health wiring feed the K8s probe trinity; and the deployment contract
generates your Helm, Dockerfile and Argo manifests from the config the app
already declares.

Attach scalo to your service and a whole class of production pain -- the kind
done wrong a hundred times elsewhere -- just goes away. Battle-tested, and
almost no code on your side **to do it properly**. It's not a bag of utility
functions you wire up yourself; it's the wiring, done right, for free.

scalo comes in two halves that share one set of conventions, idiomatic in each
language. **scalo-rs** (this crate) is the **data plane** -- the Rust hot path
where every microsecond and byte counts (`cargo add scalo`). **scalo-py** is
the **control plane** -- orchestration, APIs and integration in Python
(`pip install scalo`).

Opinionated about correctness -- backpressure, memory safety and the probe
trinity are on by default. Unopinionated about your domain -- no web framework,
no ORM, no enforced transport. Built as the foundation for PB/hr data services.

This module exists because of this --
<https://www.youtube.com/watch?v=xE9W9Ghe4Jk> -- but for the backend. And of
course, no microservices.

## Quick Start

```toml
[dependencies]
scalo = "2"
```

Default features: `config`, `logger`. Add the others you want explicitly.

```rust
use scalo::{config, logger, env};

fn main() -> anyhow::Result<()> {
    let environment = env::Environment::detect();
    logger::setup_default()?;
    config::setup(config::ConfigOptions {
        env_prefix: "MYAPP".into(),
        ..Default::default()
    })?;

    tracing::info!("Running in {environment:?}");
    Ok(())
}
```

## Features

Pick the slice you need; pay only for what you use.

| Feature | Description |
|---------|-------------|
| `env` | Environment detection (K8s, Docker, Container, BareMetal) |
| `runtime` | Runtime path resolution (XDG/container-aware) |
| `config` | 7-layer config cascade (figment-based) |
| `config-reload` | `SharedConfig<T>` + `ConfigReloader` hot-reload |
| `logger` | Structured logging, JSON/text auto-detect, sensitive-field masking |
| `metrics` | Prometheus metrics + process/container metrics |
| `otel-metrics` | OpenTelemetry metrics export (OTLP) |
| `otel-tracing` | OpenTelemetry distributed tracing |
| `http` | HTTP client with retry middleware (reqwest) |
| `http-server` | Axum HTTP server with health probe trinity (`/healthz/{startup,live,ready}`) |
| `transport-kafka` | Kafka transport (rdkafka, dynamic-linking) |
| `transport-grpc` | gRPC transport (tonic/prost) |
| `transport-memory` | In-memory transport (testing/dev) |
| `transport-redis` | Redis / Valkey Streams transport |
| `transport-grpc-vector-compat` | Vector wire-protocol compatibility |
| `spool` | Disk-backed async FIFO queue (yaque + zstd, CRC32C integrity) |
| `tiered-sink` | Resilient delivery: hot buffer + circuit breaker + disk spillover |
| `sink-stack` | Outbound control stack: timeout / load-shed / concurrency-limit / retry / rate-limit (tower) |
| `worker` | Adaptive worker pool + `BatchEngine` (SIMD parse, pre-route, field interning) |
| `memory` | Cgroup-aware `MemoryGuard` (OOM prevention) |
| `governor` | Unified self-regulation gate (hard memory + weighted soft signals) |
| `secrets` | Secrets management core (file backend) |
| `secrets-vault` | OpenBao / HashiCorp Vault provider |
| `secrets-aws` | AWS Secrets Manager provider |
| `directory-config` | YAML directory-backed config store |
| `directory-config-git` | Git integration for directory-config (git2) |
| `scaling` | Back-pressure / scaling-pressure primitives |
| `cli` | Standard CLI framework (clap) |
| `top` | TUI metrics dashboard (ratatui) |
| `io` | File rotation, NDJSON writer |
| `dlq` | Dead-letter queue (file backend) |
| `dlq-kafka` | DLQ Kafka backend |
| `output-file` | File output sink |
| `expression` | CEL expression evaluation |
| `deployment` | Deployment-contract validation |
| `version-check` | Optional startup version check |
| `full` | Everything |

## Native System Dependencies

This crate dynamically links against system C libraries for several features.
**Both build hosts and deployment targets need the appropriate packages.**

### Build Host (CI / Development)

| Feature | Crate | Build Package | Notes |
|---------|-------|--------------|-------|
| `transport-kafka` | `rdkafka-sys` | `librdkafka-dev` (>= 2.12.1) | Requires [Confluent APT repo](https://packages.confluent.io/clients/deb) - Ubuntu's default is too old |
| `directory-config-git` | `libgit2-sys` | `libgit2-dev`, `libssh2-1-dev` | System lib avoids vendored C build |
| `spool`, `tiered-sink` | `zstd-sys` | `libzstd-dev` | System lib avoids vendored C build |
| (transitive) | `libz-sys` | `zlib1g-dev` | Used by multiple deps |
| (transitive) | `openssl-sys` | `libssl-dev` | Dynamic linking via pkg-config |
| `secrets-aws` | `aws-lc-sys` | - | C/C++ compiled from source (no system lib available); ~20-30s first build, cached by sccache |

For `librdkafka-dev` >= 2.12.1, add the Confluent APT repo:

```bash
curl -fsSL https://packages.confluent.io/clients/deb/archive.key \
  | sudo gpg --dearmor -o /usr/share/keyrings/confluent-clients.gpg
echo "deb [signed-by=/usr/share/keyrings/confluent-clients.gpg] \
  https://packages.confluent.io/clients/deb noble main" \
  | sudo tee /etc/apt/sources.list.d/confluent-clients.list
sudo apt-get update
sudo apt-get install -y librdkafka-dev libssl-dev libsasl2-dev pkg-config
```

### Deployment Host (Runtime)

The compiled binary links against `.so` files at runtime. Install the
**runtime** packages (not `-dev`) on deployment hosts or in Docker images.

| Feature | Runtime Package | Shared Object |
|---------|----------------|---------------|
| `transport-kafka` | `librdkafka1` (from Confluent repo) | `librdkafka.so.1` |
| `directory-config-git` | `libgit2-1.7` (or matching version) | `libgit2.so` |
| `spool`, `tiered-sink` | `libzstd1` | `libzstd.so.1` |
| (transitive) | `zlib1g` | `libz.so.1` |
| (transitive) | `libssl3` | `libssl.so.3` |

Only install what you use. Check the features your binary enables to
determine which runtime packages are needed.

### Docker Example

```dockerfile
# Build stage
FROM rust:1 AS builder
RUN apt-get update && apt-get install -y \
    pkg-config libssl-dev librdkafka-dev libgit2-dev libzstd-dev
COPY . .
RUN cargo build --release

# Runtime stage
FROM ubuntu:24.04
RUN apt-get update && apt-get install -y --no-install-recommends \
    librdkafka1 libssl3 libgit2-1.7 libzstd1 ca-certificates \
    && rm -rf /var/lib/apt/lists/*
COPY --from=builder /app/target/release/myapp /usr/local/bin/
```

For `librdkafka1`, add the Confluent APT repo to both build and runtime stages.

## Health Check Endpoints - The Probe Trinity

For services deployed to Kubernetes, the `http-server` feature provides
the three K8s probe types:

| Probe | Path | Checks | On failure |
|---|---|---|---|
| Startup | `/healthz/startup` | Init complete | K8s waits, then restarts |
| Liveness | `/healthz/live` | Process not deadlocked | Restart pod |
| Readiness | `/healthz/ready` | Deps healthy + ready flag set | Stop routing traffic |

Liveness MUST NEVER check downstream dependencies (a DB outage shouldn't
restart your replicas). Readiness checks dependencies AND requires an
explicit `set_ready()` call - cleared during graceful shutdown.

## Self-regulation (default vertical scaling)

A scalo data-plane app regulates its own intake. Sized for steady state, a
pod slows down or speeds up WITHIN itself first -- the default, fast, local
response to a burst, a stalled upstream, or a transform that balloons memory.
Only when that vertical headroom is exhausted does it escalate to horizontal
scale (KEDA adding pods), driven by the same pressure signal. Memory is the
hard, never-OOM authority; CPU is left to the kernel scheduler (CFS), which
the byte-budget loop reads through longer process times. It is ON by default
and opt-out via `self_regulation.enabled = false`. See
[docs/SELF-REGULATION.md](docs/SELF-REGULATION.md).

## Architecture

See [docs/](docs/README.md) for the full documentation index -
[docs/ARCHITECTURE.md](docs/ARCHITECTURE.md) for the module map and layering,
and [docs/core-pillars/CONFIG.md](docs/core-pillars/CONFIG.md) for the 7-layer
config cascade reference.

## License

[Apache-2.0](LICENSE).

## Related

- **[scalo-py](https://github.com/hyperi-io/scalo-py)** -- sister library for
  Python services. Same opinions, same patterns, expressive Python ergonomics
  for control planes, APIs, and integration layers.