Crate request_shadow 

request-shadow

Async request mirroring with sampling, divergence detection, and structured response diffs. The SRE primitive for migrations: send the same request to the production service AND a candidate, compare the responses, return the production one to the client while you collect divergence telemetry.

Why a small crate

Every service-mesh has a knob for this — Linkerd shadowing, Istio mirror, AWS App Mesh. Those are great when you own the mesh. They’re useless when the migration is in-process (binary library swap, codec change, JSON-vs- protobuf swap, ORM cutover). This crate gives you the same shape as a 30-line Tokio task:

let primary  = Arc::new(Mock(ResponseRecord::ok(b"prod".to_vec())));
let shadow   = Arc::new(Mock(ResponseRecord::ok(b"prod".to_vec())));
let shadower = Shadower::new(primary, shadow, ShadowConfig::full_sample());

let outcome = shadower.call(b"hello").await?;
assert!(outcome.primary.ok);
assert!(outcome.divergence.is_none()); // bytes match

Pieces

• Backend — the async-trait abstraction the shadower calls. Implement it over reqwest::Client for HTTP, or any in-process call.
• ResponseRecord — what a backend returns: status code, headers, body.
• ShadowConfig — sampling rate (sticky over a key hash), timeout for the shadow leg, fields to ignore in the diff.
• Shadower — picks whether to mirror based on the sampling key, fires both calls in a tokio::join!, returns a ShadowOutcome.
• Divergence — structured diff: status / headers / body each get their own bool + summary.
• DivergenceLog — bounded ring buffer so the shadower can hand operators the last N divergences without unbounded memory growth.

Composes with

• reliability-toolkit-rs — wrap the shadow Backend in a [CircuitBreaker] so a flaky candidate never bleeds into the primary path.
• slo-budget-tracker — record every divergence against an SLO so you can answer “is the candidate good enough to promote?”

Re-exports

pub use backend::Backend;

pub use backend::ResponseRecord;

pub use config::IgnoreField;

pub use config::ShadowConfig;

pub use divergence::Divergence;

pub use error::ShadowError;

pub use log::DivergenceLog;

pub use shadower::ShadowOutcome;

pub use shadower::Shadower;

Modules

backend

The async backend abstraction.

config

Shadow configuration.

divergence

Structured response diff.

error

Crate-wide error type.

log

Bounded ring buffer for divergence records.

shadower

Shadower — the main type. Fires primary + shadow legs concurrently and returns the primary response plus an optional divergence.