muxer

Deterministic, multi-objective routing primitives for “provider selection” problems.

What problem this solves

You have a small set of arms (providers/models/backends) and repeated calls that produce outcomes (success/429/junk), plus cost + latency. You want an online policy that:

• explores new or recently-changed arms
• avoids regressions (junk/429 spikes)
• is deterministic by default (same stats/config → same choice), so it’s easy to debug

What it is

The core idea is:

• maintain a small sliding window of recent outcomes per provider (ok/429/junk, cost, latency)
• compute a Pareto frontier over the objectives
• pick a single provider deterministically via scalarization + stable tie-break

This crate also includes:

• a seedable Thompson-sampling policy (ThompsonSampling) for cases where you can provide a scalar reward in [0, 1] per call
• a seedable EXP3-IX policy (Exp3Ix) for more adversarial / fast-shifting reward settings
• (feature contextual) a linear contextual bandit policy (LinUcb) for per-request routing with feature vectors

Which policy should I use?

• select_mab (Window + Pareto + scalarization): when you care about multiple objectives at once (success, 429, junk, cost, latency) and you want deterministic selection with hard constraints.
• ThompsonSampling: when you can provide a single reward per call (in [0, 1]) and want a classic explore/exploit policy (seedable, optionally decayed).
• Exp3Ix: when reward is non-stationary / adversarial-ish and you still want a probabilistic policy (seedable, optionally decayed).
• LinUcb (feature contextual): when you have a per-request feature vector (e.g. cheap “difficulty” features, embeddings, metadata) and want a contextual policy.

Unified decision records (recommended for logging/replay)

Most production routers want a single “decision object” shape regardless of policy so logging, auditing, and replay don’t depend on per-policy conventions. muxer provides a unified Decision envelope with:

• chosen: the arm name
• probs: optional probability distribution (when a policy has one)
• notes: typed audit notes (explore-first, constraint gating, numerical fallback, etc.)

Each policy has a *_decide / decide_* method that returns this.

Quick examples

Deterministic multi-objective selection (Pareto + scalarization)

use muxer::{select_mab, MabConfig, Summary};
use std::collections::BTreeMap;

let arms = vec!["a".to_string(), "b".to_string()];
let mut summaries = BTreeMap::new();
summaries.insert("a".to_string(), Summary { calls: 10, ok: 9, http_429: 0, junk: 0, hard_junk: 0, cost_units: 10, elapsed_ms_sum: 900 });
summaries.insert("b".to_string(), Summary { calls: 10, ok: 9, http_429: 0, junk: 2, hard_junk: 0, cost_units: 10, elapsed_ms_sum: 900 });

let sel = select_mab(&arms, &summaries, MabConfig::default());
assert_eq!(sel.chosen, "a"); // lower junk when all else is equal

Realistic “online routing loop” (Window ingestion)

This is closer to production usage: you maintain a Window per arm, push Outcomes as requests finish, and call select_mab each decision.

cargo run --example deterministic_router

Note: this example simulates an environment and therefore requires --features stochastic if you disabled default features.

End-to-end router demo (Window + constraints + stickiness + delayed junk)

This combines multiple production patterns in one loop: window ingestion, constraints+weights, stickiness reasons, and delayed junk labeling.

cargo run --example end_to_end_router

Note: this example simulates an environment and therefore requires --features stochastic if you disabled default features.

This same scenario has a CI-checked regression test in tests/e2e_metrics.rs and now logs whether constraint fallback was used.

Window ingestion with delayed junk labeling

If your “junk” classification is only known after downstream parsing/validation, you can update the most recent outcome:

cargo run --example window_delayed_junk_label

Constraint + trade-off tuning for select_mab

Example showing “constraints first, then weights”:

cargo run --example mab_constraints_tuning

EXP3-IX (adversarial bandit) with probabilities

use muxer::{Exp3Ix, Exp3IxConfig};

let arms = vec!["a".to_string(), "b".to_string(), "c".to_string()];
let mut ex = Exp3Ix::new(Exp3IxConfig { seed: 123, decay: 0.98, ..Exp3IxConfig::default() });

let d = ex.decide(&arms).unwrap();
// ... run request with `d.chosen` ...
ex.update_reward(&d.chosen, 0.7); // reward in [0, 1]

let probs = d.probs.unwrap();
let s: f64 = probs.values().sum();
assert!((s - 1.0).abs() < 1e-9);

Runnable:

cargo run --example exp3ix_router

Note: this example requires --features stochastic if you disabled default features.

Thompson “traffic splitting” selector (mean-softmax allocation)

use muxer::{ThompsonConfig, ThompsonSampling};

let arms = vec!["a".to_string(), "b".to_string()];
let mut ts = ThompsonSampling::with_seed(
    ThompsonConfig {
        decay: 0.99,
        ..ThompsonConfig::default()
    },
    0,
);
let d = ts.decide_softmax_mean(&arms, 0.3).unwrap();
ts.update_reward(&d.chosen, 1.0);

let alloc = d.probs.unwrap();
let s: f64 = alloc.values().sum();
assert!((s - 1.0).abs() < 1e-9);

Runnable:

cargo run --example thompson_router

Note: this example requires --features stochastic if you disabled default features.

Contextual routing (LinUCB)

Runnable:

cargo run --example contextual_router --features contextual

Notes:

• If you want a probability distribution over arms for this context (e.g. for traffic-splitting or logging approximate propensities), use LinUcb::probabilities(...) or LinUcb::decide_softmax_ucb(...).
• Algorithm reference: LinUCB (Chu et al., “Contextual bandits with linear payoff functions”).

Contextual “propensity logging” example:

cargo run --example contextual_propensity_logging --features contextual

Stickiness / switching-cost control

If you want to reduce “flapping” between arms, wrap deterministic selection with StickyMab:

cargo run --example sticky_mab_router

Usage

[dependencies]
muxer = "0.1.0"

If you only want the deterministic Window + select_mab* core (no stochastic bandits), disable default features:

[dependencies]
muxer = { version = "0.1.0", default-features = false }

Development

# If you are in a larger Cargo workspace, scope to this package:
cargo test -p muxer

# (Optional) Match CI checks:
cargo fmt -p muxer --check
cargo clippy -p muxer --all-targets -- -D warnings