Crate latch_billing 

latch-billing - Pure synchronous token billing core library.

This crate defines the core types, traits, and pricing models for token-based billing. It is purely synchronous and has zero I/O dependencies.

Architecture

• Types: UsageObservation, MeterSet, BillingSubject, etc.
• Traits: PricingSource, RatingEngine, ObservationStore, RatedRecordStore, RatedRecordExporter, QuotaAuthorizer, etc.
• Design principle: Runtime-agnostic sync library. Async I/O should be implemented by downstream consumers (e.g., gateway applications).

Quick start

use latch_billing::*;

// 1. Create an observation
let mut meter_set = MeterSet::new();
meter_set.accumulate(MeterKind::InputTokens, 1000).unwrap();

let observation = UsageObservation {
    event_id: UsageEventId::from_attempt("req-123", 0, "openai").unwrap(),
    subject: BillingSubject::default(),
    meter_set,
    model_ref: ModelRef { ... },
    provider_ref: Some(ProviderRef { provider_id: "openai".to_string() }),
    source: UsageSource::ProviderReported,
    outcome: UsageOutcome::Success,
    timing: UsageTiming { ... },
    correlation: CorrelationIds::default(),
    attributes: Attributes::new(),
};

// 2. Get a price snapshot (push mode - caller fethes)
let snapshot = ...; // fetched by caller (e.e., from DB)

// 3. Create rating context (for tier-based pricing)
let context = RatingContext::default();

// 4. Rate the observation
let engine = DefaultRatingEngine::new();
let rated = engine.rate(&observation, &snapshot, &context)?;
println!("Cost: {} {}", rated.rating.total_cost, rated.rating.currency.0);

Module structure

• observation: UsageObservation, MeterSet, MeterKind, UsageSource, UsageOutcome, Attributes
• identity: BillingSubject, UsageEventId, UsageEventIdBuilder, CorrelationIds
• pricing: ModelRef, ProviderRef, PriceSnapshot, PricingSource trait, TierConfig, TierBaseline
• rating: RatedUsageRecord, RatingResult, RatedLineItem, RatingEngine trait, RatingContext
• storage: ObservationStore trait, RatedRecordStore trait, StoreResult
• quota: QuotaAuthorizer trait, QuotaReservator trait (Phase 2)
• export: RatedRecordExporter trait

Re-exports

pub use observation::Attributes;

pub use observation::AttributeError;

pub use observation::CurrencyCode;

pub use observation::CurrencyCodeError;

pub use observation::MeterKind;

pub use observation::MeterSet;

pub use observation::MeterSetError;

pub use observation::UsageObservation;

pub use observation::UsageOutcome;

pub use observation::UsageSource;

pub use observation::UsageTiming;

pub use identity::BillingSubject;

pub use identity::CorrelationIds;

pub use identity::UsageEventId;

pub use identity::UsageEventIdBuilder;

pub use identity::UsageEventIdError;

pub use pricing::AccumulationScope;

pub use pricing::MeterPrice;

pub use pricing::ModelRef;

pub use pricing::PriceSnapshot;

pub use pricing::PricingError;

pub use pricing::PricingSource;

pub use pricing::ProviderRef;

pub use pricing::TierBaseline;

pub use pricing::TierBoundary;

pub use pricing::TierConfig;

pub use rating::RatedLineItem;

pub use rating::RatedUsageRecord;

pub use rating::RatingContext;

pub use rating::RatingEngine;

pub use rating::RatingError;

pub use rating::RatingResult;

pub use storage::ObservationStore;

pub use storage::RatedRecordStore;

pub use storage::StoreError;

pub use storage::StoreResult;

pub use quota::QuotaAuthorizer;

pub use quota::QuotaDecision;

pub use quota::QuotaError;

pub use quota::QuotaRequest;

pub use quota::QuotaReservator;

pub use quota::Reservation;

pub use quota::ReservationRequest;

pub use quota::UsageAmount;

pub use export::ExportError;

pub use export::RatedRecordExporter;

pub use chrono;

pub use rust_decimal;

Modules

export

Export module - defines the RatedRecordExporter trait.

identity

Identity module - defines billing subjects, correlation IDs, and idempotency keys.

observation

Observation module - defines the raw usage observation types.

pricing

Pricing module - defines pricing models and the PricingSource trait.

quota

Quota module - defines traits for quota enforcement.

rating

Rating module - defines rated usage records and the RatingEngine trait.

storage

Storage module - defines traits for persisting observations and rated records.