Pattern 9: Single Source of Truth

Context

Constants, toggle lists, and feature matrices are needed across modules and extensions.

Problem

Duplicating configuration (timeouts, guard limits, feature lists) invites drift. Teams change one copy and forget the rest, producing inconsistent behavior.

Solution

Centralize invariants inside the module that owns them and re-export when necessary. Examples include timeout constants in core::macros::test, guard limits in validation::guards, and feature combinations in Cargo.toml. Extensions read these definitions instead of defining their own copies.

Forces

• Accessibility vs. encapsulation: invariants must be easy to import without exposing internals
• Flexibility vs. safety: allow customization through builders or configuration rather than duplicating constants
• Documentation vs. code: comments and docs should reference the single source, not restated values

Examples

#![allow(unused)]
fn main() {
// src/core/macros/test.rs
pub const DEFAULT_UNIT_TEST_TIMEOUT_SECONDS: u64 = 1;
pub const DEFAULT_INTEGRATION_TEST_TIMEOUT_SECONDS: u64 = 30;

// src/validation/guards.rs
pub const MAX_RUN_LEN: usize = 8;
}

Related Patterns

• Pattern 8: Composition Over Duplication
• Pattern 10: Capability Grouping
• Pattern 18: Timeout Defense in Depth