config-extraction

name

config-extraction

description

Extract subsystem-specific config structs from a monolithic/global configuration using the strangler fig pattern. Each subsystem gets a focused config struct and a pure conversion function, breaking the dependency on the global config package. Use when: (1) multiple packages import a central config struct and cherry-pick fields, (2) you want to decouple a subsystem so it no longer imports the config package, (3) config has cross-referencing lookups (e.g. account name → key) that should be pre-resolved, (4) the user mentions 'extract config', 'decouple config', 'config refactor', or 'strangler fig config'.

Config Extraction — Strangler Fig Pattern

Incrementally replace a monolithic global config struct with focused, subsystem-specific config structs. Each subsystem gets exactly the fields it needs, pre-resolved and ready to use.

When to Apply

• Multiple packages import a central config.Configuration and cherry-pick deeply nested fields
• You want a subsystem to be testable without constructing the entire global config
• Config contains cross-referencing lookups (e.g. storage account name → key) that get repeated across subsystems
• You're preparing to split a monolith into separate services

Pattern Overview

See references/pattern.md for the complete implementation reference with code templates and a worked example.

Implementation Steps

Per-subsystem extraction (repeat for each subsystem, one at a time):

1. Catalog field references — read every file in the subsystem package and list every conf.X.Y.Z access. This is your work list.
2. Define the focused config struct in the subsystem package — inline fields, don't embed original sub-structs.
3. Write ConfigToXxxConfig(conf config.Configuration) (XxxConfig, error) — a pure conversion function that maps and pre-resolves all values.
4. Change the subsystem's stored config type from global to focused (struct field, constructor parameter).
5. Follow the compiler errors — mechanically update every conf.X.Y.Z → conf.Z reference.
6. Update call sites (cmd/, main()) to call the conversion function and pass the result.
7. Update test helpers to construct global config and pass through the conversion function.
8. Build all binaries and compile all tests to verify.
9. Commit — one self-contained commit per subsystem.

Ordering heuristic:

• Start with leaf subsystems (fewer dependents, simpler config surface).
• Work toward central subsystems (more call sites, cross-cutting concerns) last.
• This builds confidence and establishes the pattern before hitting complexity.