Todo CLI in Rust 4. JSON persistence, testing, and scale-ready decisions

Final implementation chapter.

Read this chapter in Spanish: ES

This is where projects either become reliable, or quietly become fragile: persistence and tests.

Code references:

• src/tasks/ports/outputs/task_repository.rs
• src/tasks/adapters/persistence/in_memory_task_repository.rs
• src/tasks/adapters/persistence/json_file_task_repository.rs

Contract first, implementation second

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We defined TaskRepository before touching disk.

Why this instead of direct JSON calls from use cases

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If use cases know JSON details, they become infrastructure scripts.

With a repository port, use cases remain focused on business operations (save, find, list, delete) and adapters handle storage concerns.

Two adapters, two jobs

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InMemoryTaskRepository:

• fast,
• great for behavior tests,
• no filesystem overhead.

JsonFileTaskRepository:

• real persistence,
• serialization,
• I/O and parsing failure paths.

This split gives faster feedback and clearer fault isolation.

JSON persistence decisions

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1. platform-aware data location via directories;
2. create parent dirs when missing;
3. missing file means empty initial state;
4. invalid JSON returns explicit error.

Why not auto-heal corrupted JSON

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Because silent healing often means silent data loss.

Failing with context is less convenient in the moment, but much safer long term.

Operation semantics

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• save: upsert by id.
• list: All or ByStatus.
• find_by_id: Option<Task>.
• delete: bool (deleted vs not found).

This keeps normal outcomes (like “not found” on delete) separate from real technical failures.

Test strategy

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Coverage includes:

• save/find by id,
• list all / list by status,
• delete existing / non-existing,
• persistence across repository instances,
• explicit error on invalid JSON.

JSON tests use tempdir, so user data remains untouched.

Commits that complement this chapter

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• implement JSON file task repository adapter
• add tests for JSON file repository persistence behavior

Explicit technical debt

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Still intentionally open:

• RepoError is broad (InternalError + message),
• no file locking for multi-process concurrency,
• no JSON schema versioning/migration layer.

Reasonable for current scope, important to tackle for heavier usage.

Closing

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The project started as a “simple todo CLI” and became a full Rust architecture exercise:

• domain invariants,
• use-case orchestration,
• adapter isolation,
• stable CLI contract,
• behavior-focused tests.

That is the real payoff: training scalable engineering decisions in a small, controlled project.