deprecation-and-migration

# Deprecation and Migration

## Overview

Code is a liability, not an asset. Every line of code has ongoing maintenance cost — bugs to fix, dependencies to update, security patches to apply, and new engineers to onboard. Deprecation is the discipline of removing code that no longer earns its keep, and migration is the process of moving users safely from the old to the new.

Most engineering organizations are good at building things. Few are good at removing them. This skill addresses that gap.

## When to Use

- Replacing an old system, API, or library with a new one
- Sunsetting a feature that's no longer needed
- Consolidating duplicate implementations
- Removing dead code that nobody owns but everybody depends on
- Planning the lifecycle of a new system (deprecation planning starts at design time)
- Deciding whether to maintain a legacy system or invest in migration

## Core Principles

### Code Is a Liability

Every line of code has ongoing cost: it needs tests, documentation, security patches, dependency updates, and mental overhead for anyone working nearby. The value of code is the functionality it provides, not the code itself. When the same functionality can be provided with less code, less complexity, or better abstractions — the old code should go.

### Hyrum's Law Makes Removal Hard

With enough users, every observable behavior becomes depended on — including bugs, timing quirks, and undocumented side effects. This is why deprecation requires active migration, not just announcement. Users can't "just switch" when they depend on behaviors the replacement doesn't replicate.

### Deprecation Planning Starts at Design Time

When building something new, ask: "How would we remove this in 3 years?" Systems designed with clean interfaces, feature flags, and minimal surface area are easier to deprecate than systems that leak implementation details everywhere.

## The Deprecation Decision

Before deprecating anything, answer these questions:

```
1. Does this system still provide unique value?
   → If yes, maintain it. If no, proceed.

2. How many users/consumers depend on it?
   → Quantify the migration scope.

3. Does a replacement exist?
   → If no, build the replacement first. Don't deprecate without an alternative.

4. What's the migration cost for each consumer?
   → If trivially automated, do it. If manual and high-effort, weigh against maintenance cost.

5. What's the ongoing maintenance cost of NOT deprecating?
   → Security risk, engineer time, opportunity cost of complexity.
```

## Compulsory vs Advisory Deprecation

| Type | When to Use | Mechanism |
|------|-------------|-----------|
| **Advisory** | Migration is optional, old system is stable | Warnings, documentation, nudges. Users migrate on their own timeline. |
| **Compulsory** | Old system has security issues, blocks progress, or maintenance cost is unsustainable | Hard deadline. Old system will be removed by date X. Provide migration tooling. |

**Default to advisory.** Use compulsory only when the maintenance cost or risk justifies forcing migration. Compulsory deprecation requires providing migration tooling, documentation, and support — you can't just announce a deadline.

## The Migration Process

### Step 1: Build the Replacement

Don't deprecate without a working alternative. The replacement must:

- Cover all critical use cases of the old system
- Have documentation and migration guides
- Be proven in production (not just "theoretically better")

### Step 2: Announce and Document

```markdown
## Deprecation Notice: OldService

**Status:** Deprecated as of 2025-03-01
**Replacement:** NewService (see migration guide below)
**Removal date:** Advisory — no hard deadline yet
**Reason:** OldService requires manual scaling and lacks observability.
            NewService handles both automatically.

### Migration Guide
1. Replace `import { client } from 'old-service'` with `import { client } from 'new-service'`
2. Update configuration (see examples below)
3. Run the migration verification script: `npx migrate-check`
```

### Step 3: Migrate Incrementally

Migrate consumers one at a time, not all at once. For each consumer:

```
1. Identify all touchpoints with the deprecated system
2. Update to use the replacement
3. Verify behavior matches (tests, integration checks)
4. Remove references to the old system
5. Confirm no regressions
```

**The Churn Rule:** If you own the infrastructure being deprecated, you are responsible for migrating your users — or providing backward-compatible updates that require no migration. Don't announce deprecation and leave users to figure it out.

### Step 4: Remove the Old System

Only after all consumers have migrated:

```
1. Verify zero active usage (metrics, logs, dependency analysis)
2. Remove the code
3. Remove associated tests, documentation, and configuration
4. Remove the deprecation notices
5. Celebrate — removing code is an achievement
```

## Migration Patterns

### Strangler Pattern

Run old and new systems in parallel. Route traffic incrementally from old to new. When the old system handles 0% of traffic, remove it.

```
Phase 1: New system handles 0%, old handles 100%
Phase 2: New system handles 10% (canary)
Phase 3: New system handles 50%
Phase 4: New system handles 100%, old system idle
Phase 5: Remove old system
```

### Adapter Pattern

Create an adapter that translates calls from the old interface to the new implementation. Consumers keep using the old interface while you migrate the backend.

```typescript
// Adapter: old interface, new implementation
class LegacyTaskService implements OldTaskAPI {
  constructor(private newService: NewTaskService) {}

  // Old method signature, delegates to new implementation
  getTask(id: number): OldTask {
    const task = this.newService.findById(String(id));
    return this.toOldFormat(task);
  }
}
```

### Feature Flag Migration

Use feature flags to switch consumers from old to new system one at a time:

```typescript
functi