impact-analysis

What this skill does

# Impact Analysis Skill

When analyzing the impact of a proposed change, follow this structured process. The goal is to map the full blast radius before anyone writes a line of code — catching surprises early is far cheaper than catching them in production.

**IMPORTANT**: Always save the output as a markdown file in the `project-decisions/` directory at the project root. Create the directory if it doesn't exist.

## 0. Output Setup
```bash
# Create project-decisions directory if it doesn't exist
mkdir -p project-decisions

# File will be saved as:
# project-decisions/YYYY-MM-DD-impact-[kebab-case-topic].md
```

## 1. Understand the Proposed Change

### Parse the Request

Extract from the question or discussion:

- **What's changing?** — specific code, system, API, database, service, or process
- **Why is it changing?** — new feature, bug fix, refactor, migration, deprecation
- **What's the desired outcome?** — what should be different after the change
- **What's NOT changing?** — explicitly call out what stays the same

### Classify the Change Type

| Change Type | Typical Blast Radius | Risk Level |
|-------------|---------------------|------------|
| **Config change** | Narrow — one service | 🟢 Low |
| **Bug fix** | Narrow — one function/module | 🟢 Low |
| **New feature (additive)** | Narrow to medium — new code, no existing changes | 🟡 Medium |
| **API change (backward compatible)** | Medium — consumers may need updates | 🟡 Medium |
| **API change (breaking)** | Wide — all consumers must update | 🔴 High |
| **Database schema change** | Wide — queries, models, migrations, backfill | 🔴 High |
| **Service migration/replacement** | Wide — integrations, config, monitoring, runbooks | 🔴 High |
| **Shared library update** | Wide — all consumers of the library | 🟠 High |
| **Infrastructure change** | Wide — deployment, networking, DNS, secrets | 🔴 High |
| **Authentication/authorization change** | Very wide — every authenticated endpoint | 🔴 Critical |
| **Data model/domain change** | Very wide — every layer of the application | 🔴 Critical |

## 2. Map the Blast Radius

### 2a. Code-Level Impact
```bash
# Find all files that reference the changing code
grep -rn "[changing-module-or-function]" --include="*.ts" --include="*.js" --include="*.py" --include="*.go" --include="*.java" --include="*.rb" --include="*.php" src/ app/ lib/ 2>/dev/null | grep -v "node_modules\|\.git\|test\|spec\|mock"

# Find all imports of the changing module
grep -rn "import.*from.*[module]\|require.*[module]\|from [module] import" --include="*.ts" --include="*.js" --include="*.py" src/ app/ 2>/dev/null | grep -v "node_modules\|\.git"

# Count affected files
grep -rln "[changing-module-or-function]" --include="*.ts" --include="*.js" --include="*.py" src/ app/ 2>/dev/null | grep -v "node_modules\|\.git" | wc -l

# Find downstream dependencies (what uses the thing that uses the changing code)
# Build a two-level dependency chain
for file in $(grep -rln "[module]" --include="*.ts" --include="*.js" src/ 2>/dev/null | grep -v "node_modules"); do
  basename "$file"
  grep -rln "$(basename $file .ts)\|$(basename $file .js)" --include="*.ts" --include="*.js" src/ 2>/dev/null | grep -v "node_modules" | sed 's/^/  → /'
done

# Check for re-exports (the change may propagate through barrel files)
grep -rn "export.*from.*[module]\|module\.exports.*require.*[module]" --include="*.ts" --include="*.js" src/ 2>/dev/null | grep -v "node_modules"

# Find interface/type usages (TypeScript)
grep -rn "interface\|type " [changing-file] 2>/dev/null | while read line; do
  typename=$(echo "$line" | grep -oP '(?:interface|type)\s+\K\w+')
  echo "Type: $typename"
  grep -rn "$typename" --include="*.ts" --include="*.tsx" src/ 2>/dev/null | grep -v "node_modules" | wc -l
  echo " references"
done
```

### 2b. API Impact
```bash
# Find API routes that use the changing code
grep -rn "[module-or-function]" --include="*.ts" --include="*.js" --include="*.py" src/api/ src/routes/ app/api/ 2>/dev/null

# List all endpoints in affected route files
for file in $(grep -rln "[module]" src/api/ src/routes/ 2>/dev/null); do
  echo "=== $file ==="
  grep -n "router\.\(get\|post\|put\|delete\|patch\)\|app\.\(get\|post\|put\|delete\|patch\)\|@app\.route\|@GetMapping\|@PostMapping" "$file" 2>/dev/null
done

# Check for API versioning
find . -path "*/api/v*" -o -path "*/v1/*" -o -path "*/v2/*" 2>/dev/null | head -20

# Check for OpenAPI/Swagger specs
find . -name "openapi*" -o -name "swagger*" -o -name "*.api.yaml" -o -name "*.api.json" 2>/dev/null | head -10

# Check for API consumers (external clients, mobile apps, other services)
grep -rn "fetch\|axios\|http\.\|HttpClient\|requests\." --include="*.ts" --include="*.js" --include="*.py" src/ 2>/dev/null | grep -i "[affected-endpoint]"
```

### 2c. Database Impact
```bash
# Find models/schemas related to the change
grep -rn "model\|schema\|entity\|@Entity\|@Table\|class.*Model\|class.*Schema" --include="*.ts" --include="*.js" --include="*.py" --include="*.java" src/db/ src/models/ src/entities/ app/models/ 2>/dev/null | grep -i "[keyword]"

# Find all queries that reference affected tables/columns
grep -rn "SELECT\|INSERT\|UPDATE\|DELETE\|FROM\|JOIN\|WHERE" --include="*.ts" --include="*.js" --include="*.py" --include="*.sql" src/ 2>/dev/null | grep -i "[table-or-column]"

# Find ORM queries referencing affected models
grep -rn "findOne\|findMany\|findAll\|create\|update\|delete\|where\|include\|select" --include="*.ts" --include="*.js" --include="*.py" src/ 2>/dev/null | grep -i "[model]"

# Check for existing migrations
ls -la src/db/migrations/ db/migrations/ migrations/ prisma/migrations/ 2>/dev/null | tail -10

# Check migration history count
find . -path "*/migrations/*" -name "*.ts" -o -name "*.js" -o -name "*.py" -o -name "*.sql" 2>/dev/null | wc -l

# Check for views, functions, triggers that reference the table
grep -rn "CREATE VIEW\|CREATE FUNCTION\|CREATE TRIGGER" --include="*.sql" . 2>/dev/null | grep -i "[table]"

# Check for indexes on affected columns
grep -rn "INDEX\|@@index\|add_index\|createIndex" --include="*.ts" --include="*.js" --include="*.py" --include="*.sql" --include="*.prisma" src/ prisma/ 2>/dev/null | grep -i "[column]"
```

### 2d. Service & Integration Impact
```bash
# Find external service calls related to the change
grep -rn "fetch\|axios\|http\.\|requests\.\|HttpClient\|RestTemplate" --include="*.ts" --include="*.js" --include="*.py" --include="*.java" src/ 2>/dev/null | grep -i "[service-keyword]"

# Find message queue producers/consumers
grep -rn "publish\|subscribe\|emit\|on(\|producer\|consumer\|queue\|topic\|channel" --include="*.ts" --include="*.js" --include="*.py" src/ 2>/dev/null | grep -i "[keyword]"

# Find webhook endpoints
grep -rn "webhook\|callback\|notify" --include="*.ts" --include="*.js" --include="*.py" src/ 2>/dev/null

# Find gRPC/protobuf definitions
find . -name "*.proto" 2>/dev/null | xargs grep -l "[keyword]" 2>/dev/null

# Find GraphQL schema references
grep -rn "type \|input \|query \|mutation \|subscription " --include="*.graphql" --include="*.gql" . 2>/dev/null | grep -i "[keyword]"

# Check for shared libraries/packages (monorepo)
cat package.json 2>/dev/null | grep -E "workspace|lerna|nx|turbo"
find . -name "package.json" -maxdepth 3 | xargs grep -l "[module]" 2>/dev/null
```

### 2e. Infrastructure Impact
```bash
# Check Docker configuration
grep -rn "[keyword]" Dockerfile docker-compose.yml docker-compose.yaml 2>/dev/null

# Check CI/CD pipelines
grep -rn "[keyword]" .github/workflows/*.yml .gitlab-ci.yml Jenkinsfile bitbucket-pipelines.yml 2>/dev/null

# Check environment configuration
grep -rn "[keyword]" .env.example .env.sample .env.template 2>/dev/null
grep -rn "[keyword]" --include="*.yaml" --include="*.yml" --include="*.toml" --include="*.ini" config/ infra/ k8s/ terraform/ 2>/dev/null

# Check Kubernetes manifests
grep -rn "[keyword]" --include="*.yaml" --include="*.yml" k8s/ kubernetes/ hel