node-backend

Node.js Backend Patterns for Medusa v2

Before writing code

Fetch live docs:

1. Web-search site:docs.medusajs.com api routes middlewares express for middleware configuration
2. Web-search site:mikro-orm.io docs for MikroORM entity and query patterns
3. Web-search site:docs.medusajs.com error handling MedusaError for error types
4. Fetch https://docs.medusajs.com/learn/fundamentals/api-routes for API route conventions
5. Web-search site:node.org docs async_hooks performance for Node.js async best practices

Express Middleware Chain

Medusa Request Lifecycle

Medusa v2 is built on Express. Every HTTP request flows through a defined middleware chain:

Incoming Request
├── 1. CORS middleware
├── 2. Body parser (JSON)
├── 3. Cookie parser / session
├── 4. Authentication middleware
├── 5. Custom middlewares (middlewares.ts)
├── 6. Route handler (route.ts)
└── 7. Error handler

Custom Middleware Registration

Define custom middleware in src/api/middlewares.ts:

// Fetch live docs for defineMiddlewares
// and MiddlewareRoute type signatures
import { defineMiddlewares } from "@medusajs/medusa"

| Middleware Property | Purpose | |--------------------|---------| | matcher | Route pattern to match (glob or regex) | | method | HTTP method filter (GET, POST, etc.) | | middlewares | Array of Express middleware functions | | bodyParser | Configure or disable body parsing | | authenticate | Apply auth requirements |

Middleware Execution Order

Middleware executes in the order defined in the routes array. Global middleware (matcher *) runs before route-specific middleware.

MikroORM Entity Patterns

DML Abstraction Layer

Medusa v2 abstracts MikroORM behind DML. However, understanding the underlying ORM helps with advanced queries:

| DML Layer | MikroORM Layer | Purpose | |-----------|---------------|---------| | model.define() | @Entity() decorator | Define database entity | | DML fields | @Property() decorators | Column definitions | | DML relationships | @OneToMany(), @ManyToOne() | Relationship mappings | | DML .index() | @Index() decorator | Database indexes | | Generated migrations | MikroORM migration files | Schema changes |

Query Patterns via Service Methods

Module services generated by MedusaService provide typed query methods:

| Operation | Service Method | Underlying ORM Action | |-----------|---------------|----------------------| | Find many | list(filters, config) | em.find() with criteria | | Find one | retrieve(id, config) | em.findOneOrFail() | | Create | create(data) | em.create() + em.persist() | | Update | update(data) | em.assign() + em.flush() | | Delete | delete(id) | em.removeAndFlush() | | Soft delete | softDelete(id) | Set deleted_at timestamp |

MikroORM uses the Unit of Work pattern — changes are tracked and flushed together per request.

PostgreSQL Connection Pooling

Default Connection Settings

Medusa uses MikroORM's built-in PostgreSQL driver with connection pooling:

| Setting | Default | Production Recommendation | |---------|---------|--------------------------| | Pool minimum | 2 | 5-10 | | Pool maximum | 10 | 20-50 (based on load) | | Idle timeout | 30s | 10-30s | | Connection timeout | 5s | 5-10s | | SSL mode | Disabled | require or verify-full |

Pool Management

• Configure via DATABASE_URL query parameters or medusa-config.ts database options
• Track active vs idle connections via PostgreSQL's pg_stat_activity
• Set pool max below PostgreSQL max_connections (reserve for admin)
• Use PgBouncer for external pooling in high-concurrency scenarios

Async Patterns

Async/Await Best Practices

| Pattern | Use Case | Avoid | |---------|----------|-------| | async/await | Sequential async operations | Callback-based patterns | | Promise.all() | Parallel independent operations | Sequential awaits for independent tasks | | Promise.allSettled() | Parallel ops where some may fail | Promise.all() when partial success is OK | | for await...of | Async iterators, streams | Manual iterator consumption | | try/catch in async | Error handling in async functions | Unhandled promise rejections |

Medusa Workflow Steps

Workflow steps are inherently async. Each step receives context and returns a StepResponse:

// Fetch live docs for createStep async
// signature and StepResponse typing

Event-Driven Async (Subscribers)

Subscribers handle events asynchronously. They execute in the worker process when running in separate mode:

| Concept | Implementation | |---------|---------------| | Event emission | Automatic on entity changes or manual via event bus | | Subscriber registration | Export config with event and handler function | | Execution context | Runs in worker process (async from request) | | Error handling | Subscriber errors do not affect the triggering request |

Error Handling

Medusa Error Types

| Error Class | HTTP Status | Use Case | |------------|-------------|----------| | MedusaError | Varies | Base error class with type codes | | NOT_FOUND | 404 | Entity not found | | INVALID_DATA | 400 | Validation failure | | UNAUTHORIZED | 401 | Authentication required | | NOT_ALLOWED | 403 | Insufficient permissions | | CONFLICT | 409 | Duplicate or conflicting state | | UNEXPECTED_STATE | 500 | Internal logic error |

Error Handling in API Routes

// Fetch live docs for MedusaError import
// and error type enum values
import { MedusaError } from "@medusajs/framework/utils"

Error Handling Strategy

• Throw MedusaError with appropriate type in service methods
• Medusa's error handler middleware converts errors to HTTP responses
• Never swallow errors silently — log and re-throw or handle explicitly
• Use try/catch in workflow steps and return compensation data on failure

Logging

Built-in Logger

Medusa provides a structured logger accessible via the DI container:

| Log Level | Method | Use Case | |-----------|--------|----------| | error | logger.error() | Unrecoverable errors, exceptions | | warn | logger.warn() | Degraded state, deprecated usage | | info | logger.info() | Significant events, startup, shutdown | | debug | logger.debug() | Detailed diagnostic information |

Resolve the logger from the container via container.resolve("logger"). Log at appropriate levels, include contextual data (entity IDs, operation names), and configure structured JSON output in production.

Node.js Runtime Considerations

| Concern | Recommendation | |---------|---------------| | Node.js version | Use Node.js 20+ (LTS) for Medusa v2 | | Memory | Set --max-old-space-size for large catalogs | | Event loop | Avoid blocking the event loop with synchronous operations | | Graceful shutdown | Handle SIGTERM to close connections and finish in-flight requests | | Clustering | Use multiple server instances (not Node cluster) behind a load balancer |

Best Practices

• Middleware discipline — define all custom middleware in src/api/middlewares.ts; order middleware from general to specific; use route matchers to scope middleware narrowly
• Database access — always go through module services, never access the entity manager directly; use list with filters instead of raw queries; leverage soft deletes for data preservation
• Async safety — always await async operations; use Promise.all for parallel independent work; handle errors in every try/catch block with meaningful error types
• Performance — configure connection pool size for expected concurrency; use Redis for caching frequently accessed data; avoid N+1 queries by using service config.relations to eager-load relationships

Fetch the Medusa API route and middleware documentation for exact handler signatures, error types, and logging configuration before implementing.