Freelance Backend Engineer

Location: London (Hybrid, 3 days/week in office - located in London Liverpool Street)
Duration:  3-6 months - Outside of IR35
Start date: ASAP

Elsewhen is a London based consultancy designing digital products & services for the likes of Spotify, Inmarsat and Zego. Over the past 11 years, Elsewhen has created a working environment that is impactful, driven, open and friendly. We value outcomes over hours and agility over rigid processes.

Join the team — .

About the Role:
We’re looking for an experienced, self-driven Senior Backend Engineer to join a fast-paced project with a brand new client in the telecoms industry, building a new product which is aimed at doubling their customer base.

What you’ll do

• Own microservices in NestJS (TypeScript): define module boundaries, version APIs, and ship resilient, well-tested services.
• Design internal communication: implement REST/ gRPC / event-driven patterns (e.g., request/response, pub/sub), handle retries, timeouts, backoff, and circuit breakers.
• Implement secure JWT auth: access/refresh lifecycles, rotation, audience/issuer validation, revocation/blacklisting, and token storage best practices.
• Deliver payment integrations: integrate with gateways (e.g., Stripe/Adyen/Braintree), implement idempotent operations, handle captures/refunds/disputes, and reconcile via signed webhooks .
• Build and consume webhooks: define payload contracts, verify signatures/HMAC, ensure idempotency, and implement robust retry and DLQ strategies.
• Enable real-time features with WebSockets: use NestJS Gateways/Socket.IO or ws for presence, notifications, and fan-out at scale; secure and rate-limit connections.
• Use Redis/Valkey effectively: design cache keys/TTLs, apply cache-aside/write-through, implement distributed locks, queues/streams, pub/sub, and mitigate hot keys.
• Containerize and run locally: craft efficient multi-stage Docker builds, add healthchecks, and orchestrate multi-service dev stacks with docker-compose .
• Quality & reliability: write unit/integration/e2e tests (Jest/Supertest), add schema validation (class-validator/OpenAPI), and instrument with logs/metrics/traces.
• Collaborate across teams: review PRs, write concise design docs, and partner with product, frontend, and data to deliver measurable outcomes.

What you bring

• 5+ years building backend systems; 3+ years NestJS + TypeScript in production.
• Strong Docker and docker-compose skills (multi-stage builds, image slimming, healthchecks).
• Solid microservice design: domain boundaries, API versioning, idempotency, saga/choreography, and eventual consistency (outbox/inbox).
• Hands-on JWT implementation in distributed systems.
• Demonstrated payment experience (idempotency keys, reconciliation jobs) and webhooks (HMAC/signatures, replay protection, retries).
• Built and scaled WebSockets features in production.
• Proficiency with Redis/Valkey for caching and real-time patterns; strong SQL fundamentals (PostgreSQL/MySQL) and practical ORM experience (TypeORM/Prisma).
• Testing mindset with meaningful coverage and CI checks; working knowledge of security best practices (OWASP, secrets management, least privilege).
• Comfortable with telemetry (structured logs, metrics, tracing) and on-call ownership.

Nice to have

• Kong Gateway (or similar API gateway) for routing, auth, rate limiting, and observability.
• Kubernetes/Helm; service meshes (Istio/Linkerd).
• Event platforms (NATS/Kafka/RabbitMQ) and streaming patterns.
• GraphQL (Apollo/NestJS) including subscriptions.

Our Commitment to Diversity:

Diverse thoughts, backgrounds, and perspectives create stronger teams and better technology. We welcome everyone, regardless of culture, appearance, or perspective, fostering individuality. We empower our team to challenge norms, grow ideas, and produce their best work.