# Custom API development and the integration layer under it.

> Enterprise system integration for businesses whose software has grown into separate islands: REST and GraphQL API development, third-party API integration, legacy system modernization and the backend services that hold them together — designed so a failure is visible, retryable and survivable.

- **Email:** hello@zhajmohamad.com
- **URL:** https://zhajmohamad.com/system-integration

## The integration work itself.

An integration is not a script that runs at night. It is a contract between two systems that will both keep changing.

- **Custom API development** — REST and GraphQL APIs designed around the operations a business actually performs, versioned, authenticated and documented for the teams that will consume them.
- **Third-party API integration** — Payment providers, carriers, marketplaces, messaging, accounting and government e-invoicing endpoints, wrapped so their outages do not become your outages.
- **Integration architecture** — Queues, idempotency keys, dead-letter handling and reconciliation jobs — the difference between an integration that works and one that works on the sixth month.
- **Legacy system modernization** — Putting a documented API in front of a system nobody wants to touch, so new software can be built without a rewrite that the business cannot afford to pause for.
- **Backend services and data flow** — The services that sit between systems: transformation, validation, scheduling and the observability that tells an operator what moved and what did not.

## What a disconnected system costs.

Almost always paid in someone's afternoon, every day, until the integration exists.

- **The same data is entered three times** — Once in the shop system, once in accounting, once in a spreadsheet nobody officially owns.
- **Connect ERP to e-commerce** — Two systems that both hold products, prices and customers, with no agreed contract between them.
- **An API that changed without warning** — A provider ships a new version, the nightly job fails silently, and the gap is found a week later.
- **Reports that never agree** — Sales, stock and finance each produce a different number for the same week, and the meeting is about which one to believe.
- **A system nobody dares to replace** — It runs the operation, the person who wrote it left, and every new requirement has to route around it.
- **Growth is blocked by manual capacity** — Doubling the orders would mean doubling the people who retype them.

## Integration work in production.

- **Edip Saat (edipsaat.com)** — An integration layer between commerce, ERP, warehouse and accounting — one source of truth per field, and a reconciliation job that proves it.
- **DORTEK (dortek.com.tr)** — PLC lines through a gateway into cloud dashboards — an integration where one end of the wire is a machine.
- **Hujuzatk (hujuzatk.com)** — A multi-tenant platform whose booking, pricing and operations modules talk over internal APIs rather than shared tables.

## Questions people ask.

**Do you build the API, or connect to one?**
Both, and usually in the same project. Most integrations need a new API on one side, an adapter on the other, and a service in the middle that owns retries and reconciliation.

**How do you handle the other system going down?**
The work is queued rather than fired directly. Messages carry an idempotency key, failures move to a dead-letter queue with the reason attached, and a scheduled reconciliation compares both sides so nothing quietly disappears.

**Can you work alongside our in-house team?**
Yes. A common arrangement is that I own the integration layer and its architecture while your team owns the product they already know — with weekly deployments and a written changelog so nobody has to ask what changed.

## Related specialisms.

- [Magento ERP & payment integrations](https://zhajmohamad.com/magento-integrations)
- [Business process automation](https://zhajmohamad.com/business-automation)
- [Industrial IoT & PLC integration](https://zhajmohamad.com/industrial-iot)
- [SaaS & multi-tenant platforms](https://zhajmohamad.com/saas-platforms)
- [Custom e-commerce development](https://zhajmohamad.com/ecommerce)

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Tell me what you are trying to build, automate or scale. You get a straight answer on fit within a day — and a name to call if it is not me.