Backend Development and Integration

Backend development is the part of an app that users never see but depend on: accounts and logins, the database, file storage, server-side logic and the connections to other systems. It is for businesses that need a new backend on Firebase or Supabase, or an app connected to the ERP, invoicing or warehouse system they already run. CompletApp is a Budapest-based product studio that delivers this in Hungarian and English, to a fixed scope and a fixed price.

Sometimes the job is building the whole backend from scratch. Often it means fitting an app around software you have used for years and are not about to replace. We do both, and we are equally happy either way.

Most of what makes software feel reliable lives in the backend, and most of what makes it fail does too. We build this layer with the care it deserves: clean data models, proper security, and infrastructure that holds up under real traffic, so your product earns trust instead of losing it at the worst possible moment.

We build the full backend behind apps and platforms: authentication and user accounts, databases and data models, file and media storage, server-side logic, and the integrations that connect your product to the systems and services it relies on. Two examples. For Tudatosság Klub, a sound therapy membership app, we built the Firebase Cloud Functions workflows that keep Stripe subscriptions, Billingo invoicing and member access in step. Behind Apartmanom sits a Supabase and PostgreSQL backend, including the scheduled jobs for mail and calendar refreshes.

Each backend is shaped around what your product actually needs to do. We design the data, the rules, and the infrastructure to serve that, with nothing fragile or over-engineered in the way. The screens on top are a separate job: internal business apps when your own staff log in, web app development when your customers do.

Most businesses do not need their software replaced. They need one more thing added to it, and that thing has to speak to what is already there. So we treat your existing system as the source of truth and build around it, rather than asking you to move your operation onto something new.

In practice that means REST and SOAP APIs, webhooks, scheduled data sync in one or both directions, and reading from more than one source so a single screen can answer a question that no single database can. It also means being careful about what we write back: data your system owns stays yours to own, and we agree exactly what the app is allowed to change before we build it.

We have done this on live products. Apartmanom, a property management platform running a real hosting business since June 2026, issues genuine Hungarian invoices through the Billingo and Számlázz.hu APIs and sends guest email through Resend, all triggered from the app. The medical education app we rebuilt had to plug into the client proprietary backend rather than a stock backend-as-a-service, which is a very different discipline from greenfield work: the product has to fit the existing system, not the other way around.

Cloud data storage and connecting existing systems are target areas of the Hungarian DIMOP Plusz digitalisation programme, so for a Hungarian SME part of the build can be grant funded. What is eligible in yours comes out of a free pre-screening. The details are on grant-funded development.

Plenty of in-house business software was never built to be connected to anything, and that is not a dead end. There are three routes, and which one fits depends on your system rather than on our preference.

We can work through the exports your system can already produce, syncing on a schedule in both directions. We can connect at the database level where that is safe and permitted. Or we can build a thin service that sits in front of your system and gives it the API it never had, so everything you add later has something clean to talk to.

We look at your real setup before quoting, and we will tell you plainly if the honest answer is that one of these is a bad idea in your case. And for the data your system has nowhere to put, photos from a delivery, a note about a difficult address, a signature, we build a store beside it, linked by the reference number you already use, so nothing has to be forced into a field that was never meant for it.

For the parts we do build from scratch, we work on Firebase and Supabase and pick the right foundation for your case. Firebase shines for real-time features and fast mobile development, with authentication, real-time data, and push handled cleanly. Supabase gives you a full relational database with the structure and flexibility many web and SaaS products need. We recommend the right fit honestly, then build on it so it serves you for years rather than boxing you in.

Security is part of the design, not a checklist at the end. We set up proper authentication, access rules so each user reaches only their own data, and safeguards around storage and server logic. Where we integrate with your systems, credentials live server-side and never ship inside the app. And because we build on managed platforms with clean data models from the start, your backend scales with your usage. Growth means adding capacity, not tearing the foundation out and starting over.

We agree the full scope and a fixed price up front, so you know what you are getting and what it costs, even for the work users never see. With an integration we do the discovery first, looking at what your system can actually expose, because a price quoted before that is a guess. We deliver in reviewable stages and hand over clean, documented infrastructure you fully own.