Many digital projects do not fail at the user interface. They fail at the interfaces between systems. Customer data sits in the CRM, orders in the ERP, documents in a DMS, payments with a provider and reports in a data warehouse. When those systems do not communicate reliably, teams fall back to manual work, duplicate data entry and error-prone coordination.
That is why companies search for custom API development, REST API development or ERP integration. The task is rarely just technical. In practice, it is about mapping business processes cleanly across systems.
This article explains the main API types, when REST, GraphQL, webhooks or events make sense, what costs to expect and what companies should consider in professional backend development.
What is an API?
An API, or application programming interface, is a defined interface between software systems. It determines which data can be read, written or triggered. An API can, for example:
- retrieve customer data from a CRM
- send orders to an ERP system
- create invoices
- authenticate users in an app
- display availability from another system
- upload documents
- notify other systems about status changes
A good API describes more than endpoints. It also defines data formats, permissions, error cases, limits, versioning and responsibilities.
When does a company need a custom integration?
A custom integration is useful when standard connectors are not enough or when a process connects several systems. Typical situations:
- a web app needs to communicate with CRM or ERP
- a customer portal needs documents, contract data or status information
- a mobile app needs to display data from existing systems
- internal tools should replace manual Excel or email workflows
- a SaaS product should offer a public API to partners
- data from several systems should be consolidated for reporting
The main point: an API should not only work technically. It needs to fit the process. Otherwise it becomes difficult to maintain because the actual business logic is spread across too many places.
REST, GraphQL, webhooks or events?
Not every integration needs the same architecture. The right choice depends on how the interface will be used.
| Approach | Good for | Strengths | Limits |
|---|---|---|---|
| REST API | classic CRUD flows, clear resources, many integrations | widely understood, robust, easy to document | can require many endpoints for complex views |
| GraphQL | flexible frontends, variable data queries, complex screens | clients request exactly the data they need | needs careful schema design and limits |
| Webhooks | notifying other systems about events | simple, fast, useful for status changes | delivery, retries and security need attention |
| Event-based architecture | decoupled systems, workflows with many follow-up steps | scalable, resilient, good for growth | more complex to operate and debug |
For many business applications, REST is still a strong foundation. GraphQL can be useful when several frontends need very different data views. Webhooks complement APIs when one system needs to inform another automatically. Event-based architectures become useful when processes grow or systems should be intentionally decoupled.
What does API development cost?
Cost depends on scope, data quality, authentication, documentation, tests and integrations. As a rough orientation:
| Project | Typical scope | Realistic range |
|---|---|---|
| Small internal API | few endpoints, simple authentication, one system | 8,000 to 25,000 EUR |
| Product API | roles, versioning, documentation, tests, monitoring | 25,000 to 70,000 EUR |
| ERP/CRM integration | data mapping, error handling, synchronization | 30,000 to 100,000 EUR |
| Integration platform | several systems, events, queues, audit logs, operations | from 100,000 EUR |
The biggest uncertainty is often not API code itself. It is usually data models, legacy systems, permissions, missing documentation or unclear process rules.
Questions to answer before development
Before development starts, companies should clarify:
- Which systems need to be connected?
- Which data is read or written?
- Who owns the data?
- Which roles may perform which actions?
- Does data need to move synchronously or asynchronously?
- What happens when a system is unavailable?
- Are there limits, maintenance windows or data quality issues?
- Which logs and audit trails are required?
- Who will use the API: internal frontend, partner, mobile app or external customer?
These questions may feel dry, but they determine stability. An API is a contract between systems. If the contract is vague, errors appear at the edges.
Security: API development without open gaps
APIs often connect directly to critical data. Security should therefore be designed from the start. Important elements include:
- authentication, for example OAuth 2.0, API keys or sessions
- authorization on role and record level
- input validation
- rate limits and abuse protection
- safe error responses without internal details
- encrypted transport
- logging and monitoring
- protection against common API risks such as broken object level authorization
For portals, apps and partner integrations, checking only the login is not enough. The API must ensure that users can only see and change data they are actually allowed to access.
Documentation and developer experience
An API without documentation becomes expensive quickly. New developers need more time, integrations become more error-prone and support requests increase. Good API documentation includes:
- purpose of the API
- authentication
- endpoints or schema
- data models
- example requests and responses
- error codes
- versioning
- limits
- guidance on retries and idempotency
For REST APIs, an OpenAPI specification is often useful. It can serve as technical documentation, test foundation and sometimes even a basis for generated clients.
Typical API project workflow
A typical process looks like this:
- analyze systems and processes
- clarify data model and responsibilities
- design the API
- define authentication and permissions
- build a prototype against real or realistic data
- test error cases and limits
- create documentation
- set up monitoring, logging and deployment
- test the integration with frontend, app or partner system
- plan operations and versioning
For critical integrations, a technical spike is often useful. It checks early whether the target system is reachable, what data formats look like and which limits the external API has.
Common mistakes in integration projects
Business logic in the wrong place
If the frontend contains too much business logic, the API becomes hard to reuse. Core business logic usually belongs in the backend or a clearly defined service.
No error strategy
Errors happen: systems are offline, data is missing, limits apply, formats change. Without clear error responses, retries and logs, debugging becomes slow and expensive.
No versioning
APIs change. Without versioning or compatibility rules, existing integrations break when new requirements are added.
Too little monitoring
An API can be technically available but still deliver wrong business data. Monitoring should therefore track not only status codes, but also process metrics.
Checklist for companies
Before commissioning API development, this checklist helps:
- Is there a clear process description?
- Are source and target systems known?
- Are test accounts or sandbox environments available?
- Have data fields and data quality been checked?
- Is it clear who may see which data?
- Are privacy requirements clarified?
- Are error cases and retries defined?
- Should the API be internal, external or partner-ready?
- Is documentation required?
- Who operates and monitors the integration after launch?
Final thoughts
An API is more than a technical connection. It determines how reliably data flows between systems and how well digital products can grow. Companies should therefore treat custom API development as an architecture and process project, not only as a programming task.
A good software agency helps plan business logic, security, documentation and operations together. That creates a reliable foundation for apps, portals, automation and digital business models.
