ERP Interfaces Explained: API, EDI, Middleware & iPaaS

The short answer up front: an interface connects your ERP with other systems – shop, marketplace, warehouse, accounting. Which integration method fits depends on the partner. For modern cloud systems you usually use a REST API with webhooks, in classic B2B trade often EDI, and when many systems interact, you link them via middleware or an iPaaS platform. This guide explains the methods in practical terms and shows you when each one is the right choice.

What an ERP interface does – and why it matters

An ERP is rarely an island. Orders arise in the shop, shipments are picked in the WMS, invoices flow into the DATEV format – all of it has to stay in sync. An interface ensures that data flows automatically between systems, without manual double entry.

The benefit is concrete: fewer errors, up-to-date stock across every channel, shorter lead times. Without integration, media breaks appear – someone retypes order data from the shop into the ERP, and that is exactly where errors and overselling creep in. How resilient your integration turns out to be is decided less by the technology than by clean data logic. If you want to connect systems professionally, it pays to look at structured ERP integration.

REST API and webhooks: the standard for cloud systems

The API (Application Programming Interface) is the documented programming interface through which a system exposes functions and data to others. With modern cloud ERPs this is almost always a REST API over HTTPS that typically exchanges data in JSON format.

How REST API and webhooks work together

A REST API works on the pull principle: your system actively requests data ("Give me all orders since yesterday"). That is flexible, but can become inefficient with frequent polling.

A webhook reverses the principle (push): as soon as an event occurs – say, a new order – the source system sends the data on its own to a registered URL. That is close to real time and saves unnecessary requests. In practice you combine both: webhooks for time-critical events, REST requests for bulk data and for backfilling.

To secure APIs, methods such as OAuth come into play; with many connections, an API gateway centralizes authentication and access control. Alongside REST there are other styles like GraphQL or the older SOAP – but in the ERP world REST dominates.

EDI and EDIFACT: the classic in B2B trade

Anyone working with large retail chains, industry or automotive can hardly avoid EDI (Electronic Data Interchange). EDI is the structured, standardized exchange of business documents – orders, dispatch advices, invoices – directly between the ERP systems of two companies, without human involvement.

The most widespread international standard is EDIFACT (UN/EDIFACT), which defines fixed message types – such as ORDERS for purchase orders, DESADV for the dispatch advice or INVOIC for invoices. Alongside it there are regional and industry standards like ANSI X12 (USA) or VDA (automotive).

EDI is proven, highly standardized and often mandatory with large trading partners – but also heavyweight: formats are rigid, onboarding new partners is laborious, and you usually need an EDI service provider or converter. For pure B2B document exchange with major customers, EDI nonetheless remains the benchmark.

EDI and e-invoicing: not the same thing

An important distinction: EDIFACT INVOIC is not automatically a legally compliant e-invoice in the German sense. For the B2B sphere in Germany the following applies: the obligation to receive e-invoices has been in force since 1 January 2025. The obligation to issue is staggered – generally from 1 January 2027 for companies with more than €800,000 in prior-year revenue, and from 1 January 2028 for all remaining companies. What counts is a structured format compliant with the EN 16931 standard, such as XRechnung or ZUGFeRD. When in doubt, check with your tax advisor how your ERP outputs these formats.

Middleware and iPaaS: connecting many systems cleanly

As soon as more than two systems are involved – ERP, shop, marketplace, PIM, WMS, accounting – direct point-to-point coupling quickly turns into sprawl. Every new connection increases complexity, and swapping out one system tears holes in several integrations.

What separates middleware from iPaaS

Middleware is a mediating layer between the systems. Instead of wiring each system individually to every other, all connections run through a central layer that translates, routes and transforms data. Concepts like an Enterprise Service Bus or a message queue decouple systems in time and technically.

iPaaS (Integration Platform as a Service) is the cloud variant of this: a hosted platform that ships with prebuilt connectors for common systems and enables integrations by configuration instead of by code. That noticeably lowers the effort and is ideal when you want to link many standard systems without a large IT team.

The rule of thumb: two systems with a clean API? A direct connection is enough. Five or more systems or frequent changes? Middleware or iPaaS pays off through maintainability.

Data formats and mapping: where integrations fail

Two systems rarely speak the same language. The data format defines how information is transferred in a structured way – JSON and XML for APIs, EDIFACT for EDI, CSV for simple data import/export.

The real sticking point is the field mapping: which field in the shop corresponds to which field in the ERP? Is the item key called "SKU" there and "article number" here? Are tax rates transferred as a percentage or as a code? A clean mapping with high data quality decides whether an integration runs reliably or keeps snagging. Consistent master data on both sides is the basic prerequisite.

Delta sync and idempotency: the art of a stable integration

Two concepts separate a robust integration from an error-prone one.

Delta synchronization transfers only the records that have changed since the last reconciliation – not the entire dataset every time. That is faster, resource-friendly and, with large data volumes, practically the only viable option. The prerequisite is that the systems keep change timestamps or change markers cleanly.

Idempotency ensures that the same message can be processed multiple times without causing harm. If an order message arrives twice due to a connection drop, the order must still be created only once. This is implemented via unique reference IDs that the target system checks. Without idempotency you risk duplicates and double bookings – the most common reason why integrations cause trouble in live operation.

When to use which method? The decision table

The choice depends on the partner, the data volume and the real-time need. This overview sorts the common cases:

When you select a new system, integration capability belongs on the criteria list – a look at the ERP directory and the comparison shows which systems come with which interfaces out of the box. Cloud-native systems like xentral, weclapp or Odoo lean heavily on open APIs, while enterprise suites such as SAP S/4HANA Cloud or Dynamics 365 Business Central bring extensive EDI and middleware ecosystems. Which connection you really need is best clarified early – for instance in a neutral ERP consultation.

Conclusion

There is no single right interface, only the one that fits each use case. For connecting a shop and marketplace to a cloud ERP, the REST API with webhooks is the standard. In B2B document exchange with large partners, there is rarely a way around EDI/EDIFACT. As soon as many systems interact, middleware and iPaaS bring order and maintainability. What ultimately decides stable live operation is clean field mapping, delta synchronization and idempotency – the inconspicuous details that determine success or a constant stream of error messages. Anyone who factors in integration capability already during ERP selection saves themselves expensive fixes later on.