Break it down into smaller parts

If you are purchasing something larger than a single API, break the procurement down into smaller parts. 

Small information systems have traditionally consisted of three elements: 

1. user interface 
2. business logic implementation 
3. database. 

This division into three elements is not enough in large systems, however, as it would result in a significant volume of business logic. This type of architecture is known as monolithic. 

While monolithic architecture was previously used to develop large information systems, it has mainly been replaced by microservice architecture, in which the information system has been divided into smaller components, or services, from the perspective of data or functions. These services communicate with each other through APIs. 

Microservice architecture implements the EU's interoperability principles better than monolithic architecture, as it is technology neutral as far as APIs are concerned. It also significantly reduces the project management and supplier risks. A large system can be implemented in small parts, starting with the most important components. 

For example, purchases can be made one component or larger entity at a time, and components can be purchased from different suppliers. 

In the microservice implementation method, an API published for external parties is often a fairly straightforward combination of internal APIs. The operation of a microservice is often based on a set of functions executed by its internal APIs. 

Many organisations strive to develop and document APIs that are sufficient for the system's internal use with the same tool as external APIs. If internal APIs aim for high performance, the management system can sometimes be completely or partially by-passed, and the query can be made directly. In addition, technologies selected primarily for their performance can be used.