OpenWMS.org is made up microservices that communicate between each other in a variety of protocols either in synchronous or asynchronous style. A directed synchronous communication should always happen from north to south and west to east, whereas asynchronous events, commands and messages may be interchanged in all directions to get information populated across the whole system.

**Figure 3. Callgraph and informationflow between the four OpenWMS.org domains** **Contained Building Blocks**

Component	Responsibility
ERP	The external Enterprise Resource Planning system is the host that sends higher level warehouse orders. Not part of OpenWMS.org
WMS Domain	The Warehouse Management System is the layer that interacts with the ERP and implements all common warehouse processes, like Receiving, Shipping, Inventory, Picking etc.
TMS Domain	The Transport Management System implements the Material Flow Control (MFC) in automatic warehouses and also the processes used in manual warehouses
COMMON Domain	This layer contains all essential processes and services for every warehouse solution, no matter if automatic or manual warehouses, with or without a WMS layer. These services are always part of each customer project
CORE Domain	The CORE domain contains infrastructure components, that are not specific to warehouse projects and can also be used in other arbitrary projects. Most of these services are optional to use and depend on the deployment scenario
Subsystems	Subsystems do not belong to OpenWMS.org, but are communication partners like PLC (Programmable Logic Controllers), scale units, robots etc.