Exploring the ways to access the node properties and update them as as independent solution like Rest or Web API which can be consumed/utilized by any third party system based upon requirement.
For the fetching part I would normally recommend to use Sling Models and the Sling Model exporter, because it allows you do refine your JSON very explicitly.
If you just want to a generic POST and GET approach and write/read resources, you can use the Sling DefaultGetServlet [1] and DefaultPostServlet [2].
AEM supports OOTB GET APIs to expose content as content service (as mentioned in @Jagadeesh_Prakash comment below). For POST and other type of APIs, need to create an OSGi service layer (mostly Sling Servlet) which will accept the request and process accordingly using external APIs like Apache Sling, Oak and JCR APIs which deals with content repository. (Please find information about these API in API Guides | Adobe Experience Manager).
Note: It needs to be taken care of maintaining all the AEM instance in sync when content gets posted/deleted to any of the instance in the farm.
For the fetching part I would normally recommend to use Sling Models and the Sling Model exporter, because it allows you do refine your JSON very explicitly.
If you just want to a generic POST and GET approach and write/read resources, you can use the Sling DefaultGetServlet [1] and DefaultPostServlet [2].
