1. XML-RPC Web Services in WinCC OA An Application: online PARA JCOP FWWG Meeting 15-12-2015
Lorenzo Masetti

2. Let’s start from the result: Para Online

3. Live Demo

4. Another Example: Log Access

5. Service Architecture

6. http server also serves the required HTML and javascript
Two-layers setup
Client (Javascript)
Request
(XML-RPC)
Response
(JSON-RPC)
Server (WinCC OA)
http server
WinCC OA
http server also serves the required HTML and javascript
Service

7. Four-layers setup using Oracle WebCenter
Client (Javascript)
Request
(XML-RPC)
Response
(JSON-RPC)
J2EE Web Server ( JAVA )
DPAccess Portlet
Public Network
CMS Network
DpAccess EJB
Request
(XML-RPC)
Response
(XML-RPC)
Server (WinCC OA)
http server
Service

8. Services in WinCC OA

9. General Services in WinCC OA
Using CMSfwClass (OO-framework for WinCC OA) we implemented a general framework for exposing and consuming services
Request and response through data points
Interface of the service defined in a CMSfwClass interface

10. Exposing the service to the Web
To expose the service as a Web Service:
we use the httpServer provided with WinCC OA.
To handle the request we instantiate a ServiceClient
Forwards the request to data point
Forwards the response via http

11. Communication Format
The format can be changed by implementing an interface
Currently we have implemented:
XML-RPC
Mixed format (request in XML-RPC, response in JSON-RPC)
Request  decoded to methodName, list of parameters (dyn_mixed)
Response  decoded to anytype (depending on the return type of the method)
Client
Encode request
Decode
response
Note: decodeResponse not implemented here: can only be used on the server side
Decode
request
Encode response
Server

12. Service Router A specialized class implements a service router
Used to dispatch commands to different services depending on the prefix in the command, e.g.
dpAccess.dpGet  dispatched to DpAccess service
logAccess.getLog  dispatched to LogAccess service
All XML-RPC services implement a “system” interface that allows the client to discover which procedures are available

13. Generic Client in Javascript

14. Generic XML+JSON-RPC client in Javascript
Mixed communication format: XML for the request, JSON for the response.
JSON is easier to handle in Javascript
Using the reflection features, we can create a JS object having the same functions defined in the service
Passing the URL of the Web Service and the prefix we get an object
The implementation of the functions forwards the request to the server and gets the response via AJAX
The functions are asynchronous (the result is returned in a callback function passed as last parameter)

15. Interface to access WinCC OA functions

16. Service interface to access DPs
This is the interface used to get read-only access to the datapoints
Can access remote data points
Some convenient functions to group requests (e.g. dpGetStruct)
Some fw functions are also exposed
getDeviceChildren  fwDevice_getChildren
getPeriphAddress  fwPeriphAddress_get

17. The Javascript client to access DPs
Using the generic XML-RPC JS Client, a library (SCADA.js) exposes the methods of the DPAccess interface in javascript
SCADA.js
xmlrpc.js
On top of SCADA.JS, a library para.js is used to create the online version of the para module
para.js
SCADA.js

18. Java EJB Implementation
It is convenient to have an EJB with the same interface of the XML-RPC service
The implementation of the EJB is straightforward using an XML-RPC Proxy (this feature is implemented for example in redstone.xmlrpc library)

19. Custom Panels
The para library allows defining custom “panels” for certain data point types / data point name patterns
“Panels” are rendered using EJS (Embedded JavaScript)

20. Serialization of Asynchronous calls
SCADA.js library exposes asynchronous functions
It is much easier to write code with synchronous calls
Solution: WCCOA object offering the same functions but synchronous
The first time the function is called it adds the request to the queue and returns a default value
At the end of the execution (of the panel/function) the requests are sent to the service (multicall)
The code is executed again and the functions will return the correct value from the response
If there are dependencies, the code will be called again until the queue is empty

21. Example: WCCOA.dpGet returns “” the first time
WCCOA.wait returns true if all the prior requests were resolved
All the requests that are not dependent are grouped automatically
returnVal calls again the function until done and calls the callback at the end