Web Services & SUDS

Web Services Overview

Web services are software solutions that allow for interacting with machines residing on a network. In short, web services are nothing more than web pages for machines. They provide a standard way for a third party to request and receive data from a piece of hardware on the network without having to know anything about how that machine works. Other programs interact with the service through an interface defined by a WSDL (Web Services Description Language) file. This WSDL describes how to talk with the device and what should be expected back in response. Messages to and from the web service are formatted XML and while you need very little knowledge of XML to use the SUDS library, many times a web service will return a formatted XML string that you will have to parse through manually in order to make the data presentable.

Did you know?

SUDS doesn't actually stand for anything! It's a lightweight version of SOAP, hence the name SUDS. SOAP stands for Simple Object Access Protocol, and is commonly used for Internet communication.

What if I do not have the SUDS library?

Some installations of Ignition may not have the SUDS library, but it can easily be added following the steps below:

  1. Download the SUDS library: https://fedorahosted.org/suds/

  2. Place the library into Ignition's installation directory: Knowledge Base - Importing 3rd party modules

When working with Web Services, the SUDS documentation and the PyDocs are a good place to start.

Common Web Services Workflow

While all Web Services follow the same standards, they all do different things. They wouldn't be worth anything if you didn't get the information you need, or if they contained a lot of excess data. If you are unfamiliar with a particular Web Service, there are a few things that you can do to figure out what data is available and how to get it.

  1. Identify a Web Service that you will be using.

  2. Write a script to pull the WSDL from the Web Service.

  3. From the WSDL, identify the functions you want to use and what arguments it expects.

  4. Write a script to use that function and return your values.

  5. Parse the results and use them. This can be for display, saving to a database, or anything else you need.

Note: web services sometimes take a lot of time to return results, especially the first time they are called. If you put your Web Services script in a button, the client will freeze until the call is complete (this is because the event handlers are run on the GUI thread). It's a good idea to use system.util.invokeAsynchronous() or add a waiting image to your screen to let the user know Ignition is working as expected.

See Overview & Simple Arguments to get started.

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