Understanding GlideAjax in ServiceNow: Asynchronous Client-Server Interaction

Before I describe what Synchronous and Asynchronous mean in web development, let’s first dive into the history of these technologies.

In 1995, Brendan Eich, the creator of JavaScript, added the JavaScript engine to the web browser. Eich worked on an interpreter for the nascent language he had invented and implemented it into browsers that same year. At that time in the history of the web, there was no API for traversing and manipulating the content of a document. The only way JavaScript could manipulate and change the content of a document was by generating it on the fly while the document was loading. This method of content manipulation was possible using the document.write() method. Although this method was an innovative solution at the time, it came with its own set of problems. For instance, using document.write() to modify content on the fly could lead to issues like overwriting the entire page.

As the web matured, developers needed better ways to interact with web pages without blocking the user experience or causing performance issues. This led to the distinction between synchronous and asynchronous operations.

Synchronous Operations

Synchronous operations are tasks that are performed one after another. The program will execute a task and only move on to the next one once the current task has finished. In the early days of web development, this was the default behavior. However, it could be quite laggy, and users had to be patient enough to wait for each process to finish before starting another task on the same page.

Asynchronous Operations

Asynchronous operations, on the other hand, allow a program to initiate a task without waiting for it to finish before moving on to the next one. This means the browser can continue functioning while other processes are in progress, creating a smoother and more responsive user experience.

Asynchronous Programming in ServiceNow

Now that we understand where these terms come from, let's explore them in the context of ServiceNow, asynchronous programming, plays a crucial role in enabling smooth interactions between the client and server. The GlideAjax class allows the execution of server-side code from the client-side. By using GlideAjax, we can make asynchronous calls to Script Includes on the server, passing parameters between the client and server without blocking the user interface. This enables dynamic interactions, such as retrieving or submitting data, while keeping the page responsive.

Let’s walk through the process using an example scenario where we need to check a Configuration Item (CI) and update the Assignment Group of an Incident record in real time as the user interacts with the Incident form.

1. Client-Side Trigger:

The process starts when the user takes an action on the Incident form. For example, the user selects a CI in the Incident. This triggers a GlideAjax call to check the support group associated with the selected CI.

2. Calling GlideAjax:

In the client-side script, the GlideAjax class is used to send a request to the server. The client sends the CI record identifier (sys_id) as a parameter to a Script Include.

3. Server-Side Script Include (GlideAjax Handler):

The request sent by GlideAjax is handled by a Script Include. The Script Include contains the logic to check the support group associated with the CI.

4. Response to Client-Side:

The GlideAjax handler sends the result back to the client-side script. The result is received as an XML response within the callback function. The client script then evaluates the result and updates the Assignment Group field on the form accordingly.

5. Updating the Assignment Group:

If the result is 'true', meaning the CI belongs to the Database Support group, the client-side script updates the Assignment Group field with the corresponding sys_id for the Database Support group.

The GlideAjax class in ServiceNow allows developers to check database records and dynamically update fields on the form, such as the example of the Assignment Group. This improves the user experience by providing real-time data enabling seamless client-server interactions, making it easier for developers to build responsive, intelligent applications.