Tuesday, May 2, 2017

AJAX & JSON with CGI Programming in IBM-I

Hello Friends,


In our last article we have seen how to use CGIDEV2 to update an external HTML from RPGLE and using that we were able to see some good looking outputs in browser. Now in this article we are going to see a step further and learn some basics about the following,
  1. What is AJAX
  2. What is JSON
  3. How to use JSON in AJAX in CGI programming

AJAX: Asynchronous JavaScript and XML

In simple term, this is a technique to update the web page content without reloading the page. Also  update includes, sending request to a server in background and based on response received we can update web page content.

E.g.: Let’s take our Order Inquiry example

We call ORDINQ CGI program from web which displays the order inquiry screen and gets input (say 123) and when we click submit, it redirect to ORDLISTCGI program with parameter as 123.


Here, after we click submit the page reloads with ORDLISTCGI program and HTML output is displayed with order details of order# 123.

Now, with AJAX we are going to send HTTP request and receive the response in the background and finally going to display the order list output in the same page without reloading it.

i.e.:


Step 1: Let's create new RPGLE CGI program

I have created a simple CGI program ORDHTML which returns order details in HTML format. It is pretty straight forward. If the order is found, then I am returning the values under <H3> tag.


If we want to test this, check the url http://as400/cgi/ordhtml?order=555 in browser


Now we want this piece of information to be updated in our main page (ORDINQAJAX) when we click submit button.

The good thing is our main page (ORDINQAJAX) is not necessarily being a CGI program. Instead it can be a plain HTML/JavaScript on your desktop.

ORDINQAJAX html:



What we have done here is, when we click Retrieve Order button, it calls loadDoc() function. This is forming the url by concatenating our input number value.

Once the url is formed, a XMLHttpRequest object is created using which we are requesting the response of ORDHTML url.

And finally the output available in responseText is replaced with the paragraph tag with id = “demo”

Input:



Output:



Take a note that the page is not reloaded and result is showed in the same ordinqajax.html

Here we have achieved AJAX and eliminated the dependency of input screen coming from AS400 CGI program. Since, our input screen is our HTML file, we can play with CSS and JavaScript to make it good looking and make is responsive.

But wait… what if we want to change the display format of order details? Currently it is always coming as <H3> header directly from ORDHTML page. Here is JSON comes into play…

JSON: JavaScript Object Notation

JSON is a Java Script object. I am not going in depth about it (because there is lot of tutorial in web). But I will show the basic by giving some examples.

Have a look at the below comparison of a sample XML and converted JSON format. 



JSON string enclosed by curly braces { and } and it is made up of “key” : “value” pair and colon ( “:”) to separate both. If any set is repeated, forms an array which enclosed by square brackets [ and ].

In our example, key “id” has value “901” & phones is an array and each phones contain keys “type” & “number”. Unlike XML, we don’t have any end tag here. So this really helps to reduce the file size.

We are quite familiar with XMLs in AS400/IBM-I but not much with JSON. But this JSON will become handy when dealing with web service or with CGI programming.

Since this is a JavaScript object, we can easily parse this and get the values inside JavaScript program.

E.g.: Once we assign this JSON string into a variable (say variable X) then we can start accessing the values using dot operator.


How to use JSON in AJAX:

The basic idea is we are going to return the json format of output to browser (instead of plain html content). When we receive this result in our HTML we can assign it to a variable in the JavaScript and can split the details and use it inside HTML.

E.g.: program ORDJSON which returns the response as JSON. Please note that I have changed my content-type as text/plain in this RPGLE.


As of now the code looks ugly. I shall come back about this later.

With this program in place,  we are already ready to see the json output in the browser.

http://as400/cgi/ordjson?order=555


Now let’s change our ORDINQAJAX html to parse this json and display output.

Modified ORDINQAJAX.html



We can see, the url is changed to point ordjson CGI program. The result in this.responseText is parsed into JSON using JSON.parse JavaScript method and stores the json object in obj variable.

Then it is just simple programming to use the values inside json. Here in this example, order number and date are just displayed and item details are added into rows of table (which has id=”detail”)

Let us see the input & output.

Input:



Output:



I hope you would have learnt something interesting and useful. Now a day, AS400 business logics (existing RPLGE) is being served as web services. If you have a deeper look, we have just created a RESTful web service based on order number input and it returns order detail as output JSON.

In the world of web programming, you can very well share this URL to your clients and they can use it accordingly to their needs.

What next?

Remember I have highlighted regarding ugly json formatting in RPGLE? You may think what will be the best possible way to create JSON in rpgle. Also if we need some complex JSON to be formed, concatenation is not at all a best way. Also how do we parse a JSON inside RPGLE?

For all these questions, I shall come back with answers in my next article. Until then...

Have Fun…!!! Happy Coding..!!!


3 comments:

  1. Hi..is there a tutorial to start this, would like to give some hands on, appreciate if you can share the link

    ReplyDelete
  2. Hi Vinay... If you are visiting this blog first time, then I request you to go through my older posts. I tried explaining things from basics available in below link. kindly go through from bottom to top and let me know if you need any assistance.

    https://yusy4code.blogspot.sg/2017/04/

    ReplyDelete
  3. Thanks Yusuf..let me give a shot!

    ReplyDelete