Begin by making an initial request, and then invoke a user-defined JavaScript function called "countdown()" using the built-in JavaScript function "setTimeout("countdown()", 4000)". The number "4000" indicates the amount of time in milliseconds before executing the specified function.
Within the "countdown()" function, include the logic for an AJAX call to update the database after a designated period (4 seconds in this example), allowing the user to make a second request.
In the same AJAX call, you can output a word (such as "yes") that can be retrieved in the "countdown()" function using JavaScript's "responseText" keyword from the XMLHttpRequestObject, storing it in a temporary variable like "flag".
Next, include the following code snippet in the definition of the "countdown()" function:
function countdown() {
document.getElementById("btn_second_req").style.display = 'none';
var flag = '';
// code for preparing the AJAX call
// AJAX call made, response fetched and stored in "flag" variable
if (XMLHttpRequestObject.readyState == 4 && XMLHttpRequestObject.status == 200) {
flag = XMLHttpRequestObject.responseText;
}
if (flag == 'yes') {
document.getElementById("btn_second_req").style.display = 'block';
}
}
The HTML button triggering the second request is initially hidden and will only be displayed when the condition based on the "flag" variable is met.
Furthermore, since information about the first user call is stored in the database, server validation must be performed during the second call to ensure the existence of details from the initial request.
Implementing such logic can be done easily with JavaScript frameworks like jQuery or Prototype, or through plain JavaScript alone.
I hope this explanation proves helpful.