How to fix HTTP Error 503. The service is unavailable in Windows?  

How to Fix a 503 Error and Determine the Root Cause:

1. Evaluate resource usage. – A server employs RAM, CPU, I/O, entry processes, and website inodes as resources. You can determine whether the problem is connected to resource limitations by looking at these metrics. You may, for instance, keep an eye on incoming traffic and notice any odd spikes in it.
2. Inspect for ongoing maintenance. –  To maximize site speed and reduce security threats, regular server maintenance is essential. A web server or application is typically configured to shut down if maintenance is required.
3. Stop all active processes – The 503 service unavailable error can also be brought on by exceeding the maximum process limit. When your web server does too many tasks at once, it may become overloaded and stop working.
   You’ll need to halt running operations in order to get your site back online. Users of VPS hosting can terminate a process by using the kill command line.
4. Firewall reset – Any malicious traffic is prevented from entering your website by a web application firewall (WAF). In other words, the firewall guards your website from harmful activities like cross-site scripting and SQL injections, as well as brute force and DDoS attacks.
5. Examine server logs, then fix the code. – Check your error log if the 503 problem is still occurring.Server-side logs keep track of everything that happens on your website, from user requests for specific web pages to system failures. You can more quickly find and correct mistakes by using data from the logs.
6. Restart your server and your network hardware. –  The server chain that is hosting your application may experience connectivity problems, which can result in the 503 service unavailable error.
7. Verify Your DNS – A DNS-related problem, such as unresolved IP addresses, unresponsive DNS servers, or a misconfigured DNS cache, is another potential cause of the 503 error.DNS flushing is one practical solution to this problem. Your operating system will determine how to flush the DNS cache. We’ll demonstrate how to achieve this on Windows in the sections below.Windows by MicrosoftOpen the command prompt on Windows and type the following command:
   /flushdns in ipconfig
   The command will restart the DNS resolver cache and clean the DNS files. Once the DNS has been successfully flushed, a confirmation message will show up.