How to schedule a task in Angular

Question

I have to create a component in angular that clean the data in the database (via a service call) every hour and displays the last clean time in HTML. Couple of question

When we route from one component to another, does the component get auto destroyed and lose all its data?
Is it possible to persist data and use it in the future?

@Component({
  selector: 'demo-app',
  template:`
  <button (click)="clearData()"></button>
  <h2>Last synced time: {{ time }}</h2>
  `
})
export class AppComponent {
 
  public time;
 
  constructor(private demoService: DemoService) { }

  // ----> this should be triggered every hour <----
  clearData() {
    this.demoService.deleteData()
      .subscribe((lastCleanTime) => {this.time = lastCleanTime;});
    }
}

use timer to trigger clearData and use local storage to store that data — Rishab Vaigankar, Oct 18 '21 at 05:02
Yeah, I was thinking the same, but checking if there is a better solution. — Dan, Oct 18 '21 at 05:53

score 4 · Accepted Answer · answered Oct 18 '21 at 05:05

4

I don't prefer to do that through the client-side, it's okay to add a button to that run it, just imagine these cases:

You have 3 users, so this task will run about 3 times every hour, what about more?
You have no users, so the task will never run.

I prefer to do that from the server-side, and you can update the time through an API to get the latest time or you can add a WebSocket to make it always updated.

answered Oct 18 '21 at 05:05

Nimer Awad

3,967
3
17
31

1

Yeah, it makes more sense to add it on server-side. – Dan Oct 18 '21 at 17:15

score 0 · Answer 2 · answered Oct 18 '21 at 05:07

0

1 Yes, Component data get erased when you navigated from one component to another. if you want to do something before the component getting destroy you can use the ngOnDestroy Lifecycle method of the component.

2 you can use the service to store data while navigating between components.

answered Oct 18 '21 at 05:07

abhishek sahu

648
4
8

If you set an interval, it won't be cleaned, even if the component has been destroyed. You need to clean it manually. – Nimer Awad Oct 18 '21 at 05:14
1

Yes, need to clear Interval in ngOndestroy. – abhishek sahu Oct 18 '21 at 05:15

score 0 · Answer 3 · answered Oct 18 '21 at 05:08

Ok, instead of getting a trigger from user, it will be easier to run a database query internally every hour How to schedule a database task? Please refer to this link.

Ans 1. The component scope is only available till it is present in DOM so yes it will be destroyed once it is out of scope or DOM.

Ans 2. You can maintain data using models or state or local variables.

score 0 · Answer 4 · answered Nov 27 '21 at 17:01

A database cleanup task should indeed be run on the server side periodically. In Java projects where I have participated, we have done similar tasks with Tomcat or Jetty using ServletContextListener. Other servers, e.g. node, have their own mechanisms of course.

You create a class implementing this interface, whose methods get called when your servlet is started or destroyed. In the startup method you create a thread that cleans the db up in a loop every hour, until the servlet is destroyed:

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener
public class DBCleanupContextListener implements ServletContextListener
{
    private ExecutorService threadPool;
    private DBCleanupTask dbCleanupTask;

    public DBCleanupContextListener()
    {
        threadPool = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());
        dbCleanupTask = new DBCleanupTask();
    }

    @Override
    public void contextInitialized(ServletContextEvent arg0)
    {
        threadPool.execute(dbCleanupTask);

        // submit additional tasks here if needed...
        
    }

    @Override
    public void contextDestroyed(ServletContextEvent arg0)
    {
        dbCleanupTask.shutdown();
        threadPool.shutdown();
    }

    class DBCleanupTask implements Runnable
    {
        boolean finished = false;
        
        @Override
        public void run()
        {
            while (!finished)
            {
                
                System.out.println("db cleanup code goes here ....");
                
                try
                {
                    Thread.sleep(60 * 60 * 1000); // sleeps 1 hour = 60min*60sec*1000ms
                }
                catch (InterruptedException e)
                {
                    finished = true;
                }
            }
        }
    
        public void shutdown()
        {
            finished = true;
        }
        
    }
}

I have run this code with Tomcat, Jetty and Spring Boot. Theoretically it should run on any JavaEE server, but you need to research how to configure the servlet. On Tomcat for example it needs to be added to the web.xml, as described here:

<web-app ...>
<listener>
    <listener-class>
            com.example.DBCleanupContextListener 
        </listener-class>
</listener>
</web-app>

How to schedule a task in Angular

4 Answers4