How can I listen postgresql database with SignalR Core in .net core project?

This example is work asp.net core 3.0+. Full code is below.

Step 1. Create a trigger on PostgreSql for listening actions

 create trigger any_after_alarm_speed after
 insert
 or
 delete
 or
 update
 on
 public.alarm_speed for each row execute procedure alarm_speedf();

Step 2. Create Procedur on Postgresql

CREATE OR REPLACE FUNCTION public.alarm_speedf()
RETURNS trigger
LANGUAGE plpgsql
AS $function$
BEGIN
IF TG_OP = 'INSERT' then
PERFORM pg_notify('notifyalarmspeed', format('INSERT %s %s', NEW.alarm_speed_id, 
NEW.alarm_speed_date));
ELSIF TG_OP = 'UPDATE' then
PERFORM pg_notify('notifyalarmspeed', format('UPDATE %s %s', OLD.alarm_speed_id, 
OLD.alarm_speed_date));
ELSIF TG_OP = 'DELETE' then
PERFORM pg_notify('notifyalarmspeed', format('DELETE %s %s', OLD.alarm_speed_id, 
OLD.alarm_speed_date));
END IF;
RETURN NULL;
END;
$function$;

Step 3. Create Hub

  public class speedalarmhub : Hub
    {

        private IMemoryCache _cache;
       `private IHubContext<speedalarmhub> _hubContext;
         public speedalarmhub(IMemoryCache cache, IHubContext<speedalarmhub> hubContext)
        {
            _cache = cache;
            _hubContext = hubContext; 
        }

        public async Task SendMessage()
        {
            if (!_cache.TryGetValue("SpeedAlarm", out string response))
            {
                SpeedListener speedlist = new SpeedListener(_hubContext,_cache);
                speedlist.ListenForAlarmNotifications();
                string jsonspeedalarm = speedlist.GetAlarmList();
                _cache.Set("SpeedAlarm", jsonspeedalarm);
                await Clients.All.SendAsync("ReceiveMessage", _cache.Get("SpeedAlarm").ToString());
            }
            else
            {
                await Clients.All.SendAsync("ReceiveMessage", _cache.Get("SpeedAlarm").ToString());
            }
        }

    }

Step 4. Create Listener Controller

 public class SpeedListener :Controller
{
    private IHubContext<speedalarmhub> _hubContext;
    private IMemoryCache _cache;
    public SpeedListener(IHubContext<speedalarmhub> hubContext,IMemoryCache cache)
    {
        _hubContext = hubContext;
        _cache = cache; 
    }
    static string GetConnectionString()
    {
        var csb = new NpgsqlConnectionStringBuilder
        {
            Host = "yourip",
            Database = "yourdatabase",
            Username = "yourusername",
            Password = "yourpassword",
            Port = 5432,
            KeepAlive = 30
        };
        return csb.ConnectionString;
    }
    public void ListenForAlarmNotifications()
    {
        NpgsqlConnection conn = new NpgsqlConnection(GetConnectionString());
        conn.StateChange += conn_StateChange;
        conn.Open();
        var listenCommand = conn.CreateCommand();
        listenCommand.CommandText = $"listen notifyalarmspeed;";
        listenCommand.ExecuteNonQuery();
        conn.Notification += PostgresNotificationReceived;
        _hubContext.Clients.All.SendAsync(this.GetAlarmList());
        while (true)
        {
            conn.Wait();
        }
    }
    private void PostgresNotificationReceived(object sender, NpgsqlNotificationEventArgs e)
    {

        string actionName = e.Payload.ToString();
        string actionType = "";
        if (actionName.Contains("DELETE"))
        {
            actionType = "Delete";
        }
        if (actionName.Contains("UPDATE"))
        {
            actionType = "Update";
        }
        if (actionName.Contains("INSERT"))
        {
            actionType = "Insert";
        }
        _hubContext.Clients.All.SendAsync("ReceiveMessage", this.GetAlarmList());
    }
    public string GetAlarmList()
    {
        var AlarmList = new List<AlarmSpeedViewModel>();
        using (NpgsqlCommand sqlCmd = new NpgsqlCommand())
        {
            sqlCmd.CommandType = CommandType.StoredProcedure;
            sqlCmd.CommandText = "sp_alarm_speed_process_get";
            NpgsqlConnection conn = new NpgsqlConnection(GetConnectionString());
            conn.Open();
            sqlCmd.Connection = conn;
            using (NpgsqlDataReader reader = sqlCmd.ExecuteReader())
            {
                while (reader.Read())
                {
                    AlarmSpeedViewModel model = new AlarmSpeedViewModel();
                    model.alarm_speed_id = reader.GetInt32(0);
                  // you must fill  your model items
                    AlarmList.Add(model);
                }
                reader.Close();
                conn.Close();
            }



        }
        _cache.Set("SpeedAlarm", SerializeObjectToJson(AlarmList));
        return _cache.Get("SpeedAlarm").ToString();
    }
    public String SerializeObjectToJson(Object alarmspeed)
    {
        try
        {
            var jss = new JavaScriptSerializer();
            return  jss.Serialize(alarmspeed);
        }
        catch (Exception) { return null; }
    }
    private void conn_StateChange(object sender, System.Data.StateChangeEventArgs e)
    {

        _hubContext.Clients.All.SendAsync("Current State: " + e.CurrentState.ToString() + " Original State: " + e.OriginalState.ToString(), "connection state changed");
    }
}

Step 5 Calling Hub

<script src="~/lib/signalr.js"></script>

<script type="text/javascript">
// Start the connection.
var connection = new signalR.HubConnectionBuilder()
    .withUrl('/speedalarmhub')
    .build();


connection.on('ReceiveMessage', function (message) {

            var encodedMsg = message;
            // Add the message to the page.

});
// Transport fallback functionality is now built into start.
connection.start()
    .then(function () {

        console.log('connection started');
        connection.invoke('SendMessage');
    })
    .catch(error => {
        console.error(error.message);
    });

Step 6. Add below code Configuration Services at Startup

public void ConfigureServices(IServiceCollection services)
    {
        services.AddControllersWithViews();
        services.AddSignalR();
        services.AddMemoryCache();
    }

Step 7. add below code in Configure method

app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllerRoute(
                name: "default",
                pattern: "{controller=Home}/{action=Index}/{id?}");
              endpoints.MapHub<speedalarmhub>("/speedalarmhub");
        });

I want to listen my PostgreSQL database. And if there are any changes on table, I have to got it.

You can create a trigger associated with your specified table, and use the function pg_notify(text, text) to send a notification, like below.

Function

CREATE OR REPLACE FUNCTION mytestfunc() RETURNS TRIGGER AS $$
BEGIN
IF TG_OP = 'INSERT' then
PERFORM pg_notify('notifytesttable', 'new record inserted');
ELSIF TG_OP = 'UPDATE' then
PERFORM pg_notify('notifytesttable', 'updated');
ELSIF TG_OP = 'DELETE' then
PERFORM pg_notify('notifytesttable', 'deleted');
END IF;
RETURN NULL;
END;
$$ LANGUAGE plpgsql;

Trigger

CREATE TRIGGER any_after_testtable
AFTER INSERT OR DELETE OR UPDATE
ON testtable
FOR EACH ROW
EXECUTE PROCEDURE mytestfunc();

In your client application code, you can listen and receive notifications from PostgreSQL.

conn.Open();
conn.Notification += Conn_Notification;

using (var cmd = new NpgsqlCommand("LISTEN notifytesttable", conn))
{
    cmd.ExecuteNonQuery();
}

In Notification event handler, you can call SignalR hub method to push notifications to SignalR clients.

private static void Conn_Notification(object sender, NpgsqlNotificationEventArgs e)
{
    var notification_payload = e.Payload;

    //code logic here

    //call hub method to push PostgreSQL notifications that you received to SignalR client users

}

Test result

For detailed information about PostgreSQL LISTEN and NOTIFY features, you can check following links.

https://www.postgresql.org/docs/current/sql-notify.html

https://www.npgsql.org/doc/wait.html#processing-of-notifications