PQEventmonitor not working (LISTEN/NOTIFY)
lazarus / 16-Databases

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heejit · 2018-07-28 19:52

No notification received

Following is code

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.FormCreate(Sender: TObject);var  epq :
PQEventMonitor.TPQEventMonitor;  pqcon :
PQConnection.TPQConnection;begin  pqcon :=
PQConnection.TPQConnection.Create(nil);  pqcon.HostName:='127.0.0.1'; 
pqcon.DatabaseName:='db-name';  pqcon.UserName:='user-name'; 
pqcon.Connected:=True;   epq :=
PQEventMonitor.TPQEventMonitor.Create(pqcon);  epq.Connection :=
pqcon;  epq.Events.Add('info');  epq.Registered:=True; 
epq.OnEventAlert:=@event_handler;  end; procedure
TForm1.event_handler(Sender: TObject; EventName: string;  EventCount:
longint; var CancelAlerts: boolean);begin  writeln(EventName);end;

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goodname · 2018-08-07 12:17

What server side triggers have you created?

https://www.postgresql.org/docs/9.3/static/event-triggers.html

https://www.postgresql.org/docs/9.3/static/triggers.html

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heejit · 2018-08-07 17:27

I thought this unit is Listener of postgresql Notify command

How is related to trigger?

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goodname · 2018-08-07 19:29

Have just looked at the Lazarus code and TPQEventMonitor uses the
LISTEN and UNLISTEN commands which means your right it has nothing to
do with event triggers. Have not used this feature so someone else
will have to answer.

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griff365 · 2021-08-28 14:28

You have to explicitly set the KeepConnection option on the Postgres
DB Connection to True. The default is False.

  pqcon := PQConnection.TPQConnection.Create(nil);
  pqcon.HostName:='127.0.0.1';
  pqcon.DatabaseName:='db-name';
  pqcon.UserName:='user-name';
  pqcon.KeepConnection := true;
  pqcon.Connected:=True;


The PQEventMonitor.Poll code only checks for NOTIFY events if the
database is currently connected.
Setting KeepConnection := true  ensures that the connection will
remain open even if there are no open datasets.

This shouldn't be required, because PQEventMonitor creates it's own
dedicated persistent connection anyway.
PQEventMonitor only reads the connection information from the Postgres
DB Connection component for establishing it's own connection, and
doesn't use it for any other purpose.


The problem in the PQEventMonitor code is here:

procedure TPQEventMonitor.Poll;
var
  notify:PpgNotify;
  CancelAlerts:boolean;
begin
  if FConnection.Connected and FRegistered and
(PQconsumeInput(FDBHandle)=1) then
    begin
    CancelAlerts:=false;
    repeat
      notify:=PQnotifies(FDBHandle);
      if assigned(notify) then


I'm thinking this line of code could be changed as follows and the
KeepConnection flag wouldn't need to be set.

  if FRegistered and (PQconsumeInput(FDBHandle)=1) then