I'm trying (with a lot of help from this community) to put together my first client/server app.
I am using Indy 10 and Delphi Xe2, but suspect my problem does not lie with those, but rather with how soockets work (wizardy and black arts, if you ask me).
Because I often hit breakpoints of exceptions and step through my code before pressing Alt+F2 to halt, my next runs often hit exception "already connected" and eventually I get "too many connections".
How can I tidy this up?
(also, does anyone have a demo which uses try ... except, ratehr than try ... finally as in the Indy demos?)
"already connected" occurs on the client side when you call Connect() while Connected() still returns true. That usually occurs if you disconnect and leave unread data in the IOHandler.InputBuffer. Try clearing the InputBuffer before reconnecting. This is commonly encountered, so a near-future update to Indy may address that issue.
"too many connections" means that you set the server's MaxConnections property to a positive non-zero value and that many simultaneous clients are already connected to the server when a new client tries to connect. If you do not think that you are making that many simultaneous connections, then it usually means that you are not managing the connections correctly in your server code so disconnected clients get cleaned up correctly. The most common cause of that is putting exception handlers in your code that catch and swallow Indy's internal exceptions to itself. If you do catch exceptions, be sure to re-raise any that derive from EIdException and let the server handle them internally.
Related
We have had this error recently reported by our clients and we on't know how we can fix this. We are using Delphi XE6 to develop our application and it connect to Firebird (v2.5) database as back-end. We also have used IBObjects for working with Firebird database in Delphi.
We have managed to replicate the error on the dev machine by stopping Firebird windows service before running a query but we haven't found a way to detect the connection lost in the code (e.g. by having an event) so as we don't know when this happens so we cannot reconnect to the database in the code either.
By the way, this is full error message if it helps:
ISC ERROR CODE:335544721
ISC ERROR MESSAGE:
Unable to complete network request to host "BON-VFS-01".
Error writing data to the connection.
Any help really appreciated.
From IBOBjects FAQ
Is there a way to detect a lost connection and try to reconnect
automatically, without user action?
Hook into the OnError event and look for the ERRCODE that denotes a
lost connection. Then, you can take whatever action you deem necessary
to deal with the problem. If the connection is lost you need to do a
disconnect and then connect again.
And from one of the base members of the IBObjects:
However, perhaps the "something" you are missing is that, if the
connection is broken by an external cause, the client application has
no way to know that it is not still connected. Its first knowledge of
that fact will come the next time it tries to access the server. The
API will report "Connection lost to database" and return GDSCODE
335544741, which is identified by the constant
isc_lost_db_connection.
At the point where this exception occurs, the TIB_Connection still
thinks it is connected - the Connected property will be true. If you
try to reconnect by calling Connect, you will get an IBO exception.
It is necessary to call Disconnect. This does not simply reset a
property. The Disconnect method performs all of the necessary cleanup
to invalidate the broken transactions and cancel any now invalid
postings, datasets and caches. Once Disconnect has completed its
work, you can then place a Connect call inside a retry loop and
attempt to get going again.
I do not know of a working example, but the simplest way to deal with
this is to write a RestoreConnection handler procedure that you can
call from your IB_Session.OnError handler whenever argument ERRCODE
returns isc_lost_db_connection.
Have your RestoreConnection procedure do whatever you need to do,
trying to call Connect and handling the exception that occurs if the
request fails, until no exception occurs. Test the Connected property
after each iteration. When Connected is finally True, you are in
business. You can drop out of the retry code and inform the user that
the connection has been restored - perhaps with a sound and/or a
message in the status bar, to avoid having to show a dialog box that
she has to respond to. (if you like the idea of sound and status bar
cues, you could devise "connection lost" warning sound and status bar
message code to run at the beginning of your handler procedure as
well...)
If these broken connections are a frequent occurrence, you might like
to consider making a distinctive custom cursor that you can display
while your procedure is running, and enclose the retry code in a
non-yielding BeginBusy...EndBusy block with UseCursor enabled and
BusyCursor set to use this special cursor image.
And if re-establishing a connection is likely to take a long time, or
to be temporarily impossible, you would need to provide the ability
for the user to intervene and choose not to keep trying. You can use
the session timer for this, enclosing your "busy" block inside
another iterative block the prompts the user to "Cancel" or "Keep
Trying", at reasonable intervals.
Source
Check out if their database file is located on mapped network drive. Even if database file path appear to be local to file system, when using embedded Firebird server, function isc_attach_database will return error code 335544721 on attempt to establish connection. Exactly that was happening on my VirtualBox guest Windows XP when I first share entire host D drive and then mapped it again as D drive in virtual guest OS.
Workaround will be to move database file to local partition drive.
check your query length
max query length is 8191 chars UTF-8
its solved my problem
connect your pc to the internet, and the problem will be fixed,but i don't how it works
I'm using Indy's FTP.
It works perfectly with what I want, my only issue is that if the system isn't connected to the Internet or the server is down then it displays a TimeOut exception on screen.
I can't seem to be able to find out how to catch the exception.
I just want it to, instead of show an error, be able to show my own message to reassure the user that there is no issue. If that made sense.
I've used try excepts before and they work just not here it seems.
Could someone give me an idea or some example code of where and how to write the exception catcher here?
Thanks
EDIT:
Sorry, I was away from my normal PC so couldn't post code.
Actually I just retried and it did catch it, thanks for the note about the debugger always showing the exception.
However, after my custom message I also get a 'Connection Closed Gracefully' message. (I ran it outside of debugger)
How/Where can I catch/stop that one?
Also, it sometimes returns a message from my server, such as 'Cant connect more than 3 times on same account' or whatever. Can I stop/catch that aswell?
Thanks
Here:
Form1.ftp.Host := 'HOSTNAME';
Form1.ftp.User := 'USERNAME';
Form1.ftp.password := 'PASSWORD';
Try
Form1.ftp.Connect;
Except on E : Exception do
begin
ShowMessage('Timeout Error, dont worry');
end;
end;
Like most components in Indy, TIdFTP does everything synchronously, and errors are reported as exceptions. Standard try/except blocks work just fine. Indy is designed for that.
If you are seeing a Connection Closed Gracefully message appear when running your app outside of the debugger, it means you tried to perform a socket operation after the socket was already disconnected, and you did not catch the EIdConnClosedGracefully exception that was raised into your code. For instance, if Connect() fails, it calls Disconnect() internally before raising an exception to you. Don't call any other TIdFTP methods in that situation, other than Connect() again if needed.
As for error messages sent by the FTP server, they are usually reported by raising EIdReplyRFCError (or derived) exceptions, which you can catch in your code.
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All Indy SMTP examples I have seen show TIdSMTP being instantiated at the start of the email send routine and freed at the end.
I have written an SMTP email sender prototype application, based upon Indy 10 components, that normally works with no issues. (It can send in the clear or via explicit or implicit TLS.) It instantiates TIdSMTP on startup and frees it on closing.
I have just had a situation where IdSMTP.Send threw an exception saying that SMTP was not connected. This would be unremarkable except for the fact that I have a test for IdSMTP.Connected, immediately prior to my IdSMTP.Send, that reported that SMTP was connected. My program could no longer be made to send emails because it thought it was connected but in reality it wasn't. The only solution was to restart the program. When testing of this prototype application is completed the code will be incorporated into a server service that cannot just be restarted to fix this sort of problem.
The problem could have been avoided or, at least, worked around if I had instantiated my TIdSMTP class within my email send procedure; in the unlikely event of this re-occurring the IdSMTP object would be freed at the end of the procedure and re-instantiated the next time it gets called.
The reason I didn't architect my solution as in the examples was that re-instantiation of TIdSMTP for each email send also requires that the IdSMTP object reconnect to the email server for each send. This can be a very slow process - of the order of 5 to 10 seconds for external email servers - and this overhead can be avoided by not re-instantiating TIdSMTP each time.
So my question is: is this (or similar issues) the reason why all examples show re-instantiation each time? Or, is it simply to show a complete, contained example? The latter was how I interpreted it.
If you have experience with the pros and cons of this issue, please provide your opinions, and thoughts.
If there exists a definitive best practice, that would useful to hear about too.
All Indy SMTP examples I have seen show TIdSMTP being instantiated at the start of the email send routine and freed at the end.
They are just examples. Obviously production code can be more complex. You can reuse a single TIdSMTP object multiple times.
I have a test for IdSMTP.Connected, immediately prior to my IdSMTP.Send, that reported that SMTP was connected.
Let Send() fail and raise an exception if the connection is not available. Indy uses exceptions for error reporting, so make use of them, don't avoid them. Connected() can report a false positive. If the IOHandler.InputBuffer has unread data in it, Connected() will return true, even if the underlying socket is closed. This is by design. Don't rely on Connected() to drive your logic.
My program could no longer be made to send emails because it thought it was connected but in reality it wasn't.
After Connect() succeeds, if you get any exception that is not derived from EIdRFCReply, you should Disconnect() the connection and Clear() the InputBuffer (if there is still an IOHandler assigned - Disconnect() will free it if it was created internally by Connect()) before calling Connect() again. Connect() raises an exception if Connected() returns true, so you have to manually clear the condition that causes that (unread data) if an unexpected error occurs.
Try
If not SMTP.Connected then SMTP.Connect;
...
Except
On E: EIdRFCReply do
Begin
// an SMTP command failed, but the connection is still stable
...
End;
On E: Exception do
Begin
SMTP.Disconnect(False);
If SMTP.IOHandler <> nil then SMTP.IOHandler.InputBuffer.Clear;
...
End;
End;
In general, you should keep connections to the outside for as short a time as possible. I assume that this program/service doesn't send emails 24h/7d continously, but in response to some external event (a timer or other event that triggers "now it is time to send"), and then it sends one (or more) emails at that point.
The way I would code this is to instantiate the TIdSmtp at the start of this event and then free it at the end. This way, you would have a freshly instantiated connection to the outside every time.
If your program only sends a single email on every event, then you could perhaps code it such that if the sending fails, then signal that on the next event, you should re-instantiate the TIdSmtp variable, something along these lines:
PROCEDURE SendMsg(...)
BEGIN
IF NOT Assigned(SMTP) THEN SMTP:=TIdSmtp.Create(NIL);
TRY
... Your code to send one or more emails ...
EXCEPT
FreeAndNIL(SMTP);
... Perhaps Re-Raise exception, if outer layer needs to know ...
END
END;
This way, the SendMsg routine is self-restarting in case of error. Of course, some more error checking (what kind of exception was caught) would be advisable, but I expect you get the general idea...
Our Delphi 5 popular application is running fine on almost all client computers. On some computers it constantly throws EFCreateError exceptions during write-to-disk operations like TFileStream.Create and TStrings.SaveToFile. These computers run McAfee, so it seems plausible that McAfee is background-scanning the file while our app tries to write to the file.
Its a sporadic problem as it doesn't occur every time, but like in one out of 20 save operations. Since these saves are background actions our users trigger indirectly, they cannot try again to save.
The calls are from frameworks we use, such as JEDI's JvFormStorage. while we could change it to try to save up to 5 times, it still seems rather brittle to me.
The software has never been identified as bad by any antivirus (checked by VirusTotal). Instead it seems like the antivirus is using a blocking read operation to do its job.
I am loosing my patience with "delay write failed" errors. It silently disconnects the database from the application so nothing gets saved in the database while using it. Is there a way to detect the occurrence itself so I can flash a warning ? Or perhaps monitoring the connection itself for a disconnection ? Everyone seems to miss the balloon tip from the Windows XP so I figured to flash a more visible warning that the application must be restarted. It seems Microsoft has found a way to force people to upgrade....
I suppose this could be done with a timer and constantly check connected users:
cxlabel1.Caption := IntToStr(DataModule2.ABSDatabase1.GetDBFileConnectionsCount);
But I was thinking more of checking/detecting for the occurence itself. Is there something in Delphi that can detect this?
I would like to hear your ideas on this...
Putting this as an answer because the comment length is limited.
I have seen this before. IIRC, the problem you have is that the Delayed Write Error is an OS error, it has nothing to do with your application. Windows has already told you that the write has been committed correctly to disk. You would have to try and hook into the OS errors to see when this is happening.
You need to get the network issues resolved because that's where the problem is. In our situation it was a faulty router that was causing the problem.
It's unfair to expect users to check for the error message and then handle it. They could be out at lunch when it occurs as it's not immediate. They also have no way of know what has been saved and what hasn't. It's only a matter of time before your database is corrupted.
The problem with a timer is that it might tell you everything is fine because it triggers after the network resolves the problems.
A far better approach would be to switch to a Client/Server database. You can do this by setting up your own server that listens for web service or another remote call or switch to a database that supports client/server instead of using a file based database. This will tell you immediately when there is a problem with the save of data.