Le 25/08/2014 09:32, Richard Mace a écrit :
Hi all,
I have the following code that I use to connect to a Firebird
database, which works fine, however, if the Firebird server becomes
unavailable, fIBConnection.Connected always returns true. What's the
best way of checking to make sure that the Firebird server is still
available before I attempt to connect to the database?
AFAIK the TIbConnection is based upon TSQLConnection itself based upon
the TDatabase component. In all this chain the Connected property is
defined in the latest component, the TDatabase. And the Connected
property has a read function wich sends a boolean variable content
(FConnected variable). That's why once the first connection has been
established with the Firebird server (whether is succeeds or not), the
connection status is kept thru all component life until next disconnect
or re-connect operation takes place.
To solve the situation when the link to the databse server is broken I
rely on the transaction mechanism. In fact I always control the
transaction state allowing any query to be executed, as the Firebird
server requires that any database access has to be done under the
control of a transaction. This means thar when the
MyTrans.StartTransaction fails I know (most of the times) that the link
to the FB server is broken (omitting programmatic errors which can be
controlled or checkced in another way). This does not protect your code
from a connection exception between two consecutive database access (two
consecutive calls to the fbclient library anyway). When I need a fine
tuned control over a database access, after a initial MyDB.Connected :=
True, I always use the same sequence:
try
MyDatabase.Connedted := True;
except
on ...(exception process) do ... connection fault
end
[...] prepare data for the database
try
MyTransaction;StartTransaction
try
do whatever has to be done with the database
finally
MyTransaction.Commit
end
except
do whatever needed when the exception rises: check whether its
is an ordinary exception or a server link fault
if MyTransaction.InTransaction then
MyTransaction.RollBack;
end;
This sequence does not protect against the server disconnection between
two consecutive database calls. I easily can imagine that the same
process occurs with any other database server hosted in a remote
machine. Even when the database server is hosted in the same machine of
the applicaton, TCP/IP communication applies.
Antonio.
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