[Lazarus] Handling Firebird connection

[Richard Mace]

Antonia,
Many thanks for your reply.
I'll re-code my app and do some more testing.

Richard


On 25 August 2014 09:19, SPRL AFOR <aforsprl at gmail.com> wrote:

>  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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