In my attempt to extract data (dumps and selective reading of columns) from a diverse collection of edb databases I got faced with a fundamental problem. I have an edb database coming with a couple of log files. I know what information there is within the database, but I just get half of it extracted. I fear that the remaining half sleeps somewhere in the log files. I assumed the EDB engine knows where the log files are and automagically loads them when attaching the database (JET_paramSystemPath, JET_paramLogFilePath and JET_paramBaseName are properly set). Is that a wrong assumption? If so, what should I do to have the logs loaded as well?
Alternatively, would it be possible to simply commit the transactions to the EDB file and get rid of the logs?
If there are uncommitted transactions then the database will be marked as 'inconsistent'. You can check this using ESENTUTL /MH against the database. Calling JetAttachDatabase against an inconsistent database will always fail.
So, if your program is able to attach and open the database then it is consistent. There are two ways a database can be made consistent:
The first thing that JetInit does is to look for the logfiles specified by JET_paramLogFilePath and JET_paramBaseName. Logfiles contain the full paths of the database(s) they reference and the transactions in the logfiles are then committed to the database(s). So, if you set the system parameters properly then ESENT will load the logs when attaching the database.
On the other hand, if you don't set the parameters properly then your program will actually work on databases that don't require recovery. JetInit won't find any logfiles so it won't do anything and the attach will succeed because the database is consistent.
One further twist is that the logfiles contain the full path to the database. This means that if you have copied/moved the database then recovery will not work. Starting with Windows Server 2003 you can deal with this by setting JET_paramAlternateDatabaseRecoveryPath to the directory containing the database.
Important: to be safe you should always attach and open the database using the read-only flags. This will avoid any problems caused by bad logfile settings. A common problem is that read-only applications end up creating a set of logfiles in a different directory which prevent the database from being recovered properly.