I think this patch just takes care of the read only connection side.
What happens if another connection comes into the same directory that
does writes? Without locking the readonly connection can get serious
errors because the ondisk version of files can become out of date with
the in cache version in the other JVM.
The safest approach seems to create some sort of ADMIN interface which
would mark the database as read only, and then only read only
connections would be allowed. That seems most straight forward.
Currently as I understand it the store looks to see if the actual
files are readonly to put the db in readonly mode, so at that point if
it could check some sort of marker in the database - maybe in the
log control file it could do something similar. Putting this in the
log is natural as it is the first module to boot when coming up on a
new database, and owns whether or not writes can be done.
Another approach that might work depending on the java file locking
support may be to not avoid the lock file but enhance it. Allow
multiple read locks for the readonly db's while excluding
For context the current readonly support was meant for running derby
from some sort of readonly material like a CD. An example
application was a catalog application reading the products from a
readonly db or another example would be a computer learning product
where each lesson was data delivered in a static derby database.
One could allow any connection but fail update operations but that would
be a lot more work, and does not seem as user friendly.
Trejkaz (JIRA) wrote:
[ http://issues.apache.org/jira/browse/DERBY-393?page=all ]
Trejkaz updated DERBY-393:
--------------------------
Attachment: readonly.patch
This patch creates a new URL sub-scheme "readonlydirectory" which can be used
to load a database from a directory in read-only mode.
It isn't ideal... for one, the URL syntax is a little unwieldy. The syntax is
currently like this:
jdbc:derby:readonlydirectory:C:\Databases\MyDatabase
I would rather the syntax be like this:
jdbc:derby:directory:C:\Databases\MyDatabase;readonly=true
But I couldn't figure out how to do the latter. My patch only contains the
Java 1.4 version of the read-only store. For completeness, I suppose you'd
need a 1.3 and a 1.4 version. Either that or a decorator class which wraps any
other store. I suppose that the decorator approach would make re-use a little
easier.
Allow multiple JVMs to have read-only access to the same directory-based
database
---------------------------------------------------------------------------------
Key: DERBY-393
URL: http://issues.apache.org/jira/browse/DERBY-393
Project: Derby
Type: New Feature
Components: Store
Reporter: Trejkaz
Attachments: readonly.patch
For an application I'm building, we needed to permit multiple JVMs to access
the same database.
We couldn't easily use a network server configuration, as it would be difficult
to figure out who to connect to since either user might want to view the
database while the other database is offline.
We couldn't just dump all the data in a JAR file, as our databases often end up
being several gigabytes in size.
So what we really need is a version of the directory store which is treated as
if it were read-only.
