Daniel John Debrunner wrote:
Derby's client and engine may be in the same classpath and at different versions if the JVM is hosting more than one application, or the application installers have modified the system/user's classpath to add their required jars.
In this model, you are assuming that there is a single classpath shared by all applications run by the system/user. In this mode you cannot have multiple versions of the client or engine present - you can only have one version of the client and one version of the engine.
In reality this scenario is useless in anything but the most trivial installation. As an application can be impacted by any other application that affects the system/user's classpath reliability requires that they be isolated from each other. You end up with two modes of operation:
* an application overides the classpath in some script before the JVM is started to ensure that only the jars it expects are present * an application runs inside a server that controls the classpath for it and that server ensures the appropriate libraries are present
The issue is that today this is fully supported because the client and engine do not share code. Some of the code sharing approaches regress Derby in this area by not supporting this, or require class path ordering for it to be supported.
Some of the others support this by defining compatibility contracts and eliminate the need for classpath ordering by not duplicating classes.
While it is true that multiple class loaders solve the issue, this approach is not always possible, I believe, for example, some of the major application servers do not support different class loaders for different JDBC providers (eg. the Derby client at 10.3 and engine and 10.2).
Those application servers also define which versions are supported by the application server vendor. They also do not support client 10.2 and client 10.3 which is an equally likely combination.
Thus the argument really is, are we willing to accept regression in this area to gain code sharing, or should the code sharing solution not regress Derby?
Let's recharacterize this a little. What we are contemplating with code sharing is extracting common functionality out into a library. By saying that we are not willing to accept any solution where a component depends on a library we are shutting ourselves off from using any external library or any functionality not provided by Derby itself. This dooms us forever to reinvent any functionality that could be provided by other projects.
For example, there are libraries out there that support bytecode generation, JMX for management, high-performance concurrency on Java 1.4, regexp processing to support SQL patterns, ... By saying we are not prepared to incorporate them but instead need our own versions that can be morphed for client and server we dramatically reduce the functionality that can be made available to users.
So let me ask this: do our users want more functionality faster by allowing the use of libraries, or a completely standalone solution with tight control over the entire implementation?
-- Jeremy
