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It's a real shame that modularization focused almost entirely on solving the JDK vendor's problems rather than solving the application author's problems. Speci

It's a real shame that modularization focused almost entirely on solving the JDK vendor's problems rather than solving the application author's problems. Specifically, modularization can and should have been taken as a chance to allow multiple versions of the same dependency to exist within a single scope within an application. At present, this is sort of achievable by taking control of classloading and invoking defineModulesWithManyLoaders, but not many people will do this and key features are missing (it still does not allow a single module to directly reference two versions of the same dependency).

pron 4y ago
> Specifically, modularization can and should have been taken as a chance to allow multiple versions of the same dependency to exist within a single scope within an application.... Modules do it to the best extent possible, but they don't make it prominent because, quite simply, it's impossible to do in a way that's good enough to be advisable. In order for a library to be properly isolatable, it has to be written in a specific way and be quite limited. Any kind of input or output, including the reliance on configuration outside of the source code can immediately make the library susceptible to horrible problems when multiple copies of it exist in the same process, even if isolated. So modules allow you to use class loader isolation if you really need it, but they don't try to pretend it can actually work in general. The main (though not only) thing that modules exist to provide is strong encapsulation, which is crucial for security and makes code evolution much easier.
- clhodapp 4y ago
  It seems to work just fine without anything horrific happening in Rust. Direct dependencies can be renamed and indirect dependencies simply don't clash with each other. While libraries directly parsing some sort of conf file may be a little bit of a pattern on the JVM, it doesn't have to be... And the libraries could certainly define a new pattern to allow the user to define different versions of their configs for different library versions. I agree that, as it stands, modules enhance security by providing a stronger version of the visibility system. IMO that is important but it's just such a low bar to shoot for since it doesn't really improve the situation much more than everyone just following the rules
  - pron 4y ago
    > It seems to work just fine without anything horrific happening in Rust. Direct dependencies can be renamed and indirect dependencies simply don't clash with each other. It suffers from the same problems -- doesn't work if a library has an implicit assumption it's the only copy, and does otherwise. It's the best anyone can do, and modules do it as well as it can be done, it's just not something you want to encourage because there are too many cases where even the best is just not good enough. I don't think that instantiating multiple instances of a library whenever there's a version conflict is the right default. It turns an accidental configuration situation into a significant runtime choice. Still, if you want a tool that makes this best-effort attempt easier, take a look at layrry: https://github.com/moditect/layrry https://github.com/moditect/layrry. It basically exposes module's "multi-versioning" as a configuration rather than programmatically. > IMO that is important but it's just such a low bar to shoot for since it doesn't really improve the situation much more than everyone just following the rules Actually, it's something that can only be done deep in the VM. Modules exist to provide some runtime guarantees that nothing else can (except the SecurityManager, but that was so problematic that few used it and fewer still did it correctly). Without modules, there can be no strong security guarantees made.
    - clhodapp 4y ago
      As far as I can tell, Layrry does not allow two versions of the same library to be in scope at the same time.
      - pron 4y ago
        I don't know what you mean by "in scope" but modules do allow multiple versions to exist in the same process in different layers. That's what module's entire layer mechanism is about (internally, it uses class-loader isolation), and Layrry configures module layers (that's where it gets its name). It's in one of the first examples: https://github.com/moditect/layrry https://github.com/moditect/layrry So modules absolutely do support that by design, but again, for that to work well you need to really know what that duplicated library does (same goes for Rust or any language), and it's not a recommended practice in general because it can fail in really horrible ways if you're unaware of some implicit assumptions in the library.

pron 4y ago

> Specifically, modularization can and should have been taken as a chance to allow multiple versions of the same dependency to exist within a single scope within an application.... Modules do it to the best extent possible, but they don't make it prominent because, quite simply, it's impossible to do in a way that's good enough to be advisable. In order for a library to be properly isolatable, it has to be written in a specific way and be quite limited. Any kind of input or output, including the reliance on configuration outside of the source code can immediately make the library susceptible to horrible problems when multiple copies of it exist in the same process, even if isolated. So modules allow you to use class loader isolation if you really need it, but they don't try to pretend it can actually work in general. The main (though not only) thing that modules exist to provide is strong encapsulation, which is crucial for security and makes code evolution much easier.

clhodapp 4y ago
It seems to work just fine without anything horrific happening in Rust. Direct dependencies can be renamed and indirect dependencies simply don't clash with each other. While libraries directly parsing some sort of conf file may be a little bit of a pattern on the JVM, it doesn't have to be... And the libraries could certainly define a new pattern to allow the user to define different versions of their configs for different library versions. I agree that, as it stands, modules enhance security by providing a stronger version of the visibility system. IMO that is important but it's just such a low bar to shoot for since it doesn't really improve the situation much more than everyone just following the rules
- pron 4y ago
  > It seems to work just fine without anything horrific happening in Rust. Direct dependencies can be renamed and indirect dependencies simply don't clash with each other. It suffers from the same problems -- doesn't work if a library has an implicit assumption it's the only copy, and does otherwise. It's the best anyone can do, and modules do it as well as it can be done, it's just not something you want to encourage because there are too many cases where even the best is just not good enough. I don't think that instantiating multiple instances of a library whenever there's a version conflict is the right default. It turns an accidental configuration situation into a significant runtime choice. Still, if you want a tool that makes this best-effort attempt easier, take a look at layrry: https://github.com/moditect/layrry https://github.com/moditect/layrry. It basically exposes module's "multi-versioning" as a configuration rather than programmatically. > IMO that is important but it's just such a low bar to shoot for since it doesn't really improve the situation much more than everyone just following the rules Actually, it's something that can only be done deep in the VM. Modules exist to provide some runtime guarantees that nothing else can (except the SecurityManager, but that was so problematic that few used it and fewer still did it correctly). Without modules, there can be no strong security guarantees made.
  - clhodapp 4y ago
    As far as I can tell, Layrry does not allow two versions of the same library to be in scope at the same time.
    - pron 4y ago
      I don't know what you mean by "in scope" but modules do allow multiple versions to exist in the same process in different layers. That's what module's entire layer mechanism is about (internally, it uses class-loader isolation), and Layrry configures module layers (that's where it gets its name). It's in one of the first examples: https://github.com/moditect/layrry https://github.com/moditect/layrry So modules absolutely do support that by design, but again, for that to work well you need to really know what that duplicated library does (same goes for Rust or any language), and it's not a recommended practice in general because it can fail in really horrible ways if you're unaware of some implicit assumptions in the library.