Re: [xwiki-devs] [Proposal] Choosing the default component implementation

On 04/05/2012 12:51 PM, Anca Luca wrote: > > On 04/05/2012 06:42 PM, Vincent Massol wrote: >> >> Hi Sergiu, >> >> On Apr 5, 2012, at 5:55 PM, Sergiu Dumitriu wrote: >> >>> Hi devs, >>> >>> Currently, requesting a component instance without a hint will look >>> for the implementation that uses the "default" hint, which makes it >>> difficult to change the implementation in an XWiki instance. Sure, it >>> is easy as long as all the implementations use the "default" hint, >>> but choosing the default between alternative implementations that >>> should all still be usable by themselves is not possible. >>> >>> Also, "default" is not really a good hint, since it describes the >>> state of the implementation, not the technology, the aspect that >>> makes it different from the others. It would be better to name each >>> implementation with a proper hint. >>> >>> I propose to define a mapping that can specify which hint is the >>> default for a component. In a text file, >>> META-INF/component-defaults.txt, we'll keep >>> componentinterface=defaulthint mappings. For example: >>> >>> com.xpn.xwiki.store.XWikiStoreInterface=hibernate >>> com.xpn.xwiki.store.migration.DataMigrationManager=hibernate >>> >>> And then, when we lookup the current storage implementation, we don't >>> need to check what is the configured hint in xwiki.cfg (or >>> xwiki.properties), we can just request the default implementation. >>> >>> If there's no mapping for a component, we'll continue to use the >>> "default" hint. >>> >>> I'm not sure where exactly to keep such files. We bundle a >>> components.txt file in each jar containing component implementations. >>> We could do the same for the components we consider the platform >>> defaults, and allow overrides in the >>> WEB-INF/classes/META-INF/component-defaults.txt file. Still, this >>> means that whenever platform defaults change, we need to keep another >>> special section in the release notes, to let users know about these >>> changes, so that they can manually revert to the old default if they >>> need to. >>> >>> In the future we could change existing components to give proper >>> hints instead of "default", where such a change is applicable. >>> >>> Another idea is to not use "default" at all, and instead go for a >>> generic "xwiki", "xe", "xwiki-platform" or something like that >>> whenever there's just one implementation for a component and we can't >>> find another hint to describe it. >>> >>> WDYT? >> >> This is not really how it's been designed ATM. Whenever you wish to >> use a different implementation of a component you use a component >> implementation with the same role and same hint. You then make it >> available in your classpath. (Of course you can also do this at >> runtime simply by registering a new implementation over the old one). >> >> To decide which implementation is used you use a priority order, as >> described on: >> >> http://extensions.xwiki.org/xwiki/bin/view/Extension/Component+Module#HOver… >> >> >> I'd be curious to know your exact use case and understand why the >> current mechanism doesn't work for it. "...choosing the default between alternative implementations that should all **still be usable by themselves** is not possible" The overrides mechanism allows to change which component will be returned for the "default" hint, but all the others will be invisible. > One usecase I see is that you have multiple implementations and you want > to change the default one for a specific running instance of xwiki. > > Overwrite mechanism only allows you to say which impl should be used > from the _components with the same hint_. However, you cannot change the > hint of a component at configuration time, so if you have a standard > distr of xwiki and you want to use ldap authentication, let's say (if > only auth was impl with components), unless you do some java to add the > default hint to the ldap implementation and then to specify that this > one has priority over all the default ones, I don't see how you can > re-wire the default. Exactly. For most of the "services" the XWiki platform currently has (storage, cache), we don't have a "default" implementation, but we rely on a kind of factory to lookup the configured default. That is an actual factory class in the case of the cache service, but just more code in the old XWiki.java class for the storage initialization.

A standard way of selecting the default means that we'll need less factories, and less code is always a good thing.

Now, suppose one of the older components that had only one implementation, "default", gets alternative implementations, and we want to be able to allow more than one to be active in a wiki, and let the administrator decide which one should be considered the default. How can we approach this? The only way is indeed to have multiple hints, but last time I checked this resulted in more than one instance, even for @Singleton implementations.

Another approach is to deprecate the direct dependency declaration and instead introduce a factory that is responsible for selecting the default, but this breaks backwards compatibility.

The "I don't care, just give me the default" strategy works as long as the component implementation is self-contained and doesn't involve data communication. Let's take an example, PDF export. Currently the PDF export is only possible via FOP. If we were to convert the current interface + implementation class into a proper component, we'd name it "default", since it's the only one and who needs a factory for only one implementation.

People use it and they're happy because it just works. But we might soon add support for export via an office server. Suppose we want either FOP or Office to be usable as the default PDF export implementation. And suppose we'll want to keep both types of export active, so that we can use either one as the implementation for the default export of documents, the FOP implementation for exporting some kinds of documents like scientific articles, and the office connector for generating PDFs for presentations. Using the overrides mechanism we can only have one active at the same time, unless we introduce yet another factory which we can bypass using manual component lookup.

If at some point we decide that the office implementation works better, we might consider changing the default implementation for the pdf component. This means that we'll change the hint of the FOP implementation from "default" to "fop", change the hint of the Office implementation from "office" to "default", and our new version of XE works great if people read the installation guide and properly configure the office connector.

But what about those that want to upgrade, but are happy with the older FOP implementation and don't want to add support for the office connector?

They'll have to use patched versions of the two component implementations where their hints are reverted back to the old values. Easy? No. And even though "default" means "I don't care what the implementation does"

, here it does matter a lot which implementation you're using. They have different requirements, and it's important to know if the implementation will require an office instance or not. Saying that this won't happen since we're all really careful when designing our components is wishful thinking. We've refactored other more critical pieces of code than providing alternative implementations for a component, so we will find ourselves in situations where we'll have to switch from only one "default" implementation to several.

And "default" adds another assumption: XWiki Enterprise is the ultimate target. Our defaults are the only ones that matter. As an example, all the *Configuration components have just one "default" implementation, which relies on xwiki.properties, XWiki.XWikiPreferences etc. Doesn't that tie the platform to the XWiki Enterprise wiki?

It's not a direct dependency visible at compilation time, it's worse, and invisible assumption about the final runtime. It's certainly not the default for other types of users that want to embed xwiki-commons or xwiki-platform components in a different type of end product. To me this isn't the default configuration, this is the default configuration used by XWiki Enterprise, thus my proposal of using something else as the generic component hint instead of "default".