I On 04/06/2012 09:35 AM, Vincent Massol wrote:
Hi Sergiu,
Note that below I'm going to play the devil's advocate since I think it's
important we really think hard before changing anything and verify if our
current implementation is not enough.
See below.
On Apr 5, 2012, at 8:44 PM, Sergiu Dumitriu wrote:
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#HOverrides
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.
Yep that's how it works.
A standard way of selecting the default means that we'll need less factories,
and less code is always a good thing.
Yes but it has more limitation to what we have since with a proper Factory you
can imagine all kind of logic to decide which implementation to use (hour of
the day, whether the user is a premium user or not, etc).
BTW we do support Providers and the goal of the Provider is to be a factory for
a given Role. So from now on, there should be no need to implement a Factory
proper. Implementing a Provider is the new best practice for this.
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.
Correct, ATM we support only one hint per implementation.
? I think Marius needed that for some wysiwyg stuff and we actually
support multiple hints per implementation. It's just gonna give you a
new instance for each hint, which in my view is a bug, not a missing
feature.
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.
Not completely true. Imagine you have (Role, "default") as your current
component and you wish to be able to choose various implementation from now on. What you
could do is this:
* Write a new component, a Factory, that overrides the current component, i.e. by using
the same (Role, "default"). This factory will then delegate to whatever other
implementation it wishes.
That's only possible if the public API of the CM allows you to grab
overriden components (non-default defaults :) )
From the user of (Role, "default")'s POV he won't see a single change.
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.
Yes but we could also be thorough and instead implement a Provider<PDFExporter>
instead.
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.
Based on your use case you'll need a UI to ask the user what he wants to export, i.e. either a
"scientific article" or a "presentation" and from his choice you'll pick the
right implementation. You could also try to guess that dynamically by looking at what is being
exported but that'll require a Factory to hold that logic.
I know what you mean though: for some reason you don't want that to be dynamic
and you wish it to be static.
There's a problem with dynamicity though. Imagine an extension that wants to replace an
implementation. With your proposal the best practice would be to introduce a new Hint
since the "default" is chosen statically at configuration time. So that
wouldn't work. After you install extensions you'd need to stop the wiki, change the
binding to the new implementation from the extension and restart it. Of course you'd need
to read the documentation of the extension to know you have to use it in replacement.
Basically it would mean that extensions cannot override behaviors. They would just be
able to add new components (like new Macros) but not modify behaviors at runtime.
But this only means we need to be able to change configuration without
stopping the wiki and make it reload configs (might prove useful for
other things as well), it doesn't mean we cannot do it "static".
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.
In practice we would have 2 choices with our current component impl.:
Choice 1:
* Add 2 new implementations with hint1 and hint2 (hint1 being what was
"default" before)
* Keep the implementation with "default" hint but deprecate it and move it to
legacy. Refactor it so that it delegates yo hint1
* Add a Provider to decide which impl to use based on whatever conditions we
want
Choice 2:
* Move "default" impl to "hint1"
* Add "hint2"
* Change "default" implementation to be a Composite that delegates to hint1 or
hint2
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?
Yes, in this case this is transparent for them (and it's a good thing in most
caes!). This means they get autoupgrade to something better.
No, it means that for some reason we decided to change the underlying
technology (e.g. change of licence), it doesn't mean that everybody is
happy with the change.
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"
It doesn't really mean this. We use "default" only when there's a single
implementation. When there are more than 1 each one has a hint that is a qualifier to the
implementation since there's no reason one is more default than the other.
, 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.
I agree that we've refactored. And it has worked so far right? ;)
Designing our component manager to make it easy to transition is the right
thing to do.
Still, my major problem is not about overrides, but about the semantics of "default" (or lack of
it). This says nothing about the actual mechanisms behind the implementation, it just reflects the state of
that particular component implementation: it's the default at the moment. Not caring what the implementation
actually does works only when there is indeed just one possible implementation that is straight-forward. But
in most cases, we do rely on another library that does the work for us, and libraries die, better
alternatives come along, and changing that internal aspect of the implementation will sometimes be backwards
incompatible, or have a different behavior. Sure, it does the same job, but it does it so differently that
some will prefer to use the other approach. We have to let users decide which is their "default",
and having multiple implementations with the "default" hint but different priorities is not very
intuitive. Why not make everything default and remove hints completely if we don't really put any meaning
into the hint?
This also means that we should only have components where there is a
chance that another implementation might exist, because to the limit you
can separate the interface from the implementation for any little piece
of code that you write.
I sort of feel you for this default thing, but at the same time, it's
also a matter of education of the developer, which needs to make sure
that they put a technology hint to the component, besides the default
hint. The pb is that, as long as there is only one implementation,
regardless of the technology it's based on, you also need to put the
default hint since otherwise you'll have to hardcode the reference to
the technology everywhere if you wanna use that service. so in this case
default would mean 'this is the one that should be used because there's
no other, you dumb CM that is not capable of seeing that'.
This brings back some memories, but I don't know from what, about a
system that was giving the available implementation regardless of its
name. For example, we could make the CM return the only implementation,
if only one exists, when asking for a component, regardless of its hint,
so we don't have to put default everywhere. But then we need a strategy
for the case when there are more.
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?
This not true in xwiki commons and rendering because I've made sure that we
could use them outside of the XWiki Platform. They have default implementation
that don't use XWiki Configuration module.
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".
Ok thanks for the explanations. I understand better now what you mean.
Actually what you suggest could already be implemented using a best practice of
always using Providers when you want a Component injected. In this manner by
default you'll get the Generic Provider but anyone could implement a specific
Provider implementation for it that would choose between various implementation
based on whatever (a value in a META-INF/role-bindings.txt file, data from DB,
etc).
<side note>Only issue with having Providers everywhere is that you get late
verification of your system coherence since dependencies will be resolved only when they're
used. OTOH this is a necessity in a fully-dynamic system ;)</side note>
Also, the notion of default doesn't always have a meaning. There are lots of
cases when there are NO default. For example Macros or Transformations or…
Let's continue the discussion it's interesting :)
I'd like to review a bit the other Component system out there again to see what
they do for this. It's important that they have support for this since we want
to be able to switch to them one day. The 3 that I would review are:
* Guice
* CDI
* OSGi
yes, we should learn from others. Maybe even use one?
Thanks,
Anca
Thanks
-Vincent
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