17 Oct
2012
17 Oct
'12
3:01 p.m.
Hi,
I've made some changes to the implementation. I changed the name of the
"read-only" attribute to "final", because it seems more approprate.
https://github.com/xwiki/xwiki-commons/compare/master...feature-execution-c…
Also I have integrated it in core and the functional tests seem to
work. (I had some errors running them, but the errors didn't seem to be
related to this feature.)
https://github.com/xwiki/xwiki-platform/commit/4ceb225fe31846d4f9080eb8351f…
It would be much nicer if we had an execution context initializer for
the "real" xwikicontext. Maybe we need the ability to order the
initializers for that to work?
More comments below...
2012-10-17 09:20, Vincent Massol skrev:
I still think a single semantic with
setProperty is simpler to understand for the user. He can either pass an untyped value or
a type one.
I would prefer different method names to clearly separate declaration
from setting the value.
Yes, getProperty returns the value of the
property. I have not added
any method to obtain the declared ECP instance as I don't see any need
for it right now.
Of course, it is only the reference to the value that is guaranteed to
be read-only by the execution context. Depending on the application, it
may be desirable to make the object immutable, but that is not up to the
execution context to enforce.
I don't understand this. You mean that
we don't have a single use case where we wish to update a property put in the EC? Like
for example a counter that we want to increment… IMO there's a valid use case in
modifying an existing property in the EC.
But the property values can still be updated, unless they have been
declared 'final' (what I previously called 'read-only').
I'm pretty sure I'm right. We create a
new EC for each Request and then there are parts of the code that require a clean context
and for that they clean the current EC, push it and then pop it.
But it is this pushing and popping (and sometimes just replacing) of new
contexts that I am worried about. I want to be able to ensure that a
property value is propagated to all execution contexts that are
activated during a request cycle.
I do not want to clone my authorization
data. It wouldn't hurt cloning
it, but it would serve no purpose. But I can also imagine that some
applications might want to put objects that are expensive to clone in
the execution context. Ah ok i think I understand what you mean now. You want to
create a relationship between EC instances in the EC stack so that when reading a property
value from the current EC, if the property is inherited it's taken from the next EC in
the stack…
So basically you wish to remove the need for cloning and replace it with inheritance. I
get it now! :)
You are getting close. ;) The properties are copied or cloned
(depending on the clonedValue attribute) to the new context, if they are
marked as 'inherited'.
No, it just means that we
can perform run-time type checking when
setting the value of a declared property. It would catch some errors
earlier, but not as early as compile time.
You can let the execution
context provide some basic validation when
setting it. If you mark a value read-only, you will not need to worry
about if any component suddenly replaces or remove it in the middle of
the request.
Yes it is. ;)
The proposal adds two methods and one class, while beeing backwards
compliant with current code. To me it seems that we have plenty of time
to change our minds before releasing 4.3-rc1 if I were to merge this
with master right now. Or is our policy that we have to commit to any
API that we have added in a milestone release? Well IMO it's much better to
get an agreement before committing but you run the high risk to be asked to remove it in
hurry at the last moment before the release and fix everything that's beed modified
because of it + the stabilization risk when we're getting close to the release due to
the rollback.
We have a lazy commit rule but this rule means that any committer can ask for a rollback
at any time. I'd have personally asked for one because I think this is a critical API
and I would have liked to understand it before we put it in.
Now, I've just understood your proposal and for me the winning argument is that your
proposal makes it faster to create a new sub-EC for the same request and since we create
an insane number of EC in the course of a request it can only be a good thing. Note: we
need to check the place where we create those EC and fix them since I believe creation of
a new EC is not always required. One such place for example is $doc.display() calls. I
think each one create a new EC.
Yes, that is another advantage of this proposal. But my main
motivation, for the time beeing, is having the ability to enforce the
combination 'inherited' and 'final' for the duration of a request.
On Oct 16, 2012, at 3:08 PM, Andreas Jonsson
<aj(a)member.fsf.org> wrote:
I'm not sure I understand what you mean. In my solution you can
explicitly declare that you want the values cloned.
Hi,
2012-10-16 11:03, Vincent Massol skrev:
That's always
true, except where there are bugs (i.e. code that doesn't clone the context but create
a new one when it shouldn't) but that would be the same even with your proposal.
Hi Andreas,
It would be great if you could explain your real use case and the reason you wish to
introduce this (I guess you have a use case where you need this :)).
I want to put
authorization information in the execution context, and I
want to trust that this information is carried over to any new execution
context that is activated in the scope of the current request. It is
also great if the object that holds the information cannot be replaced.
So, if you look at the code you will see that I have implemented a
rather aggressive enforcment of the combination of the attributes
read-only and inherited.
Ok I'll need to read your code… :)
But also, the need for the inherited property
already exists for
managing the properties "xwikicontext" and "velocityContext". Just
look
at the TODO and FIXME in DefaultExecutionContextManager in the master
branch.
I wrote that code and those TODO/FIXME :)
There's only one issue and you'll still have it: it's about putting objects
that can be cloned in the EC (that includes the velocitycontext and xwikicontext). But the
general idea is just to be able to associate any kind of
metadata with each property. It can be compared to how interpreted
languages handle variables. Take for instance bash, where you can just
set a variable to a value, wich implicitly declares the variable. But
you can also explicitly declare a variable, and in the declaration you
can provide flags that sets various attributes of the variable. (e.g,
declare -r variable='readonly value'.)
I'd be fine if it solved a
problem but so far I fail to see how your solution will solve the real issue we have now
which is that the elements put in the EC are not cloneable...
See
below.
On Oct 12, 2012, at 9:27 AM, Andreas Jonsson <aj(a)member.fsf.org> wrote:
Setting and declaring
a property are completely different concepts. In
a purely declarative model, properties would have to be declared before
being set. But what I'm proposing is more in line with what bash does
with variables. Setting a value on an undefined property will
implicitly declare it with default values for all attributes. You can
also provide an initial value when declaring the property (which of
course is required for read-only properties). Hi everyone,
I would like to propose to introduce a possibility to associate
properties in the execution context with metadata by declaring them. We
introduce the class 'ExecutionContextProperty' that will store the
metadata attributes, in addition to the actual property value.
/**
* Declare a property in the execution context.
*
* @param key The key that names the property
* @param property The property to declare.
*/
public void declareProperty(String key, ExecutionContextProperty property)
Does
this mean the current ExecutionContext.setProperty() method would be deprecated/removed?
Why not use instead: setProperty(String key, ECP property)? BTW what would the default property type? Cloned?
I have set the default to not cloning, but I'm open to changing that.
Maybe the default should be 'clone if Cloneable'?
The metadata
governs how the property should be treated when managing
the execution contexts. For instance, these attributes could be supported:
* Read-only - The value may not be updated within the the execution
context.
How do you ensure this? So getProperty() would continue to return the
value and not an ECP instance right? So when the user calls declareProperty() with the
same key as an existing key it would be refused if it's been declared Readonly?
A property may only be declared once in an execution context, so it will
be refused if the property is already declared. It doesn't matter if it
was an explicit or implicit declaration. Attempting to declare an
already existing property is an indication that something is wrong, so
raising an exception seems appropriate. If the
property is read-only, then also setProperty will refuse to
replace the value.
That one makes sense.
Now
imagine that I pass an ECP instance with a value of List. How do you prevent some from
doing: getProperty("mylistkey").add(somenewelement)?
No, you
are mistaken. We allow the creation and activation of several
execution contexts per request. That's why we have the kludges in
DefaultExecutionContextManager for copying and cloning "xwikicontext"
and "velocityContext". * Inherited - The property will be inherited
from the current context
when replacing the execution context within the scope of a request.
I don't
understand this one, we create a new Execution Context for each Request. * Clone -
Also clone the value when the execution context is cloned.
What are the cases when
we don't want to clone a property? I don't see any, for me we always want to clone
them and in practice we would instead need to change our current signature to:
setProperty(String key, Cloneable value)
It's interesting since it could make creating a new EC faster… I now understand the
need for readonly. It's a bit dangerous though since it means there's no way of
ensuring a clean EC and there's always the risk that you'd modify a value located
in the previous EC in the stack. Unless we don't allow this, which would make sense
(that's probably what you're doing I guess… :))...
When putting a value into the execution context, you will have four
options: 1) don't make it "inherited", 2) make it "inherited" and
"cloned", 3) make it "inherited" and "final" 4) just make it
"inherited".
For options 3) and 4), the value should preferrably be immutable, but it
is still up to the developer to decide. If you are making an
application for tracing something, you will want a mutable instance to
be propagated to new contexts.
* Type -
The class of the value, for typechecking when replacing
the value.
Can you explain more? Does it mean you wish that getProperty() returns
a typed value? We can already do this by using generics. *
Non-null - The value may not be null
So, the arguments for this proposal are at least two:
1. Catch errors early and simplify debugging.
Can you explain more this point, I
don't see the benefit over what we can already do. 2.
Convenience of having properties automatically maintained across
execution contexts within the same request cycle.
This is probably related to the
inherit item which I don't understand. What do
you think?
Honestly I don't see the need for this.
What we lack IMO is proper Execution Context cloning and for this to happen we need to
ensure we only cloneable items in the EC. As for the other "types" I don't
really see a strong need but maybe I've misunderstood a few things (hence my questions
above).
I would like to implement and merge this ASAP.
We need to be very careful because the EC is a key class and introducing this means
we're changing an important public API and we cannot go back easily (except through
some deprecation cycles). IMO, seen the importance this should be a VOTE instead of a
PROPOSAL.
So while your proposal adds a lot of complexity it's also interesting. I'd like
to have other's opinion on this too.
I want to object to that. I do not think that it adds complexity, on
the contrary it simplifies some use cases.
One thing that I'd like to see deprecated is the
default setProperty() without Typing information.
I disagree, setting and declaring are different things. But we could
disallow implicit declarations (i.e., setting the value of an undeclared
property.)
/Best Regards,
Andreas
IMO we need to explicitly control each property with
this proposal and not use a default. It becomes very important that each property's
type is controlled since it can lead to important bugs.
Thanks
-Vincent
I will propose a vote for it later in any event.
Best Regards,
/Andreas
> We would really need some feedback from several devs before going on with this IMO.
ATM I'd like to understand it more before voting.
>
> Thanks
> -Vincent
>
> PS: Didn't get the time to read your implementation code yet
>
>> Best Regards,
>>
>> /Andreas
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