11:07 a.m.
On 10/30/14, 10:23 AM, David M. Lloyd wrote:
The main advantage of polymorphism (which is really just a way of
saying
"decoupling model location from node type") is (perhaps obviously) that
you can use the same model location for more than one type.
There's a separate branch of this thread that looks to be getting into
discussing use cases for polymorphism, so here I'll focus on details a bit.
Right now our types are tied to addresses. The address defines the type,
and is used to find the description of the type. This is even true with
complex attributes, just with a slightly different mechanism.
To get away from having the address define the type, we would need some
other mechanism for declaring a type. In a hangout today, we
brainstormed about using special attributes[1] in any complex ModelNode
(i.e. any resource or a complex attribute of OBJECT type).
We'd need some clear mechanism for resolving type definitions based on
the data in those special attributes.
The description of the parent would have to declare what the valid types
are, in some combination with what the valid addresses are. That would
allow tooling like the console to guide the user when creating new items
or radically changing existing ones.
For something like the CLI it gets more cumbersome, as our UX (which I
like a lot) is based on tab completion. Tab completion could still
suggest a type, but the user's choice would no longer get recorded in
the current input.
This is quite a bit more flexible than the current approach, but also a
fair bit more cumbersome for the many cases where there's no need for
polymorphism, so the use case discussion is important.
[1] We discussed using ".xxx" e.g. ".type" and ".namespace",
with
attributes whose names begin with the "." prefix becoming reserved. The
'.' is kind of nice because it's intuitively like a hidden file on a
filesystem, and if we use dot syntax to separate between levels in a
complex attribute or between an attribute group and an item, this case
fits in.
This means that user interface frameworks (for example) could know
that
there is only one possible sub-resource address, say "service=ha-policy"
(to use Heiko's original naming example mixed with Jeff's practical
example). Then it only has to make a decision as to how to render the
resource, rather than having to analyze the model to know that, of the
six possible children for "ha-policy", only one can ever be present.
On 10/30/2014 09:06 AM, Jason Greene wrote:
> Is it really polymorphism though if you do not share attributes or operations?
>
> It sounds like these are all distinct types just sharing the same portion of an
address.
>
> The primary justification for this modeling approach seems to be:
>
> 1) Avoid the user having to assign a name
> 2) Enforce a single occurrence of a “type”
>
>
>> On Oct 30, 2014, at 8:01 AM, David M. Lloyd <david.lloyd(a)redhat.com>
wrote:
>>
>> I think that polymorphism is a new use case for 'squatters'. I wonder
>> if we have any existing code which enforces single children?
>>
>> On 10/30/2014 05:40 AM, Jeff Mesnil wrote:
>>> I’m integrating HornetQ 2.5 in WildFly and I have a new use case for
resources that is related to singleton/squatter resources.
>>>
>>> In HornetQ 2.5 they have completely rewritten the HA configuration.
Basically, a server can be configured as live-only, replicated (master, slave, or
colocated) or using shared-store (again as a master, slave or colocated).
>>>
>>> To represent this in the management model, I have added several resources
under hornetq-server:
>>>
>>> /subsystem=messaging/
>>> hornetq-server=*/
>>> ha-policy=live-only
>>> ha-policy=replicated-master
>>> ha-policy=replicated-slave
>>> ha-policy=replicated-colocated
>>> ha-policy=shared-store-master
>>> ha-policy=shared-store-slave
>>> ha-policy=shared-store-colocated
>>>
>>> I have constraints for this ha-policy resource:
>>> * There can at most one child for this type of resource (no child means
no HA). This is enforces during the MODEL stage.
>>> * The child can only be named using one of the 7 values above (i.e. there
is no resource definition for ha-policy=*, using any other name would fail)
>>>
>>> Each ha-policy definition has a different set of attributes. Using an
attribute group to represent the HA policy does not seem a good fit as some of them have
subresources too.
>>>
>>> I wonder if that representation fits with our roadmap and whether it can be
considered as a singleton (as there can only be one resource of that type among). I have
the additional constraints of having only one chile for that type that is not covered by
your proposal though.
>>>
>>> I especially wonder how the console (and to a lesser extent the cli) can deal
with this resource.
>>>
>>> Heiko, is it something that would make sense for the console based on this
resource description:
>>>
>>> [standalone@localhost:9990 hornetq-server=default]
./ha-policy=*:read-resource-description(recursive-depth=1)
>>> {
>>> "outcome" => "success",
>>> "result" => [
>>> {
>>> "address" => [
>>> ("subsystem" => "messaging"),
>>> ("hornetq-server" => "default"),
>>> ("ha-policy" =>
"replication-colocated")
>>> ],
>>> "outcome" => "success",
>>> "result" => {
>>> ...
>>> }
>>> },
>>> {
>>> "address" => [
>>> ("subsystem" => "messaging"),
>>> ("hornetq-server" => "default"),
>>> ("ha-policy" =>
"replication-master")
>>> ],
>>> "outcome" => "success",
>>> "result" => {
>>> ...
>>> }
>>> },
>>> {
>>> "address" => [
>>> ("subsystem" => "messaging"),
>>> ("hornetq-server" => "default"),
>>> ("ha-policy" =>
"shared-store-slave")
>>> ],
>>> "outcome" => "success",
>>> "result" => {
>>> ...
>>> }
>>> },
>>> {
>>> "address" => [
>>> ("subsystem" => "messaging"),
>>> ("hornetq-server" => "default"),
>>> ("ha-policy" => "live-only")
>>> ],
>>> "outcome" => "success",
>>> "result" => {
>>> ...
>>> }
>>> },
>>> {
>>> "address" => [
>>> ("subsystem" => "messaging"),
>>> ("hornetq-server" => "default"),
>>> ("ha-policy" =>
"shared-store-master")
>>> ],
>>> "outcome" => "success",
>>> "result" => {
>>> ...
>>> }
>>> },
>>> {
>>> "address" => [
>>> ("subsystem" => "messaging"),
>>> ("hornetq-server" => "default"),
>>> ("ha-policy" =>
"replication-slave")
>>> ],
>>> "outcome" => "success",
>>> "result" => {
>>> ...
>>> }
>>> },
>>> {
>>> "address" => [
>>> ("subsystem" => "messaging"),
>>> ("hornetq-server" => "default"),
>>> ("ha-policy" =>
"shared-store-colocated")
>>> ],
>>> "outcome" => "success",
>>> "result" => {
>>> ...
>>> }
>>> }
>>> ]
>>> }
>>>
>>> jeff
>>>
>>
>> --
>> - DML
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>
> --
> Jason T. Greene
> WildFly Lead / JBoss EAP Platform Architect
> JBoss, a division of Red Hat
>
--
Brian Stansberry
Senior Principal Software Engineer
JBoss by Red Hat