Excellent write-up, Aled.
I wholly support this, as someone who fastidiously avoids the
'setFromFlag' names having been tripped up too many times by the
inconsistent inheritance and other "surprises" you mention.
A.
--
Alasdair Hodge
Principal Engineer,
Cloudsoft Corporation
On 23/11/2016 01:30, Aled Sage wrote:
Alex,
Sticking with the two proposals being discussed separately...
The problems that deprecating @SetFromFlag (and moving the naming into
config key) is solving are:
1. The behaviour is currently inconsistent and thus confusing - e.g.
the name from @SetFromFlag will not be inherited, but the config key
name will be (so "env" is not inherited but "shell.env" will be).
People can't easily tell how things will behave when writing
blueprints based on existing examples.
2. The name in @SetFromFlag is not part of the entity definition - it
cannot be discovered on the entity type, so could never be used by a
YAML composer. Moving aliases (deprecated or otherwise) into the
config key definition makes it a proper part of the type. It can
thus be included in docs as well.
3. We have no way to rename a YAML config key, defined in a
brooklyn.parameters section, deprecating the old name.
If a better name is agreed upon (e.g. correcting spelling etc) then
one either has to break backwards compatibility or jump through
hoops in bash/freemarker to respect both names.
4. Our code for handling @SetFromFlag is pretty horrible - mostly
because it is set via a different code path from config keys,
leading again to inconsistencies and difficult-to-maintain code.
This proposal is complementary/separate from
https://github.com/apache/brooklyn-server/pull/363 - I don't think that
PR (or continuation of that work) would solve these problems. YOML does
not change the nature of config keys, or EntitySpec's flag/config.
Aled
On 22/11/2016 21:44, Aled Sage wrote:
Hi all,
TL;DR: deprecate `@SetFromFlag`; instead explicitly set
deprecatedAlias (and/or alias?) on the ConfigKey.
We should change this *after* releasing 0.10.0, to decrease risk.
_*Current Situation*_
When writing a Java entity, one can declare a config key type as a
static field. One can also use the annotation `@SetFromFlag` on this.
The annotation allows one to include an alternative name for the key.
This alternative name is often confusing. It is not part of the formal
"config key" definition (i.e. not available via
entity.getEntityType()), and is only respected in some situations when
supplying config values (e.g. when supplying config directly to an
entity, but not when using config inheritance).
The `@SetFromFlag` annotation also supports "defaultVal", but this is
(hopefully!) never used. There is a better way to specify a default
value, which is to pass it into the Config Key's definition.
The other options on the `@SetFromFlag` annotation are "immutable" and
"nullable". These are ignored in some/many situations, so are (I
believe) a bad idea to use (certainly bad to rely on). It's better to
use `.constraint(Predicates.notNull())` on the config key, rather than
"nullable".
The reason for supporting multiple names can be split into (at least?)
three different use-cases (we'll discuss the second and third in a
separate email thread):
1. Backwards compatibility (e.g. because we already support two names,
so need to keep doing that; or because we want to rename a config
key, such as correcting its spelling).
This use-case could be reworded as the need to support deprecated
names.
2. Aliases (i.e. a deliberate desire to support different names,
because those different names are seen as good).
3. Hints on blueprint validation in a composer (e.g.
"environment.variables not valid; did you mean env?")
_*Proposal*_
We should stop using `@SetFromFlag` on config keys entirely -
deprecating its use, and warning anywhere it's used.
Instead, we should add support for multiple names on the ConfigKey
definition itself (i.e. to the ConfigKey interface). This will make it
part of the entity's type. This will make it easier to support in yaml
blueprints, and also possible to deprecate config key names in YAML
brooklyn.parameters.
I suggest we support something like this:
ConfigKey<String> PRE_INSTALL_COMMAND =
ConfigKeys.builder(String.class, "my.toy.example")
.description("...")
.defaultVal("myDefaultVal")
.deprecatedAlias("my.toyy.example") // e.g. deprecated to correct
the misspelling
.deprecatedAliasSince("my.old.example", "0.10.0") // second arg is
version when it was deprecated
.build();
Or in YAML:
brooklyn.parameters:
- name: my.toy.example
type: String
description: ...
default: myDefaultVal
deprecatedAliases:
- name: my.old.example
deprecatedSince: 0.10.0
# The version above is optional; it will also accept the name
by itself:
- my.toyy.example
Here is a real-world example in Java (getting rid of
`@SetFromFlag("preInstallCommand")`):
ConfigKey<String> PRE_INSTALL_COMMAND =
ConfigKeys.builder(String.class, "pre.install.command")
.description("Command to be run prior to the install method
being called on the driver")
.runtimeInheritance(BasicConfigInheritance.NOT_REINHERITED)
.deprecatedName("preInstallCommand")
.build();
When parsing the entity's Java config keys, we should be able to
translate the `@SetFromFlag` into an equivalent "deprecated aliases"
on the Config Key object (while preserving backwards compatibility).
When we can delete the `@SetFromFlag` support entirely, it will really
clean up several areas of ugly/fiddly code!
==========
Part 2
==========
We should discuss *in a different thread* whether we want to support
multiple aliases that are deliberately not deprecated (e.g.
"shell.env" and "env"). See email thread "[PROPOSAL] Deprecate config
key aliases" to follow shortly.
If desired, we could support `.alias("my.toy.example.otherName")` on
the config key.
==========
Part 3
==========
We currently also support the `@SetFromFlag` annotation on fields
(let's focus here on entity, location, enricher and policy).
The value of the field is persisted, and is set on rebind. It behaves
like an attribute that can be set via a config key. Its use has been
discouraged (on entities, at least) for quite some time, but not
formally deprecated.
_Proposal_
For entities, we should formally deprecated this - warning in any
entity that uses it - and delete support for it in a couple of
releases time.
For locations/policies/enrichers, I think we should also deprecated
it. Unfortunately they don't support "attributes", but they do support
reconfigurable config so that can be used instead.
==========
Part 4
==========
One can use @SetFromFlag on fields of other objects, which allows the
fields to be set when using the DSL `$brooklyn:object`.
I think this will become redundant with Alex's YOML. Let's not discuss
such usage of @SetFromFlag in this email thread.
