[Engine-devel] [RFC] ovirt-engine - vdc_config default options

[Yair Zaslavsky]

On 09/10/2012 12:58 AM, Alon Bar-Lev wrote:
> Hello All,
>
> I would like to discuss the method we use to manage default options. I believe it can be significantly simplified.
>
> Please read though and comment.
>
> Thank you,
> Alon Bar-Lev
>
> ---
>
> CURRENT STATE
>
> Most options are located in three different locations.
>
> Let's explain by example... The FenceQuietTimeBetweenOperationsInSec option.
>
> ---
> ConfigValues.java
> ---
> public enum ConfigValues {
> <snip>
>      @Reloadable
>      @TypeConverterAttribute(Integer.class)
>      @DefaultValueAttribute("180")
>      FenceQuietTimeBetweenOperationsInSec(30),
> <snip>
> }
> ---
>
> ---
> 0000_config.sql
> ---
> <snip>
> select fn_db_add_config_value('FenceQuietTimeBetweenOperationsInSec','180','general');
> <snip>
> ---
>
> ---
> engine-config.properties
> ---
> <snip>
> FenceQuietTimeBetweenOperationsInSec.type=Integer
> FenceQuietTimeBetweenOperationsInSec.validValues=60..600
> FenceQuietTimeBetweenOperationsInSec.description="Fence quiet time between operations (in seconds)"
> FenceQuietTimeBetweenOperationsInSec.alternateKey=Fence_Quiet_Time
> <snip>
> ---
>
> ConfigValues.java is the base, it defines all options and appropriate metadata as annotations. It defines the option name, type, default value, and if reloadable.
>
> 0000_config.sql adds all parameters into the database using their default value, we actually duplicate the parameter name and default value into the sql script. Please note that even if we do not add the parameter to the database the engine infrastructure will use the default value specify at ConfigValues.
>
> engine-config.properties is used as an interface to engine-config utility, a command-line utility that can manipulate engine options. engine-config manages only options that are specified in this property files. Property files specifies user friendly description, valid values, alias, but duplicate the option name and type that already specified in ConfigValues. engine-config will only manage options that exists in database.
>
> QUESTIONS
>
> 1. Why do we store default values in database?
>
>  From what I managed to gather, we store default values in database under the assumption that we need to keep defaults when we upgrade from one version to another.
>
> However, in most cases when upgrading an application the new defaults should apply as long as they were not overridden by user. Keeping old default as a new value is an exception that can be taken care of during the upgrade process for selected options.
>
> 2. Why do we keep properties file?
>
> In practice we could specify the metadata within the property files as annotations in ConfigValues.java. So why do we actually need the properties file?
>
>  From what I managed to gather, we use the property file as an interface to support personal, to allow adding/removing options exposed to the engine-config utility. An addition of option to the property file exposes it.
>
> SUGGESTED STATE
>
> Establish a single place to manage options.
>
> Do not store default values in database.
>
> Provide alternate method of exposing internal options to engine-config utility.
>
> ---
> ConfigValues.java
> ---
> public enum ConfigValues {
> <snip>
>      @Reloadable
>      @Public
>      @TypeConverterAttribute(Integer.class)
>      @Restriction.IntegerRange(60, 600)
This definition will probably not work, should be
@IntegerRestrictionRange(60,600)
>      @DefaultValueAttribute("180")
>      @Description("Fence quiet time between operations (in seconds)")
>      FenceQuietTimeBetweenOperationsInSec(30),
> <snip>
> }
> ---
>
> BENEFITS
>
> No duplication of option name, type, default value, etc... between multiple files. One place to role them all.
>
> Simpler upgrade sequence, in most cases as we do want the new defaults to apply.
>
> METHOD
>
> 1. Add @Public annotation for ConfigValues, to expose options to engine-config instead of the properties file.

I would still recommend in this case to have an optional properties file 
that will allow you to override the behaviour of the @Public annotation 
(i.e - cancel it).
In evolution of java frameworks from the stage where metadata was 
provided via external configuration file (XML, properties file ,etc...) 
to a stage where metadata was provided via annotations, in order to 
support changes at metadata without needing to recompile and to support 
backward compatibility, many frameworks simply made the configuration 
file to be optional for purpose of overriding the annotation deceleration

>
> 2. Add @Restrict annotation for ConfigValues, instead of validValues of properties file.

Bare in mind that annotations are limited in the values they can accept. 
Most likely it will work, but we need to check it.

>
> 3. Add @Description annotation for ConfigValues, instead of description of properties file.
>
> 4. Add engine-config --internal parameter to allow get/set/list of none @Public option, instead of the need to update the properties file.
>
> 5. Modify enigne-config [set] to add option if does not exist in database and does not match default value.

>
> 6. Modify engine-config [set] to delete option if value matches default value.
>
> 7. Modify engine-config [get] to retrieve default from ConfigValues if value is missing from database.

I would recommend as a future feature to consider modifying [set] to 
provide an option to reset the value to the default value, as presented 
at ConfigValue
>
> 8. Create upgrade script which deletes all options with value that matches the default from the database.
>
> IMPLICATIONS
>
> The option alias is removed. Can be added at later time if required using own @Alias annotation. Not sure it is actually required.
>
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