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https://issues.apache.org/jira/browse/SOLR-15960?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17483660#comment-17483660
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Jan Høydahl commented on SOLR-15960:
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Looking into solr.xml for another jira. We have a generic class "XmlConfigFile"
that does this "${solr.foo:default}" expansion in several xml files. Part of
the unification plan here could be to first look for sys.prop "solr.foo", but
if not found, look for env.var (our own static map version) "SOLR_FOO", and
substitute that instead. Our whole bootstrapping and throwing props around
between various classes during init is crazy so it may turn out to be a tad
more complex than it seems on the surface.. But this centralized XmlConfigFile
class gave me some hope :)
> Unified use of system properties and environment variables
> ----------------------------------------------------------
>
> Key: SOLR-15960
> URL: https://issues.apache.org/jira/browse/SOLR-15960
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Jan Høydahl
> Priority: Major
>
> We have a lot of boiler-plate code in Solr related to resolving configuration
> from config-files, system properties and environment variables.
> The main pattern so far has been to load a config from an xml file, which
> uses system property variables like {{{}$\{myVar{}}}}. All the environment
> variables that we expose in {{solr.in.sh}} are converted to system.properties
> in {{bin/solr}} and inside of Solr we only care about sys.props. This has
> served us quite well, but is also has certain disadvantages:
> * Naming mismatches. You have one config name in the xml file, one as system
> property and yet another for environment variable.
> * Duplicate code to deal with type conversion, and converting comma
> separated lists from env.var into Java lists
> * Every new config option needs to touch {{{}bin/solr{}}}, {{bin/solr.cmd}}
> and often more.
> In the world of containers and k8s, we want to configure almost every aspect
> of an app using environment variables. It is sometimes also more secure than
> passing sys.props on the cmdline since they won't show up in a "ps".
> So this is a proposal to unify all Solr's configs in a more structured way
> * Make naming a convention. All env.variable should be uppercase with format
> {{SOLR_X_Y}} and all sys.propos should be lowercase with the format
> {{solr.x.y}}. Perhaps {{solr.camelCase}} should map to {{SOLR_CAMEL_CASE}},
> or we discourage camel case in favour of dots.
> * Add a central {{ConfigResolver}} class to Solr that can answer e.g.
> {{getInt("solr.my.number")}} and it would return either prop
> {{solr.my.number}} or {{SOLR_MY_NUMBER}}. Similar for String, bool etc, and
> with fallback-values
> * List support, e.g. {{getListOfStrings("solr.modules")}} and it would
> return a {{List<String>}} from either {{solr.modules}} or {{SOLR_MODULES}},
> supporting comma-separated, custom separator and why not also json list
> format ["foo","bar"]?
> A pitfall of using environment variables directly is testing, since env.vars
> are immutable. I suggest we solve this by reading all {{SOLR_*}}
> env.variables on startup and inserting them into a static, mutable map
> somewhere which is the single source of truth for env.vars. Then we can ban
> the use of {{System.getenv()}}.
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