Github user paul-rogers commented on a diff in the pull request:
https://github.com/apache/drill/pull/868#discussion_r126828426
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/server/options/SystemOptionManager.java
---
@@ -48,11 +50,46 @@
* Only one instance of this class exists per drillbit. Options set at the
system level affect the entire system and
* persist between restarts.
*/
+
+/**
+ * Drill has two different config systems each with its own
namespace.First being the HOCON based boot time config
+ * system.This is a hierarchical system where the top layers override the
bottom ones in the following order
+ *
+ * Java System Options
+ * distrib.conf
+ * drill-override.conf
+ * drill-module.conf
+ *
+ * These are the options that are set before the drill starts.But once
drill starts System or session options can be
+ * modified using ALTER SYSTEM/SESSION.Even this system provides
inheritance sytle in the following order
+
+ * Session options
+ * System options
+ * Hardcoded defaults
+
+ * But system/session options have a validator and the validator has a
hard coded default value for every option. In
+ * the current system validators are registered so that system/session
options will always have a default value.
+ * So when a system/session options are not explicitly set or a user
system/session option is null the hardcoded
+ * default was applied since it checks if the option value is null and
returns the default set in the validator.But
+ * the config options set during boot time are never read and honored
since there is no linkage between the two
+ * config systems.It is also evident that there are some places where
there is some ad-hoc linkage between the
+ * two systems.For example, for the code gen compiler, config options are
supposed to be read if the system option
+ * is not null.But as the validator provides the default values config
options are never taken into consideration.
+ *
--- End diff --
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