[GitHub] [drill] sanel commented on a change in pull request #2074: DRILL-7728: SPI framework

[GitBox <gi...@apache.org>]

sanel commented on a change in pull request #2074:
URL: https://github.com/apache/drill/pull/2074#discussion_r431056640



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File path: common/src/main/java/org/apache/drill/extn/README.md
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@@ -0,0 +1,125 @@
+# Drill Service Provider Interface (SPI)
+
+The Drill SPI allows extensions (AKA plugins, providers) to extend Drill.
+This package provides the core extension mechanism and is a minor refinement
+of the standard Java
+[`ServiceLoader`](https://docs.oracle.com/javase/8/docs/api/java/util/ServiceLoader.html)
+mechanism.
+
+## Layout
+
+A _service provider_ provides one or more _services_. The provider implements a
+provider interface, defined in the Drill SPI package. That interface provides factory
+methods to implement services.
+
+The provider developer creates a jar file with the properties described below. The jar
+file is then placed in Drill's extension directory which is either:
+
+* `$DRILL_HOME/ext` if using the "classic" approach, or
+* `$DRILL_SITE/ext` if usign a separate "site" directory.
+
+### Simple Jars
+
+The simplest service is one with no dependencies other than the JDK and Drill.
+These reside in a single jar file placed in the `ext` directory. A UDF is a typical
+example:
+
+```
+$DRILL_SITE
+|- ext
+   |- coolUdfs.jar
+   |- fancyUdfs.jar
+```
+
+The name of the jar can be anything, though Drill recommends Maven-style naming:
+
+```
+collUdfs-1.3.0.jar

Review comment:
       What happens if in the same folder exists both `collUdfs-1.3.0.jar` and `collUdfs-1.2.5.jar`? Which version is guaranteed to be loaded? Maybe this use case should be mentioned here.




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