HBASE-23710 - Priority configuration for system coprocessor

Signed-off-by: Viraj Jasani <vjasani@apache.org>

hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/CoprocessorHost.java

@@ -148,8 +148,16 @@ public abstract class CoprocessorHost<E extends CoprocessorEnvironment> {
     if (defaultCPClasses == null || defaultCPClasses.length == 0)
       return;
 
-    int priority = Coprocessor.PRIORITY_SYSTEM;
+    int currentSystemPriority = Coprocessor.PRIORITY_SYSTEM;
     for (String className : defaultCPClasses) {
+      String[] classNameAndPriority = className.split("\\|");
+      boolean hasPriorityOverride = false;
+      className = classNameAndPriority[0];
+      int overridePriority = Coprocessor.PRIORITY_SYSTEM;
+      if (classNameAndPriority.length > 1){
+        overridePriority = Integer.parseInt(classNameAndPriority[1]);
+        hasPriorityOverride = true;
+      }
       className = className.trim();
       if (findCoprocessor(className) != null) {
         // If already loaded will just continue
@@ -162,10 +170,13 @@ public abstract class CoprocessorHost<E extends CoprocessorEnvironment> {
         implClass = cl.loadClass(className);
         // Add coprocessors as we go to guard against case where a coprocessor is specified twice
         // in the configuration
-        this.coprocessors.add(loadInstance(implClass, priority, conf));
+        int coprocPriority = hasPriorityOverride ? overridePriority : currentSystemPriority;
+        this.coprocessors.add(loadInstance(implClass, coprocPriority, conf));
         LOG.info("System coprocessor " + className + " was loaded " +
-            "successfully with priority (" + priority + ").");
+            "successfully with priority (" + coprocPriority + ").");
-        ++priority;
+        if (!hasPriorityOverride) {
+          ++currentSystemPriority;
+        }
       } catch (Throwable t) {
         // We always abort if system coprocessors cannot be loaded
         abortServer(className, t);

src/main/asciidoc/_chapters/cp.adoc

@@ -107,9 +107,24 @@ Therefore, the jar file must reside on the server-side HBase classpath.
 
 Coprocessors which are loaded in this way will be active on all regions of all tables.
 These are the system coprocessor introduced earlier.
-The first listed coprocessors will be assigned the priority `Coprocessor.Priority.SYSTEM`.
-Each subsequent coprocessor in the list will have its priority value incremented by one (which reduces its priority, because priorities have the natural sort order of Integers).
 
+The first listed Coprocessors will be assigned the priority `Coprocessor.Priority.SYSTEM`.
+Each subsequent coprocessor in the list will have its priority value incremented by one (which
+reduces its priority, because priorities have the natural sort order of Integers).
+
++
+These priority values can be manually overriden in hbase-site.xml. This can be useful if you
+want to guarantee that a coprocessor will execute after another. For example, in the following
+configuration `SumEndPoint` would be guaranteed to go last, except in the case of a tie with
+another coprocessor:
++
+[source,xml]
+----
+<property>
+    <name>hbase.coprocessor.region.classes</name>
+    <value>org.myname.hbase.coprocessor.endpoint.SumEndPoint|2147483647</value>
+</property>
+----
 When calling out to registered observers, the framework executes their callbacks methods in the sorted order of their priority.
 Ties are broken arbitrarily.