HBASE-20839 Fallback to FSHLog if we can not instantiated AsyncFSWAL when user does not specify AsyncFSWAL explicitly

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/AsyncFSWAL.java

@@ -1,4 +1,4 @@
-/*
+/**
  * Licensed to the Apache Software Foundation (ASF) under one
  * or more contributor license agreements.  See the NOTICE file
  * distributed with this work for additional information

hbase-server/src/main/java/org/apache/hadoop/hbase/wal/AsyncFSWALProvider.java

@@ -22,6 +22,9 @@ import java.io.IOException;
 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.fs.FileSystem;
 import org.apache.hadoop.fs.Path;
+import org.apache.hadoop.hbase.io.asyncfs.FanOutOneBlockAsyncDFSOutput;
+import org.apache.hadoop.hbase.io.asyncfs.FanOutOneBlockAsyncDFSOutputHelper;
+import org.apache.hadoop.hbase.io.asyncfs.FanOutOneBlockAsyncDFSOutputSaslHelper;
 import org.apache.hadoop.hbase.regionserver.wal.AsyncFSWAL;
 import org.apache.hadoop.hbase.regionserver.wal.AsyncProtobufLogWriter;
 import org.apache.hadoop.hbase.regionserver.wal.WALUtil;
@@ -123,4 +126,18 @@ public class AsyncFSWALProvider extends AbstractFSWALProvider<AsyncFSWAL> {
       throw new IOException("cannot get log writer", e);
     }
   }
+
+  /**
+   * Test whether we can load the helper classes for async dfs output.
+   */
+  public static boolean load() {
+    try {
+      Class.forName(FanOutOneBlockAsyncDFSOutput.class.getName());
+      Class.forName(FanOutOneBlockAsyncDFSOutputHelper.class.getName());
+      Class.forName(FanOutOneBlockAsyncDFSOutputSaslHelper.class.getName());
+      return true;
+    } catch (Throwable e) {
+      return false;
+    }
+  }
 }

hbase-server/src/main/java/org/apache/hadoop/hbase/wal/WALFactory.java

@@ -123,7 +123,19 @@ public class WALFactory {
   @VisibleForTesting
   public Class<? extends WALProvider> getProviderClass(String key, String defaultValue) {
     try {
-      return Providers.valueOf(conf.get(key, defaultValue)).clazz;
+      Providers provider = Providers.valueOf(conf.get(key, defaultValue));
+      if (provider != Providers.defaultProvider) {
+        // User gives a wal provider explicitly, just use that one
+        return provider.clazz;
+      }
+      // AsyncFSWAL has better performance in most cases, and also uses less resources, we will try
+      // to use it if possible. But it deeply hacks into the internal of DFSClient so will be easily
+      // broken when upgrading hadoop. If it is broken, then we fall back to use FSHLog.
+      if (AsyncFSWALProvider.load()) {
+        return AsyncFSWALProvider.class;
+      } else {
+        return FSHLogProvider.class;
+      }
     } catch (IllegalArgumentException exception) {
       // Fall back to them specifying a class name
       // Note that the passed default class shouldn't actually be used, since the above only fails