Keep at least one DataSource in the pool when update by events.

src/main/java/com/alibaba/druid/pool/ha/HighAvailableDataSource.java

@@ -97,6 +97,7 @@ public class HighAvailableDataSource extends WrapperAdapter implements DataSourc
     private String dataSourceFile = DEFAULT_DATA_SOURCE_FILE;
     private String propertyPrefix = "";
     private int poolPurgeIntervalSeconds = PoolUpdater.DEFAULT_INTERVAL;
+    private boolean allowEmptyPoolWhenUpdate = false;
 
     private volatile boolean inited = false;
 
@@ -113,6 +114,7 @@ public class HighAvailableDataSource extends WrapperAdapter implements DataSourc
             }
             if (dataSourceMap == null || dataSourceMap.isEmpty()) {
                 poolUpdater.setIntervalSeconds(poolPurgeIntervalSeconds);
+                poolUpdater.setAllowEmptyPool(allowEmptyPoolWhenUpdate);
                 poolUpdater.init();
                 createNodeMap();
             }
@@ -304,6 +306,14 @@ public class HighAvailableDataSource extends WrapperAdapter implements DataSourc
         nodeListener.update(); // Do update in the current Thread at the startup
     }
 
+    public boolean isAllowEmptyPoolWhenUpdate() {
+        return allowEmptyPoolWhenUpdate;
+    }
+
+    public void setAllowEmptyPoolWhenUpdate(boolean allowEmptyPoolWhenUpdate) {
+        this.allowEmptyPoolWhenUpdate = allowEmptyPoolWhenUpdate;
+    }
+
     // Getters & Setters
 
     public int getPoolPurgeIntervalSeconds() {

src/main/java/com/alibaba/druid/pool/ha/node/PoolUpdater.java

@@ -50,6 +50,7 @@ public class PoolUpdater implements Observer {
     private ScheduledExecutorService executor;
     private int intervalSeconds = DEFAULT_INTERVAL;
     private volatile boolean inited = false;
+    private boolean allowEmptyPool = false;
 
     public PoolUpdater(HighAvailableDataSource highAvailableDataSource) {
         setHighAvailableDataSource(highAvailableDataSource);
@@ -213,6 +214,11 @@ public class PoolUpdater implements Observer {
         if (nodeName == null || nodeName.isEmpty() || !map.containsKey(nodeName)) {
             return;
         }
+        map = highAvailableDataSource.getAvailableDataSourceMap();
+        if (!allowEmptyPool && map.size() == 1 && map.containsKey(nodeName)) {
+            LOG.warn(nodeName + " is the only DataSource left, don't remove it.");
+            return;
+        }
         blacklistNode(nodeName);
     }
 
@@ -247,4 +253,12 @@ public class PoolUpdater implements Observer {
     public void setIntervalSeconds(int intervalSeconds) {
         this.intervalSeconds = intervalSeconds;
     }
+
+    public boolean isAllowEmptyPool() {
+        return allowEmptyPool;
+    }
+
+    public void setAllowEmptyPool(boolean allowEmptyPool) {
+        this.allowEmptyPool = allowEmptyPool;
+    }
 }

src/test/java/com/alibaba/druid/pool/ha/node/FileNodeListenerTest.java

@@ -55,11 +55,11 @@ public class FileNodeListenerTest {
         DruidDataSource foo = (DruidDataSource) dataSource.getAvailableDataSourceMap().get("foo");
         assertEquals("jdbc:derby:memory:foo;create=true", foo.getUrl());
 
-        // Remove all
+        // try to emove all but we have to keep one left
         writePropertiesFile(file, new Properties());
 
         Thread.sleep(6000);
-        assertTrue(dataSource.getAvailableDataSourceMap().isEmpty());
+        assertEquals(1, dataSource.getAvailableDataSourceMap().size());
 
         dataSource.destroy();
     }

src/test/java/com/alibaba/druid/pool/ha/node/PoolUpdaterTest.java

@@ -39,6 +39,7 @@ public class PoolUpdaterTest {
     @Test
     public void testUpdate() {
         haDataSource.getDataSourceMap().put("foo", new MockDataSource("foo"));
+        haDataSource.getDataSourceMap().put("bar", new MockDataSource("bar"));
 
         NodeEvent event = new NodeEvent();
         event.setNodeName("foo");
@@ -91,6 +92,18 @@ public class PoolUpdaterTest {
         assertTrue(haDataSource.getDataSourceMap().isEmpty());
     }
 
+    @Test
+    public void testUpdate_onlyOneLeftToRemove() {
+        haDataSource.getDataSourceMap().put("foo", new MockDataSource("foo"));
+
+        NodeEvent event = new NodeEvent();
+        event.setNodeName("foo");
+        event.setType(NodeEventTypeEnum.DELETE);
+
+        updater.update(new FileNodeListener(), new NodeEvent[] { event });
+        assertTrue(haDataSource.getDataSourceMap().containsKey("foo"));
+    }
+
     @Test
     public void testAddNode() {
         String url = "jdbc:derby:memory:foo;create=true";
@@ -127,6 +140,7 @@ public class PoolUpdaterTest {
     @Test
     public void testDeleteNode() {
         haDataSource.getDataSourceMap().put("foo", new MockDataSource("foo"));
+        haDataSource.getDataSourceMap().put("bar", new MockDataSource("bar"));
 
         NodeEvent event = new NodeEvent();
         event.setNodeName("foo");

src/test/java/com/alibaba/druid/pool/ha/node/ZookeeperNodeListenerTest.java

@@ -56,10 +56,11 @@ public class ZookeeperNodeListenerTest {
 
         // 2. Register one Node
         ZookeeperNodeRegister register = registerNodeAndReturnRegister();
+        ZookeeperNodeRegister register2 = registerAnotherNodeAndReturnRegister();
 
         Thread.sleep(3000); // Wait for the Node to be created.
         assertFalse(dataSource.getDataSourceMap().isEmpty());
-        assertEquals(1, dataSource.getAvailableDataSourceMap().size());
+        assertEquals(2, dataSource.getAvailableDataSourceMap().size());
         DruidDataSource ds = (DruidDataSource) dataSource.getAvailableDataSourceMap().get("foo.test-foo");
         assertEquals("jdbc:derby:memory:foo;create=true", ds.getUrl());
         assertNotNull(dataSource.getConnection());
@@ -70,15 +71,16 @@ public class ZookeeperNodeListenerTest {
         for (int i = 0; i < 10; i++) {
             Thread.sleep(500);
             if (dataSource.isInBlackList("foo.test-foo")) {
-                assertTrue(dataSource.getAvailableDataSourceMap().isEmpty());
+                assertEquals(1, dataSource.getAvailableDataSourceMap().size());
                 assertFalse(dataSource.getDataSourceMap().isEmpty());
-                assertNull(dataSource.getConnection());
+                assertNotNull(dataSource.getConnection());
                 checkBlackList = true;
                 break;
             }
         }
         assertTrue(checkBlackList);
-
+        register2.destroy();
+        Thread.sleep(3000); // Wait for the destory to be finished
         dataSource.destroy();
     }
 
@@ -187,6 +189,25 @@ public class ZookeeperNodeListenerTest {
         return register;
     }
 
+    private ZookeeperNodeRegister registerAnotherNodeAndReturnRegister() {
+        ZookeeperNodeRegister register = new ZookeeperNodeRegister();
+        register.setZkConnectString(server.getConnectString());
+        register.setPath(PATH);
+        register.init();
+
+        List<ZookeeperNodeInfo> payload = new ArrayList<ZookeeperNodeInfo>();
+        ZookeeperNodeInfo node = new ZookeeperNodeInfo();
+        node.setPrefix("bar");
+        node.setHost("127.0.0.1");
+        node.setPort(5678);
+        node.setDatabase("bar");
+        node.setUsername("sa");
+        node.setPassword("");
+        payload.add(node);
+        register.register("test-bar", payload);
+        return register;
+    }
+
     @Test
     public void testInitFailure() {
         ZookeeperNodeListener listener = new ZookeeperNodeListener();