[GitHub] [helix] Marcosrico commented on a diff in pull request #2515: Distributed Semaphore Implementation

["Marcosrico (via GitHub)" <gi...@apache.org>]

Marcosrico commented on code in PR #2515:
URL: https://github.com/apache/helix/pull/2515#discussion_r1213816608


##########
meta-client/src/main/java/org/apache/helix/metaclient/recipes/lock/DistributedSemaphore.java:
##########
@@ -0,0 +1,174 @@
+package org.apache.helix.metaclient.recipes.lock;
+
+/*
+ * 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
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import org.apache.helix.metaclient.api.MetaClientInterface;
+import org.apache.helix.metaclient.datamodel.DataRecord;
+import org.apache.helix.metaclient.exception.MetaClientException;
+import org.apache.helix.metaclient.factories.MetaClientConfig;
+import org.apache.helix.metaclient.impl.zk.factory.ZkMetaClientConfig;
+import org.apache.helix.metaclient.impl.zk.factory.ZkMetaClientFactory;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.concurrent.TimeUnit;
+
+public class DistributedSemaphore {
+  private final MetaClientInterface<DataRecord> _metaClient;
+  private final String _path;
+  private static final String INITIAL_CAPACITY_NAME = "INITIAL_CAPACITY";
+  private static final String REMAINING_CAPACITY_NAME = "REMAINING_CAPACITY";
+  private static final long DEFAULT_REMAINING_CAPACITY = -1;
+  private static final Logger LOG = LoggerFactory.getLogger(DistributedSemaphore.class);
+
+  public DistributedSemaphore(MetaClientConfig config, String path, int capacity) {
+    if (MetaClientConfig.StoreType.ZOOKEEPER.equals(config.getStoreType())) {
+      ZkMetaClientConfig zkMetaClientConfig = new ZkMetaClientConfig.ZkMetaClientConfigBuilder()
+          .setConnectionAddress(config.getConnectionAddress())
+          .setZkSerializer(new DataRecordSerializer()) // Currently only support ZNRecordSerializer.
+          // Setting DataRecordSerializer as DataRecord extends ZNRecord.
+          .build();
+      _metaClient = new ZkMetaClientFactory().getMetaClient(zkMetaClientConfig);
+      _metaClient.connect();
+    } else {
+      throw new MetaClientException("Unsupported store type: " + config.getStoreType());
+    }
+
+    _path = path;
+    if (_metaClient.exists(path) == null) {
+      DataRecord dataRecord = new DataRecord(path);
+      dataRecord.setLongField(INITIAL_CAPACITY_NAME, capacity);
+      dataRecord.setLongField(REMAINING_CAPACITY_NAME, capacity);
+      _metaClient.create(path, dataRecord);
+    }
+  }
+
+  public DistributedSemaphore(MetaClientInterface<DataRecord> client, String path) {
+    _metaClient = client;
+    _path = path;
+    _metaClient.connect();
+  }
+
+  /**
+   * Acquire a permit. If no permit is available, block until when it was able to acquire.
+   * @return a permit
+   */
+  public Permit acquire() {
+    if (getRemainingCapacity() > 0) {
+      updateAcquirePermit(1);
+      return retrievePermit(_path);
+    } else {
+      throw new MetaClientException("No sufficient permits available");
+    }
+  }
+
+
+  /**
+   * Try to acquire a permit. If not enough permit is available, wait for a specific time or return when it was able to acquire.
+   * @param count number of permits to acquire
+   * @return a collection of permits
+   */
+  public Collection<Permit> acquire(int count) {
+    if (getRemainingCapacity() < count) {
+      throw new MetaClientException("No sufficient permits available");
+    } else {
+      updateAcquirePermit(count);
+      Collection<Permit> permits = new ArrayList<>();
+      for (int i = 0; i < count; i++) {
+        permits.add(retrievePermit(_path));
+      }
+      return permits;
+    }
+  }
+
+  /**
+   * Try to acquire a permit. If no enough permit is available, wait for a specific time or return when it was able to acquire.

Review Comment:
   Oh i see, then I must have gotten both acquire and tryAcquire mixed up. Should acquire then be blocking?



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