sharded-counter-java/src/ShardedCounterV1.java

   1 import com.google.appengine.api.datastore.DatastoreService;
   2 import com.google.appengine.api.datastore.DatastoreServiceFactory;
   3 import com.google.appengine.api.datastore.Entity;
   4 import com.google.appengine.api.datastore.EntityNotFoundException;
   5 import com.google.appengine.api.datastore.Key;
   6 import com.google.appengine.api.datastore.KeyFactory;
   7 import com.google.appengine.api.datastore.Query;
   8 import com.google.appengine.api.datastore.Transaction;
   9
  10 import java.util.Random;
  11
  12 /**
  13  * This initial implementation simply counts all instances of the
  14  * SimpleCounterShard kind in the datastore. The only way to increment the
  15  * number of shards is to add another shard by creating another entity in the
  16  * datastore.
  17  */
  18 public class ShardedCounterV1 {
  19
  20   private static final DatastoreService ds = DatastoreServiceFactory
  21       .getDatastoreService();
  22
  23   /**
  24    * Default number of shards.
  25    */
  26   private static final int NUM_SHARDS = 20;
  27
  28   /**
  29    * A random number generating, for distributing writes across shards.
  30    */
  31   private Random generator = new Random();
  32
  33   /**
  34    * Retrieve the value of this sharded counter.
  35    *
  36    * @return Summed total of all shards' counts
  37    */
  38   public long getCount() {
  39     long sum = 0;
  40
  41     Query query = new Query("SimpleCounterShard");
  42     for (Entity e : ds.prepare(query).asIterable()) {
  43       sum += (Long) e.getProperty("count");
  44     }
  45
  46     return sum;
  47   }
  48
  49   /**
  50    * Increment the value of this sharded counter.
  51    */
  52   public void increment() {
  53     int shardNum = generator.nextInt(NUM_SHARDS);
  54     Key shardKey = KeyFactory.createKey("SimpleCounterShard",
  55         Integer.toString(shardNum));
  56
  57     Transaction tx = ds.beginTransaction();
  58     Entity shard;
  59     try {
  60       shard = ds.get(tx, shardKey);
  61       long count = (Long) shard.getProperty("count");
  62       shard.setUnindexedProperty("count", count + 1L);
  63     } catch (EntityNotFoundException e) {
  64       shard = new Entity(shardKey);
  65       shard.setUnindexedProperty("count", 1L);
  66     }
  67     ds.put(tx, shard);
  68     tx.commit();
  69   }
  70 }