在MongoDB里实现循环序列功能
db.counters.save({_id:"SerialNo1", val:0, maxval:99})
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然后我们想system.js里添加一个Javascript函数
db.system.js.save({_id:"getNextUniqueSeq",value:function (keyName) { var seqObj = db.counters.findOne({_id:keyName}); if (seqObj == null) {print("can not find record with key: " + keyName); return -1; } // the max value of sequence var maxVal = seqObj.maxval; // the current value of sequence var curVal = seqObj.val;while(true){// if curVal reach max, reset itif(curVal >= maxVal){db.counters.update({_id : keyName, val : curVal}, { $set : { val : 0 }}, false, false);var err = db.getLastErrorObj();if( err && err.code ) {print( "unexpected error reset data: " + tojson( err ) ); return -2; } else if (err.n == 0){// fail to reset value, may be reseted by othersprint("fail to reset value: ");} // get current value again.seqObj = db.counters.findOne({_id:keyName});maxVal = seqObj.maxval;curVal = seqObj.val;continue;} // if curVal not reach the max, inc it;// increase db.counters.update({_id : keyName, val : curVal}, { $inc : { val : 1 }}, false, false);var err = db.getLastErrorObj();if( err && err.code ) {print( "unexpected error inc val: " + tojson( err ) ); return -3; } else if (err.n == 0){// fail to reset value, may be increased by othersprint("fail to inc value: ");// get current value again.seqObj = db.counters.findOne({_id:keyName});maxVal = seqObj.maxval;curVal = seqObj.val;continue;} else {var retVal = curVal + 1;print("success to get seq : " + retVal);// increase successfulreturn retVal;}}}});上面这段会把指定的序列号的val值+1，如果val达到上限则从0开始。所以叫循环序列。
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其实上面的实现在原理上和Java里的AtomicInteger系列的功能实现是类似的，利用循环重试和原子性的CAS来实现。这种实现方式在多线程的环境里由于锁（Monitor）的范围很小，所以并发性上比排他锁要好一些。
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下面我们用Java来测试一下这个函数的正确性。 即在多线程的情况下会不会得到重复的序列号。
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第一个测试，val=0， maxval=2000， Java端20个线程每个线程循环调用100次。 共2000次。 所以正确的情况下，从0到1999应该每个数字只出现一次。
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@Test public void testGetNextUniqueSeq1() throws Exception { final int THREAD_COUNT = 20; final int LOOP_COUNT = 100; Mongo mongoClient = new Mongo("172.17.2.100", 27017); DB db = mongoClient.getDB("im"); db.authenticate("imadmin", "imadmin".toCharArray()); BasicDBObject q = new BasicDBObject(); q.put("_id", "UNIQUE_KEY"); BasicDBObject upd = new BasicDBObject(); BasicDBObject set = new BasicDBObject(); set.put("val", 0); set.put("maxval", THREAD_COUNT * LOOP_COUNT); upd.put("$set", set); db.getCollection("counters").update(q, upd); Thread[] threads = new Thread[THREAD_COUNT]; final int[][] results = new int[THREAD_COUNT][LOOP_COUNT]; for (int i = 0; i < THREAD_COUNT; i++) { final int temp_i = i; threads[i] = new Thread("" + i) { @Override public void run() { try { Mongo mongoClient = new Mongo("172.17.2.100", 27017); DB db = mongoClient.getDB("im"); db.authenticate("imadmin", "imadmin".toCharArray()); for (int j = 0; j < LOOP_COUNT; j++) { Object result = db.eval("getNextUniqueSeq("UNIQUE_KEY")"); System.out.printf("Thread %s, seq=%d\n", Thread.currentThread().getName(), ((Double) result).intValue()); results[temp_i][j] = ((Double) result).intValue(); } } catch (UnknownHostException e) { e.printStackTrace(); } } }; } for (Thread thread : threads) { thread.start(); } for (Thread thread : threads) { thread.join(); } for (int num = 1; num <= LOOP_COUNT * THREAD_COUNT; num++) { // every number appear 1 times only! int times = 0; for (int j = 0; j < THREAD_COUNT; j++) { for (int k = 0; k < LOOP_COUNT; k++) { if (results[j][k] == num) times++; } } assertEquals(1, times); } }?
然后我们再测试一下循环的情况。 val=0, maxval=99。 同样是Java端20个线程每个线程循环调用100次。 共2000次。这次从0到99的数字每个应该取得20次。
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@Test public void testGetNextUniqueSeq2() throws Exception { final int THREAD_COUNT = 20; final int LOOP_COUNT = 100; Mongo mongoClient = new Mongo("172.17.2.100", 27017); DB db = mongoClient.getDB("im"); db.authenticate("imadmin", "imadmin".toCharArray()); BasicDBObject q = new BasicDBObject(); q.put("_id", "UNIQUE_KEY"); BasicDBObject upd = new BasicDBObject(); BasicDBObject set = new BasicDBObject(); set.put("val", 0); set.put("maxval", LOOP_COUNT); upd.put("$set", set); db.getCollection("counters").update(q, upd); Thread[] threads = new Thread[THREAD_COUNT]; final int[][] results = new int[THREAD_COUNT][LOOP_COUNT]; for (int i = 0; i < THREAD_COUNT; i++) { final int temp_i = i; threads[i] = new Thread("" + i) { @Override public void run() { try { Mongo mongoClient = new Mongo("172.17.2.100", 27017); DB db = mongoClient.getDB("im"); db.authenticate("imadmin", "imadmin".toCharArray()); for (int j = 0; j < LOOP_COUNT; j++) { Object result = db.eval("getNextUniqueSeq("UNIQUE_KEY")"); System.out.printf("Thread %s, seq=%d\n", Thread.currentThread().getName(), ((Double) result).intValue()); results[temp_i][j] = ((Double) result).intValue(); } } catch (UnknownHostException e) { e.printStackTrace(); } } }; } for (Thread thread : threads) { thread.start(); } for (Thread thread : threads) { thread.join(); } for (int num = 1; num <= LOOP_COUNT; num++) { // every number appear 20 times only! int times = 0; for (int j = 0; j < THREAD_COUNT; j++) { for (int k = 0; k < LOOP_COUNT; k++) { if (results[j][k] == num) times++; } } assertEquals(20, times); } }?
这个测试跑了几次都是正确的。
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由于没有可以进行对比其他的实现方式（例如排他锁）所以没有做性能测试。
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写在最后。 虽然MongoDB支持类似于存储过程的Stored Javascript，但是其实不建议使用这个来解决复杂问题。主要原因是没法调试，维护起来太不方便。而且在2.4之前MongoDB对服务端Javascript支持并不是很好， 一个mongod进程同时只能执行一段Javascript。如果能在应用层解决掉还是在应用层里实现逻辑比较好。