MongoDB：count结果不准确的原因与解决方法

本人博客同步发表，排版更佳
教训：MongoDB在分片后的集合上进行db.collection.count()操作时，出现结果不准确的现象，需要采用聚合的方法获取集合的count结果
在使用MongoDB-Java客户端做简单的插入操作（10W条）以后，使用Studio 3T查看插入结果时，发现显示的count结果与插入的数据不一致，偶然会多出几条或十几条，插入操作很简单，其中table3被分片

//创建客户端连接
MongoClient mongoClient = new MongoClient( "xxx.xxx.xxx.xxx" , 20000 );
MongoDatabase database = mongoClient.getDatabase("testdb");
MongoCollection<Document> collection = database.getCollection("table3");

//插入文档
long start = System.currentTimeMillis();
Random r = new Random();
for (int i=0;i<100000;i++){
    Document doc = new Document("id", i)
            .append("type", "database"+i)
            .append("test","testval"+i)
            .append("name", "name"+i);

    System.out.println("insertOne = " + i);
    collection.insertOne(doc);
}
System.out.println("use time = " + (System.currentTimeMillis()- start));
System.out.println("count = " + collection.count());

发现问题后，通过在shell里面查询count，命令如下

db.table3.count()

使用该命令依然会出现统计信息不准确的现象，通过谷歌发现，官方文档——(https://docs.mongodb.com/manual/reference/method/db.collection.count/)中有解释这种现象：

• On a sharded cluster, db.collection.count() can result in an inaccurate count if orphaned documentsexist or if a chunk migration is in progress.

• To avoid these situations, on a sharded cluster, use the $group stage of the db.collection.aggregate() method to $sum the documents. For example, the following operation counts the documents in a collection:

db.collection.aggregate(
   [
      { $group: { _id: null, count: { $sum: 1 } } }
   ]
)

官方文档解释了这种现象的原因以及解决方法：
不准确的原因：

• 操作的是分片的集合（前提）；
• shard分片正在做块迁移，导致有重复数据出现
• 存在孤立文档（因为不正常关机、块迁移失败等原因导致）

解决方法
使用聚合aggregate的方式查询count数量，shell命令如下：

db.collection.aggregate(
   [
      { $group: { _id: null, count: { $sum: 1 } } }
   ]
)

java代码
所以在Java中也可以采用聚合的方式获取count结果，使用聚合aggregate的方式可以准确的获取sharding后的集合的count结果。

DBObject groupFields = new BasicDBObject("_id", null);
groupFields.put("count", new BasicDBObject("$sum", 1));
BasicDBObject group = new BasicDBObject("$group", groupFields);

List<BasicDBObject> aggreList = new ArrayList<BasicDBObject>();
aggreList.add(group);
AggregateIterable<Document> output = collection.aggregate(aggreList);

for (Document dbObject : output)
{
      System.out.println("Aggregates count = "+ dbObject);
}

完整代码

        //创建客户端连接
        MongoClient mongoClient = new MongoClient( "xxx.xxx.xxx.xxx" , 20000 );
        MongoDatabase database = mongoClient.getDatabase("testdb");
        MongoCollection<Document> collection = database.getCollection("table3");

        //插入文档
        long start = System.currentTimeMillis();
        Random r = new Random();
        for (int i=0;i<100000;i++){
            Document doc = new Document("id", i)
                    .append("type", "database"+i)
                    .append("test","testval"+i)
                    .append("name", "name"+i);

            System.out.println("insertOne = " + i);
            collection.insertOne(doc);
        }

        System.out.println("use time = " + (System.currentTimeMillis()- start));

        System.out.println("count = " + collection.count());

        DBObject groupFields = new BasicDBObject("_id", null);
        groupFields.put("count", new BasicDBObject("$sum", 1));
        BasicDBObject group = new BasicDBObject("$group", groupFields);

        List<BasicDBObject> aggreList = new ArrayList<BasicDBObject>();
        aggreList.add(group);
        AggregateIterable<Document> output = collection.aggregate(aggreList);

        for (Document dbObject : output)
        {
            System.out.println("Aggregates count = "+ dbObject);
        }