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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
$group
stage of thedb.collection.aggregate()
method to$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);
}