db.test.insert( {count: 1}, {writeConcern: {w: "majority"}})
db.test.insert( {count: 1}, {writeConcern: {w: 3 }})
db.test.insert( {count: 1}, {writeConcern: {w: 4 }})

通过 MongoDB 的连接串参数：
• mongodb://host1:27107,host2:27107,host3:27017/?replicaSet=rs&readPreference=secondary
通过 MongoDB 驱动程序 API：
• MongoCollection.withReadPreference(ReadPreference readPref)
Mongo Shell：
• db.collection.find({}).readPref( “secondary” ) 

注意事项：
• 虽然看上去总是应该选择 local，但毕竟对结果集进行过滤会造成额外消耗。在一些无关紧要的场景（例如统计）下，也可以考虑 available；
• MongoDB <=3.6 不支持对从节点使用 {readConcern: "local"}；
• 从主节点读取数据时默认 readConcern 是 local，从从节点读取数据时默认readConcern 是 available（向前兼容原因）。

#注意配置文件内server参数开启 
enableMajorityReadConcern:ture
#这种方式读取不到刚写入的数据
db.orders.insert({ oid: 101, sku: ”kite", q: 1})  //主节点写入
db.orders.find({oid:101}).readPref("secondary")   //从节点读取

#使用 writeConcern + readConcern majority来解决 
db.orders.insert({ oid: 101, sku: "kiteboar", q: 1}, {writeConcern:{w: "majority”}})   //主节点写入
db.orders.find({oid:101}).readPref(“secondary”).readConcern("majority") 　　　　　　　　//从节点读取

final MongoClient client = MongoClients.create(uri);
/*
    Prereq: Create collections. CRUD operations in transactions must be on existing collections.
 */
client.getDatabase("mydb1").getCollection("foo")
        .withWriteConcern(WriteConcern.MAJORITY).insertOne(new Document("abc", 0));
client.getDatabase("mydb2").getCollection("bar")
        .withWriteConcern(WriteConcern.MAJORITY).insertOne(new Document("xyz", 0));
/* Step 1: Start a client session. */
final ClientSession clientSession = client.startSession();
/* Step 2: Optional. Define options to use for the transaction. */
TransactionOptions txnOptions = TransactionOptions.builder()
        .readPreference(ReadPreference.primary())
        .readConcern(ReadConcern.LOCAL)
        .writeConcern(WriteConcern.MAJORITY)
        .build();

/* Step 3: Define the sequence of operations to perform inside the transactions. */
TransactionBody txnBody = new TransactionBody<String>() {
    public String execute() {
        MongoCollection<Document> coll1 = client.getDatabase("mydb1").getCollection("foo");
        MongoCollection<Document> coll2 = client.getDatabase("mydb2").getCollection("bar");
        /*
           Important:: You must pass the session to the operations.
         */
        coll1.insertOne(clientSession, new Document("abc", 1));
        coll2.insertOne(clientSession, new Document("xyz", 999));
        return "Inserted into collections in different databases";
    }
};
try {
    /*
       Step 4: Use .withTransaction() to start a transaction,
       execute the callback, and commit (or abort on error).
    */

    clientSession.withTransaction(txnBody, txnOptions);
} catch (RuntimeException e) {
    // some error handling
} finally {
    clientSession.close();
}

//MongoDB 多文档事务的使用方式与关系数据库非常相似：
try (ClientSession clientSession = client.startSession()) {
    clientSession.startTransaction();
    collection.insertOne(clientSession, docOne);
    collection.insertOne(clientSession, docTwo);
    clientSession.commitTransaction();
}