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mongodb的读写分离使用Replica Sets来实现: om.mongodb.MongoException: not talking to master and retries used up第一种方法：在java代码中调用dbFactory.getDb().slaveOk(); 第二种方法：在java代码中调用 dbFactory.getDb().setReadPreference(ReadPreference.secondaryPreferred());//在复制集中优先读secondary，如果secondary访问不了的时候就从master中读

Convert转变a Secondary to an Arbiter: 如果你的副本集一个次副本节点不需要保持数据了，但是为了选举主副本，需要把次副本转变成arbiter 1、修改程序以致以不能从这个次副本查询数据。比如改变读取策略 2、停止次副本 3、在主副本中执行rs.remove("<:port>") 4 mv mongodb mongodb-old 5 mkdir mongodb 6 mongod --port 27021 --dbpath /data/db --replSet rs 7 rs.addArb("host2:27017") 8 验证rs.conf()

mongostat-Failed: error connecting to db server: no reachable servers: Failed: error connecting to db server: no reachable serversmongodb没有部署副本集的时候执行mongostat查看mongodb状态的时候没有问题。最近将mongodb部署成了副本集，再次执行mongostat的时候，就报错了。解决办法指定要查看mongodb的ip

如何设置android studio让程序运行在真机中: 点击菜单栏的“Run”->“Edit Configurations...”。然后会打开Run/Debug Configuration窗口。在窗口右侧找到“target device”部分，勾选“USB device”前面的单选框。点击“ok”。

Deploy a Replica Set部署副本集: Three member replica sets provide enough redundancy冗余 to survive 生存most network partitions and other system failures. These sets also have sufficient capacity 充足能力 for many distributed read operations. Replica sets should always have an odd number of members副本集应该总是有一个奇数的成员. This ensures that elections will proceed smoothly 顺利. For more about designing replica sets, see the Replication overview.

使用BigDecimal保留小数点位数不对: 数据格式化1.545使用下面方法格式化结果是1.54，这个四舍五入的结果不对啊。发现BigDecimal bg = new BigDecimal(d)，bg值是1.544999999999999. 2位四舍五入结构是1.54.这不科学啊。

mongodb Create a User Administrator: use admindb.createUser( { user: "siteUserAdmin", pwd: "password", roles: [ { role: "userAdminAnyDatabase", db: "admin" } ] })

Deploy a Replica Set for Testing and Development单机器测试副本集: 在一台电脑测试副本集，副本集成员开启不同的端口号27017，设置不同的mongodb数据路径

Read Preference: By default, an application directs its read operations to the primary member in a replica set. Because write operations are issued to the single primary, reading from the primary returns the latest version of a document[1]. For an application that does not require fully up-to-date data, you can improve read throughput or reduce latency by distributing some or all reads to secondary members of the replica set.

Write Concern for Replica Sets: For a replica set, the default write concern confirms write operations only on the primary. You can, however, override this default write concern, such as to confirm write operations on a specified number of the replica set members.

Replica Set Read and Write Semantics: Users may configure read preference on a per-connection basis to prefer that the read operations return results from the secondary members. If clients configure the read preference to permit 许可证secondary reads, read operations can return data from secondary members that have not replicated more recent write operations 没有复制的最近的写操作

Rollbacks During Replica Set Failover: Rollback Limitations 大于300M数据，您必须手动介入以恢复数据

Factors and Conditions that Affect Elections影响选举的因素和条件: Heartbeats心跳 10s Priority Comparisons优先级比较 Network Partitions网络分区

Three Member Replica Sets: The minimum architecture of a replica set has three members. A three member replica set can have either three members that hold data, or two members that hold data and an arbiter.

Replica Set Deployment Architectures: The architecture of a replica set affects the set’s capacity and capability. This document provides strategies for replica set deployments and describes common architectures. The standard replica set deployment for production system is a three-member replica set. These sets provide redundancy and fault tolerance. Avoid complexity when possible, but let your application requirements dictate the architecture.

Replica Set Arbiter: 只能把arbiter 增加到偶数个成员的副本集，如果你把arbiter 增加到奇数成员的副本集中，可能造成捆绑选举，造成选择不出primary副本集。

Delayed Replica Set Members: Delayed members contain copies of a replica set’s data set. However, a delayed member’s data set reflects an earlier, or delayed, state of the set. For example, if the current time is 09:52 and a member has a delay of an hour, the delayed member has no operation more recent than 08:52.

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