数据库恢复服务 微信: dbservice1234

Mongolab目前提供的免费的MongoDB实例供广大用户测试使用，免费实例的限制是最大500MB。

首先到https://mongolab.com 注册一个帐号，之后Login登录， 点击create new

plan中要选择single-node才可以看到free plan的sanbox , cloud provide直接选Amazon AWS

之后点击create new mongodb deployment即可

需要手动创建一个mongodb用户：

之后就可以利用该云端mongodb测试实例了：

例如 使用mongo shell连接

[root@ocm ~]# mongo ds041432.mongolab.com:41432/dbdao -u dbdao -p dbdao
MongoDB shell version: 3.0.3
connecting to: ds041432.mongolab.com:41432/dbdao
rs-ds041432:PRIMARY> 

rs-ds041432:PRIMARY> 
rs-ds041432:PRIMARY> 

rs-ds041432:PRIMARY> db.dbdao.insert({"hello":"world!"});
WriteResult({ "nInserted" : 1 })
rs-ds041432:PRIMARY> db.dbdao.find();
{ "_id" : ObjectId("55672f4076e23efacff7cc07"), "hello" : "world!" }

C100DBA认证讲解 MongoDB replication 原理

题目正文：

which of following is true of mechanics of replication in MongoDB?

1. Clients read from nearest member of a replica set by default
2. Members of a replica set may replicate data from any other data-bearing member of set .
3. Operations on the primary are recorded in a capped collection called oplog

翻译：

下面关于Mongodb的replication复制技术细节描述正确的是？

1. 复制集成员会默认从复制集中最近的成员哪里读取数据
2. replica set 复制集中的成员可能从其他任何承载数据的节点上复制数据
3. 在primary上的操作将会被记录到一个capped collection(受限集合)中称之为oplong

Replica set members replicate data continuously after the initial sync. This process keeps the members up to date with all changes to the replica set’s data. In most cases, secondaries synchronize from the primary.

http://docs.mongodb.org/manual/core/replica-set-sync/

==>默认情况下从Primary同步数据，而不是复制集中最近的成员

For a member to sync from another, both members must have the same value for the buildIndexessetting.

Beginning in version 2.2, secondaries avoid syncing from delayed members and hidden members.

从2.2开始secondaries会避免从延迟成员和隐藏成员哪里同步数据，所以复制集中的成员可能从其他任何承载数据的节点上复制数据是错误的

The oplog (operations log) is a special capped collection that keeps a rolling record of all operations that modify the data stored in your databases. MongoDB applies database operations on the primary and then records the operations on the primary’s oplog. The secondary members then copy and apply these operations in an asynchronous process. All replica set members contain a copy of the oplog, in thelocal.oplog.rs collection, which allows them to maintain the current state of the database.

http://docs.mongodb.org/manual/core/replica-set-oplog/

选项Operations on the primary are recorded in a capped collection called oplog是正确的。

C100DBA认证讲解 MongoDB 讲解-quick initiating new replica set member?

题目正文

which of the following is the recommended method for quickly initiating a new replica set member?

1. Empty the dbpath directory for the new member to force an initial sync
2. Mongodump a existing replica set member’s data and then mongorestore it to the new member
3. Use the seed command
4. Step up the new member to be the replica set primary
5. Use a recent file system snapshot of an existing member with journaling turned on ,assuming the data files in the snapshot are more recent than the oldest operation in the primary’s oplog .

翻译：

以下哪个选项是被推荐的初始化一个新的replica set 复制集成员的？

1. 为新成员清空dbpath路径下的文件，强制一个初始化同步
2. mongodump一个replica set Member复制集成员的数据，并mongorestore到新成员
3. 使用seed 命令
4. 将新成员配置成replica set中的primary
5. 使用一个现有成员的文件系统快照且需要启用了journaling ,假设快照中的数据文件要比primary的oplog中的最老的操作更新

官方文档对应的解释：

• Initial sync occurs when MongoDB creates new databases on a new or restored member, populating the member with the replica set’s data. When a new or restored member joins or rejoins a set, the member waits to receive heartbeats from other members. By default, the member syncs from theclosest member of the set that is either the primary or another secondary with more recent oplog entries. This prevents two secondaries from syncing from each other.
• Replication occurs continually after initial sync and keeps the member updated with changes to the replica set’s data.

http://docs.mongodb.org/v2.2/core/replication-internals/

Initial Sync

Initial sync copies all the data from one member of the replica set to another member. A member uses initial sync when the member has no data, such as when the member is new, or when the member has data but is missing a history of the set’s replication.

When you perform an initial sync, MongoDB:

1. Clones all databases. To clone, the mongod queries every collection in each source database and inserts all data into its own copies of these collections. At this time, _id indexes are also built. The clone process only copies valid data, omitting invalid documents.

2. Applies all changes to the data set. Using the oplog from the source, the mongod updates its data set to reflect the current state of the replica set.

3. Builds all indexes on all collections (except _id indexes, which were already completed).

   When the mongod finishes building all index builds, the member can transition to a normal state, i.e.secondary.

http://docs.mongodb.org/manual/core/replica-set-sync/

mongolab提供了一个flip-flop 正反器的replica set复制集演示，其通过一个真实的replica set来演示自动的failover。 该真实环境包括三个节点：replica member “flip”和”flop”。每60秒，primary将会自己让自己不可用”step-down”，之后集群将failover到另一个节点上，60秒后再反转回来。

URL: http://mongolab.org/flip-flop/

同时mongolab还提供了一个可视化的replica set  的election过程

样例日志：

Logs will start streaming when there is replica set activity. Please wait...

arbiter: Sun May 24 20:43:03.373 [rsHealthPoll] replSet member flop.mongolab.com:54117 is now in state SECONDARY
arbiter: Sun May 24 20:43:07.386 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state PRIMARY
arbiter: Sun May 24 20:44:03.460 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state SECONDARY
arbiter: Sun May 24 20:44:09.545 [rsHealthPoll] replSet member flop.mongolab.com:54117 is now in state PRIMARY
arbiter: Sun May 24 20:45:03.641 [rsHealthPoll] replSet member flop.mongolab.com:54117 is now in state SECONDARY
arbiter: Sun May 24 20:45:13.669 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state PRIMARY
arbiter: Sun May 24 20:46:01.764 [rsHealthPoll] DBClientCursor::init call() failed
arbiter: Sun May 24 20:46:01.804 [rsHealthPoll] replSet info flip.mongolab.com:53117 is down (or slow to respond): DBClientBase::findN: transport error: flip.mongolab.com:53117 ns: local.$cmd query: { getnonce: 1 }
arbiter: Sun May 24 20:46:01.805 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state DOWN
arbiter: Sun May 24 20:46:03.815 [rsHealthPoll] replSet member flip.mongolab.com:53117 is up
arbiter: Sun May 24 20:46:03.816 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state SECONDARY
arbiter: Sun May 24 20:46:07.774 [rsHealthPoll] replSet member flop.mongolab.com:54117 is now in state PRIMARY
arbiter: Sun May 24 20:47:03.863 [rsHealthPoll] replSet member flop.mongolab.com:54117 is now in state SECONDARY
Sun May 24 20:47:03.863 [rsMgr] replSet I don't see a primary and I can't elect myself
arbiter: Sun May 24 20:47:13.910 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state PRIMARY
arbiter: Sun May 24 20:48:03.990 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state SECONDARY
arbiter: Sun May 24 20:48:07.992 [rsHealthPoll] replSet member flop.mongolab.com:54117 is now in state PRIMARY
arbiter: Sun May 24 20:49:04.080 [rsHealthPoll] replSet member flop.mongolab.com:54117 is now in state SECONDARY
arbiter: Sun May 24 20:49:14.141 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state PRIMARY
arbiter: Sun May 24 20:50:04.252 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state SECONDARY
arbiter: Sun May 24 20:50:14.265 [rsHealthPoll] replSet member flop.mongolab.com:54117 is now in state PRIMARY
arbiter: Sun May 24 20:51:04.350 [rsHealthPoll] replSet member flop.mongolab.com:54117 is now in state SECONDARY
arbiter: Sun May 24 20:51:14.456 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state PRIMARY
arbiter: Sun May 24 20:52:04.569 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state SECONDARY
arbiter: Sun May 24 20:52:08.573 [rsHealthPoll] replSet member flop.mongolab.com:54117 is now in state PRIMARY
arbiter: Sun May 24 20:53:02.660 [rsHealthPoll] replSet member flop.mongolab.com:54117 is now in state SECONDARY
arbiter: Sun May 24 20:53:08.675 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state PRIMARY
arbiter: Sun May 24 20:54:02.839 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state SECONDARY
arbiter: Sun May 24 20:54:12.951 [rsHealthPoll] replSet member flop.mongolab.com:54117 is now in state PRIMARY
arbiter: Sun May 24 20:55:03.255 [rsHealthPoll] replSet member flop.mongolab.com:54117 is now in state SECONDARY
arbiter: Sun May 24 20:55:08.948 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state PRIMARY
arbiter: Sun May 24 20:56:03.017 [rsHealthPoll] replSet member flip.mongolab.com:53117 is now in state SECONDARY
arbiter: Sun May 24 20:56:09.024 [rsMgr] replSet I don't see a primary and I can't elect myself

flip-flop 的初衷是为了可视化演示复制集的election过程，让开发者能更清楚了解这一过程，以便能写出更好的客户端连接代码。同时mongolab还给出了一个连接URL:

mongodb://testdbuser:testdbpass@flip.mongolab.com:53117,flop.mongolab.com:54117/testdb

C100DBA MongoDB 讲解 replica set vote

题目中文：

Given a replica set with five data-bearing members, suppose the primary goes down with operations in its oplog that have been copied to only one secondary .

Assuming no other problems occur, which of the following describes what is most likely to happen?

1. The primary will rollback the operations
2. The secondary with the most recent oplog will be elected primary
3. The most recent secondary will roll back the operations following the election
4. Reads will stale until primary comes backup up

翻译：

对于一个有五个承载数据的成员的replica set 而言，假设primary服务器宕机了，且其oplog中最新的操作信息只成功拷贝到一个secondary上了。

假设没有其他问题发生，以下哪个选项是正确的？

1. Primary将在恢复时回滚部分操作
2. 拥有最新的oplog的这个secondary服务器将赢得election并成为primary
3. 在election后最新的这个secondary会回滚部分操作
4. 直到primary重新可用，之前读操作将读到过时的数据

关于此问题 官方文档有较明确解释：

A rollback does not occur if the write operations replicate to another member of the replica set before the primary steps down and if that member remains available and accessible to a majority of the replica set.     http://docs.mongodb.org/manual/core/replica-set-rollbacks/

只要primary上的写操作已经复制到某个secondary成员上，secondary成员可用，就可以保证rollback不会发生。