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DB Replication (HA)下部

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Network partitions

bdb replication 的实现可能被网络隔离的问题影响。


例如,考虑replication组有n个成员。网络隔离让master在一边,多于一半(n/2)的站点在另外一边。和master在一边的站点将继续前进,master继续接受数据库的写请求。不幸的是,隔离在另一边的站点,意思到他们的master不在了,将举行一个选举。这个选举将取得成功,因为这儿有总数n/2以上的站点在这边,然后这个组内将会有两个master。既然两个master都可能潜在地接受写请求,那么数据库将可能产生分歧,使得数据不一致。

如果曾经在一个组内发现了多个master,一个master检测到这个问题的时候将会返回 DB_REP_DUPMASTER。如果一个应用程序看到这个返回,它应该重新配置自己作为一个client(通过调用ENV-> rep_start),然后发起一场选举(通过调用DB_ENV->rep_elect)。赢得这次选举的可能是先前的两个master之一,也可能完全就是另外的站点。无论如何,这个胜出的系统将引导其它系统达到一致。

作为另外一个例子,考虑一个replication组有一个 master环境和两个client,A和B,在那A可能会升级为master地位而B不可能。然后,假设client A从其他的两个数据库环境中被隔离出来了,它的数据变的过期。然后假设这个master倒掉了,而且不再上线。随后,网络隔离被修复了,client A和B进行了一次选举。因为client B不能赢得选举,client A将会默认地赢得这次选举,为了重新和B同步,可能在B上提交的事务将不能回滚直到这两个站点能再次地一起前进。

在这两个例子中,都有一步就是新选举出的master引导组内的成员和它自己一致,以便它可以开始发送新信息给它们。这可能会丢失信息,因为以前提交的事务没有回滚。

在体系结构上网络隔离是个问题,应用程序可能想实现一个心跳协议以最小化一个糟糕的网络隔离的影响。只要一个master至少可以和组内一半的站点通信的时候,就不可能出现两个master。如果一个master不再能和足够的站点取得联系的时候,它应该重新配置自己作为一个client,和举行一次选举。

这儿有另外一个工具应用程序可以用来最小化网络隔离情况下的损失。通过指定一个 nsites 参数给DB_ENV->rep_elect ,也就是说,比组内的实际成员的数目大,应用程序可以阻止系统宣布他们自己成为master,除非它们可以和组内绝大部分站点通话。例如,如果组内有20 个数据库环境,把参数30指定给DB_ENV->rep_elect方法,那么这个系统至少要和16个站点通话才可以宣布自己为master。

指定一个小于组内世界成员数目的nsites参数给DB_ENV->rep_elect,也有它的用处。例如,考虑一个组有只有两个数据库环境。如果他们被隔离了,其中任何一个都不能取得足够的选票数成为master。一个合理的选择是,指定一个系统的nsites 参数为2,另一个为1。那样,当被隔离的时候,其中一个系统可以赢得选举权,而另一个不能。这能允许当网络被隔离的时候其中一个系统能继续接受写请求。

这些关卡强调了bdb replicated环境中好的网络底层构造的重要性。当replicating数据库环境在严重丢包的网络环境中,最好的解决可能是拣选一个单一的master,只有当人工干涉决定这个被选择的master不能再恢复上线时,才举行选举。

Replication FAQ


Does Berkeley DB provide support for forwarding write queries from clients to masters?
No, it does not. The Berkeley DB RPC server code could be modified to support this functionality, but in general this protocol is left entirely to the application. Note, there is no reason not to use the communications channels the application establishes for replication support to forward database update messages to the master, since Berkeley DB does not require those channels to be used exclusively for replication messages.


Can I use replication to partition my environment across multiple sites?
No, this is not possible. All replicated databases must be equally shared by all environments in the replication group.


I'm running with replication but I don't see my databases on the client.
This problem may be the result of the application using absolute path names for its databases, and the pathnames are not valid on the client system.


How can I distinguish Berkeley DB messages from application messages?
There is no way to distinguish Berkeley DB messages from application-specific messages, nor does Berkeley DB offer any way to wrap application messages inside of Berkeley DB messages. Distributed applications exchanging their own messages should either enclose Berkeley DB messages in their own wrappers, or use separate network connections to send and receive Berkeley DB messages. The one exception to this rule is connection information for new sites; Berkeley DB offers a simple method for sites joining replication groups to send connection information to the other database environments in the group (see Connecting to a new site for more information).

How should I build my send function?
This depends on the specifics of the application. One common way is to write the rec and control arguments' sizes and data to a socket connected to each remote site. On a fast, local area net, the simplest method is likely to be to construct broadcast messages. Each Berkeley DB message would be encapsulated inside an application specific message, with header information specifying the intended recipient(s) for the message. This will likely require a global numbering scheme, however, as the Berkeley DB library has to be able to send specific log records to clients apart from the general broadcast of new log records intended for all members of a replication group.


Does every one of my threads of control on the master have to set up its own connection to every client? And, does every one of my threads of control on the client have to set up its own connection to every master?
This is not always necessary. In the Berkeley DB replication model, any thread of control which modifies a database in the master environment must be prepared to send a message to the client environments, and any thread of control which delivers a message to a client environment must be prepared to send a message to the master. There are many ways in which these requirements can be satisfied.

The simplest case is probably a single, multithreaded process running on the master and clients. The process running on the master would require a single write connection to each client and a single read connection from each client. A process running on each client would require a single read connection from the master and a single write connection to the master. Threads running in these processes on the master and clients would use the same network connections to pass messages back and forth.

A common complication is when there are multiple processes running on the master and clients. A straight-forward solution is to increase the numbers of connections on the master -- each process running on the master has its own write connection to each client. However, this requires only one additional connection for each possible client in the master process. The master environment still requires only a single read connection from each client (this can be done by allocating a separate thread of control which does nothing other than receive client messages and forward them into the database). Similarly, each client still only requires a single thread of control that receives master messages and forwards them into the database, and which also takes database messages and forwards them back to the master. This model requires the networking infrastructure support many-to-one writers-to-readers, of course.

If the number of network connections is a problem in the multiprocess model, and inter-process communication on the system is inexpensive enough, an alternative is have a single process which communicates between the master the each client, and whenever a process' send function is called, the process passes the message to the communications process which is responsible for forwarding the message to the appropriate client. Alternatively, a broadcast mechanism will simplify the entire networking infrastructure, as processes will likely no longer have to maintain their own specific network connections.

Tags:DB Replication
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