MongoDB写关注同步级别

Question

I am trying to understand what exactly are the limitations of using MongoDB as the primary database for a project I am working on, it can be hard to wade through the crap online to properly understand how it compares to a more traditional database choice of say MySQL.

From what I understand from reading about HADR configuration of

IBM DB2 - http://pic.dhe.ibm.com/infocenter/db2luw/v9r7/index.jsp?topic=%2Fcom.ibm.db2.luw.admin.ha.doc%2Fdoc%2Fc0011724.html ,

MySQL - http://dev.mysql.com/doc/refman/5.5/en/replication-semisync.html

and MongoDB - http://docs.mongodb.org/manual/core/write-concern/

It seems that Replica Acknowledged http://docs.mongodb.org/manual/core/replica-set-write-concern/ is the highest level of write concern in a replica set.

Is replica acknowledged the equivalent to the synchronous level in DB2 and Semisynchronous level in MySQL?

Answer 1

No they are not.

IBM DB2 provides a way to make sure that all members of a replica set are upto speed at the same time, it is the same as MySQLs own synchronous replication. It ensures full consistentcy at all times throughout the slave set.

Semisynchronous replication again is not replica set majority either; from the documentation page:

The master waits after commit only until at least one slave has received and logged the events.

But then:

It does not wait for all slaves to acknowledge receipt, and it requires only receipt, not that the events have been fully executed and committed on the slave side.

In other words you have no idea whether or not any slaves actually performed the command. It is the same as w:0 or "unsafe" writes in MongoDB.

With majority you have an idea that every member you send to has actually performed your command as can be seen by a cute little diagram in the documentation: http://docs.mongodb.org/manual/core/replica-set-write-concern/#verify-write-operations

and if that doesn't convince you then the quote:

The following sequence of commands creates a configuration that waits for the write operation to complete on a majority of the set members before returning:

From the next paragraph should.

So MySQL semisynchronous is similar to majority but it isn't the same. DB2 is totally different.

The IBM documentation sums up the differences in replica/slave wirte concern quite well:

The more strict the synchronization mode configuration parameter value, the more protection your database solution has against transaction data loss, but the slower your transaction processing performance. You must balance the need for protection against transaction loss with the need for performance.

This applies to DB2, MySQL and MongoDB alike. You must choose.