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Replication modules
Posted by: sharding
Date: November 29, 2004 10:28AM

What I would like to see from MySQL replication would be something similar to how they implemented their storage engines. I'd like to be able to choose which replication types the server should allow. Normal replication (like it is today) and at least Multi-Master replication where a slave can replicate from 2 masters (same databases/tables if wanted) but don't provide excessive checks for data overlap; let the user's software do that.

Granted it would be nice if MySQL could catch data collisions in multi-master replication, but this issue is a complex one and not easily solved except by providing various methods and let the user choose which they want to use. Should data from Master A preside over data from Master B? Should replication simply stop until the data can be manually corrected?

Also the ability to delay replication would be great. I know this sounds counter-intuitive to the entire concept of replication, but delayed backups via replication would then be possible. I guess we could do it by writing some code to toggle the backup slave via 'slave stop/start' but that's an extra piece of code to break. With delayed replication, the functionality would be built into the database itself and provide greater recovery when errors occur. Also it protects against user error like forgetting a where clause on update/delete or something of this nature. Doesn't happen often but when it does, it is bad. With delayed replication (which should be configurable by query type (i.e. update, delete, replace, etc) then there is a window to stop the delayed replication feed so evil queries do not get replicated. The only other option is hope you have replication logs to replay and recover all the changed data.

Without mutli-master replication, creating a redundant system is extremely difficult since the single master will always be a point of failure. Currently we have implemented master failover at the application level but it's just a crutch. Hopefully these features will come in version 5.x.

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