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Re: MySQL Proxy: Scenarios
Posted by: Gerry Reno
Date: July 10, 2007 11:21AM

Edwin,
I have to agree with Rick regarding MySQL Cluster (NBD). We tried using this as a backend to our webapps and the result was disasterous. There are many architectural limitations with MySQL Cluster that make it unusable in a webapp scenario. Just for example, all the system tables are stored in MyISAM and not NDB therefore all your user tables remain strictly local to each SQL node. So when a user wants to change a password it only gets changes with respect to the one SQL node where he is currently connected. Another serious limitation is when a user install a new webapp and runs its setup page which in turn creates the databases for the webapp. Well, if one of the nodes is down for maintenance when this webapp is installed it will never know about this database even when it comes back online so if the user is directed to this node by the load balancer then everything fails. I don't mean to pick on MySQL Cluster but it appears it was not architected with anything web related in mind and that is where the world is headed today.

So after we backed out MySQL Cluster we went looking for a way to cluster MySQL in more traditional ways such as replication. This is what we have now but there were some serious tradeoffs. The main one is that the vast majority of webapps have not been written with separate read and write connection handles so therefore you are forced to direct these webapps to only the master mysql server. This severely limits your ability to be able to balance traffic over a replication cluster. I was investigating writing a proxy for MySQL when I read an article on MySQL about Universal Server which was talking about DBIx::MyServer. At the end of the article there was an announcement regarding MySQL Proxy so I began investigating it. Turns out that MySQL Proxy is exactly the right tool for bringing load balancing to non-replication aware applications. So it will solve the load-balancing and HA as far as mysqld is concerned. But what about HA for the proxy itself. I would have preferred that the proxy was an implementation of a VRRP router. This way it would take care of HA for itself automatically. The idea is that all upstream clients would use a VIP to address the proxy. You would install two proxies on different machines and assign one of these the VIP. Being a VRRP router the failover router would monitor the active router and if it could not make contact it would then take over the VIP and continue service for upstream clients. The upstream clients would never know that they had been switched to the failover proxy. So this is what MySQL Proxy needs for its own HA. Now I've only been working with MySQL Proxy for two days so maybe it has a way to do this that is transparent to upstream clients. The issue is that it is not enough that MySQL Proxy provide HA to mysqld. It must also make sure that it does not become a SPOF and therefore must provide HA for itself. In the alternative, I have been mapping out some ways that I could externally provide HA for MySQL Proxy but things get ugly quickly when you have to provide complete transparency to upstream clients.

Gerry

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