MySQL load balancing

I'm looking into ways to balance the load on our MySQL infrastructure, and can't seem to find an answer that works for me... :)

So, I have one big and fat server, that handles everything. Many DBs, many reads, many writes, etc. It handles it pretty well, but it is a single point of failure.

We've set up a couple of slaves to redirect reads to them, but faced 2 problems: it takes a lot of effort to rebuild all programs to split reads and writes; and sometimes slaves get behind, which leads to very interesting artifacts in the application.

Problems with slaves getting behind: because many databases are mixed - there are both heavy 10-20 minute queries done on data mining side, as well as atomic queries that take no time. But Slave runs one query at a time, so all atomic queries have to wait until heavy one finishes.

To resolve these 2 problems, I was thinking about something like a proxy, that would consider this:

• split read/writes automatically
• serve as a single point of entry and then redirect request to appropriate server that has needed database (e.g. separate db1 and db2 on the back-end, but have it transparent to the application)
• be aware of slave lag, and send reads to the master, when slave lag occurs (would be ideal if this can be done, say, per database; but server-wide would be pretty awesome as well)
• load balance reads between all eligible slaves (either by simple round robin, or by monitoring LA)

One problem that still remains, but which I want to consider - is fail-over. If master fails - would be nice if slave would take responsibilities of a master, and when master is back up - it would become a slave.

Any pointers to RTFM or case studies on this subject would be welcome =)

EDIT: Googled some more, and in addition to Tungsten enterprise - found dbShards, and Schooner. While looking deeper into that - does anyone have experience with these solutions? Any feedback?