"Here's a big reason to use MemCached: expiry!
Let's say you only want to do a complicated query once every fifteen minutes. Do it once, put it in a cache by key with an expiry of 15 minutes. Let memcached worry about when to take it out for you."
Yes, this is a good reason to use memcached. I used this pattern for the aggroGator, with Google's version of memcached. Which reminds me that Part 3 of the series I wrote on GAE is out... Anyways, in that app, RSS feed results are cached in memcached with a five minute poll (initiated client-side, so only polling for logged in users.)
Expiry is the cache eviction policy for memcached, where as any database cache is going to be more of an LRU policy. There will be cases where expiry is more useful, but I would actually guess that LRU is appropriate for the majority of use cases...
"MySQL memory table is not as fast as memcached. Depending on your data, memcached is 3X times or more as fast for get/set (select/insert)."
Really? I would love to see some objective results for this. Of course it would have to be an apples-to-apples comparison. The data would need to be retrieved from a cache node on separate physical machine and for the MySQL cache, it would need to be a select by primary key. Now I wouldn't be terribly surprised if memcached was slightly faster, but 3X? I would be even more surprised if a put/insert was faster at all.
"This has been said many times. MySQL and memcached serve different purposes. Memcached is used to store processed data, while MySQL generally contains raw, normailized data, which needs lots of complex queries and other processing."
I actually mentioned this at the end of my post... So obviously I agree. But I have a feeling that people use memcached to cache a lot of data that is not very processed at all. Also, the last line is very misleading. You do not need to do much normalizing of your data. I can tell you that anybody doing federated database systems have to do a lot of de-normalizing of their data. And complex queries and other processing? That is just silly.
"Facebook needs memcache for the obvious reason that it's pages are highly complex and include many pictures."
Eh? Don't see how pictures would matter... But if Facebook is using memcache for HTML fragments, then I would agree that this is the right kind of cache. I don't know if this is the case or not. Other things like my list of friends or my contact info would be a poor choice for memached. Something like the Facebook feed... That is a lot tougher. There are limits to what you can cache, since the feed changes a lot and you might have a low tolerance for stale data. You might be able to create HTML fragments for the stories and cache those?
" Also, fewer of Facebooks pages are time-critical when compared to eBay. On eBay you basically can't cache a page rendering (memcache) if it has less than a minute
of auction time left"
Item listings are certainly time-critical, i.e you expect the price to be accurate when you are looking at a listing and considering bidding on it. This is true regardless of the time remaining, being less than a minute doesn't matter too much. However, that is just one page, many other pages are not so sensitive, but they are very dynamic.
When it comes to picking between MySQL and memcached, I would first say: are you using an ORM but need caching? If the data is being accessed through ORM, then your cache layer should be a database, not memcached. Again the only exception I could see to this would be a graph, i.e. data that is hard to describe relationally (requires self referential foreign keys, etc.)