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Comment by amix | Hacker News Reader

memcached is a memory based key-value database. LightCloud is persistent i.e. data is saved to disk. I'll make this more clear on the website.

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I will be more excited to see how this key-value database actually helps to scale plurk. I am interested to integrate this into Rails if this really works very well.

I use an in-memory tokyo cabinet as a memcached replacement on my rails system. (well actually it runs merb, but close enough)

Do you therefore see LightCloud as a possible alternative/complement to memcached? Certainly buying 500 GB of disk space is cheaper than buying 500 GB of RAM.

At Plurk.com we use both LightCloud and memcached. They complement each other. I.e. memcached is used for caching stuff - to reduce the load to MySQL. LightCloud is used to store persistent data - such as how many times a plurk has been read.

You could also store this in MySQL, but generally, storing key-value data is not the force of a relational database such as MySQL and LightCloud (and other key-value databases) are optimized for this kind of storage.

there is memcachedb (http://memcachedb.org/) which provides persistence through berkeleydb.

memcachedb is not distributed, meaning that you can only scale vertically (i.e. by buying bigger machines). With LightCloud you can scale by adding extra servers to the system (i.e. horizontal scaling).

This said, Tokyo Tyrant performs better than memcachedb and offers more features (such as master-master replication and Lua scripting).

I posted a comment a while back on Tokyo Tyrant vs. memcachedb: http://news.ycombinator.com/item?id=480055

This is not a correct observation! Memcachedb is nothing but Memcached with BerkeleyDB underneath. Distribution of data in Memcached(also MemcachedDB) is achieved by client side hashing of keys. If your argument were true then, Facebook would not scale vertically - they are the biggest users of memcached. MemcacheDB can be used in exactly the same - and we have in production BTW.

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