What is Memcachedb?
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Memcachedb is a distributed key-value storage system designed for persistent. It is NOT a cache solution, but a persistent storage engine for fast and reliable key-value based object storage and retrieval. It conforms to memcache protocol(not completed, see below), so any memcached client can have connectivity with it. Memcachedb uses Berkeley DB as a storing backend, so lots of features including transaction and replication are supported. 

Memcachedb Features
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* High performance read/write for a key-value based object
* High reliable persistent storage with transcation
* High availability data storage with replication
* Memcache protocol compatibility

Supported memcache commands
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get(also mutiple get) 
set, add, replace
incr, decr
delete
stats(malloc, maps)

Private commands
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db_checkpoint
db_archive
rep_ismaster
rep_whoismaster
rep_set_priority
rep_set_ack_policy
rep_set_ack_timeout
rep_set_bulk
rep_set_request
stats(bdb, rep)

Some Warning
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Expire time has been discarded in memcachedb(we are for persistent:p), so you should not use any corresponding features of clients. The daemon does nothing while you give a expire time of item.

For more info, see: http://memcachedb.org