Memcached

Question 1

Characteristics

Answer 1

Distributed key-value store (hashtable)
Limited size – key,value pairs are discarded when cache is full
* Cache eviction policy: LRU (least-recently used).
* More details: cache divided in HOT, WARM and COLD; new
items enter the HOT; cache eviction moves object to the
lower level.

Question 2

Designed for

Answer 2

• High-throughput servers – accessing memory is much faster
  than disk.
• High-latency queries – avoid repeating costly queries.

Question 3

Architecture

Answer 3

• Clients know about all servers.
• Clients know the hash function to assign a key to a server.
  Can use consistent hashing.
• Clients send operations to the server that will hold a given
  key directly.
• Servers maintain a “key-value” store.

Question 4

Memcached limitations:

Answer 4

On a cache miss, multiple clients may end up trying to set the
value of a cache entry.
For a value updated very frequently, writes may be reordered
and the old value get written in the cache.
Solution [from Facebook]:
* Clients get leases for writing the value of a key. This
guarantees that a single client writes at a time.

Question 5

Memcached Limitations: No Replication