Concurrency Control

Not Supported

Memcached doesn't have the concept of transactions. The get/set operations are atomic, though. For example, multiple sets on the same key can be issued concurrently, but only the last will stay.

Data Model

Key/Value

Memcached servers maintain a key-value map. Keys are up to 250 bytes long and values are up to 1 MB large. There are no relational schemas.

Indexes

Hash Table

Memcached stores a key-value hash table on every server. As of May 2017, it uses MurmurHash 3 as the hash function.

Joins

Not Supported

Memcached only supports get/set and related operations on a key-value store. There is no support for SQL queries. Therefore, there is no need to support joins.

Logging

Not Supported

There is no logging in Memcached, since it is intended for being a caching system. The cache is not persistant between restarts.

Query Compilation

Not Supported

Memcached only supports get/set and related operations on a key-value store. There is no support for SQL queries. Therefore, there is no need for query compilation.

Query Execution

Tuple-at-a-Time Model

Query Interface

Custom API

Memcached provides get/set and related operations. It also allows users to specify how long a cache item should last.

Storage Architecture

In-Memory

In Memcached, everything is stored in memory, since it is intended for caching objects stored in a disk-oriented database. By storing everything in memory, it achieved the needed latency and speed. The cache is not persitent between restarts, which is an intended feature.

Storage Model

Custom

Memcached stores a hash table. As of May 2017, it uses MurmurHash 3 as the hash function. Values items are slab-allocated.

Stored Procedures

Not Supported

Memcached only supports get/set and related operations on a key-value store. There is no support for SQL queries. Therefore, there is no need to support stored procedures.

System Architecture

Shared-Nothing

In Memcached, a client knows all servers. Servers do not communicate with each other. Each key is hashed to determine which server is used to store the key-value pair. If all clients use the same hash function, then a client can read other clients' cached data.

Views

Not Supported

Memcached only supports get/set and related operations on a key-value store. There is no support for SQL queries. Therefore, there is no need to support views.