# Gache notions

## Basic idea

Like Memcached, plus...

* Usability:
  * True FLUSHALL
  * Single process for multiple bins
* Management
  * Complete locking dump/restore
  * Snapshots
* Tag support
* Pluggable storage
  * Journaling
  * Write-back/write-through replication
  * Chained storage (fast → slow)

But probably slower overall, just more convenient.

## Components

* Protocol handling (FSMs)
  * Application: Memcache protocol proper
  * Presentation: ASCII / Binary
  * Transport:
    * TCP with persistent connections
    * TCP without persistent connections
    * UDP
    * raw socketpair (stdin/stdout)
* Readline equivalence for ASCII presentation interactions
* Storage
  * Pluggable storage
    * Journaling
  * Storage engines
    * Memory
    * Memcached, for use as a debugging proxy in front of a standard instance
    * Others ?
  * Memory
      * Main memory allocation
      * Slab allocator
  * Reporting
  * Background expiration lurker

## Memory allocation

### Memcache slabs model

* http://www.mikeperham.com/2009/06/22/slabs-pages-chunks-and-memcached/ (2009)
  * Older versions of memcached used slabs sized based on powers of two, so you'd have a 1KB slab, 2KB slab, 4KB slab, ..., all the way to 1MB.
  * Newer (2006: 1.2.0-RC1 ?) versions use 1.25^n for the chunk size
* Allocate slabs for elements of a given size range ("chunks")
* Slabs are made of fixed-size (1MB) pages, split according to the chunk size
* Slab chunk sizes are in a geometric progression: (1 + x)^n