doc/Programming-Guide/StorageManager.dox

   1 /*
   2  * Copyright (C) 1996-2020 The Squid Software Foundation and contributors
   3  *
   4  * Squid software is distributed under GPLv2+ license and includes
   5  * contributions from numerous individuals and organizations.
   6  * Please see the COPYING and CONTRIBUTORS files for details.
   7  */
   8
   9 /**
  10  \defgroup StorageManager Storage Manager
  11  \ingroup Components
  12  *
  13  \par
  14         The Storage Manager is the glue between client and server
  15         sides.  Every object saved in the cache is allocated a
  16         StoreEntry structure.  While the object is being
  17         accessed, it also has a MemObject structure.
  18
  19 \par
  20         Squid can quickly locate cached objects because it keeps
  21         (in memory) a hash table of all StoreEntry's.  The
  22         keys for the hash table are MD5 checksums of the objects
  23         URI.  In addition there is also a storage policy such
  24         as LRU that keeps track of the objects and determines
  25         the removal order when space needs to be reclaimed.
  26         For the LRU policy this is implemented as a doubly linked
  27         list.
  28
  29 \par
  30         For each object the StoreEntry maps to a cache_dir
  31         and location via sdirn and sfilen. For the "ufs" store
  32         this file number (sfilen) is converted to a disk pathname
  33         by a simple modulo of L2 and L1, but other storage drivers may
  34         map sfilen in other ways.  A cache swap file consists
  35         of two parts: the cache metadata, and the object data.
  36         Note the object data includes the full HTTP reply---headers
  37         and body.  The HTTP reply headers are not the same as the
  38         cache metadata.
  39
  40 \par
  41         Client-side requests register themselves with a StoreEntry
  42         to be notified when new data arrives.  Multiple clients
  43         may receive data via a single StoreEntry.  For POST
  44         and PUT request, this process works in reverse.  Server-side
  45         functions are notified when additional data is read from
  46         the client.
  47
  48  */