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1 /*!
2 \ingroup FileBackend
3 \page file_backend_design File backend design
4 
5 Some notes on the FileBackend architecture.
6 
7 \section intro Introduction
8 
9 To abstract away the differences among different types of storage media,
10 MediaWiki is providing an interface known as FileBackend. Any MediaWiki
11 interaction with stored files should thus use a FileBackend object.
12 
13 Different types of backing storage media are supported (ranging from local
14 file system to distributed object stores). The types include:
15 
16 * FSFileBackend (used for mounted file systems)
17 * SwiftFileBackend (used for Swift or Ceph Rados+RGW object stores)
18 * FileBackendMultiWrite (useful for transitioning from one backend to another)
19 
20 Configuration documentation for each type of backend is to be found in their
21 __construct() inline documentation.
22 
23 
24 \section setup Setup
25 
26 File backends are registered in LocalSettings.php via the global variable
27 $wgFileBackends. To access one of those defined backends, one would use
28 FileBackendStore::get( <name> ) which will bring back a FileBackend object
29 handle. Such handles are reused for any subsequent get() call (via singleton).
30 The FileBackends objects are caching request calls such as file stats,
31 SHA1 requests or TCP connection handles.
32 
33 \par Note:
34 Some backends may require additional PHP extensions to be enabled or can rely on a
35 MediaWiki extension. This is often the case when a FileBackend subclass makes use of an
36 upstream client API for communicating with the backing store.
37 
38 
39 \section fileoperations File operations
40 
41 The MediaWiki FileBackend API supports various operations on either files or
42 directories. See FileBackend.php for full documentation for each function.
43 
44 
45 \subsection reading Reading
46 
47 The following basic operations are supported for reading from a backend:
48 
49 On files:
50 * stat a file for basic information (timestamp, size)
51 * read a file into a string or several files into a map of path names to strings
52 * download a file or set of files to a temporary file (on a mounted file system)
53 * get the SHA1 hash of a file
54 * get various properties of a file (stat information, content time, MIME information, ...)
55 
56 On directories:
57 * get a list of files directly under a directory
58 * get a recursive list of files under a directory
59 * get a list of directories directly under a directory
60 * get a recursive list of directories under a directory
61 
62 \par Note:
63 Backend handles should return directory listings as iterators, all though in some cases
64 they may just be simple arrays (which can still be iterated over). Iterators allow for
65 callers to traverse a large number of file listings without consuming excessive RAM in
66 the process. Either the memory consumed is flatly bounded (if the iterator does paging)
67 or it is proportional to the depth of the portion of the directory tree being traversed
68 (if the iterator works via recursion).
69 
70 
71 \subsection writing Writing
72 
73 The following basic operations are supported for writing or changing in the backend:
74 
75 On files:
76 * store (copying a mounted file system file into storage)
77 * create (creating a file within storage from a string)
78 * copy (within storage)
79 * move (within storage)
80 * delete (within storage)
81 * lock/unlock (lock or unlock a file in storage)
82 
83 The following operations are supported for writing directories in the backend:
84 * prepare (create parent container and directories for a path)
85 * secure (try to lock-down access to a container)
86 * publish (try to reverse the effects of secure)
87 * clean (remove empty containers or directories)
88 
89 
90 \subsection invokingoperation Invoking an operation
91 
92 Generally, callers should use doOperations() or doQuickOperations() when doing
93 batches of changes, rather than making a suite of single operation calls. This
94 makes the system tolerate high latency much better by pipelining operations
95 when possible.
96 
97 doOperations() should be used for working on important original data, i.e. when
98 consistency is important. The former will only pipeline operations that do not
99 depend on each other. It is best if the operations that do not depend on each
100 other occur in consecutive groups. This function can also log file changes to
101 a journal (see FileJournal), which can be used to sync two backend instances.
102 One might use this function for user uploads of file for example.
103 
104 doQuickOperations() is more geared toward ephemeral items that can be easily
105 regenerated from original data. It will always pipeline without checking for
106 dependencies within the operation batch. One might use this function for
107 creating and purging generated thumbnails of original files for example.
108 
109 
110 \section consistency Consistency
111 
112 Not all backing stores are sequentially consistent by default. Various FileBackend
113 functions offer a "latest" option that can be passed in to assure (or try to assure)
114 that the latest version of the file is read. Some backing stores are consistent by
115 default, but callers should always assume that without this option, stale data may
116 be read. This is actually true for stores that have eventual consistency.
117 
118 Note that file listing functions have no "latest" flag, and thus some systems may
119 return stale data. Thus callers should avoid assuming that listings contain changes
120 made my the current client or any other client from a very short time ago. For example,
121 creating a file under a directory and then immediately doing a file listing operation
122 on that directory may result in a listing that does not include that file.
123 
124 
125 \section locking Locking
126 
127 Locking is effective if and only if a proper lock manager is registered and is
128 actually being used by the backend. Lock managers can be registered in LocalSettings.php
129 using the $wgLockManagers global configuration variable.
130 
131 For object stores, locking is not generally useful for avoiding partially
132 written or read objects, since most stores use Multi Version Concurrency
133 Control (MVCC) to avoid this. However, locking can be important when:
134 * One or more operations must be done without objects changing in the meantime.
135 * It can also be useful when a file read is used to determine a file write or DB change.
136  For example, doOperations() first checks that there will be no "file already exists"
137  or "file does not exist" type errors before attempting an operation batch. This works
138  by stating the files first, and is only safe if the files are locked in the meantime.
139 
140 When locking, callers should use the latest available file data for reads.
141 Also, one should always lock the file *before* reading it, not after. If stale data is
142 used to determine a write, there will be some data corruption, even when reads of the
143 original file finally start returning the updated data without needing the "latest"
144 option (eventual consistency). The "scoped" lock functions are preferable since
145 there is not the problem of forgetting to unlock due to early returns or exceptions.
146 
147 Since acquiring locks can fail, and lock managers can be non-blocking, callers should:
148 * Acquire all required locks up font
149 * Be prepared for the case where locks fail to be acquired
150 * Possible retry acquiring certain locks
151 
152 MVCC is also a useful pattern to use on top of the backend interface, because operations
153 are not atomic, even with doOperations(), so doing complex batch file changes or changing
154 files and updating a database row can result in partially written "transactions". Thus one
155 should avoid changing files once they have been stored, except perhaps with ephemeral data
156 that are tolerant of some degree of inconsistency.
157 
158 Callers can use their own locking (e.g. SELECT FOR UPDATE) if it is more convenient, but
159 note that all callers that change any of the files should then go through functions that
160 acquire these locks. For example, if a caller just directly uses the file backend store()
161 function, it will ignore any custom "FOR UPDATE" locks, which can cause problems.
162 
163 \section objectstore Object stores
164 
165 Support for object stores (like Amazon S3/Swift) drive much of the API and design
166 decisions of FileBackend, but using any POSIX compliant file systems works fine.
167 The system essentially stores "files" in "containers". For a mounted file system
168 as a backing store, "files" will just be files under directories. For an object store
169 as a backing store, the "files" will be objects stored in actual containers.
170 
171 
172 \section file_obj_diffs File system and Object store differences
173 
174 An advantage of object stores is the reduced Round-Trip Times. This is
175 achieved by avoiding the need to create each parent directory before placing a
176 file somewhere. It gets worse the deeper the directory hierarchy is. Another
177 advantage of object stores is that object listings tend to use databases, which
178 scale better than the linked list directories that file sytems sometimes use.
179 File systems like btrfs and xfs use tree structures, which scale better.
180 For both object stores and file systems, using "/" in filenames will allow for the
181 intuitive use of directory functions. For example, creating a file in Swift
182 called "container/a/b/file1" will mean that:
183 - a "directory listing" of "container/a" will contain "b",
184 - and a "file listing" of "b" will contain "file1"
185 
186 This means that switching from an object store to a file system and vise versa
187 using the FileBackend interface will generally be harmless. However, one must be
188 aware of some important differences:
189 
190 * In a file system, you cannot have a file and a directory within the same path
191  whereas it is possible in an object stores. Calling code should avoid any layouts
192  which allow files and directories at the same path.
193 * Some file systems have file name length restrictions or overall path length
194  restrictions that others do not. The same goes with object stores which might
195  have a maximum object length or a limitation regarding the number of files
196  under a container or volume.
197 * Latency varies among systems, certain access patterns may not be tolerable for
198  certain backends but may hold up for others. Some backend subclasses use
199  MediaWiki's object caching for serving stat requests, which can greatly
200  reduce latency. Making sure that the backend has pipelining (see the
201  "parallelize" and "concurrency" settings) enabled can also mask latency in
202  batch operation scenarios.
203 * File systems may implement directories as linked-lists or other structures
204  with poor scalability, so calling code should use layouts that shard the data.
205  Instead of storing files like "container/file.txt", one can store files like
206  "container/<x>/<y>/file.txt". It is best if "sharding" optional or configurable.
207 
208 */