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| 1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> | ||
| 2 | <html> | ||
| 3 | <head> | ||
| 4 | <title>Xdelta3 delta compression library (BETA)</title> | ||
| 5 | <meta http-equiv="content-type" content="text/html; charset=ISO-8859-1"> | ||
| 6 | <link rel="stylesheet" type="text/css" href="xdelta3.css"/> | ||
| 7 | </head> | ||
| 8 | <body> | ||
| 9 | |||
| 10 | <!-- $Format: "$WWWLeftNavBar$" $ --!> | ||
| 11 | <table cellpadding="20px" width=700> <tr> <td class="leftbdr" valign=top height=600 width=100> <div class="leftbody"> <h1>Xdelta</h1> <a href="xdelta3.html">overview</a><br> <a href="xdelta3-cmdline.html">command line</a><br> <a href="xdelta3-api-guide.html">api guide</a><br> <br><a href="http://xdelta.org">xdelta.org</a></h2> </div> </td> <td valign=top width=500> | ||
| 12 | |||
| 13 | <!-- Copyright (C) 2003 and onward. Joshua P. MacDonald --!> | ||
| 14 | |||
| 15 | <h1>version three?</h1> | ||
| 16 | |||
| 17 | Xdelta3 is the third and latest release of Xdelta, which is a set of tools and | ||
| 18 | APIs for reading and writing compressed <em>deltas</em>. Deltas encode the | ||
| 19 | differences between two versions of a document. This release features a | ||
| 20 | completely new compression engine, several algorithmic improvements, a fully | ||
| 21 | programmable interface modelled after zlib, in addition to a command-line | ||
| 22 | utility, use of the RFC3284 (VCDIFF) encoding, a python extension, and now | ||
| 23 | 64-bit support.<p> | ||
| 24 | |||
| 25 | Xdelta3 is <em>tiny</em>. A minimal, fully functional VCDIFF decoder library | ||
| 26 | pipes in at 16KB. The command-line utility complete with encoder/decoder | ||
| 27 | tools, external compression support, and the <code>djw</code> secondary | ||
| 28 | compression routines, is just under 60KB, slightly larger than a | ||
| 29 | <code>gzip</code> executable.<p> | ||
| 30 | |||
| 31 | Xdelta3 has few dependencies because it's capable of stand-alone file | ||
| 32 | compression (i.e., what zlib and gzip do). The stand-alone compression of | ||
| 33 | Xdelta3/VCDIFF is 10-20% worse than <code>gzip</code>, you may view this as | ||
| 34 | paying for the convenience-cost of having a single encoding, tool, and api | ||
| 35 | designed to do both <em>data-compression</em> and <em>differencing</em> at | ||
| 36 | once.<p> | ||
| 37 | |||
| 38 | The Xdelta3 command-line tool, <code>xdelta3</code>, supports several | ||
| 39 | convenience routines. Delta compression works when the two inputs are | ||
| 40 | similar, but often we would like to compute the difference between two | ||
| 41 | compressed documents. <code>xdelta3</code> has (optional) support to | ||
| 42 | recognize externally compressed inputs and process them correctly. This | ||
| 43 | support is facilitated, in part, using the VCDIFF <em>application header</em> | ||
| 44 | field to store <code>xdelta3</code> meta-data, which includes the original | ||
| 45 | file names (if any) and codes to incidate whether the inputs were externally | ||
| 46 | compressed. Applications may provide their own application header.<p> | ||
| 47 | |||
| 48 | <h1>what are version one and version two?</h1> | ||
| 49 | |||
| 50 | Many shortcomings in the Xdelta1.x release are fixed in its replacement, | ||
| 51 | Xdelta3. Xdelta1 used both a simplistic compression algorithm and a | ||
| 52 | simplistic encoding. For example, Xdelta1 compresses the entire document at | ||
| 53 | once and thus uses memory proportional to the input size.<p> | ||
| 54 | |||
| 55 | The Xdelta1 compression engine made no attempt to find matching strings | ||
| 56 | smaller than say 16 or 32 bytes, and the encoding does not attempt to | ||
| 57 | efficiently encode the <code>COPY</code> and <code>ADD</code> instructions | ||
| 58 | which constitute a delta. For documents with highly similar data, however, | ||
| 59 | these techniques degrade performance by a relatively insignificant amount. | ||
| 60 | (Xdelta1.x compresses the delta with Zlib to improve matters, but this | ||
| 61 | dependency stinks.)<p> | ||
| 62 | |||
| 63 | Despite leaving much to be desired, Xdelta1 showed that you can do well | ||
| 64 | without great complexity; as it turns out, the particulars of the compression | ||
| 65 | aengine are a relatively insignificant compared to the difficulty of | ||
| 66 | programming an application that uses delta-compression. Better solve that | ||
| 67 | first.<p> | ||
| 68 | |||
| 69 | What we want are <em>systems</em> that manage compressed storage and network | ||
| 70 | communication. The second major release, Xdelta2, addresses these issues. | ||
| 71 | Xdelta2 features a storage interface -- part database and part file system -- | ||
| 72 | which allows indexing and labeling compressed documents. The feature set is | ||
| 73 | similar to RCS. The Xdelta2 interface supports efficient algorithms for | ||
| 74 | <em>extracting</em> deltas between any pair of versions in storage. The | ||
| 75 | extraction technique also does not rely on hierarchy or centralizing the | ||
| 76 | namespace, making the techniques ideal for peer-to-peer communication and | ||
| 77 | proxy architectures. I am grateful to Mihut Ionescu for implementing the | ||
| 78 | Xproxy HTTP delta-compressing proxy system based on this interface and | ||
| 79 | studying the benefits of delta-compression in that context. Xdelta2 stressed | ||
| 80 | the Xdelta1 compression engine beyond its limits; so Xdelta3 is designed as | ||
| 81 | the ideal replacement. The Xdelta2 techniques are yet to be ported to the new | ||
| 82 | implementation.<p> | ||
| 83 | |||
| 84 | </td> | ||
| 85 | </tr> | ||
| 86 | </table> | ||
| 87 | |||
| 88 | </body> | ||
| 89 | </html> | ||