Incremental update of client software

 Incremental updates use an open source library bsdiff,bsdiff It's a difference algorithm , The principle is that old documents are compared with new documents , Use as much as possible old What is already in the file , Add as little new content as possible to build new file .
 The common practice is to match the old file with the new file or use hash Technology extracts the common part , Then type the rest of the new document into patch package （ The offset address of the new content relative to the old content is recorded in the differential package ）,
 stay Patch Use in stage copying and insertion Two operations combine old files with patch File composition new file .
 Incremental update process ：
	 On the server side , Use bsdiff The tools put the old apk And the new apk Compare and get the differential package patch package ,
	 Download to local via network , adopt bspatch Tools to put local old apk and patch Package synthesis new apk package . Finally, install the new apk


 shortcoming ：
 We can't guarantee that all users can upgrade , Like our latest patch Bag is 2.0 Version and 3.0 Version difference ,
 If the user uses 1.0 edition , Then you cannot upgrade successfully , So we have to make a 1.0 and 3.0 Differential subcontracting between .
 With more and more versions , There are more and more subcontracts to be done . Can be in Linux Write an automatic script to complete .
 If the differential package is tampered with during downloading, it cannot be synthesized successfully , You can download it through md5  Or in some other way patch Check the integrity of the package .


bsdiff Incremental updating 
BSDiff Is a difference update algorithm , It runs on the server side BSDiff Algorithm generation patch package , Run on client BSPatch Algorithm , Combine old files with patch Package composition new file .
 The core idea of difference updating algorithm 
 Use as much as possible old What is already in the file , Add as little new content as possible to build new file . The usual practice is to old Document and new File to do substring matching or use hash technology , Extract public part , take new The rest of the file is packaged into patch package , stay Patch In phase , use copying and insertion Two basic operations can make old Document and patch Package synthesis new file .

BSDiff Algorithm improvement 
Insertion Operation will cause a lot of pointer changes and modifications , These values need to be recorded before they can be used in Patch The stage relocates the modified area , Because these pointer control words must be in BSDiff Stage join patch package , Produced patch The bag will be bigger .BSDiff By introducing diff string The concept of , The number of pointer control words to be recorded is greatly reduced , Thus making patch The bag is smaller .

BSDiff The three basic steps are as follows ：
1． Yes old All substrings in the file form a dictionary ;
2． contrast old Document and new file , produce diffstring and extra string;
3． take diffstring  and extra string  And the corresponding control word zip Compress into one patch package .

BSPatch Basic steps ：
 Client synthesis patch The basic steps are as follows ：
1． receive patch package ;
2． decompression patch package ;
3． Restore new file .




 Dual directory update idea :
 As the update package gets bigger and bigger , It takes longer and longer to download and install updates , Cause users to wait for a long time , Some software adopts the way of background download and background update . The so-called background download is to update immediately no matter whether the user clicks or not ,
 As long as there is a new version, it will be secretly downloaded to users in the background , A little rogue , But this is also for the user experience . What about background updates , Running programs , Every file is occupied , It can't be updated . still Chrome Think out ,
 Dual directory update , Copy the version to another directory first , Then update this new copy , The next time the user starts, start the new version directly . such as  Chrome There is the following directory structure  
Chrome
    +Application
        +35.0.1916.153
        +35.0.1916.114
         chrome.exe 
 It uses the version number as the directory name , Update the new version every time you upgrade , The old version running in another directory is not affected . Next time it starts  Chrome.exe Always load the latest version of dll Just run it .
chrome.exe It's a small program , The logic inside is to detect the version number and load the latest version dll, The program itself hardly needs to be updated .


1.csdn post 
https://blog.csdn.net/darling757267/article/details/80652267
2. Simple books 
https://www.jianshu.com/p/ca40bfc4a81f?utm_campaign=maleskine&utm_content=note&utm_medium=writer_share&utm_source=weibo
3.github Address 
https://github.com/mendsley/bsdiff
4. You know 
https://www.zhihu.com/question/24263552