python增量计算

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在rsync同步数据后，因业务需要以及文件大小限制，需要对数据做增量分析，而每次都需要拿出文件的增量部分。
    linux有差异计算命令diff以及补丁工具patch，都不是很符合预期。这两种工具都是需要对两个文件进行对比，而若如此做的话，其一计算增量耗时，其二需要有一个原数据的副本文件。多余的副本文件会导致额外的存储开销，以及数据移动成本。
    又因同步过来的数据是多个application的数据，需要针对不同的app进行增量计算，提交给相应的计算任务。希望每次在rsync后直接做增量计算，这里就考虑到直接使用python脚本编写。
   
#! /usr/bin/env python
# -*- coding: utf-8 -*-
import os
import getopt
import sys
import fnmatch  
from datetime import datetime,timedelta,date
#判断是否为新一天的开始,数据文件是按日保存,凌晨定时任务需要对前日的数据进行增量计算
def is_newday_begin(app,daystr):
file_day_file='/var/local/diff_file_storage/day_'+app
if os.path.exists(file_day_file):
try:
day_file=open(file_day_file,'r')
oldday=day_file.read()
if oldday != daystr:
return True
finally:
day_file.close()
return False
def read_position(app,daystr,file):
file_day_file='/var/local/diff_file_storage/day_'+app
if os.path.exists(file_day_file):
try:
day_file=open(file_day_file,'r')
oldday=day_file.read()
if oldday != daystr:
day_file.close()
day_file=open(file_day_file,'w+')
day_file.write(daystr)
write_position(daystr,file,0)
finally:
day_file.close()
else:
try:
day_file=open(file_day_file,'w+')
day_file.write(daystr)
finally:
day_file.close()

position=None
open_file=None
position_file=file+'_'+daystr
try:
if os.path.exists(position_file):
open_file=open(position_file,'r')
position = long(open_file.read())
else:
position=0
finally:
if open_file is not None:
open_file.close()
print 'read from ',position_file
return position
def write_position(daystr,file,position):
open_file=None
position_file=file+'_'+daystr
try:
open_file=open(position_file,'w+')
open_file.write(str(position))
finally:
if open_file is not None:
open_file.close()
print 'write to ',position_file
def read_diff(app,day,outpath,times):
filepath='/data/dc_files/'+app+day.strftime('/%Y/%m')
if is_newday_begin(app,day.strftime('%Y-%m-%d')) and times <= 0:
read_diff(app,day+timedelta(days=-1),outpath,times+1)
print 'read_diff old day : ',day
#get old position of file
file_position=0
file_position_dir='/var/local/diff_file_storage/position/'
filenames=os.listdir(filepath)
if not filepath.endswith('/'):
filepath = filepath + '/'
filepre = app+'_'+day.strftime('%Y-%m-%d')+'_'
target_file=open(outpath,'a')
try:
for line in filenames:
if line.startswith(filepre):
print 'read_diff: flush file ',line
file_position_path=file_position_dir+app+'_'+line.replace(filepre,'')
#read position from temp
oldposition = read_position(app,day.strftime('%Y-%m-%d'),file_position_path)
position=oldposition
print 'read_diff: old position is ',position
#read data,change position
try:
source_file=open(filepath+line,'r')
#target_file=open(outpath,'w')
source_file.seek(oldposition)
for temp_line in source_file:
target_file.write(temp_line)
position = position+len(temp_line)
finally:
source_file.close()
if position==oldposition:
#os.remove(outpath)
print outpath,' empty data'
#write position back to temp
write_position(day.strftime('%Y-%m-%d'),file_position_path,position)
print 'read_diff: new position is ',position
finally:
target_file.close()

    如上所示，是diff计算的demo代码。对每次增量计算的位移量进行保存，下次再进行增量计算时，就可以从位移量处直接读取。
    比之用diff,path少了副本相应开销，速度也可观。增加main后也可以直接在shell中调用。

    实现为module，则如下代码所示：
   
#! /usr/bin/env python
# Filename:diffmodule.py
# -*- coding: utf-8 -*-
import os
import getopt
import sys
import fnmatch
#read diff from sfile to ofile by position_file cached
def read_diff(position_file,sfile,ofile):
old_position=read_position(position_file)
tmp_position=0
source_file=None
target_file=None
try:
source_file=open(sfile,'r')
source_file.seek(old_position)
target_file=open(ofile,'a')
for temp_line in source_file:
target_file.write(temp_line)
tmp_position = tmp_position+len(temp_line)
finally:
if source_file is not None:
source_file.close()
if target_file is not None:
target_file.close()
if not tmp_position==0:
write_position(position_file,tmp_position+old_position)
return tmp_position
# read position from file
def read_position(position_file):
position=None
open_file=None
try:
if os.path.exists(position_file):
open_file=open(position_file,'r')
position = long(open_file.read())
else:
position=0
finally:
if open_file is not None:
open_file.close()
return position
#write position to file
def write_position(position_file,position):
open_file=None
result = False
try:
open_file=open(position_file,'w+')
open_file.write(str(position))
finally:
if open_file is not None:
open_file.close()
result = True
return result


   如上代码可供参考，在python 2.7.3下测试OK。