Metakit is a reliable/lightweight/fast database library that python can use. Metakit has an extend mode that enables many simultaneous readers and one writer w/out needing a database server to synchronize things. Here is an example showing how to use that cool feature.
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# 0 = open read-only, most recently committed contents
# 1 = open in exclusive read-write mode
# 2 = open in "commit-extend" mode
main='test.mk';aside='test.mka'
#initalize
if not os.path.exists(main):
print 'initalizing'
db = metakit.storage(main, 1)
db.getas("a[i:S]")
db.commit()
dba = metakit.storage(aside, 1)
db.aside(dba)
del db
del dba
#commits w/extend do not
#change old read
old_read = metakit.storage(main, 0)
old_reada = metakit.storage(aside, 0)
old_read.aside(old_reada)
vw = old_read.view("a")
old_len=len(vw)
print 'old length=',len(vw)
#now do commits with extend on aside file
#main is read-only aside is commit-extend
db = metakit.storage(main, 0)
dba = metakit.storage(aside, 2)
db.aside(dba)
vw = db.view("a")
vw.append(i=str(time.asctime()))
vw.append(i=str(time.asctime()))
db.commit()
dba.commit()
del db
del dba
#a new read should see the update
#extended in the aside file
new_read = metakit.storage(main, 0)
new_reada = metakit.storage(aside, 0)
new_read.aside(new_reada)
vw = new_read.view('a')
print "new length(should be %i)="%(old_len+2),len(vw)
#the old view should not be changed
vw = old_read.view('a')
print "old length(should still be %i)="%(old_len),len(vw)
#empty aside into main
#this is unsafe what extend enables
#you to do, is to control when
#to do this operation
empty_aside=1
if empty_aside:
print 'Empty aside into main:',
print 'not safe for readers!'
db = metakit.storage(main, 1)
dba = metakit.storage(aside, 1)
db.aside(dba)
db.commit(1)
main_only = metakit.storage(main, 0)
vw=main_only.view('a')
print 'main now has',len(vw)
del main_only
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Ultimately, the extend mode allows you to control when you want to do an unsafe write which empties the extended aside file into the main db and compacts it.
Otherwise, you can simply extend the aside file. The trade-off is that extended files can become unwieldly.
Tags: database
