> GAE only provides transaction at the level of the single record.
Appengine supports transactions at the level of an entity group.
> GAE does not support (and in my view it not support anytime soon)
> multi record transactions (the kind of thing you really need for any
> reliable business app).
An entity group could contain any number of records. Within a
transaction, you can get(), put(), or delete() by key.
Transactions could be added to web2py dal:
def mytxn(db,id):
record = db(db.table.id==id).select()[0] #model.get(id)
record.update_record(count=record.count+1) #entity.put()
return record.count
txn = db.build_transaction(mytxn)
if not txn(id):
raise Exception("aborted transaction")
print "success"
count = txn.result
The db could be passed into the transaction by default, and the dal
should catch any transaction related exceptions.
db.build_transaction() would be this on GQLDB:
class GQLDB(...):
def build_transaction(self,txn):
from google.appengine.ext import db as google_db
from google.appengine.ext.db import google_exception
class TryTxn(object):
def __init__(self,db,txn):
self.db = db
self.txn = txn
self.result = None
def __call__(self,*args):
try:
self.result = google_db.run_in_transaction
(self.txn,self.db,*args)
except google_exception.TransactionFailedError, e:
return False
return True
return TryTxn(self,txn)
And like this on SQL databases:
class SQLDB(...):
def build_transaction(self,txn):
class TryTxn(object):
def __init__(self,db,txn):
self.db = db
self.txn = txn
self.result = None
def __call__(self,*args):
self.result = self.txn(self.db,*args)
return True
return TryTxn(self,txn)
For any database it is used like this:
txn = db.build_transaction(mytxn)
if not txn(id):
raise Exception("aborted transaction")
print "success"
count = txn.result
Calling the txn(...) returns whether the txn suceeded or failed, so
you do not have to handle any custom exceptions outside the dal. For
SQLDB the txn(...) will allways return True.
Robin
On Jan 18, 12:55 am, mdipierro <mdipie...@cs.depaul.edu> wrote:
> GAE only provides transaction at the level of the single record. Robin
> has a patch that allows that and it will be in 1.56. This is only
> usefult for things like incrementing a counter.
>
> GAE does not support (and in my view it not support anytime soon)
> multi record transactions (the kind of thing you really need for any
> reliable business app).
>
> Massimo
>
> On Jan 17, 10:38 pm, Jonathan Benn <jonathan.b...@gmail.com> wrote:
>
> > On Jan 15, 5:10 pm, Robin B <robi...@gmail.com> wrote:
>
> > > Transactions and list properties are also not supported, until
> > > transactions are officially added to web2py, you can still do
> > > transactions this way:
>
> > Woah, transactions are not supported in GAE? That's news to me!
>
> > From what you're writing, it sounds like this is a limitation with
> > web2py and not with GAE, right? It's just that we haven't figured out
> > a generic way to do it right in web2py yet?
>
> > Thanks,
>
> > --Jonathan
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