Re: [google-appengine] App Datastore Update timing issue
I think you can handle this with a datastore transaction. See http://code.google.com/appengine/articles/transaction_isolation.html for more details. The increment_counter example at http://code.google.com/appengine/docs/python/datastore/transactions.htmlshould be adaptable to your example. -Chris On Wed, Jul 14, 2010 at 10:17 AM, Sam couponsys...@gmail.com wrote: In working with the data store, I've been able to solve all of my issues (coming from a RDBMS background, had to rethink a few things!), save for one. In the application I'm working on, it's necessary to reserve a record at a moment in time and then display that record to the end user, ensuring that each user gets a single record and no duplicates are issued. Working with the record prior, and using SQL as an engine, I was able to use the following query: UPDATE CCode SET SessionID = ' + strSessionID + ', Assigned = 1 WHERE Assigned = 0 AND PartnerID = + str(intPartnerID) + AND ExpireDate = ' + str(dtCompareDate) + ' Limit 1 This would update the record in one pass and effectively reserve the record for the established session, with no time lost inbetween issuing the update and recording the change while SQL handled any conflict resolution should two people request a record at the same time. However, dealing with datastore, I seem to be running into some problems with timing and wanted to know if anyone had any creative ways around it. I realize that I can run a standard GqlQuery to return the results I need and then update based on the returned result and add the session to the record without a problem. What concerns me is the timing factor. During the time it takes for the query to execute and the resulting put() statement, it would be possible for another user to arrive and make the same basic query recalling the same record set. I know I could check for updates against the datastore before writing back (used to do this years ago...), but this could potentially cause a deadlock condition when the load on the system became greater and multiple simultaneous users needed access to the records for selection and updating. Any thoughts on how to work around this? -- You received this message because you are subscribed to the Google Groups Google App Engine group. To post to this group, send email to google-appeng...@googlegroups.com. To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.comgoogle-appengine%2bunsubscr...@googlegroups.com . For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en. -- You received this message because you are subscribed to the Google Groups Google App Engine group. To post to this group, send email to google-appeng...@googlegroups.com. To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
[google-appengine] App Datastore Update timing issue
In working with the data store, I've been able to solve all of my issues (coming from a RDBMS background, had to rethink a few things!), save for one. In the application I'm working on, it's necessary to reserve a record at a moment in time and then display that record to the end user, ensuring that each user gets a single record and no duplicates are issued. Working with the record prior, and using SQL as an engine, I was able to use the following query: UPDATE CCode SET SessionID = ' + strSessionID + ', Assigned = 1 WHERE Assigned = 0 AND PartnerID = + str(intPartnerID) + AND ExpireDate = ' + str(dtCompareDate) + ' Limit 1 This would update the record in one pass and effectively reserve the record for the established session, with no time lost inbetween issuing the update and recording the change while SQL handled any conflict resolution should two people request a record at the same time. However, dealing with datastore, I seem to be running into some problems with timing and wanted to know if anyone had any creative ways around it. I realize that I can run a standard GqlQuery to return the results I need and then update based on the returned result and add the session to the record without a problem. What concerns me is the timing factor. During the time it takes for the query to execute and the resulting put() statement, it would be possible for another user to arrive and make the same basic query recalling the same record set. I know I could check for updates against the datastore before writing back (used to do this years ago...), but this could potentially cause a deadlock condition when the load on the system became greater and multiple simultaneous users needed access to the records for selection and updating. Any thoughts on how to work around this? -- You received this message because you are subscribed to the Google Groups Google App Engine group. To post to this group, send email to google-appeng...@googlegroups.com. To unsubscribe from this group, send email to google-appengine+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
Re: [google-appengine] App Datastore Update timing issue
Hi Sam,
Is there any way you could slightly refactor the query so that it
can be run in a transaction? Transactions can only operate on one
entity group at a time; so, perhaps you can set the a partner as the
parent of CCode.
So your query (in Python) could look more like:
def txn():
partner_key = db.Key.from_path('Partner', intPartnerID)
ccode = CCode.all().filter('Assinged', 0)\
.filter('ExpireDate =', dtCompareDate)\
.ancestor(partner_key).get()
if not ccode:
# handle no ccode
ccode.SessionID = strSessionID
ccode.Assigned = 1
db.run_in_transaction(txn)
On Wed, Jul 14, 2010 at 11:17 AM, Sam couponsys...@gmail.com wrote:
In working with the data store, I've been able to solve all of my
issues (coming from a RDBMS background, had to rethink a few
things!), save for one. In the application I'm working on, it's
necessary to reserve a record at a moment in time and then display
that record to the end user, ensuring that each user gets a single
record and no duplicates are issued. Working with the record prior,
and using SQL as an engine, I was able to use the following query:
UPDATE CCode SET SessionID = ' + strSessionID + ', Assigned = 1
WHERE Assigned = 0 AND PartnerID = + str(intPartnerID) + AND
ExpireDate = ' + str(dtCompareDate) + ' Limit 1
This would update the record in one pass and effectively reserve the
record for the established session, with no time lost inbetween
issuing the update and recording the change while SQL handled any
conflict resolution should two people request a record at the same
time.
However, dealing with datastore, I seem to be running into some
problems with timing and wanted to know if anyone had any creative
ways around it.
I realize that I can run a standard GqlQuery to return the results I
need and then update based on the returned result and add the session
to the record without a problem. What concerns me is the timing
factor. During the time it takes for the query to execute and the
resulting put() statement, it would be possible for another user to
arrive and make the same basic query recalling the same record set. I
know I could check for updates against the datastore before writing
back (used to do this years ago...), but this could potentially cause
a deadlock condition when the load on the system became greater and
multiple simultaneous users needed access to the records for selection
and updating.
Any thoughts on how to work around this?
--
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Google App Engine group.
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--
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