thank you for reply > You can let app engine choose a unique key for you I have several fields which must be unique because of business logic.how can app engine make the unique key for the several fields?
>or use a transaction to first query for an existing value and then put it >only if it does not already exist For example, in a Server "A" ,I query a value and find there is no existing value , so I insert value into Server "A" successfully. But almost the same time , another one can do the same thing(insert the same record) successfully in Server "B". Because at this time , Server "A" and Server "B" are not consistent, the prior inserts will all be successful. how can I avoid to insert the same records? On Feb 5, 10:06 pm, John Patterson <[email protected]> wrote: > You can let app engine choose a unique key for you or use a > transaction to first query for an existing value and then put it only > if it does not already exist. In order to do the query and put in a > transaction the entities must all be in the same Entity group which > will reduce concurrency. If you can partition your unique values > somehow (e.g. A-D, E-H...) then you can have a root entity for each > partition to decrease contention. > > On 5 Feb 2010, at 17:41, smile laugh wrote: > > > would someone be so kind to give me an answer. > > > thanks > > > On Feb 4, 1:20 am, smile laugh <[email protected]> wrote: > >> I have another question. > > >> to a unique field which cannot have same value, before I insert a > >> record , I must check if there is same value in the field. > > >> it seems impossible to make the constraint in cloud platform. > > >> because when I insert the record , maybe other one can insert the > >> same > >> record at the same time. that will break uniqueness of the field . > > >> thanks in advance > > >> laughsmile > > > -- > > You received this message because you are subscribed to the Google > > Groups "Google App Engine for Java" group. > > To post to this group, send email to [email protected] > > . > > To unsubscribe from this group, send email to > > [email protected] > > . > > For more options, visit this group > > athttp://groups.google.com/group/google-appengine-java?hl=en > > . -- You received this message because you are subscribed to the Google Groups "Google App Engine for Java" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/google-appengine-java?hl=en.
