GAE HR is one step towards scalability, separating commit and visibility,
but one step back in usability. This separation creates a problem (the
"eventual consistency"?) that kills its use in some application designs.
Usually, I commit inside a http request and redirect to another page that
shows the results, but I have to make a small delay (many milliseconds) to
try to get the changes of that commit. This is the easiest workaround I
find for this, not a fix. Unfortunately, I do not have a guarantee that I
got the changes of that commit. Due to multiple computing synchronism
paradigm, I think it is not possible to know how long the change updates
will take, what could take milliseconds, minutes or even days (in an
improbable case).
I was wondering I could "fix" this by using timestamps, what are easy pass
to another request in a query string or to store in a session memcache
variable. If I have the timestamp of my last commit, any later query to the
database could use this timestamp to check if the retrieved data was
committed after that timestamp, if not, it retries the query again and
again. This way I have the guarantee of the correct visibility ("strong
consistency"?), even if takes a long time. It can eventually read data
committed by another user/process after that timestamp, what I do not
consider a problem. Is this approach correct?
I can implement this with a timestamp field in every table, but a lib would
be better (built in timestamp fields, automatic tuning of polling time,
internal cache session timestamp of last commit, and maybe some low level
database callback instead of the polling). Is there a third party lib like
this? I know ORM libs that use GAE low level database APIs, but I have no
idea how to start something like that.
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