According to the post mortem from the last outage, the eventually consistent
option will have higher latency (=bad) than the strongly consistent option,
in exchange for higher availability during an unexpected failure (=good).
>From the post mortem:
"In response to this outage, we have also decided to make a major
infrastructural change in App Engine. Currently, App Engine provides a
one-size-fits-all Datastore, that provides low write latency combined
with strong consistency, in exchange for lower availability in
situations of unexpected failure in one of our serving datacenters. In
response to this outage, and feedback from our users, we have begun
work on providing two different Datastore configurations:
- The current option of low-latency, strong consistency, and lower
availability during unexpected failures (like a power outage)
- A new option for higher availability using synchronous replication
for reads and writes, at the cost of significantly higher latency"
Can someone from Google please answer these questions:
With the eventually consistent option, will both Datastore reads and writes
have higher latency, or just writes?
Does the higher availability of the eventually consistent option only apply
to Datastore reads (ie. writes will have the same availability as with the
strongly consistent option during outages)?
Nick
2010/3/18 Alkis Evlogimenos ('Αλκης Ευλογημένος) <evlogime...@gmail.com>
> Strong means all replicas will have the updates after a write returns and
> all reads after the write will "see" those updates.
>
> Consistent (or eventually consistent) means that all replicas will
> eventually have the updates after a write and some reads after the write
> will "see" stale/previous versions of the data.
>
> Basically it boils down to trading correctness (possibility of stale reads)
> for better performance.
>
> - alkis
>
>
>
> On Wed, Mar 17, 2010 at 10:48 PM, prgmratlarge <yossiele...@gmail.com>wrote:
>
>> What are strong vs consistent reads?
>>
>> On Mar 16, 11:31 pm, "Jason (Google)" <apija...@google.com> wrote:
>> > Hi Everyone. Just a quick note that we just uploaded pre-release 1.3.2
>> > SDKs for Python and Java to our Google Code project page:
>> >
>> > http://code.google.com/p/googleappengine/downloads/list
>> >
>> > Both pre-release SDKs include RELEASE_NOTE files that indicate what's
>> > new, but the App Engine back-ends have not yet been updated, so please
>> > don't try to use these new features in production just yet. Please
>> > test your existing applications locally using the new SDK and report
>> > any bugs as soon as possible. Our next general release will likely
>> > follow in the next couple of weeks barring any unforeseen issues.
>> >
>> > Thanks,
>> > - Jason
>>
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