Thanks... while I didn't follow it exactly, I get the gist of what's going on. Sounds like I should expect five- or six-sigma probabilities of minute+ eventuality in global query indexes.
Jeff On Tue, Sep 20, 2011 at 1:28 PM, Alfred Fuller <arfuller+appeng...@google.com> wrote: > Ikai is correct to think about replication in this case. In a single replica > you could have one of three states: > Applied - fully visible > Committed - has the log entry, but has yet to apply it > Missing - the log entry has yet to be replicated > Only in the first case is it visible to a global query. When you write > something, the log is committed to at least a majority of replicas. The > datastore returns success, then immediately tries to apply the write > everywhere it committed the log entry. It usually takes a couple hundred ms > to apply. This is why the majority of cases take O(100 ms) to become > visible. For a very small % of writes, the write either cannot commit to the > local replica or cannot be applied after the commit. In these cases the > datastore will still return success, but the write won't be visible until a > background process picks it up and applies it. In these case it can take > O(minutes) to be picked up and replicated/applied. If there is something > wrong in the replica you are querying (for example replication is backed up > or the bigtabale is unavailable or the background processes in that replica > are having issues), then it could take a deal longer (this becomes very very > unlikely very quickly, but not impossible). There really is no hard upper > bounds because distributed systems will have pieces that fail (and are > designed to still function when they do). > - Alfred > On Tue, Sep 20, 2011 at 10:10 AM, Ikai Lan (Google) <ika...@google.com> > wrote: >> >> Well, indexes are just Bigtable rows, so replication lag does apply to >> them as well. >> -- >> Ikai Lan >> Developer Programs Engineer, Google App Engine >> plus.ikailan.com | twitter.com/ikai >> >> >> On Tue, Sep 20, 2011 at 7:42 AM, Mike Wesner <mbwes...@gmail.com> wrote: >>> >>> And then I went and used the word replication... i meant index lag. >>> >>> On Sep 20, 9:40 am, Mike Wesner <mbwes...@gmail.com> wrote: >>> > I don't think Ikai read your post... >>> > >>> > Robert and I wanted to write a little HRD status site to track this >>> > and get real data, but we haven't done so yet. I have never seen the >>> > replication take more than about 1s. I think 1s will cover about four >>> > 9's, but that is just an educated guess. Until we (the users) >>> > actually measure this over time I don't think we can know for sure. >>> > >>> > -Mike >>> > >>> > On Sep 19, 7:16 pm, Jeff Schnitzer <j...@infohazard.org> wrote: >>> > >>> > >>> > >>> > >>> > >>> > >>> > >>> > > I know that an index update in the HRD will typically be visible >>> > > within a couple seconds. That's the average case. What is the >>> > > worst-case? >>> > >>> > > Assuming something in the datacenter goes wacky, how long might it >>> > > take for an index to update? Tens of seconds, minutes, hours, days? >>> > >>> > > Thanks, >>> > > Jeff >>> >>> -- >>> 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-appengine@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. >>> >> >> -- >> 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-appengine@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. > > -- > 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-appengine@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. > -- 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-appengine@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.
