Thanks John.  I appreciate the various ways of accessing this data, but when
you guys make updates to any of these, can you either do it in a beta
environment we can test in first, or earlier in the week?  Where there are
very few Twitter engineers monitoring these lists during the weekends, and
we ourselves often have other plans, this really makes for an interesting
weekend for all of us when changes go into production that break code.  It
happens, but it would be nice to have this earlier in the week, or in a beta
environment we can test in.
Also, when things like this do happen, is there a way you can lift following
limits for specific users so we can correct the wrong with out customers?


On Sun, Sep 6, 2009 at 8:59 AM, John Kalucki <> wrote:

> I can't speak to the policy issues, but I'll share a few things about
> social graph backing stores.
> To put it politely, the social graph grows quickly. Projecting the
> growth out just 3 or 6 months causes most engineers to do a spit-
> take.
> We have three online (user-visible) ways of storing the social graph.
> One is considered canonical, but it is useless for online queries. The
> second used to handle all queries. This store began to suffer from
> correctness and internal inconsistency problems as this store was
> pushed well beyond its capabilities. We recognized this issue long
> before the issues became critical, allocated significant resources,
> and built a third store. This store is correct (eventually
> consistent), internally consistent, fast, efficient, very scalable,
> and we're very happy with it.
> As the second system was slagged into uselessness, we had to cut over
> the majority of the site to the third system when the third reached a
> good, but not totally perfect, state. As we cut over, all sorts of
> problems, bugs and issues were eliminated. Hope was restored, flowers
> bloomed, etc. Yet, the third store has two minor user-visible flaws
> that we are fixing. Note that working on a large critical production
> data store with heavy read and write volume takes time, care and
> resources. There is minor pagination jitter in one case and a certain
> class of row-count-based queries have to be deprecated (or limited)
> and replaced with cursor-based queries to be practical. For now, we're
> sending the row-count-queries queries back to the second system, which
> is otherwise idle, but isn't consistent with the first or third
> system.
> We also have follower and following counts memoized in two ways that I
> know about, and there's probably at least one more way that I don't
> know about.
> Experienced hands can intuit the trade-offs and well-agonized choices
> that were made when we were well-behind a steep growth curve on the
> social graph.
> These are the cards.
> -John Kalucki
> Services, Twitter Inc.

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