On 17.02.2016 08:16, Stas Malyshev wrote:

Hi!

(2) Shouldn't BlazeGraph do the caching (too)? It knows how much a query
costs to re-run and it could even know if a query is affected by a data

BlazeGraph does a lot of caching, but it's limited by the memory and it
AFAIK does not do whole query caching (like mysql does, for example) -
which means if you run two big queries one after another, the latter
could remove from cache what the former put there. Its caching, AFAIK,
is on much lower level. Which is helpful too since different queries
share a lot of underlying data, but not exactly our case here.

update (a cache might still be the same as a current result even after
many data changes). Having several caching layers is useful, but the
more elaborate (query-structure dependent) caching strategies should
maybe be left to the database.

I don't think Blazegraph does anything like resolving changes to see if
query results changed, that sound like pretty hard thing to do in triple
store. You can manually store specific query result AFAIK but that's
just form of writing data as I understand and may not be very scalable.

Yes, in general this would be extremely hard. There are some easy cases one could catch, but it is not clear how effective this would be for our load. I am just saying we should not try to build a query-aware caching strategy that would better be done on a lower level.

The points (3)-(5) are based on guessing. As Magnus said, some analysis
could help to confirm or refute this. On the other hand, caching should
not just focus on current usage patterns only, but consider a bit what
could happen in the future.

Well, again the problem is that one use case that I think absolutely
needs caching - namely, exporting data to graphs, maps, etc. deployed on
wiki pages - is also the one not implemented yet because we don't have
cache (not only, but one of the things we need) so we've got chicken and
egg problem here :) Of course, we can just choose something now based on
educated guess and change it later if it works badly. That's probably
what we'll do.

Yes, it is hard to predict what load this will create. The caching levels around Wikipedia prevent re-computation of the page on most page views, so maybe there would not actually be very many repeated requests for the same query coming from tOne option could be a dedicated caching layer just for such wiki uses. On the one hand, the set of all embedded queries is known upfront (so, in contrast to other uses, you already know which queries will be asked). On the other hand, users may wish to do a forced refresh his side. The main danger again seems to be bursts of activity (a page getting a lot of edits in a short time, and each edit invalidates the ParserCache and requires refetching query results). On the positive side, this specific usage of WDQS can pass its own caching parameters (which we can control), so if there is a caching layer in place, one could react to issues on short notice by being more conservative there than for other queries.

The interesting thing about the wiki-embedding usage is that it requires quick propagation of changes. Scenario: a user visits a Wikipedia page with a map created from a query; the user finds an outdated item on the map; she goes to Wikidata to fix it, and refreshes (edits) the page to see the change. Now if she is too quick, the change will not have made it into the query result yet -- she could try in a minute or so. However, if we have a long caching period, her first query will have populated the cache and prevent the update from showing for the maximal amount of time (the whole cache period). This seems like a case where long caching would be rather bad for user experience.

Markus

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