Hey Ori!

Am 02.10.2014 06:45, schrieb Ori Livneh:

I'm embarrassed to say that I don't know nearly enough about Wikidata to be able to make a recommendation. Where would you recommend I look if I wanted to understand the caching architecture?

And I'm embarressed to say that we have very little high level documentation. There is no document on the overall caching architecture.

The use case in question is accessing data Items (and other Entities, like Properties) from client wikis like Wikipedia. Entities are accessed through an EntityRevisionLookup service; CachingEntityRevisionLookup is an implementation of EntityRevisionLookup that takes an "actual" EntityRevisionLookup (e.g. a WikiPageEntityRevisionLookup) and a BagOStuff, and implements a caching layer.

We use two CachingEntityRevisionLookup nested into each other: the outer-most uses a HashBagOStuff to implement in-process caching, the second level uses memcached. The objects that are cached there are instances of EntityRevision, which is a thin wrapper around an Entity (usually, an Item) plus a revision ID.

Please let me know if you have further questions!

-- daniel

PS: what do you think, where should this info go? Wikibase/docs/caching.md or some such?

Am 02.10.2014 17:55, schrieb Jeroen De Dauw:

Hey,

We use two CachingEntityRevisionLookup nested into each other: the outer-most
uses a HashBagOStuff to implement in-process caching, the second level uses
memcached.

It is odd to have two different decorator instances for caching around th EntityRevisionLookup. I suggest to have only a single decorator for caching, which writes to a caching interface. Then this caching interface can have an implementation what uses multiple caches, and perhaps have a decorator on that level.

Went that way first. Didn't work out nicely. I forget why exactly.

I don't care much either way.

On 10/02/2014 09:22 AM, Nikolas Everett wrote:

On Thu, Oct 2, 2014 at 12:45 AM, Ori Livneh <ori@wikimedia.org mailto:ori@wikimedia.org> wrote:

On Wed, Oct 1, 2014 at 2:46 PM, Daniel Kinzler
<daniel.kinzler@wikimedia.de <mailto:daniel.kinzler@wikimedia.de>> wrote:

    We currently use memcached to share cached objects across wikis, most
    importantly, entity objects (like data items). Ori suggested we
    should look into
    alternatives. This is what he wrote:

    [21:15] <ori> I was wondering if you think the way you use memcached
    is optimal
    (this sounds like a loaded question but I mean it sincerely). And if
    not, I was
    going to propose that you identify an optimal distributed object
    store, and I
    was also going to offer to help push for procurement and deployment
    of such a
    service on the WMF cluster.
    [21:17] <ori> memcached is a bit of a black box. it is very
    difficult to get
    comprehensible metrics about how much space and bandwidth you're
    utilizing,
    especially when your data is mixed up with everything else that goes
    into memcached
    [21:18] <ori> and the fact that you're serializing objects using php
    serialize()
    rather than simple values makes it even harder, because it means
    that you can
    only really poke around from php with wikidata code available


The other major problem with memcache is that it doesn't support complex
data structures like lists, queues, sets, or maps, so when you want to
do things like, say, push or pop an item from a queue, you end up having
to retrieve the entire collection, unserialize it, manipulate it
locally, re-serialize it, and transmit it back in its entirety.

Use more Redis maybe?

Or use actual storage.