Hi Alexander,
thank you for sharing with us details about data.ox.ac.uk and pointing
me at https://github.com/oucs/humfrey (who needs documentation when you
have the source code? ;-)).
I have been thinking about pros/cons of having a custom/additional index
coupled with a TDB dataset and keeping it up-to-date (and/or in sync with
TDB).
I see two approaches:
1. internal to Jena
- pros
- simplicity for users, it works out of the box
- ...
- cons
- it requires an internal notification sub-system (which we have,
but it does not covers all possible paths and it might impact
performances)
- it might create expectations that indexes will never go out of
sync (while it might happen)
- ...
2. external to Jena
- pros
- relatively easy to implement assuming SPARQL and external index
APIs
- ...
- cons
- it requires an additional service
- it isn't simple (or possible) with certain SPARQL Update requests
- ...
Re: sanity/insanity, I don't comment.
We have a system which intercept all the update requests, it puts into a
sort of key-value store in S3 with a cache in front of it. We have a
queuing/messaging system which applies changes to replicas on different
nodes taking stuff from there. Nodes can be different types: RDF stores,
free-text indexes, etc. In this scenario, update requests cannot be
unconstrained SPARQL queries, but you can replay updates and apply them
to different type of nodes/indexes. Some of the stuff is available here:
https://github.com/talis/
I imagine it is the case for you as well, it's not something you can
just download, unzip and run as it is. This sort of simplicity is
something IMHO not to underestimate and it is what drives me towards
option 1. above.
Knowing what others are doing it is certainly useful to better understand
what's needed.
Thanks,
Paolo
PS:
I am not going to ask you what you do for: monitoring, backups,
high-availability, load balancing, etc. ;-)
Alexander Dutton wrote:
> Hi Paolo,
>
> On 06/03/12 14:56, Paolo Castagna wrote:
>> Alexander Dutton wrote:
>>> This is the way we're going with our site, data.ox.ac.uk. After
>>> each update to the triplestore we'll regenerate an ElasticSearch
>>> index from a SPARQL query. […]
>> interesting...
>>
>> How do you update your triplestore (SPARQL Update, Jena APIs via
>> custom code, manually from command line, ...)?
>
> Our administration interface manages grabbing data from elsewhere,
> transforming it in various ways, and then uses the graph store HTTP
> protocol to push it into Fuseki. Once that's done it fires off a
> notification on a redis pubsub channel to say "this update just completed".
>
> There's then something that listens on the relevant channel which will
> perform the ElasticSearch update. (There are other things that handle
> uploading dataset metadata to thedatahub, and archiving datasets for
> bulk download).
>
> There's code at https://github.com/oucs/humfrey, but it's a bit of a
> nightmare to set up and (surprise, surprise) lacks documentation. The
> ElasticSearch stuff is still in development on the elasticsearch branch.
> At some point I'll find the time to make it easier to install and create
> a demo site. (as you may have noticed, the whole thing is an eclectic
> mix of technologies; Django, ElasticSearch, redis, PostgreSQL, Apache
> httpd…)
>
>> We (still) have two related JIRA 'issues':
>>
>> - LARQ needs to update the Lucene index when a SPARQL Update request
>> is received https://issues.apache.org/jira/browse/JENA-164
>>
>> - Refactor LARQ so that it becomes easy to plug in different indexes
>> such as Solr or ElasticSearch instead of Lucene
>> https://issues.apache.org/jira/browse/JENA-17
>>
>> I am still unclear how to intercept all the possible update routes
>> (i.e. SPARQL Update, APIs, bulk loaders, etc...).
>
> Our approach is to limit the ways in which updates can happen (i.e.
> things will become inconsistent if it doesn't happen through our admin
> interface). This obviously doesn't work in the general case, but could
> be a useful half-way house (e.g. say "'INSERT … WHERE …' will leave you
> with a stale index. If you care, use 'CONSTRUCT' and the graph store
> protocol instead").
>
>> But, I think it would be useful to allow people to use Apache Solr
>> and/or ElasticSearch indexes (and/or other custom indexes) and keep
>> those up-to- date when changes come in.
>
> For external indexes presumably you either need something that gets
> hooked into the JVM and listens for updates there, or a way to push
> notifications to external applications/services when things happen.
>
>> What do you store in ElasticSearch?
>
> Technically, nothing yet, as I'm still implementing it ;-). Once it's
> implemented it'll build indexes tailored to the types of modelling
> patterns we expect to have in the store. For example, we might SPARQL
> for organisations like <http://is.gd/gsc1Zs> and for each create a chunk
> of JSON to feed into ElasticSearch. Targets for indexing so far include
> organisations, people, vacancies, courses, and equipment. We'll add more
> indexes as we add new types of things.
>
>
> All the best,
>
> Alex
>
> PS. I'd be interested to know whether our approach is generally
> considered sane…
