Wow dude, this is blowing my mind just a little.
Ok, sticking with the twitter example, I'm concerned about the edge cases.
I'd say it's easy to optimize with a relational db or any other storage for
that matter if I make the assumption that people only follow a few hundred
people and only want recent messages. However some people follow hundreds
of thousands of people. If Guy Kawasaki uses my app, I'd run into a problem
quickly.
However I see your point that I don't have to limit myself to just the
obvious relationships, but can create relationships that serve specific
purposes and use-cases such as your day example. I'm not sure how I would
want to model my use-case to allow for Guy Kawaski, I'll have to think more
about it. Is there a threshold beyond which adding relationships between
nodes causes problems? If not, or if it's high, you could create custom
relationships for every type of query you'd want to do.
However, a secondary question comes up. If we continue with the twitter
example, and I want to be able to page through results, is that directly
supported through Neo4j's API? Coming from a more traditional storage
background I tend to think of what I'd want as a sort by time and then a
skip and limit on the results (so I could say give me messages 1-100 sorted
by time descending). Is there anything equivalent in Neo4j or is the
approach totally different?
Thanks,
Lincoln
On Wed, Mar 17, 2010 at 12:41 PM, Craig Taverner <cr...@amanzi.com> wrote:
> Hi Lincoln,
>
> So it sounds like you don't need the IS_VISIBLE relations after all. The
> traverser works by following all relationships of the specified types and
> directions from each current node (as you traverse, or walk the graph). You
> can have a complex graph and traverse to high depth very fast (thousands of
> relationships per second). The traverser will also automatically check that
> the same node is not returned twice. The test for the relationship type is
> efficient. Still reasonable, but less efficient is the custom test you
> might
> put in the returnable evaluator, but if the limiting factor is usually the
> number of relationships traversed, and if that is kept managable, the
> evaluator test is no concern.
>
> I think twitter is a good case in point, even with many millions of users,
> you will still only follow perhaps a hundred and they will tweet perhaps a
> hundred, or a thousand times, so your traverser will find the 10k-100k
> messages quite quickly. This can be speeded up further, but the right
> approach depends again on your use case. The idea with using a graph
> database is that the actual usage probably maps very well to the graph
> structure, so when deciding how to speed up your search, consider how it
> will be used. In twitter one normally only cares about recent messages, so
> how about not linking directly from the user to the message, but link to an
> intermediate node representing time, for example, a day-node. Then each new
> message is added to the day node for that day, and that will automatically
> become yesterday the next day. Then your traversal can have a stop
> evaluator
> to not follow old messages (unless your query is looking for old messages,
> of course). So the 100k messages might drop to only a few hundred, or even
> just a few dozen. Certainly that will be a query of the order of
> milliseconds!
>
> Moving away from the traverser, you also have the option to call directly
> the getRelationships() methods from the node. If you structure is
> predictable, like viewer-->FOLLOWS-->user-->CREATED-->message, then two
> nested for loops would work, the outer iterating over the followers and the
> inner iterating over the messages. If you changed to add a time-based
> interim node (which is a kind of graph-index), then you need to have three
> loops. If you made your time index a deeper tree (months->days->hours,
> etc.), then you would need to further refactor the code. However, if you
> stuck with a traverser, you might not need to change the traverser even of
> the graph structure changed, as long as the same relationship types were
> maintained. Does that make sense?
>
> Cheers, Craig
>
> On Wed, Mar 17, 2010 at 4:00 PM, Lincoln <linxbet...@gmail.com> wrote:
>
> > Thanks Craig,
> >
> > I'd like to clarify my question (I don't think it changes your answer
> > though).
> >
> > I wanted all messages visible to me created by users I follow. Thus, the
> > FOLLOWS relationship is not enough. I'd need to see messages that are
> > visible to me and then check if they were created by users I follow, or
> I'd
> > need to see messages created by users I follow and then see if they're
> > visible to me.
> >
> > I assume your last example still yields the result I'm looking for.
> Could
> > you describe what actually happens here though? I'm unclear on what the
> > traversal looks like. Would it first traverse every outgoing FOLLOWS
> > relationship from the viewer, yielding other users, and then traverse all
> > the CREATED relationships to get to messages?
> >
> > Also, given very large numbers of FOLLOWS and CREATED relationships (with
> > say, a twitter graph), how is this made efficient?
> >
> > Sorry for all the basic questions but I couldn't find this information in
> > the docs. If there's something I should be reading before posting these
> > questions, please point me to it.
> >
> > Thanks!
> >
> > Lincoln
> >
> > On Wed, Mar 17, 2010 at 7:06 AM, Craig Taverner <cr...@amanzi.com>
> wrote:
> >
> > > I'm uncertain about one ambiguity in your model, you are able to find
> > > messages through FOLLOWS and IS_VISIBLE_BY. These will give two
> different
> > > sets, and my first impression was that FOLLOWS gives you the right
> > answer.
> > > In other words you want to query for 'all messages by users I follow'?
> In
> > > that case you do not need IS_VISIBLE_BY. However, if there are messages
> > by
> > > people you follow, but are not allowed to see, then you also need the
> > > IS_VISIBLE_BY. But I would still reconsider linking directly from the
> > > viewer
> > > to the message for that case. I'd rather have the messages linked to
> some
> > > categorization structure for things like 'public', 'private', etc.
> > >
> > > Anyway, here are some suggestions for the various approaches above:
> > > *'all messages by users I follow'*
> > > val msgs = viewer.traverse(
> > > Order.BREADTH_FIRST, StopEvaluator.END_OF_GRAPH,
> > > (tp: TraversalPosition) => IsMessage(tp.currentNode()),
> > > Rels.FOLLOWS, Direction.OUTGOING,
> > > Rels.CREATED, Direction.OUTGOING)
> > >
> > > *'all messages visible to me'*
> > > val msgs = viewer.traverse(
> > > Order.BREADTH_FIRST, StopEvaluator.END_OF_GRAPH,
> > > ReturnableEvaluator.ALL_BUT_START_NODE,
> > > Rels.IS_VISIBLE_BY, Direction.INCOMING)
> > >
> > > *'all messages, visible to me, by people I follow'*
> > > val msgs = viewer.traverse(
> > > Order.BREADTH_FIRST, StopEvaluator.END_OF_GRAPH,
> > > (tp: TraversalPosition) => {
> > > val msg = tp.currentNode()
> > > IsMessage(msg) && IsVisibleBy(msg,viewer)
> > > },
> > > Rels.FOLLOWS, Direction.OUTGOING,
> > > Rels.CREATED, Direction.OUTGOING)
> > >
> > > Of course I assume you make the utility functions IsMessage(node: Node)
> > and
> > > IsVisibleBy(msg: Node, user: Node), and these will test the existance
> of
> > > properties and relations as appropriate to make the decision.
> > >
> > >
> > > On Wed, Mar 17, 2010 at 6:32 AM, Lincoln <linxbet...@gmail.com> wrote:
> > >
> > > > Hi, I've just started looking at Neo4j and I'm quite intrigued.
> > However,
> > > > the cognitive dissonance that I've grown so used to in modeling
> storage
> > > is
> > > > proving to be a bit difficult to let go at this early stage :)
> > > >
> > > > I was hoping that if someone could help me through an example I would
> > be
> > > > able to grok how to properly structure my data and query it in Neo4j.
> > > >
> > > > Nodes:
> > > > Message( text: String )
> > > > User( id: Long )
> > > >
> > > > Relationships:
> > > > CREATED
> > > > FOLLOWS
> > > > IS_VISIBLE_BY
> > > >
> > > > So I might have a graph with entries like so:
> > > >
> > > > User(1) --> CREATED --> Message("i woke up late today")
> > > > User(2) --> CREATED --> Message("hello")
> > > > User(3) --> CREATED --> Message("ugh, i hate mondays")
> > > >
> > > > User(1) --> FOLLOWS --> User(2)
> > > >
> > > > Let's also say all messages are visible to User 1.
> > > >
> > > > Message("i woke up late today") --> IS_VISIBLE_BY --> User(1)
> > > > Message("hello") --> IS_VISIBLE_BY --> User(1)
> > > > Message("ugh, i hate mondays") --> IS_VISIBLE_BY --> User(1)
> > > >
> > > > So, I can do a simple traversal for visible:
> > > >
> > > > val graphDb = new EmbeddedGraphDatabase( "path/to/neo4j-db" )
> > > > val index = new LuceneIndexService( graphDb )
> > > > val viewer = index.getSingleNode("id", 1)
> > > > val msgs = viewer.traverse( Order.BREADTH_FIRST,
> > > > StopEvaluator.END_OF_GRAPH,
> > > > ReturnableEvaluator.ALL_BUT_START_NODE, Rels.IS_VISIBLE_BY,
> > > > Direction.INCOMING)
> > > > msgs.toList.map(_.toJson).mkString("{ msgs : [", ",", "] }") //
> > assuming
> > > i
> > > > have the relevant functions
> > > >
> > > > But let's say that this is going to return too many messages. Just
> > > because
> > > > all the messages are possibly visible to me, doesn't mean I want to
> see
> > > > them
> > > > all. So, I'd like to additionally filter by the FOLLOWS
> relationship.
> > > I'd
> > > > like to express "get all messages that are visible and were created
> by
> > a
> > > > user that I follow." Can someone show me an example of how to do
> that?
> > > >
> > > > I'm guessing that you need to implement a custom ReturnableEvaluator,
> > but
> > > I
> > > > don't understand how you traverse multiple relationships at the same
> > > time.
> > > >
> > > > Thanks,
> > > > Lincoln
> > > > _______________________________________________
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