Re: [Neo] Traversers in the REST API
I definitely agree that limiting or paging a set of results is probably not very useful without some sort of sorting. The (only) benefit of pushing sorting to the client is that the client might be able to filter the result further before sorting it. Since sorting is generally the most expensive operation it should be done as late as possible. However the idea of semi-sorting, to get only one page of sorted results at each request, that was mentioned in some thread yesterday sounds quite compelling. I agree that an equivalent of LIMIT, OFFSET and ORDER BY is a good target. As to indexing: the structure of the graph IS the index to a large extent. This means that a well designed graph would often not need paging if the traversal is done right. There are however some cases where this is hard to accomplish and we need to work on supporting those cases better. Remember that a Graph Database is NOT a Relational Database. A lot of the ideas people have about databases are based on their knowledge of Relational Databases. I understand that it can be hard, but if that baggage could be left at the door it would make things a lot easier. Nobody is saying that Relational Databases are dead (except for some publicity stunts) far from it! What we (and a lot of other people) are saying is that the age of one database to rule them all is over. Different problems are best solved with different kinds of databases, RDBMSes are great at some, K/V stores some, and Graph Databases are great for some. Then there are some problems that are best solved with a combination of two or more (kinds of) databases, where each database brings its own strengths to the table, and is used only for the things it is good at. That's enough deviation from the topic, my conclusions remain the same as they were before this discussion started, I will state them in as few words as possible and in bullet point form to convey them as clearly as I can: * The REST API will probably need result set limiting or pagination. * Limiting and pagination will require (server side) sorting * Sorting can be better implemented if it's implemented in the core of the traversal framework * Limiting / Pagination can be deferred for a while until we know what it needs to look like (from looking at actual uses) * (Server side) Sorting can be deferred until we need it for limiting / pagination Peace, Tobias On Thu, Apr 8, 2010 at 10:17 PM, Michael Ludwig mil...@gmx.de wrote: Tobias Ivarsson schrieb am 08.04.2010 um 18:23:27 (+0200) [Re: [Neo] Traversers in the REST API]: On Wed, Apr 7, 2010 at 3:05 PM, Alastair James al.ja...@gmail.com wrote: when we start talking about returning 1000s of nodes in JSON over HTTP just to get the first 10 this is clearly sub-optimal (as I build websites this is a very common use case). So, as you say, sorting and limiting can wait, but I suspect the HTTP API would benefit from offering it. Limiting need not require changes to the core API, it could be implemented as a second stage in the HTTP API code prior to output encoding. For paging / limiting: yes, you are absolutely right, this would not effect the core API at all, only the REST API. Limiting/paging is something we would probably add to the REST API before sorting. Limiting and paging usually go hand in hand with sorting, in my experience. Why would anyone want to page through an unsorted collection? Sorting might be a similar case, but I still think the client would be better fitted to do sorting well. The server has indexes to support the sorting. (If it doesn't, it has a problem anyway.) What does the client have to support sorting? So how would it be better fitted to do sorting well? But once paging / limiting is added it would be quite natural / useful to add sorting as well. What I want to avoid is keeping state on the server while waiting for the client to request the next page. If you ensure a binary tree index is used to do the sorting, you should be fine. -- Michael Ludwig ___ Neo mailing list User@lists.neo4j.org https://lists.neo4j.org/mailman/listinfo/user -- Tobias Ivarsson tobias.ivars...@neotechnology.com Hacker, Neo Technology www.neotechnology.com Cellphone: +46 706 534857 ___ Neo mailing list User@lists.neo4j.org https://lists.neo4j.org/mailman/listinfo/user
Re: [Neo] Traversers in the REST API
Since in manycases the results of a query will need to be reformed into
their associated domain objects, we've chosen to do our sorting at that
point (and on the server). We do our (primary) filtering within the
traversal/DB-domain object processes. That seems to work well.
Pagination is kinda tricky if the data changes between subsequent
requests for pages. Since pagination is generally used for UIs, a
common approach is to place the entire dataset (or a cursor, depending
on where the data is coming from) in a session object. Regardless of
where it is kept, if you want to deal with data changes, you either
have to a) invalidate the cached dataset if data changes or b) keep a
copy of the whole dataset around in its as queried state so that
subsequent paging requests are consistent. Either case involves
keeping a fairly big duplicate data structure on the server or middle
tier and violates one of the objectives of REST-ful APIs, which is that
of statelessness. For that reason, I personally think the REST-ful API
shouldn't deal with paging. It should probably be done at some
intermediate level as needed by applications. We can certainly build a
separate API that we can all leverage if needed, but I don't think it
should be in the core REST-ful layer.
Just my $0.02, after taxes.
Original Message
Subject: Re: [Neo] Traversers in the REST API
From: Tobias Ivarsson tobias.ivars...@neotechnology.com
Date: Fri, April 09, 2010 4:00 am
To: Neo user discussions user@lists.neo4j.org
I definitely agree that limiting or paging a set of results is probably
not
very useful without some sort of sorting. The (only) benefit of pushing
sorting to the client is that the client might be able to filter the
result
further before sorting it. Since sorting is generally the most
expensive
operation it should be done as late as possible. However the idea of
semi-sorting, to get only one page of sorted results at each request,
that
was mentioned in some thread yesterday sounds quite compelling.
I agree that an equivalent of LIMIT, OFFSET and ORDER BY is a good
target.
As to indexing: the structure of the graph IS the index to a large
extent.
This means that a well designed graph would often not need paging if
the
traversal is done right. There are however some cases where this is
hard to
accomplish and we need to work on supporting those cases better.
Remember that a Graph Database is NOT a Relational Database. A lot of
the
ideas people have about databases are based on their knowledge of
Relational
Databases. I understand that it can be hard, but if that baggage could
be
left at the door it would make things a lot easier. Nobody is saying
that
Relational Databases are dead (except for some publicity stunts) far
from
it! What we (and a lot of other people) are saying is that the age of
one
database to rule them all is over. Different problems are best solved
with
different kinds of databases, RDBMSes are great at some, K/V stores
some,
and Graph Databases are great for some. Then there are some problems
that
are best solved with a combination of two or more (kinds of) databases,
where each database brings its own strengths to the table, and is used
only
for the things it is good at.
That's enough deviation from the topic, my conclusions remain the same
as
they were before this discussion started, I will state them in as few
words
as possible and in bullet point form to convey them as clearly as I
can:
* The REST API will probably need result set limiting or pagination.
* Limiting and pagination will require (server side) sorting
* Sorting can be better implemented if it's implemented in the core of
the
traversal framework
* Limiting / Pagination can be deferred for a while until we know what
it
needs to look like (from looking at actual uses)
* (Server side) Sorting can be deferred until we need it for limiting /
pagination
Peace,
Tobias
On Thu, Apr 8, 2010 at 10:17 PM, Michael Ludwig mil...@gmx.de wrote:
Tobias Ivarsson schrieb am 08.04.2010 um 18:23:27 (+0200)
[Re: [Neo] Traversers in the REST API]:
On Wed, Apr 7, 2010 at 3:05 PM, Alastair James al.ja...@gmail.com
wrote:
when we start talking about returning 1000s of nodes in JSON over
HTTP just to get the first 10 this is clearly sub-optimal (as I
build websites this is a very common use case). So, as you say,
sorting and limiting can wait, but I suspect the HTTP API would
benefit from offering it. Limiting need not require changes to
the
core API, it could be implemented as a second stage in the HTTP
API
code prior to output encoding.
For paging / limiting: yes, you are absolutely right, this would
Re: [Neo] Traversers in the REST API
Since in manycases the results of a query will need to be reformed into their associated domain objects Unlikely to be the case over the HTTP API. Its unlikely people will create domain objects in (e.g.) PHP they will just use the data directly. Pagination is kinda tricky if the data changes between subsequent requests for pages. Since pagination is generally used for UIs, a common approach is to place the entire dataset (or a cursor, depending on where the data is coming from) in a session object. Regardless of where it is kept, if you want to deal with data changes, you either have to a) invalidate the cached dataset if data changes or b) keep a copy of the whole dataset around in its as queried state so that subsequent paging requests are consistent. Either case involves keeping a fairly big duplicate data structure on the server or middle tier and violates one of the objectives of REST-ful APIs, which is that of statelessness. For that reason, I personally think the REST-ful API shouldn't deal with paging. It should probably be done at some intermediate level as needed by applications. We can certainly build a separate API that we can all leverage if needed, but I don't think it should be in the core REST-ful layer. Well, I think for my use cases (websites), its likely that users dont flick between pages that often. For example, on may sites, users will view page 1 and select an item, any very view move on to page 2. Its a very different usage pattern compared to a traditional desktop UI, so there is absolutely no need to hold the sorted set on the server in a cursor type way. A typical use case for me would be 1000+ matching rows, with 90%+ of page views for the first 10, 5% for the next 10 etc...! You can clearly see that sending the entire results set of 1000+ rows over HTTP/JSON is inefficient. Of course, caching between the web server and the neo HTTP API can help, but not in all cases, and it seems silly to rely on this. Al -- Dr Alastair James CTO James Publishing Ltd. http://www.linkedin.com/pub/3/914/163 www.worldreviewer.com WINNER Travolution Awards Best Travel Information Website 2009 WINNER IRHAS Awards, Los Angeles, Best Travel Website 2008 WINNER Travolution Awards Best New Online Travel Company 2008 WINNER Travel Weekly Magellan Award 2008 WINNER Yahoo! Finds of the Year 2007 Noli nothis permittere te terere! ___ Neo mailing list User@lists.neo4j.org https://lists.neo4j.org/mailman/listinfo/user
Re: [Neo] Traversers in the REST API
Why not just slap memcached in the middle? Would help with scalability
as well, plus you could keep cached results keyed by query params in
there if needed. Just a thought...
Original Message
Subject: Re: [Neo] Traversers in the REST API
From: Alastair James al.ja...@gmail.com
Date: Fri, April 09, 2010 8:32 am
To: Neo user discussions user@lists.neo4j.org
Since in manycases the results of a query will need to be reformed
into
their associated domain objects
Unlikely to be the case over the HTTP API. Its unlikely people will
create
domain objects in (e.g.) PHP they will just use the data directly.
Pagination is kinda tricky if the data changes between subsequent
requests for pages. Since pagination is generally used for UIs, a
common approach is to place the entire dataset (or a cursor,
depending
on where the data is coming from) in a session object. Regardless of
where it is kept, if you want to deal with data changes, you either
have to a) invalidate the cached dataset if data changes or b) keep
a
copy of the whole dataset around in its as queried state so that
subsequent paging requests are consistent. Either case involves
keeping a fairly big duplicate data structure on the server or middle
tier and violates one of the objectives of REST-ful APIs, which is
that
of statelessness. For that reason, I personally think the REST-ful
API
shouldn't deal with paging. It should probably be done at some
intermediate level as needed by applications. We can certainly build
a
separate API that we can all leverage if needed, but I don't think it
should be in the core REST-ful layer.
Well, I think for my use cases (websites), its likely that users dont
flick
between pages that often. For example, on may sites, users will view
page 1
and select an item, any very view move on to page 2. Its a very
different
usage pattern compared to a traditional desktop UI, so there
is absolutely no need to hold the sorted set on the server in a cursor
type
way.
A typical use case for me would be 1000+ matching rows, with 90%+ of
page
views for the first 10, 5% for the next 10 etc...! You can clearly see
that
sending the entire results set of 1000+ rows over HTTP/JSON is
inefficient.
Of course, caching between the web server and the neo HTTP API can
help, but
not in all cases, and it seems silly to rely on this.
Al
--
Dr Alastair James
CTO James Publishing Ltd.
[1]http://www.linkedin.com/pub/3/914/163
[2]www.worldreviewer.com
WINNER Travolution Awards Best Travel Information Website 2009
WINNER IRHAS Awards, Los Angeles, Best Travel Website 2008
WINNER Travolution Awards Best New Online Travel Company 2008
WINNER Travel Weekly Magellan Award 2008
WINNER Yahoo! Finds of the Year 2007
Noli nothis permittere te terere!
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References
1. http://www.linkedin.com/pub/3/914/163
2. http://www.worldreviewer.com/
3. https://lists.neo4j.org/mailman/listinfo/user
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Re: [Neo] Traversers in the REST API
What I want to avoid is keeping state on the server while waiting for the client to request the next page. You are quite right. However, I think for many use cases (e.g. generating a paginated list of results on a webpage) it would not be necessary to store state on the server. That would be more similar to a SQL cursor, what I am talking about is simply SQL LIMIT, OFFSET and ORDER BY. Cheers Al On 8 April 2010 17:23, Tobias Ivarsson tobias.ivars...@neotechnology.comwrote: What I want to avoid is keeping state on the server while waiting for the client to request the next page. -- Dr Alastair James CTO James Publishing Ltd. http://www.linkedin.com/pub/3/914/163 www.worldreviewer.com WINNER Travolution Awards Best Travel Information Website 2009 WINNER IRHAS Awards, Los Angeles, Best Travel Website 2008 WINNER Travolution Awards Best New Online Travel Company 2008 WINNER Travel Weekly Magellan Award 2008 WINNER Yahoo! Finds of the Year 2007 Noli nothis permittere te terere! ___ Neo mailing list User@lists.neo4j.org https://lists.neo4j.org/mailman/listinfo/user
Re: [Neo] Traversers in the REST API
Tobias Ivarsson schrieb am 08.04.2010 um 18:23:27 (+0200) [Re: [Neo] Traversers in the REST API]: On Wed, Apr 7, 2010 at 3:05 PM, Alastair James al.ja...@gmail.com wrote: when we start talking about returning 1000s of nodes in JSON over HTTP just to get the first 10 this is clearly sub-optimal (as I build websites this is a very common use case). So, as you say, sorting and limiting can wait, but I suspect the HTTP API would benefit from offering it. Limiting need not require changes to the core API, it could be implemented as a second stage in the HTTP API code prior to output encoding. For paging / limiting: yes, you are absolutely right, this would not effect the core API at all, only the REST API. Limiting/paging is something we would probably add to the REST API before sorting. Limiting and paging usually go hand in hand with sorting, in my experience. Why would anyone want to page through an unsorted collection? Sorting might be a similar case, but I still think the client would be better fitted to do sorting well. The server has indexes to support the sorting. (If it doesn't, it has a problem anyway.) What does the client have to support sorting? So how would it be better fitted to do sorting well? But once paging / limiting is added it would be quite natural / useful to add sorting as well. What I want to avoid is keeping state on the server while waiting for the client to request the next page. If you ensure a binary tree index is used to do the sorting, you should be fine. -- Michael Ludwig ___ Neo mailing list User@lists.neo4j.org https://lists.neo4j.org/mailman/listinfo/user
Re: [Neo] Traversers in the REST API
On 8 April 2010 21:17, Michael Ludwig mil...@gmx.de wrote: Limiting and paging usually go hand in hand with sorting, in my experience. Why would anyone want to page through an unsorted collection? Its quite possible that you might want the nodes in the order they were found (e.g. the closest matching nodes first), however, I agree, sorting by an arbitrary property is very useful! Al ___ Neo mailing list User@lists.neo4j.org https://lists.neo4j.org/mailman/listinfo/user
Re: [Neo] Traversers in the REST API
These two ways of traversing a graph complement each other, it's not that
one is better than the other. Would you agree on this?
I think I agree. I would hope to be able to use XPath/Gremlin style querying
for most things, and a more programatic system for more complex ones.
a JSON document describing the traverser, like:
{ order: depth first,
uniquness: node,
return evaluator:
{ language: javascript,
body: function shouldReturn( pos ) {...} },
prune evaluator:
{ language: javascript,
body: function },
relationships: [
{ direction: outgoing,
type: KNOWS },
{ type: LOVES }
],
max depth: 4 }
Looks good for my needs. Using javascript in this form looks sensible.
Any idea about the performance implications of using a javax.scripting
language here? I guess not too severe.
Is there any need for a shared context between calls to the evaluators? So I
could store custom information and access it again when traversing further
nodes. So, you could passing a 'context' object (with its initial values)
that gets passed in as a second parameter to each evaluator function. Then
again, this is probably bad practice
Any idea how you will handle pagination? Obviously sorting is an issue as
you are unlikely to want the nodes in the traversal order. In my mind it
would be nice to allow the return evaluator to return a 'sorting value' that
indicates that nodes rank in the result set. E.g. sorting on a score
attribute of the node:
function shouldReturn(pos)
{
if (!some_condition) return false;
return pos.currentNode().score;
}
But I guess this is a comment on the Neo API as a whole?
Cheers
Al
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Re: [Neo] Traversers in the REST API
Our thoughts on how to handle pagination is not yet, we'll get something
that works first, and then add pagination (in a number of places) later on.
As for sorting: yes, that is a comment on the API as a whole. We have opted
at not providing sorting, since there are good sorting facilities available
in the JRE already. Since that makes it easy for the user to implement their
own sorting it would be sub optimal for Neo4j to provide sorting. Since
sorting is a costly operation (both in time and space) it should be done as
late in the process as possible, probably with a lot of user code in between
the traversal and the place where the sorting actually takes place. This has
been our thinking in the REST API as well, meaning that sorting will be left
to the client. It is possible that we will return to this decision and add
sorting to the REST API, and that it might trickle down to the core API.
Features like this are however much easier to add than to remove, which is
why it is not implemented at the moment.
Cheers,
Tobias
On Wed, Apr 7, 2010 at 9:47 AM, Alastair James al.ja...@gmail.com wrote:
These two ways of traversing a graph complement each other, it's not
that
one is better than the other. Would you agree on this?
I think I agree. I would hope to be able to use XPath/Gremlin style
querying
for most things, and a more programatic system for more complex ones.
a JSON document describing the traverser, like:
{ order: depth first,
uniquness: node,
return evaluator:
{ language: javascript,
body: function shouldReturn( pos ) {...} },
prune evaluator:
{ language: javascript,
body: function },
relationships: [
{ direction: outgoing,
type: KNOWS },
{ type: LOVES }
],
max depth: 4 }
Looks good for my needs. Using javascript in this form looks sensible.
Any idea about the performance implications of using a javax.scripting
language here? I guess not too severe.
Is there any need for a shared context between calls to the evaluators? So
I
could store custom information and access it again when traversing further
nodes. So, you could passing a 'context' object (with its initial values)
that gets passed in as a second parameter to each evaluator function. Then
again, this is probably bad practice
Any idea how you will handle pagination? Obviously sorting is an issue as
you are unlikely to want the nodes in the traversal order. In my mind it
would be nice to allow the return evaluator to return a 'sorting value'
that
indicates that nodes rank in the result set. E.g. sorting on a score
attribute of the node:
function shouldReturn(pos)
{
if (!some_condition) return false;
return pos.currentNode().score;
}
But I guess this is a comment on the Neo API as a whole?
Cheers
Al
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www.neotechnology.com
Cellphone: +46 706 534857
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Re: [Neo] Traversers in the REST API
Cheers guys. All sounds good. One comment: As for sorting: yes, that is a comment on the API as a whole. We have opted at not providing sorting, since there are good sorting facilities available in the JRE already. Since that makes it easy for the user to implement their own sorting it would be sub optimal for Neo4j to provide sorting. Since sorting is a costly operation (both in time and space) it should be done as late in the process as possible, probably with a lot of user code in between the traversal and the place where the sorting actually takes place. This has been our thinking in the REST API as well, meaning that sorting will be left to the client. It is possible that we will return to this decision and add sorting to the REST API, and that it might trickle down to the core API. Features like this are however much easier to add than to remove, which is why it is not implemented at the moment. Well, thats the case when Neo is running in the same JVM as the user code, but when we start talking about returning 1000s of nodes in JSON over HTTP just to get the first 10 this is clearly sub-optimal (as I build websites this is a very common use case). So, as you say, sorting and limiting can wait, but I suspect the HTTP API would benefit from offering it. Limiting need not require changes to the core API, it could be implemented as a second stage in the HTTP API code prior to output encoding. Cheers Al ___ Neo mailing list User@lists.neo4j.org https://lists.neo4j.org/mailman/listinfo/user
[Neo] Traversers in the REST API
We had a little discussion about how to make a first/good implementation of
traversers in the Neo4j REST API. If we just start at the core and expose
basic traversers, as they are in the graph database API (or will be, with
the next iteration of the traversal framework finding its way into the
kernel soon) a solution like this could be handy:
POST /node/{id}/traverse
where the entity (data you post) would be a JSON document describing the
traverser, like:
{ order: depth first,
uniquness: node,
return evaluator:
{ language: javascript,
body: function shouldReturn( pos ) {...} },
prune evaluator:
{ language: javascript,
body: function },
relationships: [
{ direction: outgoing,
type: KNOWS },
{ type: LOVES }
],
max depth: 4 }
Where max depth could be a short-hand for a prune evaluator which prunes
after a certain depth, and if both max depth and prune evaluator are
specified, they are merged into one prune evaluator which checks for both
conditions. Also if nothing is supplied (an empty POST, sort of) depth 1 is
assumed, just to prevent traversing the entire graph in vain).
Looking at the prune evaluator and return evaluator it'd be nice to
define them in some language, f.ex javascript, ruby or python or whatever.
We're initially thinking of using javax.script.* stuff (ScriptEngine) for
that, it'd probably be enough, at least to get things going.
So, moving on to the result which would come back... The default would be
that nodes comes back (as an array of JSON, think an array of GET /node/{id}
data). But wait, how could I make it return relationships or paths instead?
Well, we could extend the URI (template) to:
POST /node/{id}/traverse/{returnType}
Where returnType could be node, relationship or path (or even
something else?).
We've happily ignored the whole paging issue which would be handy/necessary
for rather big traversals, but for an initial version I think we can do
without it.
So, how does all this feel? It'd be fun with feedback and suggestions!
--
Mattias Persson, [matt...@neotechnology.com]
Neo Technology, www.neotechnology.com
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Re: [Neo] Traversers in the REST API
Hi guys,
For what its worth
I have yet to use the Neo4j traversal framework because it is simply is not
expressive enough. The traverser framework is like a single-relational
traverser on a multi-relational graph. You only allow or disallow certain edge
labels--not the ordered concatenation of labels. Moreover, even with ordered
labels defined, the choices that a traverser make at every element (edge and
vertex) should be predicated on general purpose computing---predicated on the
state/history of the walker, the properties of the elements, ... anything.
relationships: [
{ direction: outgoing,
type: KNOWS },
{ type: LOVES }
],
max depth: 4 }
What if I want to find all the people that love my post popular (by eigenvector
centrality) friends who also know me? Thus, simply taking knows and loves
relationships arbitrarily doesn't tell me that. What comes into play in such
situations are edge weights, ordered paths, loops, sampling, etc.
A general purpose traverser framework requires that you be able to define
adjacency arbitrarily. The traverser must be able to ask the engineer, at every
step (edge and vertex [1]), what do you mean by adjacent where do I go
next?
[1] edges should be treated just as vertices---they have properties that can be
reasoned on. many times you want edges returned, not just vertices.
Take care,
Marko.
http://markorodriguez.com
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Re: [Neo] Traversers in the REST API
On Tue, Mar 30, 2010 at 4:20 PM, Marko Rodriguez okramma...@gmail.comwrote:
Hi guys,
For what its worth
I have yet to use the Neo4j traversal framework because it is simply is not
expressive enough. The traverser framework is like a single-relational
traverser on a multi-relational graph. You only allow or disallow certain
edge labels--not the ordered concatenation of labels. Moreover, even with
ordered labels defined, the choices that a traverser make at every element
(edge and vertex) should be predicated on general purpose
computing---predicated on the state/history of the walker, the properties of
the elements, ... anything.
Good thing we are building this on the new traversal framework that we are
working on then ;)
Some of the features you are mentioning that the current/previous traversal
framework is lacking are supported in the new framework, and others are on
the roadmap. Those features will be exposed through the REST API as well
when they are ready. This will include revising the way you declare which
relationships to traverse. What we would like to be able to say is:
First expand relationships of type A,B or C, then of type T,U,V or W, then
if the previous was T or U, the next should be X, if the previous was V or
W, the next should be an arbitrary depth of Y relationships. And of course
be able to have different kinds of filters in each step (on both nodes and
relationships), not only selection based on relationship type.
This is however not implemented yet, but as the new traversal API evolves we
plan to let the REST API follow.
relationships: [
{ direction: outgoing,
type: KNOWS },
{ type: LOVES }
],
max depth: 4 }
What if I want to find all the people that love my post popular (by
eigenvector centrality) friends who also know me? Thus, simply taking
knows and loves relationships arbitrarily doesn't tell me that. What
comes into play in such situations are edge weights, ordered paths, loops,
sampling, etc.
A general purpose traverser framework requires that you be able to define
adjacency arbitrarily. The traverser must be able to ask the engineer, at
every step (edge and vertex [1]), what do you mean by adjacent where
do I go next?
Exactly, depth first and breadth first are just two very basic
implementations of this. The new traversal framework will eventually have
support for letting the engineer provide the selector for where-to-go-next.
The first use case that comes to mind for me is for implementing best first
traversals, but I know that there are other things oen might want to write,
and I am sure there are even more that I haven't thought of. When this is
implemented we will add some means for exposing it to the users of the REST
API as well, but for now our idea is to make the REST API useful with what
we have, and to get the new traversal framework in front of users.
[1] edges should be treated just as vertices---they have properties that
can be reasoned on. many times you want edges returned, not just vertices.
Take care,
Marko.
http://markorodriguez.com
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Tobias Ivarsson tobias.ivars...@neotechnology.com
Hacker, Neo Technology
www.neotechnology.com
Cellphone: +46 706 534857
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Re: [Neo] Traversers in the REST API
Mattias Persson schrieb am 30.03.2010 um 16:06:49 (+0200):
a JSON document describing the traverser, like:
{ order: depth first,
uniquness: node,
return evaluator:
{ language: javascript,
body: function shouldReturn( pos ) {...} },
prune evaluator:
{ language: javascript,
body: function },
relationships: [
{ direction: outgoing,
type: KNOWS },
{ type: LOVES }
],
max depth: 4 }
Looking at the prune evaluator and return evaluator it'd be nice
to define them in some language, f.ex javascript, ruby or python or
whatever. We're initially thinking of using javax.script.* stuff
(ScriptEngine) for that, it'd probably be enough, at least to get
things going.
XSLT, which builds on XPath, works by having the processor traverse the
tree and the user define templates featuring a match pattern. For every
node, the processor dispatches to the best matching template, from where
you can control further processing.
Now those match patterns are a subset of XPath, and rightly so: If the
user were given the full power of XPath, it would easily get horribly
expensive to determine the best matching template for a given node.
Likewise in a graph traversal, wouldn't it be reasonable to only allow
something with restricted expressive and imperative power, like the
match patterns in XSLT?
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Michael Ludwig
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