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https://issues.apache.org/jira/browse/SOLR-8925?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15232782#comment-15232782
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Dennis Gove commented on SOLR-8925:
-----------------------------------
I like this. Just a couple of questions.
1. What does this do with duplicate nodes? ie, overlapping friend networks.
Will it prune those out, show the node twice, mark a node has having multiple
sources?
2. When using the scatter parameter will the nodes be marked as which group
they fall into? What if a node falls into multiple groups (kinda related to #1
above)?
3. Will a node include information about its source, ie - why it's included in
a graph?
4. If gatherNodes is doing a 'join' between friends and articles I'd expect the
tuple to be a join of the tuple found in articles and the tuple found in
friends. But if "The inner gatherNodes() expression then emits the friend
Tuples" I believe this is more of an intersect. Ie, give me tuples in friends
which also appear in articles, using the author->user equalitor.
> Add gatherNodes Streaming Expression to support breadth first traversals
> ------------------------------------------------------------------------
>
> Key: SOLR-8925
> URL: https://issues.apache.org/jira/browse/SOLR-8925
> Project: Solr
> Issue Type: New Feature
> Reporter: Joel Bernstein
> Assignee: Joel Bernstein
> Fix For: 6.1
>
>
> The gatherNodes Streaming Expression is a flexible general purpose breadth
> first graph traversal. It uses the same parallel join under the covers as
> (SOLR-8888) but is much more generalized and can be used for a wide range of
> use cases.
> Sample syntax:
> {code}
> gatherNodes(
> friends,
> gatherNodes(
> friends,
> search(articles, q=“body:(query
> 1)”, fl=“author”),
> walk ="author->user”,
> gather="friend"),
> walk=“friend-> user”,
> gather="friend",
> scatter=“roots, branches, leaves”
> )
> {code}
> The expression above is evaluated as follows:
> 1) The inner search() expression is evaluated on the *articles* collection,
> emitting a Stream of Tuples with the author field populated.
> 2) The inner gatherNodes() expression reads the Tuples form the search()
> stream and traverses to the *friends* collection by performing a distributed
> join between articles.author and friends.user field. It gathers the value
> from the *friend* field during the join.
> 3) The inner gatherNodes() expression then emits the *friend* Tuples. By
> default the gatherNodes function emits only the leaves which in this case are
> the *friend* tuples.
> 4) The outer gatherNodes() expression reads the *friend* Tuples and Traverses
> again in the "friends" collection, this time performing the join between
> *friend* Tuples emitted in step 3. This collects the friend of friends.
> 5) The outer gatherNodes() expression emits the entire graph that was
> collected. This is controlled by the "scatter" parameter. In the example the
> *root* nodes are the authors, the *branches* are the author's friends and the
> *leaves* are the friend of friends.
> This traversal is fully distributed and cross collection.
> Like all streaming expressions the gather nodes expression can be combined
> with other streaming expressions. For example the following expression uses a
> hashJoin to intersect the network of friends rooted to authors found with
> different queries:
> {code}
> hashInnerJoin(
> gatherNodes(friends,
> gatherNodes(friends
> search(articles,
> q=“body:(queryA)”, fl=“author”),
> walk ="author->user”,
> gather="friend"),
> walk=“friend -> user”,
> gather="friend",
> scatter=“branches, leaves”),
> gatherNodes(friends,
> gatherNodes(friends
> search(articles,
> q=“body:(queryB)”, fl=“author”),
> walk ="author->user”,
> gather="friend"),
> walk=“friend -> user”,
> gather="friend",
> scatter=“branches, leaves”),
> on=“friend”
> )
> {code}
>
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