thanks for your help, sorry for not being clearer.
I create the constraint then if I run
MERGE (headNode:HEAD {list:"mylist"})-[:LINK]->headNode
I get Error: org.neo4j.cypher.PatternException: MERGE needs at least some
part of the pattern to already be known. Please provide values for one of:
headNode, headNode
So I change it to:
MERGE (headNode:HEAD {list:"mylist"})
WITH headNode
MERGE headNode-[:LINK]->(headNode)
Then run the query:
MERGE (headNode:HEAD {list:"mylist"})
WITH headNode
MATCH (headNode)-[old:LINK]->after
DELETE old
CREATE headNode-[:LINK]->(newNode:LINKNODE { number : {nodeNumber}
})-[:LINK]->after
Then I get Error: Relationship 391112 not found
I have been doing this test work in a node.js project. (so I can test
adding multiple nodes in parallel)
If it helps, the source code is here
https://github.com/aranm/neo4j-linked-list
The relevant file is here
https://github.com/aranm/neo4j-linked-list/blob/master/tests/parallelInsertionTest.js
When I change line 40 from
async.map(nodesToInsert, function(item, callback) --inserting in parallel
to
async.mapSeries(nodesToInsert, function(item, callback) --inserting in
series
then all is well, it works when we are not hitting the db at the same time.
But as soon as we introduce the parallel stuff all goes south.
On Mon, Feb 10, 2014 at 11:13 PM, Michael Hunger <
[email protected]> wrote:
> You would also use MERGE for the initial creation
>
> What doesn't work? Did you create the constraint?
>
> Am 10.02.2014 um 12:49 schrieb Aran Mulholland <[email protected]>:
>
> Just to make it absolutely clear, are you saying when creating the initial
> node I do:
>
> MERGE (headNode:HEAD {list:"mylist"})
>
> CREATE (headNode)-[:LINK]->(headNode)
>
>
> But when inserting I do:
>
> MERGE (headNode:HEAD {list:"mylist"})
> WITH headNode //had to add this line,
> cause I got an error WITHout it :)
> MATCH (headNode)-[old:LINK]->after
> DELETE old
> CREATE headNode-[:LINK]->(newNode:LINKNODE { number : {nodeNumber}
> })-[:LINK]->after
>
> Sorry for the re-clarification (I'm feeling a little dense here). The
> reason I am asking is that it doesn't work, so I must have mis-understood
> you somewhere.
>
>
>
>
> On Mon, Feb 10, 2014 at 10:30 PM, Michael Hunger <
> [email protected]> wrote:
>
>> Only with unique constraints, merge takes a global lock on the label +
>> property.
>>
>> Michael
>>
>> Am 10.02.2014 um 12:17 schrieb Aran Mulholland <[email protected]
>> >:
>>
>> Why do you need the unique constraint?
>>
>>
>> On Mon, Feb 10, 2014 at 9:52 PM, Michael Hunger <
>> [email protected]> wrote:
>>
>>> I think you have to change it to use merge which takes a schema index
>>> lock.
>>>
>>> ie.
>>>
>>> create constraint on (h:HEAD) assert h.list is unique;
>>>
>>> MERGE (headNode:HEAD {list:"mylist"})
>>>
>>> MATCH (headNode)-[old:LINK]->after
>>>
>>> DELETE old
>>> CREATE headNode-[:LINK]->(newNode:LINKNODE { number : {nodeNumber}
>>> })-[:LINK]->after
>>>
>>>
>>>
>>> HTH
>>>
>>> Michael
>>>
>>> Am 10.02.2014 um 11:30 schrieb Aran Mulholland <[email protected]
>>> >:
>>>
>>> I've got some real headaches inserting into linked lists.
>>>
>>> If I create a node as such:
>>>
>>> CREATE (headNode:HEAD)-[:LINK]->(headNode)
>>>
>>> then insert like so
>>>
>>> MATCH (headNode:HEAD)-[old:LINK]->after
>>> DELETE old
>>> CREATE headNode-[:LINK]->(newNode:LINKNODE { number : {nodeNumber}
>>> })-[:LINK]->after
>>>
>>> then everything is fine and dandy. But when I try to run the insertion
>>> code in parallel, then I get some real issues. I either get an error like
>>> "Relationship 325457 not found" or I get two or more lists coming off the
>>> headNode. The first error I can deal with as I can rerun the query, the
>>> second is totally unacceptable as my other queries get very messed up
>>> results.
>>>
>>> I am writing for a twitter like service and am using the linked lists so
>>> I can have a news feed. There is a high possibility that the list will need
>>> to be added to by more than one user at once, and this chance will grow the
>>> more users the system has.
>>>
>>> What I want to be able to do is put a lock around the insertion so that
>>> this does not happen.
>>>
>>> How can I better craft my queries so that parallel insertion via cypher
>>> over REST is sustainable?
>>>
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