Aran,
it seems that MERGE doesn't take a write lock by default, only when it didn't
find anything in the first place and has to create it.
Can you try the following instead.
"Removal of a non-existing property causes a write lock on the node". So if we
do that on the head-node it should actually work.
> MERGE (headNode:HEAD {list:"mylist"})
REMOVE headNode._lock_
> WITH headNode
> MATCH (headNode)-[old:LINK]->after
> DELETE old
> CREATE headNode-[:LINK]->(newNode:LINKNODE { number : {nodeNumber}
> })-[:LINK]->after
Am 11.02.2014 um 11:15 schrieb Aran Mulholland <[email protected]>:
> Anyone got any more ideas on how to do this?
>
>
> On Tue, Feb 11, 2014 at 3:01 PM, Aran Mulholland <[email protected]>
> wrote:
> I have placed this question on stack overflow here:
>
> http://stackoverflow.com/questions/21692829/neo4j-how-do-you-do-parallel-insertion-into-a-linked-list-via-cypher-over-rest
>
>
>
> On Mon, Feb 10, 2014 at 11:40 PM, Aran Mulholland <[email protected]>
> wrote:
> 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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>>>
>>>
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