Why should you copy to tempdir when initiate GraphFunctions?

[giahung24]

Hello,
Thanks for creating this api, it's very interesting for my current work.
I have this question (as in title), why you must create a temp dir 'neo4j' and then copy from the content of 'neo4j' in project directory?
Thank you!

[tmills]

The neo4j library relies on lucene for its indexing, and lucene requires a lock on the index directory while operating. So you can remove the temp dir aspect and it will work fine in a single thread but will not allow you to have multiple processes accessing the index at the same time. So this is a workaround - by having each jvm make its own copy upon startup, I can run multiple processes at the same time that all use the graph api. It is obviously not optimal, especially if the index is large, but it worked for me. I of course would love to know if there is a better solution.

EDIT: multiple threads -> multiple processes.

[giahung24]

Ah ok. Thanks for the explanation. I didn't know about neo4j mechanism.

[giahung24]

I am not sure that this will help.
I have removed "static" from your class GraphFunctions and also from its functions. So every time I need to call a method, i just use one instance of GraphFunctions.
In your threads, can you use the same intance of GraphFunctions?

[tmills]

Actually I expect that won't work. If you are accessing the db from multiple threads in the same jvm it should be ok to have one static GraphDatabaseService object that multiple threads reference. The issue that I was trying to solve was having multiple jvms each with their own GraphDatabaseService object, which was not possible because the underlying directory was locked. But within one jvm using static should make sure there is only one entry point to the directory.

[giahung24]

Ok.
I dont have much experience in parallel processing.
Thanks for interesting info.
Cheers,

Le vendredi 24 juin 2016, Tim Miller notifications@github.com a écrit :

Actually I expect that won't work. If you are accessing the db from
multiple threads in the same jvm it should be ok to have one static
GraphDatabaseService object that multiple threads reference. The issue that
I was trying to solve was having multiple jvms each with their own
GraphDatabaseService object, which was not possible because the underlying
directory was locked. But within one jvm using static should make sure
there is only one entry point to the directory.

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