Re: Threaded Operations - Quarkus


Joe Obernberger
 

Sorry for the delay.

When a request comes in via REST, Quarkus creates a thread to handle it; I believe it actually comes from a thread pool.

This code now does:

GraphTraversalSource traversal = StaticInfo.getSingleton().getGraph().buildTransaction().checkExternalVertexExistence(false).consistencyChecks(false).propertyPrefetching(true).start().traversal();

for each incoming request.  It then does a tx().commit and a traversal.close() when complete.


The StaticInfo class contains a JanusGraph object (getGraph()).

What I'm wondering is there appears to be two ways to add nodes and edges.  One is using a GraphTraversalSource, and one is using a JanusGraph:

GraphTraversalSource.addV()

Or

JanusGraph.addVertex


-Joe


On 6/22/2022 4:51 PM, Boxuan Li wrote:
When you say use JanusGraph.tx().createdThreadedTx() directly, what do you mean? Can you give an example?

From: janusgraph-users@... <janusgraph-users@...> on behalf of Joe Obernberger via lists.lfaidata.foundation <joseph.obernberger=gmail.com@...>
Sent: Tuesday, June 21, 2022 4:25 PM
To: janusgraph-users@... <janusgraph-users@...>
Subject: Re: [janusgraph-users] Threaded Operations - Quarkus
 

Thanks for all the help on this.  I'm coming closer to a solution thanks to you all.

Question - I've been using GraphTraversalSource to do all the adding vertices and edges to my graph.  Example:

GraphTraversalSource traversal = JanusGraph.tx().createThreadedTx().traversal();

Is it better to use JanusGraph.tx().createdThreadedTx() directly?


-Joe


On 6/17/2022 3:03 PM, Boxuan Li wrote:
Yeah using `newTransaction()` won't make a difference in your use case. Based on your input, there are a couple of things you could try:

  1. As suggested by Kevin, you could use locking. See https://docs.janusgraph.org/advanced-topics/eventual-consistency/#data-consistency. It is slow but it will hopefully solve most race conditions you have. Based on my understanding of Cassandra's nature, I think you could still see such inconsistencies but the chance is much lower for sure.
  2. You could periodically identify and remove the inconsistencies using an offline pipeline.
  3. You could use an external locking service on client side. For example, using Redis to make sure a conflicting transaction won't start at the first place.
These solutions have their own pros & cons, so it really depends on you.

Best,
Boxuan


From: janusgraph-users@... <janusgraph-users@...> on behalf of Joe Obernberger via lists.lfaidata.foundation <joseph.obernberger=gmail.com@...>
Sent: Friday, June 17, 2022 2:24 PM
To: janusgraph-users@... <janusgraph-users@...>
Subject: Re: [janusgraph-users] Threaded Operations - Quarkus
 

So - unsurprisingly, Boxuan is correct.
Code like this:
GraphTraversalSource traversal = StaticInfo.getGraph().newTransaction().traversal();
try {
                    datasourceVertex = traversal.V().has("someID", id).next();
                } catch (java.util.NoSuchElementException nse) {
                    datasourceVertex = traversal.addV("source").property("someID", id).next();
                }

being called from multiple threads results in several vertices with the same 'someID'.

Not sure how to fix this.

-Joe

On 6/17/2022 10:28 AM, Joe Obernberger via lists.lfaidata.foundation wrote:

Good stuff - thank you Boxuan.
Backend is Cassandra running on bare metal on 15 nodes.
Race condition is rare.
When the race condition happens, I'm seeing duplicate nodes/edges; basically the graph becomes invalid.
Yes.  This is a good idea.  I could write a spark job to examine the graph and fix up discrepancies.  Smart.
Not sure what a locking services is?  Example?

My current plan (not tested yet) is to use a static class that contains the JanusGraph 'graph'.  On Quarkus when a REST call comes in, a new thread is created.  That thread will use Marc's idea of
GraphTraversalSource traversal = StaticInfo.getGraph().newTransaction().traversal();

Do stuff and then traversal.tx().commit();
That will be done in a loop so that if the commit fails, it will retry X times.

At least that's my current plan.  Not sure if it will work.

-Joe

On 6/17/2022 8:52 AM, Boxuan Li wrote:
Hi Joe,

Unfortunately the way Marc suggests won’t help with your usecase. Tbh I would have suggested the same answer as Marc before I saw your second post. If one has one JVM thread handling multiple transactions (not familiar with quarkus so not sure if that is possible), then one has to do what Marc suggested. But in your usecase, it won't be any different from your current usage because JanusGraph will automatically create threaded transaction for each thread (using ThreadLocal) when you use the traversal object.

The real issue in your use case is that you want ACID support, which really depends on your backend storage. At least in our officially supported Cassandra, HBase, and BigTable adapters, this is not (yet) supported.

There are a few workarounds, though. Before discussing that further, I would like to ask a few questions:

  1. What is your backend storage and is it distributed?
  2. How often does this “race condition” happen? Is it very rare or it’s fairly common?
  3. What is your end goal? Do you want to reduce the chance of this “race condition”, or you want to make sure this does not happen at all?
  4. Are you willing to resolve such duplicate vertices/edges at either read time or offline?
  5. Are you willing to introduce a third dependency, e.g. a distributed locking service?

Best,
Boxuan

From: janusgraph-users@... <janusgraph-users@...> on behalf of Joe Obernberger via lists.lfaidata.foundation <joseph.obernberger=gmail.com@...>
Sent: Friday, June 17, 2022 8:12:04 AM
To: janusgraph-users@... <janusgraph-users@...>
Subject: Re: [janusgraph-users] Threaded Operations - Quarkus
 

Thank you Marc.  I'm currently doing everything with a traversal, and then doing a traversal.tx().commit()
Sounds like what you suggested is what I want, but just to be clear:
Here's what I'm trying to do.

Thread 1/JVM1 gets a request that requires adding new vertices and edges to the graph.
Thread 2/JVM1 gets a similar request. 
Some of the vertices added in Thread 1 end up having the same attributes/name has vertices from Thread 2, but I only want to have one vertex if it's going to have the same attributes.
If Thread 1 adds that vertex before it does a commit, then Thread 2, when it looks up said vertex won't find it; so it will also add it.

Code example (traversal is a GraphTraversalSource gotten from JanusGraphFactory.traversal())

try {
            correlationVertex = traversal.V().has("correlationID", correlationID).next();
        } catch (java.util.NoSuchElementException nse) {
            correlationVertex = null;
        }

.
.
.

if (correlationVertex == null) {
            correlationVertex = traversal.addV("correlation").property("correlationID", correlationID).next();
            correlationVertex.property("a", blah1);
            correlationVertex.property("b", blah2);

        }

I do similar things with edges:

        try {
            dataSourceToCorrelationEdge = traversal.E().has("edgeID", edgeID).next();
        } catch (NoSuchElementException nse) {
            dataSourceToCorrelationEdge = null;
        }

Ultimately, I'd like to have several JVMs handling these requests; each which runs multiple threads.
I'll look at using a new transaction per call.  Thank you!

-Joe

On 6/17/2022 8:01 AM, hadoopmarc@... wrote:
Hi Joe,

Do you mean with threadsafe transactions that requests from different client threads should be handled independently, that is in different JanusGraph Transactions?

In that case, I think you want to use a GraphTraversalSource per request like this:

g = graph.newTransaction().traversal()

Best wishes,    Marc



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