Re: Pooling Http Client lockup

[Dave Reynolds]

Hi,

On 08/12/17 19:39, ajs6f wrote:
Dave--
Jena switched HTTP Commons versions a good bit from 3.1.1 to now, so that may 
be part of it. I will look into whether HTTP Commons Client changed its 
behavior under us.

Thanks.

I agree that just upping the maxes and hoping for the best isn't the best 
outcome at all.

Otherwise, this sounds either buggy (if all the execs really are getting 
closed) or at least not very ergonomic (since you aren't seeing any warnings).

Do I understand correctly that the behavior you would want would be that

1) after the max number of connections have been drawn out of the pool and 
used, the next request should block only until a connection is released,
2) and that closing a query execution should definitely return the connection 
underneath it to the pool, more or less immediately?

Yes.

It's almost like something above the client is blocking and not letting the 
connections get released...

Indeed.

I'll attempt to recreate the behaviour with a minimal test case with 
none of our other code stack in the way. Should be able isolate whether 
it some bad practice at our (well, my) end which just happens to be safe 
in earlier versions of http client/jena or whether it really is Jena.

If I can get it pinned down to the latter then I'll open a JIRA.

Dave

On Dec 8, 2017, at 12:55 PM, Dave Reynolds <dave.e.reyno...@gmail.com> wrote:

Hi Rob,

Thanks, that's useful to know but just raising the limit will just make it less 
frequent but not cure it.

The issue is that if you ever get more in-flight than the limit then all future 
requests are blocked. The in-flight requests return fine, the execs are closed 
but the httpclient never recovers.

I will, of course, check again that I'm successfully closing all the execs. 
However, with Jena 3.1.1 this code has successfully run for months between 
reboots with request rates in the millions per week (with occasional high 
bursts). No lock ups at all. As it stands I can't switch it to a newer Jena 
unless I can absolutely guarantee an upper limit on the number of concurrent 
requests. That is often possible (through an apache reverse proxy front end 
with request throttling) but feels like an unsatisfying resolution.

Dave

On 08/12/17 16:56, Rob Vesse wrote:
I think this relates to a HttpClient behaviour that limits the maximum 
connections to a given service
At least in how Jena sets it up the default is 5 connections per service which 
is more generous than the HTTP client defaults.  Jena appears to read this from 
a JVM property http.maxConnections OR you can construct your own client by 
calling setMaxConnPerRoute() and setMaxConnTotal() on the builder to set your 
desired settings.
Rob
On 08/12/2017, 16:44, "Dave Reynolds" <dave.e.reyno...@gmail.com> wrote:
     Hi,
          I've being updating some rather old libraries that use Jena to issue
     sparql requests to a remote endpoint and pull back results.
          These work under Jena 3.1.1 but there's a fatal problem under Jena 
3.2.0
     and later ...
          If I issue 6 concurrent execSelect calls to a remote sparql endpoint
     (happens to be fuseki) then 5 will get issued and return correctly but
     #6 will not and from then onwards no further remote calls will go
     through. This only happens if at least 5 requests are in flight with no
     response yet from the remote endpoint before the final one is issued.
          It's hard to produce a deterministic, standalone test case because by
     definition it depends on an external sparql endpoint being reliably
     there and reliably not too fast :)
          Looking at the stack trace I see:
          Unsafe.park(boolean, long) line: not available [native method]
     LockSupport.park(Object) line: 175
     AbstractQueuedSynchronizer$ConditionObject.await() line: 2039
     CPool(AbstractConnPool<T,C,E>).getPoolEntryBlocking(T, Object, long,
     TimeUnit, Future<E>) line: 377
     AbstractConnPool<T,C,E>.access$200(AbstractConnPool, Object, Object,
     long, TimeUnit, Future) line: 67
     AbstractConnPool$2.get(long, TimeUnit) line: 243
     AbstractConnPool$2.get(long, TimeUnit) line: 191
     PoolingHttpClientConnectionManager.leaseConnection(Future<CPoolEntry>,
     long, TimeUnit) line: 282
     PoolingHttpClientConnectionManager$1.get(long, TimeUnit) line: 269
     MainClientExec.execute(HttpRoute, HttpRequestWrapper, HttpClientContext,
     HttpExecutionAware) line: 191
     ProtocolExec.execute(HttpRoute, HttpRequestWrapper, HttpClientContext,
     HttpExecutionAware) line: 185
     RetryExec.execute(HttpRoute, HttpRequestWrapper, HttpClientContext,
     HttpExecutionAware) line: 89
     RedirectExec.execute(HttpRoute, HttpRequestWrapper, HttpClientContext,
     HttpExecutionAware) line: 111
     InternalHttpClient.doExecute(HttpHost, HttpRequest, HttpContext) line: 185
     InternalHttpClient(CloseableHttpClient).execute(HttpUriRequest,
     HttpContext) line: 83
     InternalHttpClient(CloseableHttpClient).execute(HttpUriRequest,
     HttpContext) line: 56
     HttpOp.exec(String, HttpUriRequest, String, HttpResponseHandler,
     HttpClient, HttpContext) line: 1081
     HttpOp.execHttpGet(String, String, HttpResponseHandler, HttpClient,
     HttpContext) line: 308
     HttpOp.execHttpGet(String, String, HttpClient, HttpContext) line: 367
     HttpQuery.execGet() line: 326
     HttpQuery.exec() line: 288
     QueryEngineHTTP.execResultSetInner() line: 348
     QueryEngineHTTP.execSelect() line: 340
     SSResultSet.<init>(BaseSparqlSource, String) line: 33
     RemoteSparqlSource(BaseSparqlSource).streamableSelect(String) line: 59
          Which suggests some problem with the connection pool locking.
          I've tried 3.4.0  and 3.5.0 which incorporate the fix to JENA-1335 (in
     case that's related) but no luck.
          I've tried both:
              HttpOp.setDefaultHttpClient(HttpClients.createMinimal());
          and
              HttpOp.setDefaultHttpClient(HttpClients.createDefault());
          right at the start of application startup.
          Which don't fix the problem. Though perhaps I'm getting the sequence 
of
     where to set those wrong.
               Any suggestions?
          Any pointers to documentation on how to configure the http client set 
up
     in current Jena versions?
          Thanks,
     Dave