Unexpected 'Number of open connections' exception

[Rubén Navarro Piris]

Hi!

we're currently using the Stardog Community Edition (4.2) for some testing purposes and ran into a problem with too many open connections.

Some words about the setup:

* We've limited the number of concurrent connections to 5 in our application (Spring-based web app)

* We're also observing the number of TCP connections on the server running Stardog (which usually is about 600-1000, including waiting connections)

After a while our application crashes with the following exception: 

com.complexible.stardog.StardogException: 

Number of open connections exceeds the limit allowed by the license: 8096

My question is, whether Stardog deliberately crashes due to license violations or if this is unintended behavior. At the exact moment the exception gets thrown, the number of TCP connection on the Stardog server barely exceed 900, and the number of user connections was capped at 5. We're not quite sure where the 8096 is coming from.

We didn't encounter this error while using a commercial license.

Cheers!

Ruben

[Stephen Nowell]

Hi Ruben,

Stardog in no way crashes deliberately; certainly not for community
license violations. This exception is just the server rejecting a new
connection because of the limit.

"Connection" here does not mean a TCP connection. It means a database
connection. Ensure that any connection your application creates is being
closed, or they will just pile up. The `stardog-admin server status`
command will display the number of open connections at any given time.
Note that open transactions also count towards this limit, as they
internally open a connection.

Cheers,
Stephen

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[Rubén Navarro Piris]

HI Stephen!

We tried using the 'server-admin server status' command, but no information on the number of open connections is shown:

$ /opt/stardog/bin/stardog-admin server status --verbose

Backup Storage Directory : .backup

Connection Timeout       : 1h

Export Storage Directory : .exports

Named Graph Security     : true

Platform Arch            : amd64

Platform OS              : Linux 4.4.8-boot2docker, Java 1.8.0_91

Query All Graphs         : true

Query Timeout            : 5m

Stardog Home             : /opt/stardog-4.2

Stardog Version          : 4.2

Strict Parsing           : true

[Stephen Nowell]

Hi Ruben,

I forgot that `stardog-admin server status` is different for the Community version and doesn't provide that information. It is still likely that the culprit is Connections or transactions not being closed/committed. Which API are you using in your application to connect to Stardog?

Cheers,
Stephen

[Rubén Navarro Piris]

Hi Stephen!

We tested this behaviour and narrowed it down to the versioning API. In our Java code, the following code starts the connection, but after executing close, the connection is still present (we used a commercial license and the `stardog-admin server status` tool as you recommended):

try (VersioningConnection conn = this.connectionConfiguration.connect()

                                                             .as(VersioningConnection.class);

     TupleQueryResult result = conn.getVersioningMetadata().select(query).execute()) {

  while (result.hasNext()) {

    BindingSet bindings = result.next();

    // read results

  }

}

We also tested directly using the command line tool and reproduced the bad behaviour as well:

$ stardog-admin db create -o versioning.enabled=true -n myDb

Successfully created database 'myDb'.

$ stardog-admin server status | grep myDb -B2

|  Name   | Conn. Open | Trans. Open | Running Queries | Query Avg. Time (ms) | Query/sec |

+---------+------------+-------------+-----------------+----------------------+-----------+

| myDb    |          0 |           0 |               0 |                  0.0 |       0.0 |

$ stardog vcs list myDb

Version:   677658fb-0a8b-4349-8b6f-c07724d51888

Committer: admin

Date:      2016-12-05 13:04:05

    Database creation

$ stardog-admin server status | grep myDb -B2

|  Name   | Conn. Open | Trans. Open | Running Queries | Query Avg. Time (ms) | Query/sec |

+---------+------------+-------------+-----------------+----------------------+-----------+

| myDb    |          1 |           0 |               0 |                  0.0 |       0.0 |

$ sleep 60

$ stardog-admin server status | grep myDb -B2

|  Name   | Conn. Open | Trans. Open | Running Queries | Query Avg. Time (ms) | Query/sec |

+---------+------------+-------------+-----------------+----------------------+-----------+

| myDb    |          1 |           0 |               0 |                  0.0 |       0.0 |

Please let me know if there is a workaround for this issue, as we rely on this versioning API.

Cheers!

Ruben