Basic Troubleshooting

Troubleshooting Checklist

The following is a short list of things to check when you have a problem.

  • Did you call mongoc_init() in main()? If not, you will likely see a segfault.

  • Have you leaked any clients or cursors as can be found with mongoc-stat <PID>?

  • Have packets been delivered to the server? See egress bytes from mongoc-stat <PID>.

  • Does ASAN show any leaks? Ensure you call mongoc_cleanup() at the end of your process to cleanup lingering allocations from the MongoDB C driver.

  • If compiling your own copy of MongoDB C Driver, consider using the cmake option -DENABLE_TRACING=ON to enable function tracing and hex dumps of network packets to STDERR and STDOUT.

Performance Counters

The MongoDB C Driver comes with an optional and unique feature to help developers and sysadmins troubleshoot problems in production. Performance counters are available for each process using the C Driver. If available, the counters can be accessed outside of the application process via a shared memory segment. The counters may be used graph statistics about your application process easily from tools like Munin or Nagios. For example, the command watch --interval=0.5 -d mongoc-stat $PID may be used to monitor an application.

Performance counters are only available on Linux platforms and macOS arm64 platforms that support shared memory segments. On supported platforms, they are enabled by default. Applications can be built without the counters by specifying the cmake option -DENABLE_SHM_COUNTERS=OFF. Additionally, if performance counters are already compiled, they can be disabled at runtime by specifying the environment variable MONGOC_DISABLE_SHM.

Performance counters keep track of the following:

  • Active and Disposed Cursors

  • Active and Disposed Clients, Client Pools, and Socket Streams.

  • Number of operations sent and received, by type.

  • Bytes transferred and received.

  • Authentication successes and failures.

  • Number of wire protocol errors.

Note

An operation is considered “sent” when one or more bytes of the corresponding message is written to the stream, regardless of whether the entire message is successfully written or if the operation ultimately succeeds or fails. This does not include bytes that may be written during the stream connection process, such as TLS handshake messages.

To access counters for a given process, simply provide the process id to the mongoc-stat program installed with the MongoDB C Driver.

$ mongoc-stat 22203
   Operations : Egress Total        : The number of sent operations.                    : 13247
   Operations : Ingress Total       : The number of received operations.                : 13246
   Operations : Egress Queries      : The number of sent Query operations.              : 13247
   Operations : Ingress Queries     : The number of received Query operations.          : 0
   Operations : Egress GetMore      : The number of sent GetMore operations.            : 0
   Operations : Ingress GetMore     : The number of received GetMore operations.        : 0
   Operations : Egress Insert       : The number of sent Insert operations.             : 0
   Operations : Ingress Insert      : The number of received Insert operations.         : 0
   Operations : Egress Delete       : The number of sent Delete operations.             : 0
   Operations : Ingress Delete      : The number of received Delete operations.         : 0
   Operations : Egress Update       : The number of sent Update operations.             : 0
   Operations : Ingress Update      : The number of received Update operations.         : 0
   Operations : Egress KillCursors  : The number of sent KillCursors operations.        : 0
   Operations : Ingress KillCursors : The number of received KillCursors operations.    : 0
   Operations : Egress Msg          : The number of sent Msg operations.                : 0
   Operations : Ingress Msg         : The number of received Msg operations.            : 0
   Operations : Egress Reply        : The number of sent Reply operations.              : 0
   Operations : Ingress Reply       : The number of received Reply operations.          : 13246
      Cursors : Active              : The number of active cursors.                     : 1
      Cursors : Disposed            : The number of disposed cursors.                   : 13246
      Clients : Active              : The number of active clients.                     : 1
      Clients : Disposed            : The number of disposed clients.                   : 0
      Streams : Active              : The number of active streams.                     : 1
      Streams : Disposed            : The number of disposed streams.                   : 0
      Streams : Egress Bytes        : The number of bytes sent.                         : 794931
      Streams : Ingress Bytes       : The number of bytes received.                     : 589694
      Streams : N Socket Timeouts   : The number of socket timeouts.                    : 0
 Client Pools : Active              : The number of active client pools.                : 1
 Client Pools : Disposed            : The number of disposed client pools.              : 0
     Protocol : Ingress Errors      : The number of protocol errors on ingress.         : 0
         Auth : Failures            : The number of failed authentication requests.     : 0
         Auth : Success             : The number of successful authentication requests. : 0

Submitting a Bug Report

Think you’ve found a bug? Want to see a new feature in the MongoDB C driver? Please open a case in our issue management tool, JIRA:

Bug reports in JIRA for all driver projects (i.e. CDRIVER, CSHARP, JAVA) and the Core Server (i.e. SERVER) project are public.