Troubleshooting CrateDB on Docker with jcmd

You can find a lot of troubleshooting guides for Java applications out there on the internet that explain how to perform a heap dump, thread dump, and so on. Since CrateDB is written in Java, these tools can of course also be used to troubleshoot CrateDB instances in case something goes awry.

Most of these guides, however, explain how to use tools (such as jcmd) on Java applications running directly as process on the operating system. Fewer of them cover how to apply jcmd commands inside a Docker container.

When it comes to troubleshooting the crate Docker container, things work a bit differently. This document explains why the ‘usual’ way to run jcmd does not work and how to solve it. It does not, however, explain how to analyze the output (since that is identical to non-containerized applications)!

Table of contents

Introduction

jcmd has been the successor of multiple tools (jstack, jinfo, jmap) since JDK 8. It can be used to perform various diagnostic tasks on a running Java application.

$ jcmd -h
Usage: jcmd <pid | main class> <command ...|PerfCounter.print|-f file>
   or: jcmd -l
   or: jcmd -h

  command must be a valid jcmd command for the selected jvm.
  Use the command "help" to see which commands are available.
  If the pid is 0, commands will be sent to all Java processes.
  The main class argument will be used to match (either partially
  or fully) the class used to start Java.
  If no options are given, lists Java processes (same as -l).

  PerfCounter.print display the counters exposed by this process
  -f  read and execute commands from the file
  -l  list JVM processes on the local machine
  -? -h --help print this help message

Running inside crate Docker container

After starting a crate Docker container you can also run jcmd inside the container.

$ docker exec -ti b768001196c /bin/bash
[root@b768001196ce data]# $JAVA_HOME/bin/jcmd -l
1 io.crate.bootstrap.CrateDB -Cpath.home=/crate -Cnode.name=debug
106 jdk.jcmd/sun.tools.jcmd.JCmd -l

However, when trying to run any command, the command fails, even though you run it as root with full privileges.

[root@b768001196ce data]# $JAVA_HOME/bin/jcmd 1 VM.version
1:
com.sun.tools.attach.AttachNotSupportedException: Unable to open socket file /proc/1/root/tmp/.java_pid1: target process 1 doesn't respond within 10500ms or HotSpot VM not loaded
     at jdk.attach/sun.tools.attach.VirtualMachineImpl.<init>(VirtualMachineImpl.java:100)
     at jdk.attach/sun.tools.attach.AttachProviderImpl.attachVirtualMachine(AttachProviderImpl.java:58)
     at jdk.attach/com.sun.tools.attach.VirtualMachine.attach(VirtualMachine.java:207)
     at jdk.jcmd/sun.tools.jcmd.JCmd.executeCommandForPid(JCmd.java:115)
     at jdk.jcmd/sun.tools.jcmd.JCmd.main(JCmd.java:99)

The same happens when you try to run it as user crate, which owns the process:

[root@b768001196ce data]# su crate -c "$JAVA_HOME/bin/jcmd 1 VM.version"
1:
com.sun.tools.attach.AttachNotSupportedException: Unable to open socket file /proc/1/root/tmp/.java_pid1: target process 1 doesn't respond within 10500ms or HotSpot VM not loaded
     at jdk.attach/sun.tools.attach.VirtualMachineImpl.<init>(VirtualMachineImpl.java:100)
     at jdk.attach/sun.tools.attach.AttachProviderImpl.attachVirtualMachine(AttachProviderImpl.java:58)
     at jdk.attach/com.sun.tools.attach.VirtualMachine.attach(VirtualMachine.java:207)
     at jdk.jcmd/sun.tools.jcmd.JCmd.executeCommandForPid(JCmd.java:115)
     at jdk.jcmd/sun.tools.jcmd.JCmd.main(JCmd.java:99)

On a different note: when looking at the Docker logs of the crate container, you can see that when trying to run the jcmd command, the CrateDB instance logs a full thread dump.

What is the problem then?

The entrypoint of the crate Docker image ensures that the CrateDB Java process runs as user crate, since CrateDB must be run as a non-root user.

This is done by chroot ing with user crate (chroot --userspec=1000 / "$@"), because this does not spawn an additional process for changing the user - unlike su crate -c "$@", where su would result in the process with PID 1 and the crate command would be a child-process with a different PID. This is not what one wants in a Docker container, where the application must (?) run as PID 1.

With that knowledge in mind, you can use chroot to execute the jcmd command as well.

[root@b768001196ce data]# chroot --userspec=1000 / $JAVA_HOME/bin/jcmd 1 VM.version
1:
OpenJDK 64-Bit Server VM version 13.0.1+9
JDK 13.0.1

jcmd <PID> help lists all available commands that you can now start using for troubleshooting CrateDB inside the Docker container.

[root@b768001196ce data]# chroot --userspec=1000 / $JAVA_HOME/bin/jcmd 1 help
1:
The following commands are available:
Compiler.CodeHeap_Analytics
Compiler.codecache
Compiler.codelist
Compiler.directives_add
Compiler.directives_clear
Compiler.directives_print
Compiler.directives_remove
Compiler.queue
GC.class_histogram
GC.class_stats
GC.finalizer_info
GC.heap_dump
GC.heap_info
GC.run
GC.run_finalization
JFR.check
JFR.configure
JFR.dump
JFR.start
JFR.stop
JVMTI.agent_load
JVMTI.data_dump
ManagementAgent.start
ManagementAgent.start_local
ManagementAgent.status
ManagementAgent.stop
Thread.print
VM.class_hierarchy
VM.classloader_stats
VM.classloaders
VM.command_line
VM.dynlibs
VM.events
VM.flags
VM.info
VM.log
VM.metaspace
VM.native_memory
VM.print_touched_methods
VM.set_flag
VM.stringtable
VM.symboltable
VM.system_properties
VM.systemdictionary
VM.uptime
VM.version
help

For more information about a specific command use 'help <command>'.

To execute one of these commands from outside of the Docker container without explicitly attaching to it, you can combine the docker exec command with the jcmd command. This would look like so:

$ docker exec -ti <ID> /bin/bash -c "chroot --userspec=1000 / \$JAVA_HOME/bin/jcmd 1 <CMD>"

For example, running GC.heap_info on Docker container with ID b768001196ce:

$ docker exec -ti b768001196ce /bin/bash -c "chroot --userspec=1000 / \$JAVA_HOME/bin/jcmd 1 GC.heap_info"
1:
 garbage-first heap   total 524288K, used 129716K [0x00000000e0000000, 0x0000000100000000)
  region size 1024K, 126 young (129024K), 22 survivors (22528K)
 Metaspace       used 57165K, capacity 59755K, committed 60080K, reserved 1099776K
  class space    used 7721K, capacity 8941K, committed 8960K, reserved 1048576K

Troubleshooting Commands

Thread Dump

Command:jcmd <PID> Thread.print

Example

$ docker exec -ti b768001196ce /bin/bash -c "chroot --userspec=1000 / \$JAVA_HOME/bin/jcmd 1 Thread.print"
1:
...

Heap Info

Command:jcmd <PID> GC.heap_info

Example

$ docker exec -ti b768001196ce /bin/bash -c "chroot --userspec=1000 / \$JAVA_HOME/bin/jcmd 1 GC.heap_info"
1:
...

Heap Dump

Command:jcmd <PID> GC.heap_dump <PATH>

Example

$ docker exec -ti b768001196ce /bin/bash -c "chroot --userspec=1000 / \$JAVA_HOME/bin/jcmd 1 GC.heap_dump /data/crate.hprof"
1:
Heap dump file created

Note

The <PATH> should be a path that resides on a mounted volume, so you can access the created heap dump from ouside of the container and the container is not “blown up”.

Java Flight Recording

Command:jcmd <PID> JFR.start name=<NAME> duration=<DURATION> filename=<PATH> settings=profile

Example

$ docker exec -ti b768001196ce /bin/bash -c "chroot --userspec=1000 / \$JAVA_HOME/bin/jcmd 1 JFR.start name=recording1 duration=60s filename=/data/recording1.jfr"
1:
Started recording 1. The result will be written to:

/data/recording1.jfr

Note

The <PATH> should be a path that resides on a mounted volume, so you can access the created jfr dump from ouside of the container and the container is not “blown up”.

These are the most common troubleshooting tasks, but of course there are many more possibilities to get diagnostic information using the jcmd command. You can find more information about the utility at the jcmd documentation.