290 lines
12 KiB
Markdown
290 lines
12 KiB
Markdown
# Troubleshooting Sidekiq
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Sidekiq is the background job processor GitLab uses to asynchronously run
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tasks. When things go wrong it can be difficult to troubleshoot. These
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situations also tend to be high-pressure because a production system job queue
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may be filling up. Users will notice when this happens because new branches
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may not show up and merge requests may not be updated. The following are some
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troubleshooting steps that will help you diagnose the bottleneck.
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> **Note:** GitLab administrators/users should consider working through these
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> debug steps with GitLab Support so the backtraces can be analyzed by our team.
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> It may reveal a bug or necessary improvement in GitLab.
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>
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> **Note:** In any of the backtraces, be wary of suspecting cases where every
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> thread appears to be waiting in the database, Redis, or waiting to acquire
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> a mutex. This **may** mean there's contention in the database, for example,
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> but look for one thread that is different than the rest. This other thread
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> may be using all available CPU, or have a Ruby Global Interpreter Lock,
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> preventing other threads from continuing.
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## Thread dump
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Send the Sidekiq process ID the `TTIN` signal and it will output thread
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backtraces in the log file.
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```
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kill -TTIN <sidekiq_pid>
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```
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Check in `/var/log/gitlab/sidekiq/current` or `$GITLAB_HOME/log/sidekiq.log` for
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the backtrace output. The backtraces will be lengthy and generally start with
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several `WARN` level messages. Here's an example of a single thread's backtrace:
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```
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2016-04-13T06:21:20.022Z 31517 TID-orn4urby0 WARN: ActiveRecord::RecordNotFound: Couldn't find Note with 'id'=3375386
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2016-04-13T06:21:20.022Z 31517 TID-orn4urby0 WARN: /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/activerecord-4.2.5.2/lib/active_record/core.rb:155:in `find'
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/opt/gitlab/embedded/service/gitlab-rails/app/workers/new_note_worker.rb:7:in `perform'
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/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/processor.rb:150:in `execute_job'
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/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/processor.rb:132:in `block (2 levels) in process'
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/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/middleware/chain.rb:127:in `block in invoke'
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/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/sidekiq_middleware/memory_killer.rb:17:in `call'
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/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/middleware/chain.rb:129:in `block in invoke'
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/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/sidekiq_middleware/arguments_logger.rb:6:in `call'
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...
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```
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In some cases Sidekiq may be hung and unable to respond to the `TTIN` signal.
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Move on to other troubleshooting methods if this happens.
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## Process profiling with `perf`
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Linux has a process profiling tool called `perf` that is helpful when a certain
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process is eating up a lot of CPU. If you see high CPU usage and Sidekiq won't
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respond to the `TTIN` signal, this is a good next step.
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If `perf` is not installed on your system, install it with `apt-get` or `yum`:
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```
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# Debian
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sudo apt-get install linux-tools
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# Ubuntu (may require these additional Kernel packages)
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sudo apt-get install linux-tools-common linux-tools-generic linux-tools-`uname -r`
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# Red Hat/CentOS
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sudo yum install perf
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```
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Run perf against the Sidekiq PID:
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```
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sudo perf record -p <sidekiq_pid>
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```
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Let this run for 30-60 seconds and then press Ctrl-C. Then view the perf report:
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```
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sudo perf report
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# Sample output
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Samples: 348K of event 'cycles', Event count (approx.): 280908431073
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97.69% ruby nokogiri.so [.] xmlXPathNodeSetMergeAndClear
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0.18% ruby libruby.so.2.1.0 [.] objspace_malloc_increase
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0.12% ruby libc-2.12.so [.] _int_malloc
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0.10% ruby libc-2.12.so [.] _int_free
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```
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Above you see sample output from a perf report. It shows that 97% of the CPU is
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being spent inside Nokogiri and `xmlXPathNodeSetMergeAndClear`. For something
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this obvious you should then go investigate what job in GitLab would use
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Nokogiri and XPath. Combine with `TTIN` or `gdb` output to show the
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corresponding Ruby code where this is happening.
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## The GNU Project Debugger (gdb)
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`gdb` can be another effective tool for debugging Sidekiq. It gives you a little
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more interactive way to look at each thread and see what's causing problems.
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NOTE: **Note:**
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Attaching to a process with `gdb` will suspends the normal operation
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of the process (Sidekiq will not process jobs while `gdb` is attached).
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Start by attaching to the Sidekiq PID:
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```
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gdb -p <sidekiq_pid>
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```
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Then gather information on all the threads:
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```
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info threads
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# Example output
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30 Thread 0x7fe5fbd63700 (LWP 26060) 0x0000003f7cadf113 in poll () from /lib64/libc.so.6
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29 Thread 0x7fe5f2b3b700 (LWP 26533) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
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28 Thread 0x7fe5f2a3a700 (LWP 26534) 0x0000003f7ce0ba5e in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
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27 Thread 0x7fe5f2939700 (LWP 26535) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
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26 Thread 0x7fe5f2838700 (LWP 26537) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
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25 Thread 0x7fe5f2737700 (LWP 26538) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
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24 Thread 0x7fe5f2535700 (LWP 26540) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
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23 Thread 0x7fe5f2434700 (LWP 26541) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
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22 Thread 0x7fe5f2232700 (LWP 26543) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
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21 Thread 0x7fe5f2131700 (LWP 26544) 0x00007fe5f7b570f0 in xmlXPathNodeSetMergeAndClear ()
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from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
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...
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```
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If you see a suspicious thread, like the Nokogiri one above, you may want
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to get more information:
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```
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thread 21
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bt
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# Example output
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#0 0x00007ff0d6afe111 in xmlXPathNodeSetMergeAndClear () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
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#1 0x00007ff0d6b0b836 in xmlXPathNodeCollectAndTest () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
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#2 0x00007ff0d6b09037 in xmlXPathCompOpEval () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
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#3 0x00007ff0d6b09017 in xmlXPathCompOpEval () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
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#4 0x00007ff0d6b092e0 in xmlXPathCompOpEval () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
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#5 0x00007ff0d6b0bc37 in xmlXPathRunEval () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
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#6 0x00007ff0d6b0be5f in xmlXPathEvalExpression () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
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#7 0x00007ff0d6a97dc3 in evaluate (argc=2, argv=0x1022d058, self=<value optimized out>) at xml_xpath_context.c:221
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#8 0x00007ff0daeab0ea in vm_call_cfunc_with_frame (th=0x1022a4f0, reg_cfp=0x1032b810, ci=<value optimized out>) at vm_insnhelper.c:1510
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```
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To output a backtrace from all threads at once:
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```
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set pagination off
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thread apply all bt
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```
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Once you're done debugging with `gdb`, be sure to detach from the process and
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exit:
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```
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detach
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exit
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```
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## Check for blocking queries
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Sometimes the speed at which Sidekiq processes jobs can be so fast that it can
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cause database contention. Check for blocking queries when backtraces above
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show that many threads are stuck in the database adapter.
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The PostgreSQL wiki has details on the query you can run to see blocking
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queries. The query is different based on PostgreSQL version. See
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[Lock Monitoring](https://wiki.postgresql.org/wiki/Lock_Monitoring) for
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the query details.
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## Managing Sidekiq queues
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It is possible to use [Sidekiq API](https://github.com/mperham/sidekiq/wiki/API)
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to perform a number of troubleshooting on Sidekiq.
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These are the administrative commands and it should only be used if currently
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admin interface is not suitable due to scale of installation.
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All this commands should be run using `gitlab-rails console`.
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### View the queue size
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```ruby
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Sidekiq::Queue.new("pipeline_processing:build_queue").size
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```
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### Enumerate all enqueued jobs
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```ruby
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queue = Sidekiq::Queue.new("chaos:chaos_sleep")
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queue.each do |job|
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# job.klass # => 'MyWorker'
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# job.args # => [1, 2, 3]
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# job.jid # => jid
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# job.queue # => chaos:chaos_sleep
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# job["retry"] # => 3
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# job.item # => {
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# "class"=>"Chaos::SleepWorker",
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# "args"=>[1000],
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# "retry"=>3,
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# "queue"=>"chaos:chaos_sleep",
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# "backtrace"=>true,
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# "queue_namespace"=>"chaos",
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# "jid"=>"39bc482b823cceaf07213523",
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# "created_at"=>1566317076.266069,
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# "correlation_id"=>"c323b832-a857-4858-b695-672de6f0e1af",
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# "enqueued_at"=>1566317076.26761},
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# }
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# job.delete if job.jid == 'abcdef1234567890'
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end
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```
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### Enumerate currently running jobs
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```ruby
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workers = Sidekiq::Workers.new
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workers.each do |process_id, thread_id, work|
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# process_id is a unique identifier per Sidekiq process
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# thread_id is a unique identifier per thread
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# work is a Hash which looks like:
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# {"queue"=>"chaos:chaos_sleep",
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# "payload"=>
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# { "class"=>"Chaos::SleepWorker",
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# "args"=>[1000],
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# "retry"=>3,
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# "queue"=>"chaos:chaos_sleep",
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# "backtrace"=>true,
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# "queue_namespace"=>"chaos",
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# "jid"=>"b2a31e3eac7b1a99ff235869",
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# "created_at"=>1566316974.9215662,
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# "correlation_id"=>"e484fb26-7576-45f9-bf21-b99389e1c53c",
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# "enqueued_at"=>1566316974.9229589},
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# "run_at"=>1566316974}],
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end
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```
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### Remove Sidekiq jobs for given parameters (destructive)
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```ruby
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# for jobs like this:
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# RepositoryImportWorker.new.perform_async(100)
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id_list = [100]
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queue = Sidekiq::Queue.new('repository_import')
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queue.each do |job|
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job.delete if id_list.include?(job.args[0])
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end
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```
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### Remove specific job ID (destructive)
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```ruby
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queue = Sidekiq::Queue.new('repository_import')
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queue.each do |job|
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job.delete if job.jid == 'my-job-id'
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end
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```
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## Canceling running jobs (destructive)
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> Introduced in GitLab 12.3.
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This is highly risky operation and use it as last resort.
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Doing that might result in data corruption, as the job
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is interrupted mid-execution and it is not guaranteed
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that proper rollback of transactions is implemented.
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```ruby
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Gitlab::SidekiqDaemon::Monitor.cancel_job('job-id')
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```
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> This requires the Sidekiq to be run with `SIDEKIQ_MONITOR_WORKER=1`
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> environment variable.
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To perform of the interrupt we use `Thread.raise` which
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has number of drawbacks, as mentioned in [Why Ruby’s Timeout is dangerous (and Thread.raise is terrifying)](https://jvns.ca/blog/2015/11/27/why-rubys-timeout-is-dangerous-and-thread-dot-raise-is-terrifying/):
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> This is where the implications get interesting, and terrifying. This means that an exception can get raised:
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>
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> - during a network request (ok, as long as the surrounding code is prepared to catch Timeout::Error)
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> - during the cleanup for the network request
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> - during a rescue block
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> - while creating an object to save to the database afterwards
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> - in any of your code, regardless of whether it could have possibly raised an exception before
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>
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> Nobody writes code to defend against an exception being raised on literally any line. That’s not even possible. So Thread.raise is basically like a sneak attack on your code that could result in almost anything. It would probably be okay if it were pure-functional code that did not modify any state. But this is Ruby, so that’s unlikely :)
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