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# Instrumenting Ruby Code

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GitLab Performance Monitoring allows instrumenting of both methods and custom
blocks of Ruby code. Method instrumentation is the primary form of
instrumentation with block-based instrumentation only being used when we want to
drill down to specific regions of code within a method.
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## Instrumenting Methods

Instrumenting methods is done by using the `Gitlab::Metrics::Instrumentation`
module. This module offers a few different methods that can be used to
instrument code:

* `instrument_method`: instruments a single class method.
* `instrument_instance_method`: instruments a single instance method.
* `instrument_class_hierarchy`: given a Class this method will recursively
  instrument all sub-classes (both class and instance methods).
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* `instrument_methods`: instruments all public and private class methods of a Module.
* `instrument_instance_methods`: instruments all public and private instance methods of a
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  Module.

To remove the need for typing the full `Gitlab::Metrics::Instrumentation`
namespace you can use the `configure` class method. This method simply yields
the supplied block while passing `Gitlab::Metrics::Instrumentation` as its
argument. An example:

```
Gitlab::Metrics::Instrumentation.configure do |conf|
  conf.instrument_method(Foo, :bar)
  conf.instrument_method(Foo, :baz)
end
```

Using this method is in general preferred over directly calling the various
instrumentation methods.

Method instrumentation should be added in the initializer
`config/initializers/metrics.rb`.

### Examples

Instrumenting a single method:

```
Gitlab::Metrics::Instrumentation.configure do |conf|
  conf.instrument_method(User, :find_by)
end
```

Instrumenting an entire class hierarchy:

```
Gitlab::Metrics::Instrumentation.configure do |conf|
  conf.instrument_class_hierarchy(ActiveRecord::Base)
end
```

Instrumenting all public class methods:

```
Gitlab::Metrics::Instrumentation.configure do |conf|
  conf.instrument_methods(User)
end
```

### Checking Instrumented Methods

The easiest way to check if a method has been instrumented is to check its
source location. For example:

```
method = Rugged::TagCollection.instance_method(:[])

method.source_location
```

If the source location points to `lib/gitlab/metrics/instrumentation.rb` you
know the method has been instrumented.

If you're using Pry you can use the `$` command to display the source code of a
method (along with its source location), this is easier than running the above
Ruby code. In case of the above snippet you'd run the following:

```
$ Rugged::TagCollection#[]
```

This will print out something along the lines of:

```
From: /path/to/your/gitlab/lib/gitlab/metrics/instrumentation.rb @ line 148:
Owner: #<Module:0x0055f0865c6d50>
Visibility: public
Number of lines: 21

def #{name}(#{args_signature})
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  if trans = Gitlab::Metrics::Instrumentation.transaction
    trans.measure_method(#{label.inspect}) { super }
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  else
    super
  end
end
```

## Instrumenting Ruby Blocks

Measuring blocks of Ruby code is done by calling `Gitlab::Metrics.measure` and
passing it a block. For example:
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```ruby
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Gitlab::Metrics.measure(:foo) do
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  ...
end
```

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The block is executed and the execution time is stored as a set of fields in the
currently running transaction. If no transaction is present the block is yielded
without measuring anything.

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3 values are measured for a block:
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1. The real time elapsed, stored in NAME_real_time.
2. The CPU time elapsed, stored in NAME_cpu_time.
3. The call count, stored in NAME_call_count.
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Both the real and CPU timings are measured in milliseconds.
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Multiple calls to the same block will result in the final values being the sum
of all individual values. Take this code for example:
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```ruby
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3.times do
  Gitlab::Metrics.measure(:sleep) do
    sleep 1
  end
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end
```
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Here the final value of `sleep_real_time` will be `3`, _not_ `1`.