Dynamic Instrumentation is a feature of the Datadog tracing library that lets you add instrumentation to your application at runtime without code changes or redeployments. Follow these instructions to set up Dynamic Instrumentation for Ruby.

Prerequisites

Before you begin, review the Dynamic Instrumentation prerequisites. Ruby applications also require:

• Tracing library: ddtrace version 2.9.0 or higher. See the installation instructions for setup details.
• Ruby version: Ruby 2.6 or higher.
• Ruby implementation: Only MRI (CRuby) is supported. JRuby and other Ruby implementations are not supported.
• Web framework: Only Rack-based applications are supported (including Rails, Sinatra, and other Rack-compatible frameworks). Background processes and jobs (including Sidekiq, Resque, etc.) are not supported.
• Environment: RAILS_ENV or RACK_ENV must be set to production. Development environments are not supported.

Installation

1. If you don’t already have APM enabled, in your Agent configuration, set the DD_APM_ENABLED environment variable to true and listening to the port 8126/TCP.
2. Run your service with Dynamic Instrumentation enabled by setting the DD_DYNAMIC_INSTRUMENTATION_ENABLED environment variable to true. Specify DD_SERVICE, DD_ENV, and DD_VERSION Unified Service Tags so you can filter and group your instrumentations and target active clients across these dimensions.
3. After starting your service with Dynamic Instrumentation enabled, you can start using Dynamic Instrumentation on the APM > Dynamic Instrumentation page.

Note: Dynamic Instrumentation initializes when the application processes its first HTTP request. Ensure your application receives at least one request after startup before creating probes.

Configuration

Configure Dynamic Instrumentation using the following environment variables:

What to do next

See Dynamic Instrumentation for information about adding instrumentations and browsing and indexing the data.

Ruby-specific considerations

Instrumentable code

Line probes can only be placed on executable lines of code. The following example shows which lines can be targeted:

def example_method(param)    # Cannot instrument (method definition)
  if param == 1              # Can instrument
    result = "yes"           # Can instrument
  else                       # Cannot instrument
    result = "no"            # Can instrument
  end                        # Cannot instrument
  result                     # Can instrument
end                          # Can instrument (method exit)

Lines that cannot be instrumented include method definition lines (def), else/elsif clauses, most end keywords (except the final end of a method), comment-only lines, and empty lines.

Expression language instance variable conflicts

Ruby’s use of @ for instance variables creates conflicts with Dynamic Instrumentation’s expression language special variables. The following variable names are reserved and cannot be accessed if used as instance variables in your Ruby code:

• @return - The return value of the method
• @duration - Method execution duration
• @exception - Any exception raised by the method
• @it - Current item in collection operations
• @key - Current key in hash operations
• @value - Current value in hash operations

If your code uses instance variables with these names, rename them to use Dynamic Instrumentation expressions that reference them.

Code loading timing

Dynamic Instrumentation tracks code as it loads. For line probes to work correctly:

• Files must be loaded after the Datadog tracer initializes
• Code loaded before the tracer starts cannot be instrumented with line probes
• Method probes can still work for classes defined before tracking starts
• Best practice: Ensure the tracer initializes early in your application boot process

Supported features

• Dynamic Logs (log probes)
• Line probes (capture variables at a specific line)
• Method probes (capture method entry and exit)
• Probe conditions using Expression Language
• Message templates using Expression Language
• PII redaction
• Source code integration

Limitations

The following features available in other languages are not supported for Ruby:

• Dynamic Metrics
• Dynamic Spans
• Dynamic Span Tags
• Local variable capture for method probes (use line probes inside the method as a workaround)

Further reading

Documentation, liens et articles supplémentaires utiles:

Getting Started with Datadog AgentDOCUMENTATION