Logging and monitoring in Ruby on Rails

Introduction

Logging helps you keep track of what your app is doing. Monitoring watches your app's health and alerts you if something goes wrong.

You want to see errors happening in your Rails app.

You need to check how long requests take to process.

You want to know if your app is running smoothly over time.

You want to get alerts if the app crashes or slows down.

You want to keep records for debugging or audits.

Syntax

Ruby on Rails

Rails.logger.level = Logger::INFO
Rails.logger.info "Your message here"
Rails.logger.error "Error message here"

Rails.logger is the built-in logger in Rails apps.

You can set the log level to control what messages are saved (DEBUG, INFO, WARN, ERROR, FATAL).

Examples

Different log levels show different importance of messages.

Ruby on Rails

Rails.logger.debug "Debugging info"
Rails.logger.info "App started"
Rails.logger.warn "This might be a problem"
Rails.logger.error "Something went wrong"

Setting log level to ERROR means only error messages are saved.

Ruby on Rails

Rails.logger.level = Logger::ERROR
Rails.logger.info "This will NOT be logged"
Rails.logger.error "This will be logged"

In config/environments/development.rb, you can set logger output and level.

Ruby on Rails

config.logger = Logger.new(STDOUT)
config.log_level = :debug

Sample Program

This sets the log level to info and prints logs to the console. Then it logs an info and an error message.

Ruby on Rails

# config/environments/development.rb
Rails.application.configure do
  config.log_level = :info
  config.logger = Logger.new(STDOUT)
end

# In any controller or model
Rails.logger.info "User signed in"
Rails.logger.error "Failed to save record"

Important Notes

Logs usually include timestamps and severity levels automatically.

Monitoring tools like New Relic or Datadog can collect and show logs and app health.

Keep log files from growing too large by rotating them regularly.

Summary

Logging records app events and errors for later review.

Monitoring watches app health and alerts you to problems.

Rails has built-in logging you can configure easily.

Practice

1. What is the main purpose of logging in a Rails application?

easy

A. To record events and errors for later review

B. To monitor real-time server performance

C. To deploy the application automatically

D. To manage database migrations

Solution

1. Step 1: Understand logging purpose

   Logging captures events and errors that happen in the app so developers can check them later.

2. Step 2: Differentiate from monitoring

   Monitoring is about watching app health live, not storing past events.

3. Final Answer:

   To record events and errors for later review -> Option A

4. Quick Check:

   Logging = record events/errors [OK]

Hint: Logging saves past events; monitoring watches live [OK]

Common Mistakes:

• Confusing logging with monitoring
• Thinking logging deploys apps
• Mixing logging with database tasks

2. Which of the following is the correct way to write a log message in Rails?

easy

A. logger.write("User signed in")

B. Rails.log('info', 'User signed in')

C. log.info('User signed in')

D. Rails.logger.info "User signed in"

Solution

1. Step 1: Recall Rails logging syntax

   Rails uses Rails.logger with methods like info, error, debug to write logs.

2. Step 2: Check each option

   Only Rails.logger.info "User signed in" uses correct Rails.logger.info syntax.

3. Final Answer:

   Rails.logger.info "User signed in" -> Option D

4. Quick Check:

   Rails.logger.info is correct syntax [OK]

Hint: Use Rails.logger.method("message") for logging [OK]

Common Mistakes:

• Using undefined logger object
• Wrong method names like write or log
• Incorrect method call syntax

3. Given this Rails code snippet, what will be the log level of the message?

Rails.logger.warn "Disk space low"

medium

A. Info

B. Warn

C. Error

D. Debug

Solution

1. Step 1: Identify the logging method used

   The method called is warn, which sets the log level to warning.

2. Step 2: Match method to log level

   Rails.logger.warn writes a warning level message.

3. Final Answer:

   Warn -> Option B

4. Quick Check:

   Rails.logger.warn = Warn level [OK]

Hint: Method name matches log level (warn = Warn) [OK]

Common Mistakes:

• Confusing warn with info or error
• Assuming default log level is info
• Mixing debug and warn levels

4. You added this line in a Rails controller but no log appears:

logger.error "Payment failed"

What is the most likely reason?

medium

A. The log level is set higher than error

B. Rails.logger must be used instead of logger

C. The logger object is not defined in controllers

D. Logging is disabled by default in Rails

Solution

1. Step 1: Understand logger availability in controllers

   In Rails controllers, logger is available as a shortcut to Rails.logger.

2. Step 2: Check log level settings

   If the log level is set higher than error (like fatal), error messages won't show.

3. Final Answer:

   The log level is set higher than error -> Option A

4. Quick Check:

   Log level filters out lower severity logs [OK]

Hint: Check log level filters if logs don't appear [OK]

Common Mistakes:

• Assuming logger is undefined in controllers
• Thinking Rails.logger must be used always
• Believing logging is off by default

5. You want to monitor your Rails app's performance in production. Which approach is best?

hard

A. Use Rails built-in logger to track all performance metrics

B. Add custom puts statements in code to watch performance

C. Integrate an external monitoring tool like New Relic or Datadog

D. Rely on Rails console logs only

Solution

1. Step 1: Understand Rails logging limits

   Rails logging records events but does not provide detailed real-time performance monitoring.

2. Step 2: Identify best monitoring practice

   External tools like New Relic or Datadog specialize in monitoring app health and performance live.

3. Final Answer:

   Integrate an external monitoring tool like New Relic or Datadog -> Option C

4. Quick Check:

   Use external tools for monitoring, not just logs [OK]

Hint: Use specialized tools for monitoring, not just logs [OK]

Common Mistakes:

• Expecting Rails logger to monitor performance live
• Using puts for monitoring which is not scalable
• Ignoring external monitoring solutions