Sidekiq is a background job processing framework for Ruby applications that uses threads to handle many jobs concurrently. It's used to move time-consuming tasks out of the web request cycle, improving application response times. Sidekiq processes jobs asynchronously using Redis as its job store, allowing web applications to respond quickly while heavy tasks run in the background.

Key benefits include:

• Improved user experience with faster page loads
• Better resource utilization through concurrent processing
• Reliable job processing with automatic retries
• Scalability through multiple worker processes

Sidekiq differs in several key ways:

• Concurrency Model: Sidekiq uses threads (lightweight), while Resque uses processes (heavyweight). DelayedJob can use either.
• Performance: Sidekiq is typically 20x faster than Resque due to threading
• Storage: Sidekiq and Resque use Redis, DelayedJob uses your application's database
• Memory Usage: Sidekiq uses less memory due to threading vs forking
• Dependencies: Sidekiq requires Redis, DelayedJob requires only a database

Example performance difference:

# Sidekiq can process 1000s of jobs/second with one process
# Resque might need 20+ processes for similar throughput

Sidekiq has three main components: Client (pushes jobs to Redis from web process), Redis (stores jobs in queues), and Server (pulls and executes jobs in worker process). The client serializes jobs to JSON, Redis maintains ordered queues, and the server uses threads to process jobs concurrently

Redis is an in-memory data structure store used as a database, cache, and message broker. Sidekiq uses Redis because:

• Speed: In-memory storage provides extremely fast read/write operations
• Data Structures: Redis lists provide perfect queue semantics
• Atomicity: Redis operations are atomic, preventing race conditions
• Persistence: Optional persistence ensures jobs survive restarts
• Pub/Sub: Enables real-time communication between components

A basic Sidekiq worker includes the Sidekiq::Worker module and defines a perform method:

class EmailWorker
  include Sidekiq::Worker
  
  def perform(user_id, subject)
    user = User.find(user_id)
    UserMailer.notification(user, subject).deliver_now
  end
end

# Enqueue the job
EmailWorker.perform_async(123, "Welcome!")

The worker must be serializable, so pass simple data types (strings, numbers, arrays, hashes) rather than complex objects.

These methods control when jobs are executed:

# Execute immediately (as soon as worker is available)
MyWorker.perform_async(arg1, arg2)

# Execute after a delay
MyWorker.perform_in(5.minutes, arg1, arg2)

# Execute at specific time
MyWorker.perform_at(DateTime.tomorrow.noon, arg1, arg2)

Scheduled jobs are stored in a separate Redis sorted set and moved to queues when their time arrives.

Queues organize jobs by priority or type. Workers can be assigned to specific queues:

class CriticalWorker
  include Sidekiq::Worker
  sidekiq_options queue: 'critical'
  
  def perform(data)
    # Process critical job
  end
end

# Start worker with queue priorities
# bundle exec sidekiq -q critical,2 -q default,1

The number after queue name indicates weight (higher = more priority). Critical queue gets checked twice as often as default.