counting_semaphore

A counting semaphore implementation for Ruby with local and distributed (Redis) variants.

When you have a metered and limited resource that only supports a certain number of simultaneous operations you need a semaphore primitive. In Ruby, most semaphores usually controls access "one whole resource":

sem = Semaphore.new
sem.with_lease do
  # Critical section where you hold access to the resource
end

This is well covered - for example - by POSIX semaphores if you are within one machine, and is known as a binary semaphore (it is either "open" or "closed"). There are also counting semaphores where you permit N of leases to be taken, which is available in the venerable redis-semaphore gem.

The problem comes if you need to hold access to a certain amount of a resource. For example, you know that you are doing 5 expensive operations in bulk, and you know that your entire application can only be doing 20 in total - governed by the API access limits. For that, you need a counting semaphore - such a semaphore is provided by concurrent-ruby for example. It allows you to acquire a certain number of permits and then release them.

This library provides both a simple LocalSemaphore which can be used across threads or fibers, and a Redis-based RedisSemaphore for coordination across processes and machines. Both implement a Lease-based API compatible with concurrent-ruby's Semaphore.

The recommended way to use the semaphore is with the with_lease method, which provides automatic cleanup:

require 'counting_semaphore'

# Create a local semaphore with capacity of 10
semaphore = CountingSemaphore::LocalSemaphore.new(10)

# Acquire 3 permits and automatically release on block exit
semaphore.with_lease(3, timeout_seconds: 10) do
  puts "Holding 3 permits"
  # Do your work here - permits are automatically released when the block exits
end

The block receives the lease object, which you can inspect:

semaphore.with_lease(3) do |lease|
  puts "Holding #{lease.permits} permits (ID: #{lease.id})"
  # Automatic cleanup on block exit
end

The Redis semaphore works identically but coordinates across processes and machines:

require 'redis'

redis = Redis.new
semaphore = CountingSemaphore::RedisSemaphore.new(
  10,                    # capacity
  "api_ratelimit",       # namespace (unique identifier)
  redis: redis,
  lease_ttl_seconds: 60  # lease expires after 60 seconds
)

# Use it the same way - works across multiple processes
semaphore.with_lease(3) do
  puts "Doing distributed work with 3 permits"
  # Permits automatically released when done
end

You can query the current state of the semaphore:

puts "Available permits: #{semaphore.available_permits}"
puts "Capacity: #{semaphore.capacity}"
puts "Currently in use: #{semaphore.currently_leased}"

For more control, you can manually acquire and release leases. This is useful when you can't use a block structure:

# Acquire permits (returns a Lease object)
lease = semaphore.acquire(2)

begin
  # Do some work
  puts "Working with 2 permits..."
ensure
  # Always release the lease
  semaphore.release(lease)
end

# Try to acquire immediately (returns nil if not available)
lease = semaphore.try_acquire(1)
if lease
  begin
    puts "Got the permit!"
  ensure
    semaphore.release(lease)
  end
else
  puts "Could not acquire permit"
end

# Try to acquire with timeout
lease = semaphore.try_acquire(2, 5.0)  # Wait up to 5 seconds
if lease
  begin
    # Work with the permits
  ensure
    semaphore.release(lease)
  end
end

# Acquire all currently available permits
drained_lease = semaphore.drain_permits

if drained_lease
  begin
    puts "Drained #{drained_lease.permits} permits for exclusive access"
    # Do exclusive work
  ensure
    semaphore.release(drained_lease)
  end
end

1. Automatic Cleanup: with_lease ensures permits are always released
2. Type Safety: Lease objects ensure you can only release what you've acquired
3. Cross-Semaphore Protection: Can't accidentally release a lease to the wrong semaphore
4. Distributed Coordination: Redis semaphore works seamlessly across processes and machines
5. Lease Expiration: Redis leases automatically expire to prevent deadlocks

This library aims for compatibility with Concurrent::Semaphore from the concurrent-ruby gem, but with a key difference to support both local and distributed implementations.

The core difference from Concurrent::Semaphore is that acquire returns a lease object that must be passed to release, rather than using numeric permit counts for both operations:

# concurrent-ruby style
semaphore.acquire(2)
# ... work ...
semaphore.release(2)  # Must remember the count!

# counting_semaphore style
lease = semaphore.acquire(2)
# ... work ...
semaphore.release(lease)  # Lease knows its own count

The Concurrent::Semaphore API where acquire(n) and release(n) use arbitrary counts works well for in-memory semaphores, but creates challenges for distributed Redis-based implementations:

1. Individual leases need TTLs: In Redis, each lease must have an expiration to prevent deadlocks from crashed processes
2. Lease tracking is essential: Distributed systems need unique identifiers for each acquired lease
3. Cross-process coordination: Releasing "2 permits" doesn't map cleanly to "which 2 leases?" across processes
4. Ownership semantics: The lease object makes it explicit what you acquired and what you're releasing

A lease is a simple struct that contains:

• semaphore - reference to the semaphore it came from
• id - unique identifier (local counter for LocalSemaphore, Redis key for RedisSemaphore)
• permits - number of permits held

This design:

• Prevents bugs: Can't accidentally release the wrong amount or to the wrong semaphore
• Works for both implementations: LocalSemaphore and RedisSemaphore use the same API
• Follows familiar patterns: Similar to file handles, database connections, and other resource management
• Maintains compatibility: The with_lease block form works identically to concurrent-ruby's usage

The library provides the same query methods as Concurrent::Semaphore:

• available_permits - returns the number of permits currently available
• capacity - returns the total capacity of the semaphore
• currently_leased - returns the number of permits currently in use

Additionally, drain_permits returns a lease object (or nil) instead of an integer, maintaining consistency with the lease-based design.

Add this line to your application's Gemfile:

gem "counting_semaphore"

And then execute:

$ bundle install

Or install it yourself as:

$ gem install counting_semaphore

There are no dependencies (but you need the redis gem for development - or you can feed a compatible object instead).

Do a fresh checkout and run bundle install. Then run tests and linting using bundle exec rake.

Bug reports and pull requests are welcome on GitHub at https://github.com/julik/counting_semaphore.

The gem is available as open source under the terms of the MIT License.