Idempo

A relatively straightforward idempotency keys gem. If your client sends the Idempotency-Key or X-Idempotency-Key header to your Rack application, and the response can be cached, Idempo will provide both a concurrent request lock and a cache for idempotent responses. If the idempotent response is already saved for this idempotency key and request fingerprint, the cached response is going to be served instead of calling your application.

Usage

Idempo supports a number of backends, we recommend using Redis if you have multiple application servers / dynos and MemoryBackend if you are only using one single Puma worker. To initialize with Redis as backend pass the backend: parameter when adding the middleware:

be = Idempo::RedisBackend.new(Rails.application.config.redis_connection_pool)
use Idempo, backend: be

and to initialize with a memory store as backend:

use Idempo

In principle, the following requests qualify to be cached used the idempotency key:

• Any request which is not a GET, HEAD or OPTIONS and...
• Provides an Idempotency-Key or X-Idempotency-Key header

The default time for storing the cache is 30 seconds from the moment the request has finished generating. The response is going to be buffered, then serialized using msgpack, then deflated. Idempo will not cache the response if its size cannot be known in advance, and if the size of the response body exceeds a reasonable size (4 MB is our limit for the time being) - this is to prevent your storage from filling up with very large responses.

Controlling the behavior of Idempo from your application

You can control the behavior of Idempo using special response headers:

• Set X-Idempo-Policy to no-store to disable retention of the response even though it otherwise could be cached
• Set X-Idempo-Persist-For-Seconds to a decimal number of seconds to store your response for. If your response contains time-sensitive data you might need to tweak the storage time.

Idempo supports a number of data stores (here they are called "backends") - MemoryBackend, ActiveRecordBackend, RedisBackend.

Using memory for idempotency keys

If you run only one Puma on one server (so multiple threads but one process) the MemoryBackend will work fine for you.

• It uses a Set with a Mutex around it to store requests in progress
• It uses a sorted array for expiration and cached responses.

Needless to say, if your server terminates or restarts all the data disappears with it. This backend will also only work if you are running one Puma process (or other single-process server, and just one instance of it).

Using your database for idempotency keys (via ActiveRecord)

The relational database you already have is a perfectly fine place to store idempotency key locks and responses. A requirement for that is that your database supports some form of advisory locking - both PostgreSQL and MySQL do. First you will need to create a table for the records. The table is going to be called idempo_responses, and you need to add a migration in your Rails project for it:

$ rails g migration add_idempo_responses

and then add a migration like this:

class AddIdempoResponses < ActiveRecord::Migration[7.0]
  def change
    Idempo::ActiveRecordBackend.create_table(self)
  end
end

Then configure Idempo to use the backend (in your application.rb):

be = Idempo::ActiveRecordBackend.new
config.middleware.insert Idempo, backend: be

In your regular tasks (cron or Rake) you will want to add a call to delete old Idempo responses (there is an index on expire_at):

Idempo::ActiveRecordBackend.new.prune!

If you need to use Idempo with PGBouncer you will need to write your own locking implementation based on fencing tokens or similar.

Using Redis for idempotency keys

Redis is a near-perfect data store for idempotency keys, but it can have race conditions with locks if your application runs for too long or crashes very often. If you have Redis, initialize Idempo using the RedisBackend:

use Idempo, backend: Idempo::RedisBackend.new

If you have a configured Redis connection pool (and you should) - pass it to the initializer:

be = Idempo::RedisBackend.new(config.redis_connection_pool)
config.middleware.insert Idempo, backend: be

All data stored in Redis will have TTLs and will expire automatically. Redis scripts ensure that updates to the stored idempotent responses and locking happen atomically.

Installation

Add this line to your application's Gemfile:

gem 'idempo'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install idempo

More advanced use cases

Check out the files in the examples/ directory to see a few customisations you can do.

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to rubygems.org.

Contributing

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

License

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