Handshaker

Handshaker is a Ruby gem for implementing multi-strategy Collaborative Transactional Validation (CTV). If you're not familiar with the concept, fear not, because it's something we just made up at Batteries 911.

CTV is a process where multiple parties cooperate to validate the integrity of a transaction. You can think of it as a very sophisticated if check, that might potentially involve tens of different conditions.

At Batteries 911, CTV is employed during the cross-docking handshake process (hence the name of the gem), where two users validate the integrity of the inventory that is being transferred.

Installation

Add this line to your application's Gemfile:

gem 'handshaker'

And then execute:

$ bundle

Or install it yourself as:

$ gem install handshaker

Usage

The simplest example we can provide of CTV is two users, Bob and Alice, confirming a transaction, each with their own password.

transaction = Handshaker::Transaction::Strict.new(steps: [
  Handshaker::Step::Literal.new(answer: 'Secret123', party: bob),
  Handshaker::Step::Literal.new(answer: 'Secret456', party: alice)
])

transaction.valid? # => false

transaction.contribute_as(alice, with: 'Secret456') # => Handshaker::Contribution
transaction.valid? # => false

transaction.contribute_as(bob, with: 'Secret789')
transaction.valid? # => false
 
transaction.contribute_as(bob, with: 'Secret123')
transaction.valid? # => true

As you can see, the transaction was only valid once both Bob and Alice had both entered their passwords.

Not impressed yet? Read on.

Transaction/step types

Transactions can be resolved in different ways. For instance, consider the following scenario: Bob and Alice are going on a trip together and they must agree on how much they want to spend. In this case, we don't care about them entering an exact value in the system, we just want them to enter the same value before we allow the transaction to continue.

This type of transaction is a joint resolution transaction, which is represented by a Handshaker::Transaction::Joint object. It will also not use the Literal step type, which expects an answer we already know, but rather the Open step type, which lets the users reply however they want and negotiate an answer:

transaction = Handshaker::Transaction::Joint.new(steps: [
  Handshaker::Step::Open.new(party: bob),
  Handshaker::Step::Open.new(party: alice)
])

transaction.valid? # => false

transaction.contribute_as(alice, with: 500) # Alice wants to spend 500 USD
transaction.valid? # => false

transaction.contribute_as(bob, with: 1000) # Bob wants to spend 1000 USD
transaction.valid? # => false

transaction.contribute_as(alice, with: 1000) # Alice agrees to spend 1000 USD
transaction.valid? # => true
transaction.resolution # => 1000

The following table details the strategies we have and how they work:

Transaction type	Description
Strict	Expects all users to provide the answer configured in their step.
Joint	Expects all users to provide the same answer.
AllIn	Expects all users to reply true.
AllOut	Expects all users to reply false.

The following table details the steps we have and how they work:

Step type	Description
Literal	Expects the exact answer configured in its step.
Boolean	Expects a boolean answer.
Open	Accepts any answer.

Note that, while you can mix and match transaction types and step types in any way you want (because steps share the same interface), some step types don't make much sense in certain contexts. For instance, it would not make much sense to create a joint transaction with literal steps, because it would either be non-resolvable or have a predictable outcome.

Similarly, it is not recommended to mix different step types in the same transaction, as the results would be pretty unpredictable. Handshaker might raise an error in the future about this.

Enforcing order

We can enforce the order in which the steps are executed. This can be useful, for instance, if you want to collect decisions in a specific order. For instance, in a sale transaction you might want to verify that the seller confirms delivery before the buyer confirms reception.

To accomplish this, we can use the ordered property, which forbids a user to contribute if the previous user has not contributed yet.

transaction = Handshaker::Transaction::AllIn.new(
  steps: [
    Handshaker::Step::Boolean.new(party: seller),
    Handshaker::Step::Boolean.new(party: buyer)
  ],
  ordered: true
)

transaction.valid? # => false

transaction.contribute_as(buyer, with: true) # raises Handshaker::OrderError

transaction.contribute_as(seller, with: true)
transaction.valid? # => false

transaction.contribute_as(buyer, with: true)
transaction.valid? # => true

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 tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/batteries911/handshaker.

License

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

Todo

• AR/Rails/Pragma integration