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ocean-dynamo

OceanDynamo is a massively scalable Amazon DynamoDB near drop-in replacement for ActiveRecord.

OceanDynamo requires Ruby 2.0 and Ruby on Rails 4.0.0 or later.

<img src=“https://badge.fury.io/rb/ocean-dynamo.png” alt=“Gem Version” />

Features

As one important use case for OceanDynamo is to facilitate the conversion of SQL databases to no-SQL DynamoDB databases, it is important that the syntax and semantics of OceanDynamo are as close as possible to those of ActiveRecord. This includes callbacks, exceptions and method chaining semantics. OceanDynamo follows this pattern closely and is of course based on ActiveModel.

The attribute and persistence layer of OceanDynamo is modeled on that of ActiveRecord: there’s save, save!, create, update, update!, update_attributes, find_each, destroy_all, delete_all, read_attribute, write_attribute and all the other methods you’re used to. The design goal is always to implement as much of the ActiveRecord interface as possible, without compromising scalability. This makes the task of switching from SQL to no-SQL much easier.

OceanDynamo uses only primary indices to retrieve related table items and collections, which means it will scale without limits.

OceanDynamo is fully usable as an ActiveModel and can be used by Rails controllers. Thanks to its structural similarity to ActiveRecord, OceanDynamo works with FactoryGirl.

Current State

  • Secondary indices are now fully supported! See below for more information.

  • Version 2 of the AWS Ruby SDK is now used.

  • Work begun on association proxies, etc.

Future milestones

  • Direct support for the DynamoDB JSON attribute types for arrays and hashes

  • Collection proxies, to implement ActiveRecord-style method chaining, e.g.: blog_entry.comments.build(body: "Cool!").save!

  • The has_and_belongs_to_many assocation.

Current use

OceanDynamo is used as a central component in Ocean, a Rails framework and development pipeline for creating massively scalable HATEOAS microservice SOAs in the cloud.

Ocean uses OceanDynamo to implement highly scalable job queues and authentication. It will be used increasingly as features are added to OceanDynamo and will eventually replace all ActiveRecord tables in Ocean.

However, OceanDynamo can of course also be used stand-alone.

Documentation

Contributing

Contributions are welcome. Fork in the usual way. OceanDynamo is developed using TDD: the specs are extensive and test coverage is very near to 100 percent. Pull requests will not be considered unless all tests pass and coverage is equally high or higher. All contributed code must therefore also be exhaustively tested.

Examples

Basic syntax

The following example shows the basic syntax for declaring a DynamoDB-based schema.

class AsyncJob < OceanDynamo::Table

  dynamo_schema(:guid) do
    attribute :credentials,          :string
    attribute :token,                :string,     default: Proc { SecureRandom.uuid }
    attribute :steps,                :serialized, default: []
    attribute :max_seconds_in_queue, :integer,    default: 1.day
    attribute :default_poison_limit, :integer,    default: 5
    attribute :default_step_time,    :integer,    default: 30
    attribute :started_at,           :datetime
    attribute :last_completed_step,  :integer
    attribute :finished_at,          :datetime
    attribute :destroy_at,           :datetime,   local_secondary_index: true
    attribute :created_by
    attribute :updated_by
    attribute :succeeded,            :boolean,    default: false
    attribute :failed,               :boolean,    default: false
    attribute :poison,               :boolean,    default: false

    global_secondary_index :token, projection: :all
  end

end

Attributes

Each attribute has a name, a type (:string, :integer, :float, :datetime, :boolean, or :serialized) where :string is the default. Each attribute also optionally has a default value, which can be a Proc. The hash key attribute is by default :id (overridden as :guid in the example above) and is a :string.

The :string, :integer, :float and :datetime types can also store sets of their type. Sets are represented as arrays, may not contain duplicates and may not be empty.

All attributes except the :string type can take the value nil. Storing nil for a string value will return the empty string, "".

Schema args and options

dynamo_schema takes args and many options. Here’s the full syntax:

dynamo_schema(
  table_hash_key = :id,                   # The name of the hash key attribute
  table_range_key = nil,                  # The name of the range key attribute (or nil)
  table_name: compute_table_name,         # The basename of the DynamoDB table
  table_name_prefix: nil,                 # A basename prefix string or nil
  table_name_suffix: nil,                 # A basename suffix string or nil
  read_capacity_units: 10,                # Used only when creating a table
  write_capacity_units: 5,                # Used only when creating a table
  connect: :late,                         # true, :late, nil/false
  create: false,                          # If true, create the table if nonexistent
  locking: :lock_version,                 # The name of the lock attribute or nil/false
  timestamps: [:created_at, :updated_at]  # A two-element array of timestamp columns, or nil/false
) do
  # Attribute definitions
  ...
  ...
end

has_many and belongs_to

Example

The following example shows how to set up has_many / belongs_to relations:

class Forum < OceanDynamo::Table
  dynamo_schema do
    attribute :name
    attribute :description
  end
  has_many :topics, dependent: :destroy
end

class Topic < OceanDynamo::Table
  dynamo_schema(:guid) do
    attribute :title
  end
  belongs_to :forum
  has_many :posts, dependent: :destroy
end

class Post < OceanDynamo::Table
  dynamo_schema(:guid) do
    attribute :body
  end
  belongs_to :topic, composite_key: true
end

The only non-standard aspect of the above is composite_key: true, which is required as the Topic class itself has a belongs_to relation and thus has a composite key. This must be declared in the child class as it needs to know how to retrieve its parent.

Restrictions

Restrictions for belongs_to tables:

  • The hash key must be specified and must not be :id.

  • The range key must not be specified at all.

  • belongs_to can be specified only once in each class.

  • belongs_to must be placed after the dynamo_schema attribute block.

Restrictions for has_many tables:

  • has_many must be placed after the dynamo_schema attribute block.

These restrictions allow OceanDynamo to implement the has_many / belongs_to relation in a very efficient and massively scalable way.

Implementation

belongs_to claims the range key and uses it to store its own id, which normally would be stored in the hash key attribute. Instead, the hash key attribute holds the id of the parent. We have thus reversed the roles of these two fields. As a result, all children store their parent id in the hash key, and their own id in the range key.

This type of relation is even more efficient than its ActiveRecord counterpart as it uses only primary indices in both directions of the has_many / belongs_to association. No scans.

Furthermore, since DynamoDB has powerful primary index searches involving substrings and matching, the fact that the range key is a string can be used to implement wildcard matching of additional attributes. This gives, amongst other things, the equivalent of an SQL GROUP BY request, again without requiring any secondary indices.

It’s our goal to use a similar technique to implement has_and_belongs_to_many relations, which means that secondary indices won’t be necessary for the vast majority of DynamoDB tables. This ultimately means reduced operational costs, as well as reduced complexity.

Secondary Indices

Local Secondary Indices

Up to five attributes can be declared as local secondary indices, in the following manner:

class Authentication < OceanDynamo::Table
  dynamo_schema(:username, :expires_at) do
    attribute :token,       :string,   local_secondary_index: true
    attribute :max_age,     :integer
    attribute :created_at,  :datetime
    attribute :expires_at,  :datetime
    attribute :api_user_id, :string
  end
end

The items of the above table can be accessed by using the hash key :username and the range key :expires_at. The local_secondary_index declaration makes it possible to access items using the same hash key :username but with :token as an alternate range key. Local secondary indices all use the same hash key as the primary index, substituting another attribute instead as the range key.

NB: The primary index [:username, :expires_at] requires all items to have a unique combination of keys. Secondary indices don’t require the range key to be unique for the same hash key. This means that secondary index searches always will return a collection.

Local secondary indices are queried through find_local_each and find_local. They take the same arguments; the former yields to a block for each item, the other returns all items in an array.

The following finds all Authentications where :username is “joe” and :token is “quux”:

Authentication.find_local(:username, "joe", :token, "=", "quux")

This retrieves all Authentications belonging to Joe, sorted on :token:

Authentication.find_local(:username, "joe", :token, ">=", "0")

The same thing but with the only the item with the highest token value:

Authentication.find_local(:username, "joe", :token, ">=", "0",
                          scan_index_forward: false, limit: 1)

Global Secondary Indices

Global secondary indices are declared after all attributes, but still within the do block:

class Authentication < OceanDynamo::Table
  dynamo_schema(:username, :expires_at) do
    attribute :token,       :string,   local_secondary_index: true
    attribute :max_age,     :integer
    attribute :created_at,  :datetime
    attribute :expires_at,  :datetime
    attribute :api_user_id, :string

    global_secondary_index :token, projection: :all
    global_secondary_index :api_user_id, :expires_at, read_capacity_units: 100
    global_secondary_index :expires_at, write_capacity_units: 50
  end
end

Each global_secondary_index clause (there can be a maximum of 5 per table) takes the following arguments:

  • hash_value (required),

  • range_value (optional),

  • :projection (default :keys_only, :all for all attributes)

  • :read_capacity_units (defaults to the table’s read capacity, normally 10)

  • :write_capacity_units (default to the table’s write capacity, normally 5)

Global secondary indices are queried through find_global_each and find_global. They take the same arguments; the former yields to a block for each item, the other returns all items in an array.

The following finds all Authentications whose :token is +“quux”+:

Authentication.find_global(:token, "quux")

This retrieves all Authentications belonging to the user with the ID +“dfstw-ruyhdf-ewijf”+, sorted in ascending order of the :expires_at attribute:

Authentication.find_global(:api_user_id, "dfstw-ruyhdf-ewijf", 
                           :expires_at, ">=", 0)

To get the highest :expires_at record:

Authentication.find_global(:api_user_id, "dfstw-ruyhdf-ewijf", 
                           :expires_at, ">=", 0,
                           scan_index_forward: false, limit: 1)

Installation

gem install ocean-dynamo

Then, locate the gem’s directory and copy

spec/dummy/config/initializers/aws.rb

to your project’s

config/initializers/aws.rb

Also copy

spec/dummy/config/aws.yml.example

to both the following locations in your project:

config/aws.yml.example
config/aws.yml

Enter your AWS credentials in the latter file.

Running the specs

To run the specs for the OceanDynamo gem, you must first install DynamoDB Local. It’s a Java clone of Amazon DynamoDB which runs locally on your computer. We use it for development and testing.

Download DynamoDB Local from the following location: docs.aws.amazon.com/amazondynamodb/latest/developerguide/Tools.DynamoDBLocal.html

Next, copy the AWS configuration file from the template:

cp spec/dummy/config/aws.yml.example spec/dummy/config/aws.yml

NB: aws.yml should be excluded from source control. This allows you to enter your AWS credentials safely. On the other hand, aws.yml.example SHOULD be under source control. Don’t put sensitive information in it.

You’re now ready to start DynamoDB Local:

java -Djava.library.path=./DynamoDBLocal_lib -jar DynamoDBLocal.jar -sharedDb

Replace -sharedDb with -inMemory to run the DB in RAM.

With DynamoDB Local running, you should now be able to do

rspec

All tests should pass.

Cleaning up the DB

You might want to add the following to your spec_helper.rb file, before the RSpec.configure block:

# DynamoDB table cleaner
CHEF_ENV = "master" unless defined?(CHEF_ENV)
regexp = Regexp.new("^.+_#{CHEF_ENV}_[0-9]{1,3}-[0-9]{1,3}-[0-9]{1,3}-[0-9]{1,3}_test$")
cleaner = lambda { 
  c = Aws::DynamoDB::Client.new
  c.list_tables.table_names.each do |t| 
    begin
      c.delete_table({table_name: t}) if t =~ regexp
    rescue Aws::DynamoDB::Errors::LimitExceededException
      sleep 1
      retry
    end
  end
}

Then, inside the RSpec.configure block:

config.before(:suite) { cleaner.call }
config.after(:suite) { cleaner.call }

This will remove only those test tables created by the specs on this particular machine and environment. This is safe even on AWS and in parallel with other machines.

Rails console

The Rails console is available from the built-in dummy application:

cd spec/dummy
rails console

This will, amongst other things, also create the CloudModel table if it doesn’t already exist. On Amazon, this will take a little while. With DynamoDB Local, it’s practically instant.

When you leave the console, you must navigate back to the top directory (cd ../..) in order to be able to run RSpec again.

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A massively scalable Amazon DynamoDB near drop-in replacement for ActiveRecord.

rubygems.org/gems/ocean-dynamo

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