ReadySet Rails

A gem for caching with ReadySet within Rails applications.

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Table of Contents

What is ReadySet?

ReadySet is a database acceleration engine that acts as a read replica and implements a novel type of query caching that automatically keeps your caches up-to-date by watching your database's replication stream. ReadySet helps you scale your database by shedding load and reducing query latency.

What does this gem do?

This gem makes it easy to use ReadySet from within your Rails application by allowing you to selectively route queries to ReadySet. The high-level features of this gem include:

  • A Readyset.route method that takes a block and routes to ReadySet any queries within that block
  • A controller extension that allows you to route to ReadySet all queries that occur within specific controller actions
  • A model extension that allows you to define queries to be routed to ReadySet within the context of an existing model
  • Rake tasks that allow you to easily manage cache migration on ReadySet, ensuring that a consistent set of caches exists across all of your environments
  • Automatic failover back to your primary database in the event that your ReadySet instance is unreachable (disabled by default)

Note that ReadySet only guarantees support for PostgreSQL right now, so this gem only supports PostgreSQL.

Installing

If you run into any trouble with the below steps, please feel free to reach out via our community Slack!

  1. Follow the instructions here to install and run ReadySet
  2. Add the following line to your Gemfile and run bundle install: sh gem 'readyset' The ReadySet Rails gem currenly supports Ruby versions >= 3.0 and Rails versions >= 6.1.
  3. Add a section to your config/database.yml file with ReadySet's connection information. If you currently connect to only one database, you'll need to move your primary database connection information to be nested under a new key named primary. yaml development: primary: # This is the connection information for your primary database database: testdb username: postgres password: readyset adapter: postgresql port: 5432 readyset: # This is the connection information for ReadySet database: testdb username: postgres password: readyset adapter: readyset host: "127.0.0.1" port: 5433 # This setting tells Rails that there's no need to run migrations or other database # administration tasks against ReadySet database_tasks: false You can verify that ReadySet is up and your application is connected by running rails readyset:status: sh $ rails readyset:status +----------------------------+------------------------+ | Database Connection | Connected | | Connection Count | 1 | | Snapshot Status | Completed | | Maximum Replication Offset | (0/6DBBD78, 0/6DBBFF0) | | Minimum Replication Offset | (0/6DBBD78, 0/6DBBFF0) | | Last started Controller | 2024-01-17 18:49:02 | | Last completed snapshot | 2024-01-19 15:18:02 | | Last started replication | 2024-01-19 15:18:02 | +----------------------------+------------------------+ You can also view the tables that ReadySet knows about and their statuses by running rails readyset:tables: sh $ rails readyset:tables +---------------------------------+-------------+-------------+ | table | status | description | +---------------------------------+-------------+-------------+ | "public"."posts" | Snapshotted | | +---------------------------------+-------------+-------------+
  4. Run Readyset.configure wherever you configure other gems in your application, and set any desired configuration options: ruby Readyset.configure do |config| # Set your config options here end The list of available configuration options can be found here.
  5. FOR RAILS 7 USERS: Enable Rails's query log tags features by setting config.active_record.query_log_tags_enabled = true wherever you configure your ActiveRecord settings. This will append information to your ActiveRecord query logs that tells you where the given query was routed (e.g. to ReadySet or to your primary database)

Quickstart

  1. Follow the instructions above to set up ReadySet and install the gem
  2. Route a query to ReadySet in your application like so: ruby Readyset.route do Post.where(user_id: user_id) end
  3. Start up your application and drive traffic through the part of your application that invokes the query you routed in the previous step
  4. If you are running Rails 7 and enabled query log tags as explained in step 5 above, you should see an annotation next to the query in your application logs that denotes where the query was routed. If the destination tag has the value readyset, the query was routed to ReadySet.

If you are running Rails 6, you can validate that the query was routed to ReadySet by running rails readyset:proxied_queries. A "proxied" query is one that was served by ReadySet but was proxied to your primary database, since a cache for the query does not yet exist:

   $ rails readyset:proxied_queries
   +--------------------+-------------------------------------------------------+-------------+-------+
   | id                 | text                                                  | supported   | count |
   +--------------------+-------------------------------------------------------+-------------+-------+
   | q_281c5f9b8e4013bb | SELECT                                                | yes         | 1     |
   |                    |   *                                                   |             |       |
   |                    | FROM                                                  |             |       |
   |                    |   "posts"                                             |             |       |
   |                    | WHERE                                                 |             |       |
   |                    |   ("user_id" = $1)                                    |             |       |
   +--------------------+-------------------------------------------------------+-------------+-------+
  1. Create a cache for the query by running rails readyset:proxied_queries:cache_all_supported. This will create caches for all of the queries proxied by ReadySet that are supported to be cached. You can verify that the expected caches were created by running rails readyset:caches: sh $ rails readyset:caches +--------------------+--------------------+-------------------------------------+--------+-------+ | id | name | text | always | count | +--------------------+--------------------+-------------------------------------+--------+-------+ | q_281c5f9b8e4013bb | q_281c5f9b8e4013bb | SELECT | false | 0 | | | | "public"."posts"."user_id" | | | | | | FROM | | | | | | "public"."posts" | | | | | | WHERE | | | | | | ("public"."posts"."user_id" = $1) | | | +--------------------+--------------------+-------------------------------------+--------+-------+
  2. Drive traffic through the part of your application that invokes your cached query. The first invocation of the query will be a cache miss, but the second will be served from the cache. You can verify that the cache was successfully used by looking at the count column in the output of rails readyset:caches: sh $ rails readyset:caches +--------------------+--------------------+-------------------------------------+--------+-------+ | id | name | text | always | count | +--------------------+--------------------+-------------------------------------+--------+-------+ | q_281c5f9b8e4013bb | q_281c5f9b8e4013bb | SELECT | false | 1 | | | | "public"."posts"."user_id" | | | | | | FROM | | | | | | "public"."posts" | | | | | | WHERE | | | | | | ("public"."posts"."user_id" = $1) | | | +--------------------+--------------------+-------------------------------------+--------+-------+

Usage

Getting Started with Caching

Queries in arbitrary code blocks can be routed to ReadySet using the Readyset.route method like so:

Readyset.route do
  Post.where(user_id: user_id)
end

Any queries invoked in the given block will be routed to the ReadySet instance configured in your config/database.yml file; however, until a cache is created for a particular query, invocations of that query against ReadySet will be proxied to your database. To create a cache for a specific query, you have a few options:

  • Invoke .create_readyset_cache directly on an ActiveRecord query in the Rails console: ruby Post.where(user_id: 1).create_readyset_cache!
  • Create caches for all of the queries supported by ReadySet that ReadySet has seen and proxied to your database since it last started up using the provided Rake task: sh rails readyset:proxied_queries:cache_all_supported Note: If you route a query to ReadySet, decide you no longer want to cache that query, and stop routing that query to ReadySet, that query will still exist in ReadySet's list of queries that it has proxied to your database. This means that running the above Rake task will still create a cache for that query even though it is no longer annotated to be routed to ReadySet in your application code. The list of queries ReadySet has proxied can be cleared by restarting ReadySet or by running rails readyset:proxied_queries:drop_all.
  • View the list of queries that ReadySet has proxied by running the following in a Rails console: ruby Readyset::Query::ProxiedQuery.all You can invoke #cache! on the queries in this list for which you'd like to create caches on ReadySet.
  • View the list of queries that ReadySet has proxied and that are supported by ReadySet to be cached by running the following: sh rails readyset:proxied_queries:supported Pick a query from the list that you'd like to cache, and pass the ID to the rails readyset:create_cache command like so: sh rails 'readyset:create_cache[your_query_id]'

Once a cache has been created for a particular query, it will persist on ReadySet across restarts (although any in-memory cached data will be lost when ReadySet goes down). You can view the list of existing caches using the provided Rake task:

rails readyset:caches

To drop a given cache in the list printed by the above command, you can pass the name of the cache to the readyset:caches:drop Rake task like so:

rails 'readyset:caches:drop[my_cache]'

You can also view the list of existing caches in an interactive form via the Rails console:

Readyset::Query::CachedQuery.all

You can invoke #drop! on any of the caches in this list to remove the cache from ReadySet.

Query Routing in Controllers

The gem includes an extension to ActionController that allows you to route to ReadySet all of the queries that occur within the context of a given controller action:

class PostsController < ActionController
  route_to_readyset only: :show

  def show
    @post = Post.where(id: params[:id])
  end
end

route_to_readyset takes the same parameters as Rails's around_filters.

Query Routing in Models

The gem also includes an extension that allows you to define queries in your model that will be routed to ReadySet:

class Post < ApplicationRecord
    readyset_query :posts_for_user, ->(user_id) { where(user_id: user_id) }
end

The above example will define a .posts_for_user class method on the Post model that invokes the query Post.where(user_id: user_id) against ReadySet. Note that other invocations of this query outside of the context of the .posts_for_user method will not be routed to ReadySet.

This feature allows you to specify which queries should be routed to ReadySet in a centralized location and prevents the need to use Readyset.route everywhere a cached query is invoked.

Cache Migrations

Once you have a set of caches you are happy with in your development environment, you'll need a way to easily reproduce the same set of caches in other environments (e.g. staging, production, etc.). To facilitate this, the gem includes a "migration" feature, that allows you to dump the current set of caches to a "migration file" and re-create these caches using the same migration file.

The following Rake task dumps the current set of caches to the db/readyset_caches.rb file:

rails readyset:caches:dump

This file should be checked into version control with your application code. To update a ReadySet instance so that its set of caches matches the caches in your migration file:

rails readyset:caches:migrate

This command 1) drops any caches that exist on ReadySet that are not present in the migration file and 2) creates any caches that are present in the migration file that do not exist on ReadySet. To run the command for a particular Rails environment, you can set the RAILS_ENV environment variable.

Automatic Failover

To handle situations where ReadySet is unreachable for any reason, the gem includes an automatic failover feature. The gem tracks the number of ReadySet connection failures over a configurable window of time, and if the number of errors exceeds the configured threshold, any queries previously being routed to ReadySet will be routed to the primary database. A background task is started up that periodically attempts to establish a connection to ReadySet and check its status. When the task confirms that ReadySet is available again, it allows queries to be routed to ReadySet again.

This feature is disabled by default. To enable it, set the config.failover.enabled configuration option to true. You can read about the other available configuration options here.

Configuration Options

The gem's configuration options can be set by passing a block to Readyset.configure and setting options on the yielded Readyset::Configuration object. The available options are documented below. The values below are the default values for each of the options.

Readyset.configure do |config|
  # Whether the gem's automatic failover feature should be enabled.
  config.failover.enabled = false
  # Sets the interval upon which the background task will check
  # ReadySet's availability after failover has occurred.
  config.failover.healthcheck_interval = 5.seconds
  # Sets the time window over which connection errors are counted
  # when determining whether failover should occur.
  config.failover.error_window_period = 1.minute
  # Sets the number of errors that must occur within the configured
  # error window period in order for failover to be triggered.
  config.failover.error_window_size = 10
  # The file in which cache migrations should be stored.
  config.migration_path = File.join(Rails.root, 'db/readyset_caches.rb')
end

License

MIT License