ruby/rails: Adapter supports non-admin users, readme refresh (#114)

* ruby/rails: Adapter supports non-admin users, readme refresh

- Update the adapter to work with non-Admin users. Allows the user
  to be set with a CLUSTER_USER env var (defaults to 'admin') and will
  generate an appropriately scoped auth token for the cluster.
- Select schema based on CLUSTER_USER
- Rework the readme into:
   - ./README.md: Explains how to use the pg-aws_rds_iam plugin to
     generate DSQL tokens and necessary dialect updates. Intended
     audience is someone who has an existing Rails app and wants to
     know the necessary changes for DSQL.
   - ./petclinic/README.md: How to install the sample Rails app and
     work with the data model.

Author: trstephen-amazon

ruby/rails/README.md

@@ -1,49 +1,135 @@
-# Aurora DSQL Ruby on Rails code examples
+# Ruby on Rails with Aurora DSQL
+This example demonstrates how to use an Aurora DSQL cluster with a Ruby On Rails
+application. Aurora DSQL only supports token-based authentication so we extend the
+[`pg-aws_rds_iam`][rds-plugin-repo] plugin to generate Aurora DSQL auth tokens
+when required.
+
+It also includes changes to ActiveRecord behavior to be compatible with Aurora DSQL
+supported features.
+
+[rds-plugin-repo]: https://github.com/haines/pg-aws_rds_iam
+
+## Running this example
+See [`petclinic/README.md`](./petclinic/README.md).
+
+## Using Aurora DSQL authentication tokens with Rails
+These are the changes to make to your Rails application to be compatible with Aurora DSQL.
+
+### Add a token generator
+To modify your Rails application to work with Aurora DSQL you should reproduce the
+`DsqlAuthTokenGenerator` in [`adapter.rb`][file-adapter].
+
+```ruby
+require "aws-sdk-dsql"
+
+class DsqlAuthTokenGenerator
+  def call(host:, port:, user:)
+    # e.g. host == "<clusterID>.dsql.us-east-1.on.aws"
+    region = host.split(".")[2]
+    raise "Unable to extract AWS region from host '#{host}'" unless region =~ /[\w\d-]+/
+
+    token_generator = Aws::DSQL::AuthTokenGenerator.new(
+      credentials: Aws::CredentialProviderChain.new.resolve,
+    )
+
+    auth_token_params = {
+      endpoint: host,
+      region: region,
+      expires_in: 15 * 60 # 15 minutes, optional
+    }
+
+    case user
+    when "admin"
+      token_generator.generate_db_connect_admin_auth_token(auth_token_params)
+    else
+      token_generator.generate_db_connect_auth_token(auth_token_params)
+    end
+  end
+end
+```
 
-## Overview
+`call` will be invoked when a new database connection is requested. It will:
+1. Retrieve credentials for the running environment. The `Aws::CredentialProviderChain` discovers credentials
+   in the order described in [these docs][docs-cred-provider].
+1. Determine which token type to generate based on the database user.
 
-The code examples in this topic show you how to use the Ruby on Rails work with Aurora DSQL. 
+The retrieved credentials will need permission to `dsql:DbConnectAdmin` for the `admin` user or
+`dsql:DbConnect` for a custom user. See Aurora DSQL documentation for [IAM role connect][docs-dsql-iam]
+and [authentication token generation][docs-generate-token] for more details.
 
-## Run the examples
 
-### Prerequisites
+Finally, register the adapter with the `pg-aws_rds_iam` plugin.
+```ruby
+PG::AWS_RDS_IAM.auth_token_generators.add :dsql do
+  DsqlAuthTokenGenerator.new
+end
+```
 
-* ruby version == 3.3.5 is needed
-* AWS credentials file is configured
+[file-adapter]: ./petclinic/config/initializers/adapter.rb
+[docs-cred-provider]: https://docs.aws.amazon.com/sdk-for-ruby/v3/developer-guide/credential-providers.html
+[docs-dsql-iam]: https://docs.aws.amazon.com/aurora-dsql/latest/userguide/authentication-authorization.html#authentication-authorization-iam-role-connect
+[docs-generate-token]: https://docs.aws.amazon.com/aurora-dsql/latest/userguide/SECTION_authentication-token.html
 
-### Setup test running environment 
+### Alter ActiveRecord behavior
+Disable features not supported by Aurora DSQL. The example includes this in [`adapter.rb`][file-adapter].
 
-Install ruby 3.3.5 for Cloud Desktop:
-```
-sudo yum install -y libyaml-devel
-rbenv install 3.3.5
-```
+```ruby
+require "active_record/connection_adapters/postgresql/schema_statements"
+
+module ActiveRecord::ConnectionAdapters::PostgreSQL::SchemaStatements
+  # DSQL does not support setting min_messages in the connection parameters
+  def client_min_messages=(level); end
+end
 
-### Run the example tests
-Open a Ruby on Rails app console to manually test console commands:
+require "active_record/connection_adapters/postgresql_adapter"
 
+class ActiveRecord::ConnectionAdapters::PostgreSQLAdapter
+  def set_standard_conforming_strings; end
+
+  # Avoid error running multiple DDL or DDL + DML statements in the same transaction
+  def supports_ddl_transactions?
+    false
+  end
+end
 ```
-# Use the account credentials dedicated for Ruby on Rails
 
-# Download the Amazon root certificate from the official trust store
-# This example shows one of the available certs that can be used by the client;
-# other certs such as AmazonRootCA2.pem, AmazonRootCA3.pem, etc. can also be used.
-wget https://www.amazontrust.com/repository/AmazonRootCA1.pem -O root.pem
+### Use the adapter in the database configuration
+Refer to [`database.yml`](./petclinic/config/database.yml).
+
+```yml
+development:
+  <<: *default
+
+  # Always the database name for Aurora DSQL
+  database: postgres
+
+  # eg: admin or other postgres users
+  username: <postgres username>
+
+  # Set this value based on the access of the configured user,
+  # or omit if running as 'admin' and using the 'public' schema.
+  schema_search_path: myschema
 
-export CLUSTER_ENDPOINT="<your cluster endpoint from us-east-1>"
-cd petclinic
-bundle install
+  # Set to Aurora DSQL instance endpoint
+  # Use environment variables, etc for production values!
+  # e.g. {clusterId}.dsql.{region}.on.aws
+  host: foo0bar1baz2quux3quuux4.dsql.us-east-1.on.aws
 
-# Generate the schema from the model files in db/migrate.
-bin/rails db:migrate
+  # Use the custom token generator we created
+  aws_rds_iam_auth_token_generator: dsql
 
-# Start the rails console
-bin/rails console
+  # Provide the path to the root certificate. 
+  # Amazon's root certs can be fetched from https://www.amazontrust.com/repository/
+  sslrootcert: <replace with the path to root certificate>
+  sslmode: verify-full
 
+  # More DSQL compatibility tweaks
+  advisory_locks: false
+  prepared_statements: false
 ```
 
 ---
 
-Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved. 
+Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 
 SPDX-License-Identifier: MIT-0