You can keep reading here or jump to my blog to get the full experience, including the wonderful pink, blue and white palette.
This is part of a series:
- Rewriting to Haskell–Intro
- Rewriting to Haskell–Project Setup
- Rewriting to Haskell–Deployment
- Rewriting to Haskell–Automatic Formatting
Coming from Rails we are used to employing yaml files to configure a web application. This is why we decided to do the same with Servant. As a matter of fact, we now have a configuration.yml file:
database:
username: stream
database: stream_development
password: ""
application:
aws_s3_access_key: "ABCD1234"
aws_s3_secret_key: "EFGH5678"
aws_s3_region: us-east-1
aws_s3_bucket_name: stream-demo-bucket
That is great for development but how can we run test against the test database? Turns out that the package we use to parse the yaml file allows the use of ENV variables:
database:
username: stream
database: _env:DATABASE:stream_development
password: ""
application:
aws_s3_access_key: "ABCD1234"
aws_s3_secret_key: "EFGH5678"
aws_s3_region: us-east-1
aws_s3_bucket_name: stream-demo-bucket
That is, now we can just run DATABASE=stream_test stack test!
In the repository we actually keep a configuration.yml.example file and git ignore configuration.yml to avoid leaking credentials:
database:
username: stream
database: _env:DATABASE:stream_development
password: ""
application:
aws_s3_access_key: "REPLACE_ME"
aws_s3_secret_key: "REPLACE_ME"
aws_s3_region: us-east-1
aws_s3_bucket_name: ll-stream-demo
For production we use Ansible (with Ansible Vault) to put in place the correct configuration.yml. Plus, we instruct Hapistrano to make that file available for each deployment:
linked_files:
- haskell/configuration.yml
To read the configuration inside the Servant application we use loadYamlSettings from the yaml package:
loadYamlSettings
:: FromJSON settings
=> [FilePath] -- ^ run time config files to use, earlier files have precedence
-> [Value] -- ^ any other values to use, usually from compile time config. overridden by files
-> EnvUsage
-> IO settings
In other words, given a type settings that is an instance of FromJSON we can decode yaml files into a value of that type. And this is how we do it for Stream:
data Configuration
= Configuration
{ configurationDatabaseUser :: String,
configurationDatabaseDatabase :: String,
configurationDatabasePassword :: String,
configurationApplicationAwsS3AccessKey :: AccessKey,
configurationApplicationAwsS3SecretKey :: SecretKey,
configurationApplicationAwsS3Region :: Region,
configurationApplicationAwsS3BucketName :: BucketName
}
instance FromJSON Configuration where
parseJSON (Object x) = do
database <- x .: "database"
application <- x .: "application"
Configuration
<$> database .: "username"
<*> database .: "database"
<*> database .: "password"
<*> application .: "aws_s3_access_key"
<*> application .: "aws_s3_secret_key"
<*> application .: "aws_s3_region"
<*> application .: "aws_s3_bucket_name"
loadConfiguration :: IO Configuration
loadConfiguration =
loadYamlSettings ["./configuration.yml"] [] useEnv
Get the latest content via email from me personally. Reply with your thoughts. Let's learn from each other. Subscribe to my PinkLetter!
Top comments (0)