# Serverless Haskell

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Deploying Haskell code onto [AWS Lambda] using [Serverless].

## Prerequisites

* AWS account
* [Stack]
* [NPM]
* [Docker]

## Usage

There are two ways to start, either via the stack template, or directly modifying a project. You may want to use the manual approach as the template specifies a specific stack resolver as it needs to hardcode the `stack.yaml` file.

In either case, you will want to have [Serverless] installed, eg. `npm install -g serverless`.

### Using the stack template

* Create a [Stack] package for your code:

  ```shell
  stack new mypackage https://raw.githubusercontent.com/seek-oss/serverless-haskell/master/serverless-haskell.hsfiles
  ```

* Update the resolver in the `stack.yaml` file. This is hardcoded as the resolver number is not known at template interpolation time. You should pick either the latest resolver, or one you have used before and have thus prebuilt many of the core packages for.
  
* Install the dependencies and build the project:
 
  ```shell
  cd mypackage
  npm install
  stack build
  sls invoke local -f mypackage-func
  ```
  
  This should invoke serverless locally and display output once everything has built.
  
### Manually

* Create a [Stack] package for your code:

  ```shell
  stack new mypackage
  ```

  LTS 9-15 are supported, older versions are likely to work too but untested.

* Initialise a Serverless project inside the Stack package directory and install
  the `serverless-haskell` plugin:

  ```shell
  cd mypackage
  npm init .
  npm install --save serverless serverless-haskell@x.y.z
  ```

  The version of the NPM package to install must match the version of the
  Haskell package.

* Create `serverless.yml` with the following contents:

  ```yaml
  service: myservice

  provider:
    name: aws
    runtime: haskell

  functions:
    myfunc:
      handler: mypackage.myfunc
      # Here, mypackage is the Haskell package name and myfunc is the executable
      # name as defined in the Cabal file. The handler field may be prefixed
      # with a path of the form `dir1/.../dirn`, relative to `serverless.yml`,
      # which points to the location where the Haskell package `mypackage` is
      # defined. This prefix is not needed when the Stack project is defined at
      # the same level as `serverless.yml`.

  plugins:
    - serverless-haskell
  ```

* Write your `main` function:

  ```haskell
  import qualified Data.Aeson as Aeson

  import AWSLambda

  main = lambdaMain handler

  handler :: Aeson.Value -> IO [Int]
  handler evt = do
    putStrLn "This should go to logs"
    print evt
    pure [1, 2, 3]
  ```

* Use `sls deploy` to deploy the executable to AWS Lambda.

  The `serverless-haskell` plugin will build the package using Stack and upload
  it to AWS together with a JavaScript wrapper to pass the input and output
  from/to AWS Lambda.

  You can test the function and see the invocation results with
  `sls invoke -f myfunc`.

  To invoke the function locally, use `sls invoke local -f myfunc`.

### API Gateway

This plugin supports handling API Gateway requests. Declare the HTTP events
normally in `serverless.yml` and use
[AWSLambda.Events.APIGateway](https://hackage.haskell.org/package/serverless-haskell/docs/AWSLambda-Events-APIGateway.html)
in the handler to process them.

[Serverless Offline] can be used for local testing of API Gateway requests. You
must use `--useDocker` flag so that the native Haskell runtime works correctly.

When using [Serverless Offline], make sure that the project directory is
world-readable, otherwise the started Docker container will be unable to access
the handlers and all invocations will return HTTP status 502.

### Notes

* Only AWS Lambda is supported at the moment. Other cloud providers would
  require different JavaScript wrappers to be implemented.

See
[AWSLambda](https://hackage.haskell.org/package/serverless-haskell/docs/AWSLambda.html)
for documentation, including additional options to control the deployment.

## Development

`master` branch is the stable version. It is normally released to Hackage once
new changes are merged via Git tags.

The package is also maintained in Stackage LTS, provided the dependencies are
not blocking it.

### Testing

* Haskell code is tested with Stack: `stack test`.
* TypeScript code is linted with `eslint`.

### Integration tests

Integration test verifies that the project can build and deploy a complete
function to AWS, and it runs with expected functionality.

Integration test is only automatically run up to deployment due to the need for
an AWS account. To run manually:

* Ensure you have the required dependencies:
  - `curl`
  - [jq]
  - `libpcre` headers (`-devel` package or similar)
  - [NPM]
  - [`pkg-config`](pkg-config)
  - `pwgen`
  - [Stack]
* Get an AWS account and add the access credentials into your shell environment.
* Run `./integration-test/run.sh`. The exit code indicates success.
* To verify just the packaging, without deployment, run
  `./integration-test/run.sh --dry-run`.
* By default, the integration test is run with the LTS specified in
  `stack.yaml`. To specify a different series, use `RESOLVER_SERIES=lts-9`.
* To avoid creating a temporary directory for every run, specify
  `--no-clean-dir`. This can speed up repeated test runs, but does not guarantee
  the same results as a clean test.

### Releasing

* Ensure you are on the `master` branch.
* Ensure that all the changes are reflected in the changelog.
* Run the integration tests.
* Run `./bumpversion major|minor|patch`. This will increment the version number,
  update the changelog, create and push the Git tag and the branch.
* If you have released an LTS version, merge the version branch into `master`,
  taking care of the conflicts around version numbers and changelog, and release
  the latest version as well.

[AWS Lambda]: https://aws.amazon.com/lambda/
[Docker]: https://www.docker.com/
[jq]: https://stedolan.github.io/jq/
[NPM]: https://www.npmjs.com/
[pkg-config]: https://www.freedesktop.org/wiki/Software/pkg-config/
[Serverless]: https://serverless.com/framework/
[Serverless Offline]: https://github.com/dherault/serverless-offline
[Stack]: https://haskellstack.org