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Programming language: Haskell
License: Apache License 2.0
Tags: Web    

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shelduck is a hybrid web-server/api-client. Its main use is as an opinionated tool for QAing webhooks on remote services. Read about it here.

shelduck is made up of a few concurrent components:

  • An ngrok client can be used to forward a local service (by setting ENABLE_NGROK=true in your Env). shelduck expects ngrok in your path, and a fixed ngrok url (requires a paid account) with a configuration block like:
# ~/.ngrok2/ngrok.yml
authtoken: foobarbaz
    hostname: "yoururl.grok.io"
    proto: http
    addr: 8080

This fixed ngrok URL is what you should use to create subscriptions in the service you are testing.

  • A web-service is run to catch and record incoming webhooks.

  • An API client/data-types/DSL is used to describe what topics result from what API actions. For example the following asserts that creating a user via the Intercom API fires a webhook with topic user.created:

blank & requestEndpoint .~ "https://api.intercom.io/users"
      & requestOpts .~ options
      & requestParameters .~ object ["email" .= ("bob+{{random}}@intercom.io" :: T.Text)]
      & requestTopic .~ "user.created"
  • A templater is used to splice in different attributes. For example {{random}} injects a UUID.

Running shelduck

Check out src/Shelduck/IntercomDefinitions.hs for some descriptions of Intercom webhooks.

You can run shelduck by concurrently starting the server and request engine:

run :: TVar TopicResult -> StateT DefinitionListRun IO ()
run t = void $ do
  -- ...
  go $ blank & requestEndpoint .~ "https://api.intercom.io/users"
             & requestOpts .~ options
             & requestParameters .~ object ["email" .= ("bob+{{random}}@intercom.io" :: T.Text)]
             & requestTopic .~ "user.created"
  -- ...
  where go :: WebhookRequest -> StateT DefinitionListRun IO (W.Response L.ByteString)
        go = ((^. response) <$>) . runAssertion t

runIntercomDefinitions :: IO ()
runIntercomDefinitions = do
  info "Running Intercom definitions"
  r <- newTVarIO Nothing :: IO (TVar TopicResult)
  withAsync (server r) $ \webServer ->
    withAsync (runDefs r) $ \testRun -> wait testRun >> cancel webServer
  return ()
  where runDefs r = execStateT (run r) defaultDefinitionListRun

Web UI

Currently, shelduck writes to ~/shelduck.log (this will be configurable one day). You can boot a web app which provides a visual representation of that log using the Shelduck.WebApp module. See src/Main.hs for an example.

Keen support

Test runs are automatically sent to Keen if KEEN_PROJECT_ID and KEEN_API_KEY are set in your Env.

Slack support

Test failures are sent to Slack as a webhook if SLACK_WEBHOOK_URL is set in your Env.


I recommend running NixOS. Here's an example Vagrantfile. You generally need cabal2nix and cabal-install:

nix-env -iA nixos.cabal-install
nix-env -iA nixos.cabal2nix

I spend a lot of time in a repl:

cabal2nix --shell . > shell.nix
nix-shell --command 'cabal repl'

To deploy a new version, I write a new project.nix, build using run.nix, and use nix-copy-closure to send it to a remote NixOS machine (eg. on EC2):

cabal2nix . > project.nix
nix-build run.nix
NIX_SSHOPTS="-i /vagrant/your.pem" nix-copy-closure --to [email protected] /nix/store/abcdef-shelduck-