Handler Plugins

This document provides in-depth information about handler plugins and how you can create your own to extend the functionality of the Hooks framework for your own deployment.

Writing a Handler Plugin

Handler plugins are Ruby classes that extend the Hooks::Plugins::Handlers::Base class. They are used to process webhook payloads and can do anything you want. They follow a very simple interface that allows you to define a call method that takes four parameters: payload, headers, env, and config.

Important: The call method should return a hash by default. Since the server now defaults to JSON format, any hash returned by the handler will be automatically converted to JSON with the correct Content-Type: application/json headers set by Grape. This ensures consistent API responses and proper JSON serialization.

payload: The webhook payload, which can be a Hash or a String. This is the data that the webhook sender sends to your endpoint.
headers: A Hash of HTTP headers that were sent with the webhook request.
env: A modified Rack environment that contains a lot of context about the request. This includes information about the request method, path, query parameters, and more. See rack_env_builder.rb for the complete list of available keys.
config: A Hash containing the endpoint configuration. This can include any additional settings or parameters that you want to use in your handler. Most of the time, this won't be used but sometimes endpoint configs add opts that can be useful for the handler.

The method should return a hash that will be automatically serialized to JSON format with appropriate headers. The server defaults to JSON format for both input and output processing.

# example file path: plugins/handlers/example.rb
class Example < Hooks::Plugins::Handlers::Base
  # Process a webhook payload
  #
  # @param payload [Hash, String] webhook payload (pure JSON with string keys)
  # @param headers [Hash] HTTP headers (string keys, optionally normalized - default is normalized)
  # @param env [Hash] A modified Rack environment that contains a lot of context about the request
  # @param config [Hash] Endpoint configuration
  # @return [Hash] Response data (automatically converted to JSON)
  def call(payload:, headers:, env:, config:)
    # Return a hash - it will be automatically converted to JSON with proper headers
    return {
      status: "success",
      message: "webhook processed successfully",
      timestamp: Time.now.iso8601
    }
  end
end

After you write your own handler, it can be referenced in endpoint configuration files like so:

# example file path: config/endpoints/example.yml
path: /example_webhook
handler: example # this is the name of the handler plugin class

It should be noted that the handler: key in the endpoint configuration file should match the name of the handler plugin class, but in lowercase and snake case. For example, if your handler plugin class is named ExampleHandler, the handler: key in the endpoint configuration file should be example_handler. Here are some more examples:

ExampleHandler -> example_handler
MyCustomHandler -> my_custom_handler
Cool2Handler -> cool_2_handler

Default JSON Format

By default, the Hooks server uses JSON format for both input and output processing. This means:

Input: Webhook payloads are parsed as JSON and passed to handlers as Ruby hashes
Output: Handler return values (hashes) are automatically converted to JSON responses with Content-Type: application/json headers
Error Responses: Authentication failures and handler errors return structured JSON responses

Best Practice: Always return a hash from your handler's call method. The hash will be automatically serialized to JSON and sent to the webhook sender with proper headers. This ensures consistent API responses and proper JSON formatting.

Example response format:

{
  "status": "success",
  "message": "webhook processed successfully", 
  "data": {
    "processed_at": "2023-10-01T12:34:56Z",
    "items_processed": 5
  }
}

Note: The JSON format behavior can be configured using the default_format option in your global configuration. See the Configuration documentation for more details.

`payload` Parameter

The payload parameter can be a Hash or a String. If the payload is a String, it will be parsed as JSON. If it is a Hash, it will be passed directly to the handler. The payload can contain any data that the webhook sender wants to send.

The payload is parsed as JSON (if it can be) and returned as a pure Ruby hash with string keys, maintaining the original JSON structure. This ensures that the payload is always a valid JSON representation that can be easily serialized and processed.

TL;DR: The payload is almost always a Hash with string keys, regardless of whether the original payload was a Hash or a JSON String.

For example, if the client sends the following JSON payload:

{
  "hello": "world",
  "foo": ["bar", "baz"],
  "truthy": true,
  "coffee": {"is": "good"}
}

It will be parsed and passed to the handler as:

{
  "hello" => "world",
  "foo" => ["bar", "baz"],
  "truthy" => true,
  "coffee" => {"is" => "good"}
}

`headers` Parameter

The headers parameter is a Hash that contains the HTTP headers that were sent with the webhook request. It includes standard headers like host, user-agent, accept, and any custom headers that the webhook sender may have included.

By default, the headers are normalized (lowercased and trimmed) but kept as string keys to maintain their JSON representation. Header keys are always strings, and any normalization simply ensures consistent formatting (lowercasing and trimming whitespace). You can disable header normalization by setting the environment variable HOOKS_NORMALIZE_HEADERS to false or by setting the normalize_headers option to false in the global configuration file.

TL;DR: The headers are always a Hash with string keys, optionally normalized for consistency.

For example, if the client sends the following headers:

Host: hooks.example.com
User-Agent: foo-client/1.0
Accept: application/json, text/plain, */*
Accept-Encoding: gzip, compress, deflate, br
Client-Name: foo
X-Forwarded-For: <IP_ADDRESS>
X-Forwarded-Host: hooks.example.com
X-Forwarded-Proto: https
Authorization: Bearer <TOKEN>

They will be normalized and passed to the handler as:

{
  "host" => "hooks.example.com",
  "user-agent" => "foo-client/1.0",
  "accept" => "application/json, text/plain, */*",
  "accept-encoding" => "gzip, compress, deflate, br",
  "client-name" => "foo",
  "x-forwarded-for" => "<IP_ADDRESS>",
  "x-forwarded-host" => "hooks.example.com",
  "x-forwarded-proto" => "https",
  "authorization" => "Bearer <TOKEN>" # a careful reminder that headers *can* contain sensitive information!
}

It should be noted that the headers parameter is a Hash with string keys (not symbols). They are optionally normalized (lowercased and trimmed) to ensure consistency.

You can disable header normalization by either setting the environment variable HOOKS_NORMALIZE_HEADERS to false or by setting the normalize_headers option to false in the global configuration file.

`env` Parameter

The env parameter is a Hash that contains a modified Rack environment. It provides a lot of context about the request, including information about the request method, path, query parameters, and more. This can be useful for debugging or for accessing additional request information. It is considered everything plus the kitchen sink that you might need to know about the request.

Here is a partial example of what the env parameter might look like:

{
  "REQUEST_METHOD" => "POST",
  "PATH_INFO" => "/webhooks/example",
  "QUERY_STRING" => "foo=bar&baz=123",
  "HTTP_VERSION" => "HTTP/1.1",
  "REQUEST_URI" => "https://hooks.example.com/webhooks/example?foo=bar&baz=qux",
  "SERVER_NAME" => "hooks.example.com",
  "SERVER_PORT" => 443,
  "CONTENT_TYPE" => "application/json",
  "CONTENT_LENGTH" => 123,
  "REMOTE_ADDR" => "<IP_ADDRESS>",
  "hooks.request_id" => "<REQUEST_ID>",
  "hooks.handler" => "ExampleHandler"
  "hooks.endpoint_config" => {}
  "hooks.start_time" => "2023-10-01T12:34:56Z",
  # etc...
}

For the complete list of available keys in the env parameter, you can refer to the source code at lib/hooks/app/rack_env_builder.rb.

`config` Parameter

The config parameter is a Hash (symbolized) that contains the endpoint configuration. This can include any additional settings or parameters that you want to use in your handler. Most of the time, this won't be used, but sometimes endpoint configs add opts that can be useful for the handler.

Built-in Features

This section goes into details on the built-in features that exist in all handler plugins that extend the Hooks::Plugins::Handlers::Base class.

`#log`

The log.debug, log.info, log.warn, and log.error methods are available in all handler plugins. They are used to log messages at different levels of severity.

`#error!`

All handler plugins have access to the error! method, which is used to raise an error with a specific message and HTTP status code. This is useful for returning error responses to the webhook sender.

When using error! with the default JSON format, both hash and string responses are handled appropriately:

class Example < Hooks::Plugins::Handlers::Base
  # Example webhook handler
  #
  # @param payload [Hash, String] Webhook payload
  # @param headers [Hash<String, String>] HTTP headers
  # @param env [Hash] A modified Rack environment that contains a lot of context about the request
  # @param config [Hash] Endpoint configuration
  # @return [Hash] Response data (automatically converted to JSON)
  def call(payload:, headers:, env:, config:)

    if payload.nil? || payload.empty?
      log.error("Payload is empty or nil")
      # String errors are JSON-encoded with default format
      error!("Payload cannot be empty or nil", 400)
    end

    return {
      status: "success"
    }
  end
end

Recommended approach: Use hash-based error responses for consistent JSON structure:

class Example < Hooks::Plugins::Handlers::Base
  def call(payload:, headers:, env:, config:)

    if payload.nil? || payload.empty?
      log.error("Payload is empty or nil")
      # Hash errors are automatically converted to JSON
      error!({
        error: "payload_empty",
        message: "the payload cannot be empty or nil",
        success: false,
        custom_value: "some_custom_value",
        request_id: env["hooks.request_id"]
      }, 400)
    end

    return {
      status: "success"
    }
  end
end

This will return a properly formatted JSON error response:

{
  "error": "payload_empty",
  "message": "the payload cannot be empty or nil",
  "success": false,
  "custom_value": "some_custom_value",
  "request_id": "uuid-here"
}

`#Retryable.with_context(:default)`

This method uses a default Retryable context to handle retries. It is used to wrap the execution of a block of code that may need to be retried in case of failure.

Here is how it can be used in a handler plugin:

class Example < Hooks::Plugins::Handlers::Base
  # Example webhook handler
  #
  # @param payload [Hash, String] Webhook payload
  # @param headers [Hash<String, String>] HTTP headers
  # @param env [Hash] A modified Rack environment that contains a lot of context about the request
  # @param config [Hash] Endpoint configuration
  # @return [Hash] Response data (automatically converted to JSON)
  def call(payload:, headers:, env:, config:)
    result = Retryable.with_context(:default) do
      some_operation_that_might_fail()
    end

    log.debug("operation result: #{result}")

    return {
      status: "success",
      operation_result: result,
      processed_at: Time.now.iso8601
    }
  end
end

If Retryable.with_context(:default) fails after all retries, it will re-raise the last exception encountered.

See the source code at lib/hooks/utils/retry.rb for more details on how Retryable.with_context(:default) works.

`#failbot` and `#stats`

The failbot and stats methods are available in all handler plugins. They are used to report errors and send statistics, respectively. These are custom methods and you can learn more about them in the Instrumentation Plugins documentation.

Extra Components

If you need even more flexibility, you can pass in extra components to your Hooks application when building it. These "extra components" are available globally and can be used in your handler plugins. Here is example that demonstrates using an extra component:

# config.ru

# Define some class that you might want all your handlers to be able to call
class ExamplePublisher
  def initialize
    @published_messages = []
  end

  def call(data)
    @published_messages << data
    puts "Published: #{data.inspect}"
    "Message published successfully"
  end

  def publish(data)
    call(data)
  end

  def messages
    @published_messages
  end
end

# Create publisher instance
publisher = ExamplePublisher.new

# Create and run the hooks application with custom class
app = Hooks.build(config: "./spec/acceptance/config/hooks.yaml", publisher:)
run app

Now, in all handler plugins, you can access the publisher instance like so:

# example file path: plugins/handlers/hello.rb

class Hello < Hooks::Plugins::Handlers::Base
  def call(payload:, headers:, env:, config:)
    # call the custom publisher instance
    publisher.publish("hello")

    {
      status: "success",
      handler: self.class.name,
      timestamp: Time.now.utc.iso8601,
      messages: publisher.messages,
    }
  end
end

It should be noted that any extra components you pass in like this should be thread-safe if you are running the Hooks server in a multi-threaded environment. This is because the Hooks server can handle multiple requests concurrently, and any shared state should be properly synchronized.

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Handler Plugins

Writing a Handler Plugin

Default JSON Format

`payload` Parameter

`headers` Parameter

`env` Parameter

`config` Parameter

Built-in Features

`#log`

`#error!`

`#Retryable.with_context(:default)`

`#failbot` and `#stats`

Extra Components

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Handler Plugins

Writing a Handler Plugin

Default JSON Format

payload Parameter

headers Parameter

env Parameter

config Parameter

Built-in Features

#log

#error!

#Retryable.with_context(:default)

#failbot and #stats

Extra Components

`payload` Parameter

`headers` Parameter

`env` Parameter

`config` Parameter

`#log`

`#error!`

`#Retryable.with_context(:default)`

`#failbot` and `#stats`