How Plugins Work

Introduction

Engine plugins are HTTP servers that run alongside a Hasura DDN instance and can be written in any language capable of running an HTTP server.

They are configured in DDN using metadata. The engine sends HTTP requests to the plugin at the specified execution step, the plugin processes the request, and then sends a response back to the engine, which continues execution based on the plugin's response.

Plugins can be applied at the following steps:

Execution Step	Description	Example Usage
Pre-Parse	The first step in the execution pipeline, where custom logic can be applied before the query is parsed and its internal representation is generated.	Add an allowlist layer to restrict access to specific queries and mutations.
Pre-Response	The final step in the execution pipeline, where custom logic can be added after the query is executed but before the response is sent to the client.	Trigger Slack notifications after a mutation is executed.

Architecture

Plugin Configuration

Engine plugins are configured in DDN using metadata. The metadata specifies the URL of the engine plugin and the execution step at which the plugin should be called. The configuration also can control the request that is sent to the engine plugin.

Here is an example of a plugin configuration in DDN metadata:

kind: LifecyclePluginHook
version: v1
definition:
  name: cloudflare allowlist
  url:
    valueFromEnv: ALLOW_LIST_URL
  pre: parse
  config:
    request:
      headers:
        additional:
          hasura-m-auth:
            value: "your-strong-m-auth-key"
      session: {}
      rawRequest:
        query: {}
        variables: {}

In this example, the plugin is configured to run at the pre-parse execution step. The plugin is called cloudflare allowlist. The URL of the plugin is read from the ALLOW_LIST_URL environment variable. The plugin is configured to add a hasura-m-auth header to the request with the value your-strong-m-auth-key.

Additionally, the request sent to the plugin includes the query and variables from the incoming GraphQL request, as well as the session information from the incoming request.

Pre-Parse Plugin

The pre-parse plugin is triggered at the first step in the execution pipeline, before the query is parsed. Use this step to add custom logic before parsing begins.

For pre-parse plugin configuration click here.

Pre-Parse Plugin Request

A sample request that is sent to the pre-parse plugin is as follows:

{
  "rawRequest": {
    "query": "query MyQuery { getAuthorById(author_id: 10) { first_name } }",
    "variables": {},
    "operationName": "MyQuery"
  },
  "session": {
    "role": "user",
    "variables": {
      "x-hasura-role": "user",
      "x-hasura-user-id": "123"
    }
  }
}

Customize the request

The request sent to the plugin can be customized based on the plugin's configuration.

Pre-Parse Plugin Response

The pre-parse plugin has the ability to control the execution pipeline by returning a specific response to DDN. The response determines whether execution continues, halts, or returns an error. The possible response types are:

Response Type	HTTP Status Code	Response Body	Description
Continue	204	None	Execution continues without interruption.
Response	200	Response body	Execution halts, and the provided response is returned to the client.
User Error	400	Error object	Execution halts, and the provided error is returned to the client as a user error.
Internal Error	500	Error object	Execution halts, and the provided error is returned to the client as an internal server error.

Response Validation

DDN does not validate the plugin's response. It is the plugin's responsibility to ensure the response is valid and aligns with its intended logic.

Use Cases

The pre-parse plugin can be used to add a multitude of functionalities to DDN. Some use cases are:

• Allowlist: Add an allowlist layer to restrict access to specific queries and mutations based on the incoming request and session information.
• Basic Rate Limiting: Implement rate limiting to restrict the total number of requests that can be made to DDN in a given time period.
• Custom Query Validation: Add custom query validation logic to ensure that the incoming query is valid based on custom business logic.
• Cache Get: Implement a cache get layer to fetch the response from the cache before executing the query.

Multiple Pre-Parse Plugins

You can configure multiple pre-parse plugins in DDN metadata. These plugins execute in the order they are listed in the metadata configuration.

If a plugin returns a Response, User Error, or Internal Error, the execution stops immediately, and the response is sent back to the client. Any plugins defined after that will not run.

Let’s consider an example where the engine uses two pre-parse plugins: Pre-parse Hook 1 and Pre-parse Hook 2.

Here’s how the process works:

Case 1: Continue

If Pre-parse Hook 1 returns a Continue response (HTTP status code 204), the engine proceeds to send the request to Pre-parse Hook 2. This continues until all configured pre-parse plugins have been executed.

Continue response body

The Continue response body is ignored by DDN. Plugins returning a Continue response can safely leave the body empty.

Case 2: Response, User Error, or Internal Error

If Pre-parse Hook 1 returns any of the following:

• Response (HTTP status code 200)
• User Error (HTTP status code 400)
• Internal Error (HTTP status code 500)

The engine stops further execution and sends the response to the client and other plugins will not be called.

Will subsequent pre-response plugins execute?

Yes, even if a pre-parse plugin returns a Response, User Error, or Internal Error, the subsequent pre-response plugins will still execute.

If all pre-parse plugins return a Continue response (HTTP status code 204), the engine completes execution and sends its generated response to the client.

Can pre-parse plugins modify the request?

No, pre-parse plugins cannot alter the request itself. They only influence the execution pipeline by returning specific responses.

Pre-Response Plugin

The pre-response plugin is triggered at the final step in the execution pipeline after the query is executed. Use this step to add webhooks after the query is executed. Please note that the pre-response plugin cannot control or change the execution pipeline.

For pre-response plugin configuration click here.

Pre-Response Plugin Request

A sample request that is sent to the pre-response plugin is as follows:

{
  "response": {
    "data": {
      "getAuthorById": {
        "first_name": "John"
      }
    }
  },
  "session": {
    "role": "user",
    "variables": {
      "x-hasura-role": "user",
      "x-hasura-user-id": "123"
    }
  },
  "rawRequest": {
    "query": "query MyQuery { getAuthorById(author_id: 10) { first_name } }",
    "variables": {},
    "operationName": "MyQuery"
  }
}

Customize the request

The request sent to the plugin can be customized based on the plugin's configuration.

Pre-Response Plugin Response

The pre-response plugin cannot control the execution pipeline. The response from the plugin is ignored by DDN.

Use Cases

The pre-response plugin can be used to add a number of functionalities to DDN. Some use cases also make use of the pre-parse plugin. They are:

• Slack Notifications: Trigger Slack notifications after a query/mutation is executed.
• Cache Set: Implement a cache set layer to store the response in the cache.
• Cache Invalidation: Implement a cache invalidation layer to invalidate the cache based on the incoming request and session information.
• Audit Logs: Add audit logs to track the queries and mutations executed by the users.

Multiple Pre-Response Plugins

Multiple pre-response plugins can be configured in DDN metadata.

The engine sends requests to all configured pre-response plugins in parallel. It does not wait for responses from the plugins and immediately sends the response generated by the engine to the client.