Intercepting the Request

Introduction

In the 5.34.0 version we added a possibility for users to intercept the Fastify request in it’s initial phase. It can be done via the HandlerOnRequestPlugin .

With this plugin you can do anything with the Fastify’s Request object, even stop the request from being processed. To find out why you can stop the request from being processed, read about it in the Fastify Lifecycle and Fastify Hooks documentation.

Creating and Registering the Plugin to Intercept the Request

import { createHandlerOnRequest } from "@webiny/handler";

const handlerOnRequestPlugin = createHandlerOnRequest(async (request, reply) => {
  if (request.raw.path === "/testing") {
    // do something for this path
  }
});

Of course, as with our other plugins, you must register it in the Lambda handler plugins array:

const handler = createHandler({
  plugins: [
    // ... other plugins
    handlerOnRequestPlugin
  ]
});

Intercepting the OPTIONS Request and Stopping the Request From Being Processed

By default, Webiny intercepts the OPTIONS request and stop all of the Fastify lifecycles, after the onRequest, from being ran. We do this because we have no need to run all the code we usually run for, eg., POST request. You can see the code here .

Via the HandlerOnRequestPlugin, users can prevent our default behavior.

Why Would the User Prevent the Webiny Default Behavior?

The most simple answer would be: to output their own headers in the OPTIONS request.

How Can Users Prevent the Webiny Default Behavior on OPTIONS Request to Be Executed?

To prevent the Webiny default behavior on OPTIONS request, user would need to add a simple plugin which returns false as the result.

Here is an example of sending a custom header myCustomHeader, and prevent the Webiny default behavior, on OPTIONS request:

import { createHandlerOnRequest } from "@webiny/handler";

const handlerOnRequestPlugin = createHandlerOnRequest(async (request, reply) => {
  if (request.method !== "OPTIONS") {
    return;
  }
  reply
    .code(204)
    .headers({
      myCustomHeader: "sending the custom headers"
    })
    /**
     * There must be some output or the request will hang.
     * Also, send must be just before the hijack(), otherwise all other method calls will not have any effect.
     */
    .send("")
    /**
     * It is important to hijack() the reply, as it prevents all other Fastify lifecycle events to be executed.
     * Of course, if user wants all the events to be triggered, they should not hijack() the reply.
     */
    .hijack();
  /**
   * It is important to return false in the end because it actually prevents
   * all other HandlerOnRequestPlugin to be ran, including the Webiny default behavior.
   */
  return false;
});

Conclusion

Simply, with the HandlerOnRequestPlugin user can, in the initial stage of the Fastify Request Lifecycle:

• intercept the request and prevent it to go further down the lifecycle line
• modify the Webiny default OPTIONS headers output to suit their own need
• log the request to some outside service