Example WordPress Proxy Layer

gistfile1.php

<?php

	/**
	 * Init Proxy endpoints.
	 *
	 * @return void
	 */
	public function rest_api_init() {

		/**
		 * Proxy for GET requests.
		 */
		register_rest_route(
			$this->proxy_base, '/.*', [
				'methods'             => \WP_REST_Server::READABLE,
				'callback'            => [ $this, 'get_request' ],
				// @TODO: Add extra layers in here.
				'permission_callback' => '__return_true',
			]
		);
	}
  
  
   /**
	 * Proxy GET requests.
	 *
	 * @param \WP_REST_Request $request Proxied request.
	 *
	 * @return \WP_REST_Response
	 */
	public function get_request( \WP_REST_Request $request ) {

    /**
     * Use for whitelisting/blacklisting.
     */
    $response = apply_filters( $this->namespace . '/proxy/response_override', false, $request );
    if ( $response ) {
      return $response;
    }

		/**
		 * @param \WP_REST_Request $request Original request.
		 */
		$request = apply_filters( $this->namespace . '/proxy/request', $request );

		/**
		 * @param \WP_REST_Request $request API request.
		 */
		do_action( $this->namespace . '/proxy/request_received', $request );

		// Get the route.
		$route = $this->route( $request );

		/**
		 * Pre-fetch results. Can be from database or from cache.
		 *
		 * @param array $ Empty array.
		 * @param string           $route   The third-party API route to request.
		 * @param \WP_REST_Request $request API request.
		 */
		$result = apply_filters( $this->namespace . '/proxy/result_pre', [], $route, $request );

		// We have no results, use the third-party API.
		if ( empty( $result ) ) {

			// Proxy the request.
			$response = wp_remote_get(
				$route,
				[
					/**
					 * Filter the request headers.
					 *
					 * @param array            $        Header KV pairs.
					 * @param string           $route   Route requested.
					 * @param \WP_REST_Request $request API request.
					 */
					'headers' => apply_filters( $this->namespace . '/proxy/request_headers', [], $route, $request ),
				]
			);

			/**
			 * Raw API response received.
			 *
			 * @param object|array     $response Response from third-party API.
			 * @param string           $route    Route requested.
			 * @param \WP_REST_Request $request  API request.
			 */
			do_action( $this->namespace . '/proxy/raw_response_received', $response, $route, $request );

			$result = json_decode( $response['body'], true );

			/**
			 * Do something with the response before it is returned. E.g. Import the product(s) and/or cache it.
			 *
			 * @param array|\WP_Error  $result  Result from API call.
			 * @param string           $route   Route requested.
			 * @param \WP_REST_Request $request API request.
			 */
			do_action( $this->namespace . '/proxy/response_received', $result, $route, $request );
		}

		/**
		 * Filter the response results before returning it.
		 *
		 * @param array|\WP_Error  $result  Result from API call.
		 * @param string           $route   Route requested.
		 * @param \WP_REST_Request $request API request.
		 */
		$result = apply_filters( $this->namespace . '/proxy/result', $result, $route, $request );

		$rest_response = new \WP_REST_Response( $result );

		/**
		 * Filter the WordPress REST response before it gets dispatched.
		 *
		 * @param \WP_REST_Response $rest_response Response to send back to request..
		 * @param string            $route         Route requested.
		 * @param \WP_REST_Request  $request       API request.
		 */
		$rest_response = apply_filters( $this->namespace . '/proxy/rest_response', $rest_response, $route, $request );

		return rest_ensure_response( $rest_response );
	}
  
	/**
	 * Given a request, return the real URL.
	 *
	 * @param \WP_REST_Request $request The request.
	 *
	 * @return string
	 */
	private function route( \WP_REST_Request $request ) {
		$route = str_replace( '/' . $this->proxy_base . '/', '', $request->get_route() );
		$route = sprintf( '%s%s', trailingslashit( $this->config['api.host'] ), $route );
		$route = add_query_arg( $request->get_query_params(), $route );

		return $route;
	}