avoid file system writes and return image data directly

This should speed up the rendering process, resulting in lower
latency for image generation requests.

Author: striezel

changelog.md

@@ -10,6 +10,11 @@ Since [Plotly.js](https://plotly.com/javascript/) is the main dependency of this
 application, major version changes in Plotly.js will also trigger a major
 version change in this application.
 
+## Version 5.9.0-pre (2024-11-25)
+
+* __[improvement]__
+The image rendering process is simplified and does now use less I/O operations.
+
 ## Version 5.8.16 (2024-11-22)
 
 * __[maintenance]__

export-server/package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "plotly-node-export-server",
-  "version": "5.8.16",
+  "version": "5.9.0-pre",
   "description": "Plotly.js Node.js export server",
   "repository": {
     "url": "https://gitlab.com/striezel/plotly-node-export-server.git",

export-server/package.json

@@ -1,6 +1,6 @@
 /*
     Plotly.js offline image export server with Node.js
-    Copyright (C) 2018, 2020, 2021, 2022, 2023  Dirk Stolle
+    Copyright (C) 2018, 2020, 2021, 2022, 2023, 2024  Dirk Stolle
 
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -124,27 +124,8 @@ const server = http.createServer(function(req, res) {
     const result = await ssr.render(body, filename, req.headers["x-image-width"], req.headers["x-image-height"]);
     if (result.success) {
       res.statusCode = 200; // 200 == OK
-      var stream = fs.createReadStream(result.filename);
-      stream.on('open', function() {
-        res.setHeader('Content-Type', 'image/svg+xml');
-        stream.pipe(res);
-      });
-      stream.on('close', function() {
-        res.end();
-        fs.unlink(result.filename, function(err) {
-          if (err) {
-            console.error('Failed to unlink file ' + result.filename + '! '
-                         + err.toString());
-          }
-        });
-      });
-      stream.on('error', function(err) {
-        res.statusCode = 500; // 500 == Internal Server Error
-        res.setHeader('Content-Type', 'text/plain');
-        res.end('Failed to serve file due to I/O error.\n');
-        console.error('Failed to serve file ' + result.filename + ': '
-                     + err.toString());
-      });
+      res.setHeader('Content-Type', 'image/svg+xml');
+      res.end(result.data);
     } else {
       res.statusCode = 500; // 500 == Internal Server Error
       res.setHeader('Content-Type', 'application/json');

export-server/server.js

@@ -31,8 +31,7 @@ const { v4: uuidv4 } = require('uuid');
    Returns:
      object that contains two members:
        success - (boolean) indicates success of the rendering process
-       filename - (string) filename of the output, only present after successful
-                  rendering
+       data    - (string) SVG data, only present after successful rendering
        failure - (string) reason for render failure; only present after failed
                  rendering, may be cryptic and is not necessarily human-friendly
 */
@@ -93,12 +92,8 @@ exports.render = async function(jsonData, filename, width, height) {
     .then(win.eval)
     .then(() => win.Plotly.toImage({data: array_data, layout: layout, config: config},
                                    { format: 'svg', imageDataOnly: true }))
-    .then(function(data) {
-      fs.promises.writeFile(filename, data);
-    })
-    .then(() => {
-      result = { success: true, filename: filename };
-      return result;
+    .then(function(img_data) {
+      return { success: true, data: img_data };
     })
     .catch(function(error) {
       console.log("Error occurred: " + error);