startupSnapshot - v8 - Node

The v8.startupSnapshot interface can be used to add serialization and deserialization hooks for custom startup snapshots.

$ node --snapshot-blob snapshot.blob --build-snapshot entry.js
$ node --snapshot-blob snapshot.blob

In the example above, entry.js can use methods from the v8.startupSnapshot interface to specify how to save information for custom objects in the snapshot during serialization and how the information can be used to synchronize these objects during deserialization of the snapshot. For example, if the entry.js contains the following script:

'use strict';

import fs from 'node:fs';
import zlib from 'node:zlib';
import path from 'node:path';
import assert from 'node:assert';

import v8 from 'node:v8';

class BookShelf {
  storage = new Map();

  // Reading a series of files from directory and store them into storage.
  constructor(directory, books) {
    for (const book of books) {
      this.storage.set(book, fs.readFileSync(path.join(directory, book)));
    }
  }

  static compressAll(shelf) {
    for (const [ book, content ] of shelf.storage) {
      shelf.storage.set(book, zlib.gzipSync(content));
    }
  }

  static decompressAll(shelf) {
    for (const [ book, content ] of shelf.storage) {
      shelf.storage.set(book, zlib.gunzipSync(content));
    }
  }
}

// __dirname here is where the snapshot script is placed
// during snapshot building time.
const shelf = new BookShelf(__dirname, [
  'book1.en_US.txt',
  'book1.es_ES.txt',
  'book2.zh_CN.txt',
]);

assert(v8.startupSnapshot.isBuildingSnapshot());
// On snapshot serialization, compress the books to reduce size.
v8.startupSnapshot.addSerializeCallback(BookShelf.compressAll, shelf);
// On snapshot deserialization, decompress the books.
v8.startupSnapshot.addDeserializeCallback(BookShelf.decompressAll, shelf);
v8.startupSnapshot.setDeserializeMainFunction((shelf) => {
  // process.env and process.argv are refreshed during snapshot
  // deserialization.
  const lang = process.env.BOOK_LANG || 'en_US';
  const book = process.argv[1];
  const name = `${book}.${lang}.txt`;
  console.log(shelf.storage.get(name));
}, shelf);

The resulted binary will get print the data deserialized from the snapshot during start up, using the refreshed process.env and process.argv of the launched process:

$ BOOK_LANG=es_ES node --snapshot-blob snapshot.blob book1

Currently the application deserialized from a user-land snapshot cannot be snapshotted again, so these APIs are only available to applications that are not deserialized from a user-land snapshot.

Usage in Deno

Type