express-partial-content

About

A HTTP 206 Partial Content handler to serve any readable stream partially in Express.

Based on this blog post: https://www.codeproject.com/Articles/813480/HTTP-Partial-Content-In-Node-js.

Installation

yarn add express-partial-content

OR

npm install express-partial-content

Note: Express package is a peer dependency for express-partial-content and must be present in dependencies of the host package.

Usage

From the express-file-server example:

• Implement a ContentProvider function which prepares and returns a Content object:

  import { promisify } from "util";
  import fs from "fs";
  import { Range, ContentDoesNotExistError, ContentProvider } from "express-partial-content";
  import {logger} from "./logger";
  
  const statAsync = promisify(fs.stat);
  const existsAsync = promisify(fs.exists);
  
  export const fileContentProvider: ContentProvider = async (req: Request) => {
    // Read file name from route params.
    const fileName = req.params.name;
    const file = `${__dirname}/files/${fileName}`;
    if (!(await existsAsync(file))) {
      throw new ContentDoesNotExistError(`File doesn't exist: ${file}`);
    }
    const stats = await statAsync(file);
    const totalSize = stats.size;
    const mimeType = "application/octet-stream";
    const getStream = (range?: Range) => {
      if (!range) {
        // Request if for complete content.
        return fs.createReadStream(file);
      }
      // Partial content request.
      const { start, end } = range;
      logger.debug(`start: ${start}, end: ${end}`);
      return fs.createReadStream(file, { start, end });
    };
    return {
      fileName,
      totalSize,
      mimeType,
      getStream
    };
  };
  

• In your express code, use createPartialContentHandler factory method to generate an express handler for serving partial content for the route of your choice:

  import {createPartialContentHandler} from "express-partial-content";
  import {logger} from "./logger";
  
  const handler = createPartialContentHandler(fileContentProvider, logger);
  
  const app = express();
  const port = 8080;
  
  // File name is a route param.
  app.get("/files/:name", handler);
  
  app.listen(port, () => {
    logger.debug("Server started!");
  });
  

• Run your server and use a multi-part/multi-connection download utility like aria2c to test it:

  aria -x5 -k1M http://localhost:8080/files/readme.txt
  

Examples

There are two examples in the src/examples folder:

• express-file-server: Implements a file based ContentProvider.
• express-mongo-server: Implements a mongodb based ContentProvider.

Running the examples:

• express-file-server: Run the following commands, the server will listen on http://localhost:8080/.

  yarn build:dev
  yarn copy-assets
  yarn run:examples:file
  

• express-mongo-server:

  • Run your own Mongo instance first or create one in https://mlab.com.

  • Setup the connection string in MongoUrl environment variable:

    export MongoUrl=mongodb+srv://username:password@mongoserver/dbname?retryWrites=true
    

  • Build and run the example, the server will listen on http://localhost:8080/.

    yarn build:dev
    yarn copy-assets
    yarn run:examples:file
    

Connecting to the running server:

Browse to https://localhost:8080/files/readme.txt

Reference

createPartialContentHandler function:

This is a factory method which generates a partial content handler for express routes.

Arguments:

• contentProvider: An async function which returns a Promise resolved to a Content object (see below).
• logger: Any logging implementation which has a debug(message:string, extra: any) method. Either winston or bunyan loggers should work.

Returns:

• Express Route Handler: createPartialContentHandler returns an express handler which can be mapped to an Express route to serve partial content.

ContentProvider function:

This function needs to be implemented by you. It's purpose is to fetch and return Content object containing necessary metadata and methods to stream the content partially. This method is invoked by the express handler (returned by createPartialContentHandler) on each request.

Arguments:

• Request: It receives the Request object as it's only input. Use the information available in Request to find the requested content, e.g. through Request.params or query string, headers etc.

Returns:

• Promise<Content>: See below.

Throws:

• ContentDoesNotExistError: Throw this to indicate that the content doesn't exist. The generated express handler will return a 404 in this case.

  Note: Any message provided to the ContentDoesNotExistError object is returned to the client.

Content object:

This object contains metadata and methods which describe the content. The ContentProvider method builds and returns it.

Properties:

All the properties of this object are used to return content metadata to the client as various Response headers.

• fileName: Used as the Content-Disposition header's filename value.
• mimeType: Used as the Content-Type header value.
• totalSize: Used as the Content-Length header value.

Methods:

• getStream(range?: Range): This method should return a readable stream initialized to the provided range (optional). You need to handle two cases:

  • range is null: When range is not-specified, the client is requesting the full content. In this case, return the stream as it is.
  • range is {start, end}: When client requests partial content, the start and end values will point to the corresponding byte positions (0 based and inclusive) of the content. You need to return stream limited to these positions.