React Native

S3 Resume Download

See more Amazon S3 (new) Examples

Suppose an S3 download of a very large file failed for some reason and you have a partial file on disk. Rather than restart the entire download, you wish to download the remaining portion. This example demonstrates how to finish a previously failed download.

Note: This example requires Chilkat v9.5.0.83 or above.

React Native

import { AuthAws, FileAccess, Rest, Stream, StringBuilder } from '@chilkat/react-native'

async function chilkatExample() {
  // This example requires the Chilkat API to have been previously unlocked.
  // See Global Unlock Sample for sample code.

  const rest = new Rest();

  // Connect to the Amazon AWS REST server.
  const bTls = true;
  const port = 443;
  const bAutoReconnect = true;
  await rest.connectAsync('s3.amazonaws.com', port, bTls, bAutoReconnect);

  // ---------------------------------------------------------------------------
  // Important: For buckets created in regions outside us-east-1,
  // there are three important changes that need to be made.
  // See Working with S3 Buckets in Non-us-east-1 Regions for the details.
  // ---------------------------------------------------------------------------

  // Provide AWS credentials for the REST call.
  const authAws = new AuthAws();
  authAws.accessKey = 'AWS_ACCESS_KEY';
  authAws.secretKey = 'AWS_SECRET_KEY';
  authAws.serviceName = 's3';
  rest.setAuthAws(authAws);

  // Set the bucket name via the HOST header.
  // In this case, the bucket name is "chilkat100".
  rest.host = 'chilkat100.s3.amazonaws.com';

  // We want to continue downloading a file.
  // The relative local filepath of our previously partially downoaded file is: qa_output/hamlet.xml
  // Let's find out how many bytes are already downloaded.
  const fac = new FileAccess();
  const localFilepath = 'qa_output/hamlet.xml';
  // Note: The FileSize method returns a signed 32-bit integer.  If the file is potentially larger than 2GB, call FileSizeStr instead to return
  // the size of the file as a string, then convert to an integer value.
  const sz = fac.fileSize(localFilepath);
  if (sz > 0) {
    console.log(`sz = ${sz}`);

    // If the sz equals 42375 bytes, then we want to add a Range header that looks like this:
    // Range: bytes=42375-
    const sbRange = new StringBuilder();
    sbRange.append('bytes=');
    sbRange.appendInt(sz);
    sbRange.append('-');
    rest.addHeader('Range', sbRange.getAsString());
    console.log(`Added Range: ${sbRange.getAsString()}`);
  }

  // Send the request to download the remainder of the file.
  try {
    await rest.sendReqNoBodyAsync('GET', '/hamlet.xml');
  } catch {
    console.log(rest.lastErrorText);
    return;
  }

  // Read the response header.
  const responseStatusCode = await rest.readResponseHeaderAsync();
  if (responseStatusCode < 0) {
    console.log(rest.lastErrorText);
    return;
  }

  console.log(`Response status code = ${responseStatusCode}`);

  // We expect a 200 or 206 response status if the file data is coming.
  // Otherwise, we'll get a string response body with an error message(or no response body).
  if ((responseStatusCode === 200) || (responseStatusCode === 206)) {

    const bodyStream = new Stream();

    // The stream's sink will be a file.
    // We will append to the file..
    bodyStream.sinkFile = localFilepath;

    // Indicate that we wish to append to the output file.
    // The SinkFileAppend property was added in Chilkat v9.50.83
    bodyStream.sinkFileAppend = true;

    // Read the response body to the stream.  Given that we've
    // set the stream's sink to a file, it will stream directly
    // to the file.
    try {
      await rest.readRespBodyStreamAsync(bodyStream, true);
    } catch {
      console.log(rest.lastErrorText);
      return;
    }

    console.log('Successfully downloaded the file.');
  } else {
    try {
      const errResponse = await rest.readRespBodyStringAsync();
      console.log(errResponse);
    } catch {
      console.log(rest.lastErrorText);
    }
  }
}