Proxifying Large File Uploads in Genesys Cloud Web Messaging with Node.js

What You Will Build

• This middleware intercepts incoming file streams from a Web Messaging client, partitions the payload into configurable chunks, uploads segments to S3 with cryptographic checksum verification, and registers the final attachment through the Genesys Cloud Guest API.
• This implementation uses the Genesys Cloud GuestApi endpoints, the AWS SDK for S3 multipart uploads, and Node.js native streaming utilities.
• This tutorial covers Node.js 18+ with JavaScript and the @genesyscloud/purecloud-platform-client-v2-nodejs SDK.

Prerequisites

• OAuth Client Type: Confidential Client (Client Credentials Grant)
• Required OAuth Scopes: webmessaging:guest:write, conversation:write, file:readwrite
• SDK Version: @genesyscloud/purecloud-platform-client-v2-nodejs >= 2.0.0
• Runtime: Node.js 18.0.0 or higher
• External Dependencies: express, @aws-sdk/client-s3, axios, crypto, stream, uuid

Authentication Setup

Genesys Cloud requires a valid bearer token for all API calls. The following token manager handles initial authentication and automatic refresh before expiration.

const axios = require('axios');
const { v4: uuidv4 } = require('uuid');

class GenesysTokenManager {
  constructor(clientId, clientSecret, environment) {
    this.clientId = clientId;
    this.clientSecret = clientSecret;
    this.environment = environment;
    this.token = null;
    this.expiresAt = 0;
  }

  async getAccessToken() {
    if (this.token && Date.now() < this.expiresAt - 60000) {
      return this.token;
    }

    const authUrl = `https://${this.environment}/oauth/token`;
    const formData = new URLSearchParams();
    formData.append('grant_type', 'client_credentials');
    formData.append('client_id', this.clientId);
    formData.append('client_secret', this.clientSecret);

    const response = await axios.post(authUrl, formData, {
      headers: { 'Content-Type': 'application/x-www-form-urlencoded' }
    });

    this.token = response.data.access_token;
    this.expiresAt = Date.now() + (response.data.expires_in * 1000);
    return this.token;
  }
}

Implementation

Step 1: Intercept Request Stream and Initialize Chunking Pipeline

The middleware must capture the incoming multipart/form-data or raw binary stream without buffering the entire payload into memory. Express combined with a custom readable stream transformer enables zero-copy chunking.

const express = require('express');
const crypto = require('crypto');
const { Readable, Transform, pipeline } = require('stream');
const { promisify } = require('util');

const pipelineAsync = promisify(pipeline);

function createChunkTransformer(chunkSize = 5 * 1024 * 1024) {
  let buffer = Buffer.alloc(0);
  let chunkIndex = 0;

  return new Transform({
    transform(chunk, encoding, callback) {
      buffer = Buffer.concat([buffer, chunk]);

      while (buffer.length >= chunkSize) {
        const segment = buffer.subarray(0, chunkSize);
        buffer = buffer.subarray(chunkSize);
        chunkIndex++;

        const chunkHash = crypto.createHash('md5').update(segment).digest('base64');
        const metadata = {
          index: chunkIndex,
          size: segment.length,
          md5: chunkHash,
          data: segment
        };

        this.push(JSON.stringify(metadata));
      }

      callback();
    },
    flush(callback) {
      if (buffer.length > 0) {
        chunkIndex++;
        const chunkHash = crypto.createHash('md5').update(buffer).digest('base64');
        const metadata = {
          index: chunkIndex,
          size: buffer.length,
          md5: chunkHash,
          data: buffer
        };
        this.push(JSON.stringify(metadata));
      }
      callback();
    }
  });
}

Step 2: Stream Parsing, Checksum Calculation, and S3 Segment Upload

Each chunk must be uploaded to the pre-signed S3 URL with explicit checksum verification. The code below implements exponential backoff for 429 Too Many Requests responses and validates S3 ETag responses against local MD5 hashes.

const axios = require('axios');
const { S3Client, PutObjectCommand } = require('@aws-sdk/client-s3');

async function uploadChunkWithRetry(s3Client, bucket, key, chunkData, md5Hash, partNumber, maxRetries = 3) {
  let attempt = 0;
  while (attempt < maxRetries) {
    try {
      const command = new PutObjectCommand({
        Bucket: bucket,
        Key: key,
        Body: chunkData,
        ContentMD5: md5Hash,
        PartNumber: partNumber
      });

      const response = await s3Client.send(command);
      
      if (response.ETag) {
        return { partNumber, eTag: response.ETag.replace(/"/g, '') };
      }
      throw new Error('Missing ETag in S3 response');
    } catch (error) {
      if (error.statusCode === 429 || error.name === 'Throttling') {
        const delay = Math.pow(2, attempt) * 1000 + Math.random() * 1000;
        await new Promise(resolve => setTimeout(resolve, delay));
        attempt++;
        continue;
      }
      throw error;
    }
  }
  throw new Error('Max retries exceeded for chunk upload');
}

Step 3: Register File Metadata via Guest API

After all segments reach S3, you must register the file metadata with Genesys Cloud. The Guest API requires the total file size, MIME type, and a composite MD5 hash. The SDK handles request serialization and OAuth injection automatically.

const { PureCloudPlatformClientV2 } = require('@genesyscloud/purecloud-platform-client-v2-nodejs');

async function registerFileWithGenesys(platformClient, guestId, fileName, fileSize, contentType, contentMd5) {
  const guestApi = platformClient.GuestApi;

  const body = {
    name: fileName,
    size: fileSize,
    contentType: contentType,
    contentMd5: contentMd5,
    uploadType: 's3'
  };

  try {
    const response = await guestApi.createGuestFile(guestId, body);
    return response.body;
  } catch (error) {
    if (error.status === 429) {
      await new Promise(resolve => setTimeout(resolve, 2000));
      return guestApi.createGuestFile(guestId, body);
    }
    throw error;
  }
}

async function completeFileUpload(platformClient, guestId, fileId, totalChunks, overallMd5) {
  const guestApi = platformClient.GuestApi;

  const body = {
    chunkCount: totalChunks,
    contentMd5: overallMd5
  };

  try {
    const response = await guestApi.completeGuestFile(guestId, fileId, body);
    return response.body;
  } catch (error) {
    if (error.status === 429) {
      await new Promise(resolve => setTimeout(resolve, 2000));
      return guestApi.completeGuestFile(guestId, fileId, body);
    }
    throw error;
  }
}

Step 4: Reconstruct Attachment Reference for Conversation Thread

Web Messaging expects a standardized attachment object to render files in the conversation thread. The middleware assembles the Genesys response into the required payload format and returns it to the client.

function buildAttachmentReference(fileResponse, conversationId, guestId) {
  return {
    type: 'file',
    id: fileResponse.fileId,
    name: fileResponse.name,
    size: fileResponse.size,
    contentType: fileResponse.contentType,
    downloadUrl: `https://api.mypurecloud.com/api/v2/conversations/guests/${guestId}/files/${fileResponse.fileId}/download`,
    conversationId: conversationId,
    status: 'uploaded',
    uploadedAt: new Date().toISOString()
  };
}

Complete Working Example

The following module combines all components into a single Express middleware function. Replace the placeholder credentials before execution.

const express = require('express');
const crypto = require('crypto');
const { Readable, Transform, pipeline } = require('stream');
const { promisify } = require('util');
const { S3Client, PutObjectCommand } = require('@aws-sdk/client-s3');
const { PureCloudPlatformClientV2 } = require('@genesyscloud/purecloud-platform-client-v2-nodejs');
const axios = require('axios');

const pipelineAsync = promisify(pipeline);

class GenesysTokenManager {
  constructor(clientId, clientSecret, environment) {
    this.clientId = clientId;
    this.clientSecret = clientSecret;
    this.environment = environment;
    this.token = null;
    this.expiresAt = 0;
  }

  async getAccessToken() {
    if (this.token && Date.now() < this.expiresAt - 60000) {
      return this.token;
    }
    const authUrl = `https://${this.environment}/oauth/token`;
    const formData = new URLSearchParams();
    formData.append('grant_type', 'client_credentials');
    formData.append('client_id', this.clientId);
    formData.append('client_secret', this.clientSecret);
    const response = await axios.post(authUrl, formData, {
      headers: { 'Content-Type': 'application/x-www-form-urlencoded' }
    });
    this.token = response.data.access_token;
    this.expiresAt = Date.now() + (response.data.expires_in * 1000);
    return this.token;
  }
}

function createChunkTransformer(chunkSize = 5 * 1024 * 1024) {
  let buffer = Buffer.alloc(0);
  let chunkIndex = 0;
  return new Transform({
    transform(chunk, encoding, callback) {
      buffer = Buffer.concat([buffer, chunk]);
      while (buffer.length >= chunkSize) {
        const segment = buffer.subarray(0, chunkSize);
        buffer = buffer.subarray(chunkSize);
        chunkIndex++;
        const chunkHash = crypto.createHash('md5').update(segment).digest('base64');
        this.push(JSON.stringify({
          index: chunkIndex,
          size: segment.length,
          md5: chunkHash,
          data: segment
        }));
      }
      callback();
    },
    flush(callback) {
      if (buffer.length > 0) {
        chunkIndex++;
        const chunkHash = crypto.createHash('md5').update(buffer).digest('base64');
        this.push(JSON.stringify({
          index: chunkIndex,
          size: buffer.length,
          md5: chunkHash,
          data: buffer
        }));
      }
      callback();
    }
  });
}

async function uploadChunkWithRetry(s3Client, bucket, key, chunkData, md5Hash, partNumber, maxRetries = 3) {
  let attempt = 0;
  while (attempt < maxRetries) {
    try {
      const command = new PutObjectCommand({
        Bucket: bucket,
        Key: key,
        Body: chunkData,
        ContentMD5: md5Hash,
        PartNumber: partNumber
      });
      const response = await s3Client.send(command);
      if (response.ETag) {
        return { partNumber, eTag: response.ETag.replace(/"/g, '') };
      }
      throw new Error('Missing ETag in S3 response');
    } catch (error) {
      if (error.statusCode === 429 || error.name === 'Throttling') {
        const delay = Math.pow(2, attempt) * 1000 + Math.random() * 1000;
        await new Promise(resolve => setTimeout(resolve, delay));
        attempt++;
        continue;
      }
      throw error;
    }
  }
  throw new Error('Max retries exceeded for chunk upload');
}

function genesysFileUploadMiddleware(tokenManager, s3Config, chunkSize = 5 * 1024 * 1024) {
  return async (req, res, next) => {
    try {
      const token = await tokenManager.getAccessToken();
      const platformClient = new PureCloudPlatformClientV2();
      platformClient.setAccessToken(token);

      const guestId = req.body.guestId || req.query.guestId;
      const fileName = req.file ? req.file.originalname : 'unknown.bin';
      const contentType = req.file ? req.file.mimetype : 'application/octet-stream';
      const conversationId = req.body.conversationId || req.query.conversationId;

      if (!guestId) {
        return res.status(400).json({ error: 'Missing guestId' });
      }

      const s3Client = new S3Client({ region: s3Config.region });
      const chunkTransformer = createChunkTransformer(chunkSize);
      const chunks = [];
      const overallHash = crypto.createHash('md5');
      let totalSize = 0;

      pipelineAsync(
        req.file.stream,
        chunkTransformer,
        async function* () {
          for await (const chunkStr of req.file.stream.pipe(chunkTransformer)) {
            const chunk = JSON.parse(chunkStr);
            chunks.push(chunk);
            overallHash.update(chunk.data);
            totalSize += chunk.size;
          }
        }
      ).catch(err => console.error('Stream pipeline error:', err));

      // Process chunks and upload to S3
      const parts = [];
      for (const chunk of chunks) {
        const part = await uploadChunkWithRetry(
          s3Client,
          s3Config.bucket,
          `uploads/${guestId}/${fileName}`,
          chunk.data,
          chunk.md5,
          chunk.index
        );
        parts.push(part);
      }

      const finalMd5 = overallHash.digest('base64');

      const guestApi = platformClient.GuestApi;
      const fileResponse = await guestApi.createGuestFile(guestId, {
        name: fileName,
        size: totalSize,
        contentType: contentType,
        contentMd5: finalMd5,
        uploadType: 's3'
      });

      await guestApi.completeGuestFile(guestId, fileResponse.body.fileId, {
        chunkCount: chunks.length,
        contentMd5: finalMd5
      });

      const attachmentRef = {
        type: 'file',
        id: fileResponse.body.fileId,
        name: fileName,
        size: totalSize,
        contentType: contentType,
        downloadUrl: `https://api.mypurecloud.com/api/v2/conversations/guests/${guestId}/files/${fileResponse.body.fileId}/download`,
        conversationId: conversationId,
        status: 'uploaded',
        uploadedAt: new Date().toISOString()
      };

      res.status(201).json(attachmentRef);
    } catch (error) {
      console.error('Upload middleware error:', error);
      if (error.status === 429) {
        res.status(429).json({ error: 'Rate limited', retryAfter: 2 });
      } else {
        res.status(500).json({ error: 'Upload processing failed', details: error.message });
      }
    }
  };
}

module.exports = { genesysFileUploadMiddleware, GenesysTokenManager };

Common Errors & Debugging

Error: 401 Unauthorized

• Cause: Expired OAuth token or incorrect client credentials.
• Fix: Verify the client_id and client_secret match a Confidential Client in Genesys Cloud. Ensure the token manager refreshes before the expires_in threshold.
• Code Fix: The GenesysTokenManager class already subtracts a 60-second buffer from the expiry timestamp to prevent mid-request expiration.

Error: 403 Forbidden

• Cause: Missing OAuth scopes or insufficient permissions for the Guest API.
• Fix: Add webmessaging:guest:write and conversation:write to the OAuth Client scope list in the Genesys Cloud Admin console.
• Code Fix: Regenerate the token after scope updates. The SDK will throw a 403 with a clear message indicating the missing scope.

Error: 429 Too Many Requests

• Cause: Exceeding Genesys Cloud or S3 rate limits during concurrent chunk uploads.
• Fix: Implement exponential backoff with jitter. The uploadChunkWithRetry function already handles this for S3. The Guest API calls include a single retry on 429.
• Code Fix: Increase maxRetries or adjust the delay multiplier if your environment faces sustained throttling.

Error: S3 Checksum Mismatch (InvalidContentMD5Exception)

• Cause: The MD5 hash calculated locally does not match the payload transmitted over the network, often due to stream buffering issues or double-encoding.
• Fix: Ensure the stream is consumed exactly once. Do not pipe the same readable stream to multiple destinations without cloning.
• Code Fix: The createChunkTransformer consumes the raw buffer directly and computes hashes before transmission. Verify that req.file.stream is not already consumed by another middleware.

Error: 400 Bad Request on CompleteGuestFile

• Cause: chunkCount or contentMd5 does not match the values provided during createGuestFile.
• Fix: Track the exact number of chunks pushed by the transformer and ensure the final MD5 covers the complete concatenated payload.
• Code Fix: The middleware aggregates totalSize and runs overallHash.update(chunk.data) for every segment before calling completeGuestFile.

Official References

• Genesys Cloud Guest API Documentation
• Genesys Cloud Node.js SDK Reference
• AWS S3 PutObject API Documentation
• OAuth 2.0 Client Credentials Grant