Export Transcript via S3 Presigned URL

How the Export Flow Works

After a caption session ends, you can download all finalized bilingual rows as a plain-text file. The export flow keeps raw audio completely out of storage:

Browser (finalized rows)
  → POST /api/sessions/{session_id}/export
  → FastAPI on Fargate
  → Serialize rows to TXT format
  → PutObject to private S3 transcript bucket
  → Generate time-limited presigned URL (24 hours by default)
  → Return presigned URL to browser
  → Browser downloads the TXT file directly from S3

No raw audio is ever written to S3. Only the finalized transcript TXT is stored. The transcript object is automatically deleted after 14 days by an S3 lifecycle rule.

Step 1 – Stop the Session

Click Stop in the dashboard. The backend:

1. Closes the Transcribe stream and Translate connection
2. Unregisters the session from the in-memory registry
3. Returns a final summary with the session ID

Click Export TXT in the dashboard. The frontend sends:

POST https://dpeohr327wt9l.cloudfront.net/api/sessions/{session_id}/export
Content-Type: application/json

{
  "segments": [
    {
      "segment_id": "seg-001",
      "speaker_label": null,
      "text_vi": "Chào buổi sáng",
      "text_en": "Good morning",
      "spoken_language": "vi",
      "timestamp_start": 0.0,
      "timestamp_end": 2.1
    }
  ]
}

The backend responds with a presigned URL and expiry timestamp:

{
  "download_url": "https://livecap-transcripts-dev-720459752315.s3.ap-southeast-1.amazonaws.com/transcripts/session-abc123.txt?X-Amz-...",
  "expires_at": "2026-07-09T12:00:00Z"
}

Step 3 – Verify the S3 Object Was Created

Using the CLI:

aws s3 ls "s3://livecap-transcripts-dev-720459752315/transcripts/" `
  --region ap-southeast-1 --profile <aws-profile>

You should see a .txt file for each exported session. The URL expires after 24 hours (configurable via DOWNLOAD_LINK_EXPIRATION).

Step 4 – Verify the Lifecycle Rule

Transcript objects are automatically removed after 14 days. Check the rule:

aws s3api get-bucket-lifecycle-configuration `
  --bucket livecap-transcripts-dev-720459752315 `
  --region ap-southeast-1 --profile <aws-profile>

Expected output:

{
  "Rules": [{
    "ID": "delete-old-transcripts",
    "Status": "Enabled",
    "Filter": {"Prefix": "transcripts/"},
    "Expiration": {"Days": 14}
  }]
}

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