API Reference: Upload Widget Events and Webhooks You Need to Implement for Marketplaces

Stop losing sales to buggy uploads: essential events and webhook behaviours marketplaces must implement in 2026

Marketplaces that buy and sell user-uploaded content face a unique set of problems in 2026: handling large, resumable uploads, proving provenance for payouts, enforcing moderation and copyright checks, and doing all of this securely and reliably at scale. This API reference shows the exact upload widget events, webhook payloads, and recommended server responses you need to implement so buyers, sellers and platform teams can operate with confidence.

Why this matters now (2026 context)

Late 2025 and early 2026 saw accelerated adoption of paid creator marketplaces and AI training marketplaces (buyers pay creators for content). These trends increase the need for robust audit trails, signature-verified events, and deterministic payout triggers. Additionally, edge-first architectures and serverless compute mean webhooks are often processed at the edge — which increases the importance of idempotency, signature verification, and predictable retry semantics.

How to read this reference

We present: (1) canonical event names, (2) sample webhook payloads you can validate against a JSON schema, (3) recommended HTTP responses and retry behaviour, (4) security and header conventions, and (5) operational best practices for marketplaces.

Event model overview

Design your system so each webhook contains a small set of consistent fields for idempotency, provenance and verification. Minimal fields every webhook should include:

• event_id — globally unique UUIDv4 for deduplication
• event_type — a stable string (see list below)
• timestamp — ISO 8601 UTC
• resource — the primary object (upload_id, user_id, asset_id)
• payload — event-specific JSON
• signature — HMAC or public-key signature in header

Essential upload widget event types (canonical list)

Use these event names verbatim to make integrations predictable. Prefix events consistently (e.g., upload., asset., marketplace.).

• upload.started — user initiated an upload session
• upload.chunk_uploaded — a chunk was successfully stored (resumable uploads)
• upload.progress — optional periodic progress updates
• upload.assembled — file assembled from chunks and integrity checked
• upload.completed — file fully available and persisted
• upload.rejected — rejected by validation/moderation/virus scan
• upload.moderation — moderation result (safe, risky, rejected)
• upload.ownership_verified — ownership/rights check passed
• asset.published — asset listed for sale on marketplace
• marketplace.royalty_registered — royalty/metadata recorded
• marketplace.payout_ready — asset qualified for payout
• asset.removed — asset removed for TOS/copyright

Canonical webhook envelope (recommended)

Each webhook should use a small, predictable envelope to simplify validation and retries.

{
  "event_id": "uuid-v4",
  "event_type": "upload.completed",
  "timestamp": "2026-01-18T12:34:56Z",
  "resource": {
    "upload_id": "upl_123",
    "uploader_id": "user_456",
    "asset_id": "asset_789"
  },
  "payload": { /* event-specific */ }
}

Payload examples (practical samples)

1) upload.started

{
  "event_id": "6f8a2bde-...",
  "event_type": "upload.started",
  "timestamp": "2026-01-18T12:00:00Z",
  "resource": {"upload_id":"upl_0001","uploader_id":"user_42"},
  "payload": {
    "upload_session": "sess_abc",
    "upload_method": "resumable",
    "total_bytes": 12582912,
    "file_name": "sunset.mov",
    "client_ip": "203.0.113.5"
  }
}

2) upload.chunk_uploaded (resumable)

{
  "event_id":"...",
  "event_type":"upload.chunk_uploaded",
  "resource":{"upload_id":"upl_0001"},
  "payload":{
    "chunk_index":3,
    "chunk_size":5242880,
    "total_chunks":5,
    "checksum":"sha256:..."
  }
}

3) upload.assembled

{
  "event_type":"upload.assembled",
  "resource":{"upload_id":"upl_0001","asset_id":"asset_777"},
  "payload":{
    "bytes":12582912,
    "content_type":"video/quicktime",
    "sha256":"...",
    "integrity": "verified"
  }
}

4) upload.completed (final success)

{
  "event_type":"upload.completed",
  "resource":{"asset_id":"asset_777","upload_id":"upl_0001"},
  "payload":{
    "available_url":"https://cdn.example.com/asset_777.mp4",
    "storage_location":"s3://marketplace-prod/asset_777.mp4",
    "size_bytes":12582912
  }
}

5) upload.rejected

{
  "event_type":"upload.rejected",
  "resource":{"upload_id":"upl_0002","uploader_id":"user_99"},
  "payload":{
    "reason":"virus_detected",
    "scanner_result":"Eicar-Test-File",
    "action":"quarantined"
  }
}

6) marketplace.payout_ready

{
  "event_type":"marketplace.payout_ready",
  "resource":{"asset_id":"asset_777","seller_id":"user_42"},
  "payload":{
    "amount_cents":1500,
    "currency":"USD",
    "payout_batch_id":"payout_2026_01_17_001",
    "trigger": "sale_confirmed"
  }
}

Recommended server responses and retry semantics

Make webhook delivery deterministic by using clear response codes. The sender (platform) decides retries based on your response codes and headers.

Accepted response patterns

• 200 OK — processed synchronously and acknowledged. Body: {"received":true}. No retry.
• 202 Accepted — accepted for async processing (e.g., queued). Sender should retry until it receives 200 or configured max attempts. Include an Id for tracking in any logs.
• 204 No Content — allowed as an alias for 200 OK if you prefer empty bodies.
• 4xx (non-429) — treat as permanent failure; do not retry (e.g., 401 for invalid signature, 403 for forbidden, 404 for endpoint removed). Return a JSON error describing cause.
• 429 Too Many Requests — include Retry-After (seconds). Sender should respect and implement exponential backoff.
• 5xx — transient server error; sender should retry with exponential backoff and jitter. Use Retry-After when available.

Suggested response body for success

{
  "received": true,
  "event_id": "6f8a2bde-...",
  "processing_id": "proc_1234"
}

Security headers and signature verification (practical)

Use HMAC-SHA256 signatures with a per-webhook secret or a public-key signature (Ed25519) for higher security. Include a timestamp to prevent replay attacks. Recommended headers:

• X-Signature — hex or base64 signature (e.g., HMAC-SHA256)
• X-Timestamp — ISO8601 UTC or unix secs
• X-Webhook-Id — identifier of sending system
• X-Event-Id — event_id for quick correlation
• X-Content-SHA256 — body hash to detect tampering

Store webhook secrets in a secure secrets store (KMS/Secrets Manager). Rotate secrets regularly and support multiple active secrets to avoid downtime.

Signature verification example — Node.js (HMAC SHA256)

// Node.js express example
const crypto = require('crypto');
function verifySignature(secret, body, headerSig, timestamp) {
  const windowSec = 300; // 5 minutes for replay protection
  const age = Math.abs(Date.now()/1000 - Date.parse(timestamp)/1000);
  if (age > windowSec) return false;
  const signed = `${timestamp}.${body}`;
  const expected = crypto.createHmac('sha256', secret).update(signed).digest('hex');
  return crypto.timingSafeEqual(Buffer.from(expected), Buffer.from(headerSig));
}

Signature verification example — Python (Ed25519)

from nacl.signing import VerifyKey
from nacl.encoding import HexEncoder

def verify_ed25519(pubkey_hex, signature_hex, message_bytes):
    pk = VerifyKey(pubkey_hex, encoder=HexEncoder)
    try:
        pk.verify(message_bytes, bytes.fromhex(signature_hex))
        return True
    except Exception:
        return False

Idempotency and deduplication

Always use event_id (UUID) to deduplicate. Implement a short-lived event store (e.g., Redis with TTL 24–72 hours) keyed by event_id. For operations that must be exactly-once (payout disbursements, license transfers), use an idempotency-key per business operation and persist its final state.

• Reject duplicates early: if event_id exists, return 200 and include an explanation.
• Persist idempotency-key status with states: RECEIVED, PROCESSING, COMPLETED, FAILED.
• Combine event_id + resource to prevent cross-resource collisions.

Handling large and resumable uploads

For large media and datasets, chunked/resumable upload events are critical. Key recommendations:

1. Emit upload.chunk_uploaded for each chunk with chunk_index and checksum.
2. Emit upload.assembled only after integrity checks (aggregate checksum) pass.
3. If assembly fails, send upload.rejected with actionable reason and remediation steps for the uploader.
4. Support server-side verification of chunk order and missing chunks when processing events.

Marketplace-specific flows and event sequencing

Marketplaces require stronger ordering and proofs for payments. Example sequence for a sale-ready asset:

1. upload.completed → asset created
2. upload.ownership_verified → confirms IP/rights
3. upload.moderation(safe) → content allowed
4. asset.published → listed for sale
5. marketplace.royalty_registered → royalties metadata stored
6. marketplace.payout_ready → payout triggered after sale confirmation

Each step should include event_id and trace_id. For financial operations, persist a signed audit record of the sequence (immutable ledger or append-only store) to support disputes and compliance.

Operational best practices: scaling, queues, and dead-letter handling

Don't process webhooks synchronously if the work is heavy (e.g., transcoding, ML moderation). Instead:

• Accept webhook (200/202) and enqueue event for downstream workers (SQS/Kafka/PubSub).
• Persist event raw payload for reprocessing (GCS/S3) and store a pointer in the queue message.
• Implement dead-letter queues for permanent processing failures and monitor with alerts.
• Use batching where appropriate (e.g., group multiple progress events into one update every X seconds).

Failure modes and retry strategy

Use exponential backoff with jitter. Suggested policy:

• Retry delays: 1s, 2s, 4s, 8s, 16s, then linear up to 1h total window.
• Max attempts configurable by platform (default 12 attempts).
• Allow recipients to send 2xx to stop retries even after initial attempts.

Compliance, audit trails and retention

Marketplaces must manage PII and copyright claims. Implement:

• Retention policies per regulation (GDPR: right-to-be-forgotten workflows; HIPAA: audit logs and encryption).
• Event archival with tamper-evidence — store signatures and hashes of event batches (append-only store).
• Access control: only authorized services can retrieve raw upload contents or PII.

Real-world pattern: MediaMart case study (hypothetical)

MediaMart, a mid-size marketplace, implemented these patterns in Q4 2025 to handle creator payouts. Key wins:

• Reduced duplicate payout incidents by 99% using event_id+idempotency store.
• Cut webhook processing latency by moving heavy work to workers and returning 202 fast.
• Closed disputes faster by exposing an immutable event ledger that included content checksums and signatures.

2026 trends to plan for

• Edge-first webhook processing: Handle initial handshake/verification at edge functions; forward canonical events to centralized processing.
• Standardization: CloudEvents + JSON Schema: Many integrations now accept CloudEvents envelopes — support it as an optional format for easier interoperability.
• AI-assisted moderation: Hook your moderation pipeline to accept ML model outputs as events (upload.moderation with score and model_version).
• Proof-of-contribution for AI marketplaces: Platforms paying creators for training data require signed provenance and licensing events (upload.ownership_verified and marketplace.royalty_registered).

Checklist: implementable steps before launch

1. Define canonical event list and publish JSON Schemas for each payload.
2. Require and verify X-Signature + X-Timestamp on every webhook.
3. Record event_id in Redis with TTL for deduplication.
4. Return strict response codes: 200 for success, 202 for async, 4xx for permanent rejection, 5xx for transient failures.
5. Queue heavy work and provide a retry/dead-letter strategy.
6. Support chunked/resumable events and assembly verification.
7. Emit marketplace-specific events for royalties, ownership and payouts and persist signed audit trails.

Actionable examples: quick verification flow

Example flow for upload.completed → payout_ready:

1. Receive upload.completed, verify signature and event_id dedupe.
2. Enqueue for moderation and ownership checks; respond 202 Accepted.
3. When moderation passes, emit upload.moderation and ownership verification events.
4. When both pass, emit marketplace.royalty_registered and marketplace.payout_ready; payout worker listens to payout_ready and executes idempotent transfer using idempotency-key.

Final notes: avoid common pitfalls

• Don't depend on order of delivery — design for eventual ordering using sequence numbers if needed.
• Avoid long synchronous processing in webhook handlers — always prefer 202 + queue.
• Document every field and its types; schemas reduce integration friction.
• Monitor webhooks: success rate, latency, retries, and DLQ trends — alert on regressions.

Takeaways

For marketplaces handling paid content, implementing the canonical upload events and secure webhook practices above will dramatically reduce disputes, speed payouts, and improve reliability. Use event_id for dedupe, sign every webhook, return clear HTTP codes, and offload heavy processing to background workers. Support resumable upload events and publish JSON Schemas to make third-party integrations predictable.

Call to action

Need production-ready schemas, SDKs, and sample receivers for these events? Visit our API hub at uploadfile.pro to download JSON Schema definitions, SDK examples (Node, Python, Go), and a webhook test harness you can use to simulate delivery and signature verification locally. Implement the patterns in this guide and reduce webhook-related incidents before your next product launch.

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