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Logging, auditing & legal compliance

Log subjects​

On the billing backend we keep logs of:

  • Every state change
  • Every webhook payload (raw, immutable)
  • Every retry
  • Every error.

We use structured logs stored separately, with critical logs being written to an append-only log storage ClickHouse.

Retention​

Based on the Hungarian laws and GDPR, we store payment information and details for minimum 7 years. We also store critical logs without personal data indefinitely.

ClickHouse - Immutable Audit Logs​

High-performance columnar database for analytics and compliance.

What We Store:

  • Every payment state transition
  • All API requests and responses
  • Webhook payloads (raw and parsed)
  • Authentication events
  • System errors and warnings

Guarantees:

  • Append-only: Logs cannot be modified or deleted
  • Fast queries: Billion-row queries in <1 second
  • Data retention: Indefinite (legal compliance)
  • Tamper-evident: Cryptographic checksums

Use Cases:

  • Regulatory audits
  • Fraud investigation
  • Business intelligence
  • Incident forensics