How do I log and audit API traffic at the gateway?

To log and audit API traffic at the gateway, you typically want request/response observability + security auditability without exposing sensitive data.

1) Decide what you need to capture

At the gateway, log enough to answer:

• Who called it? (client ID, user ID, API key, JWT subject, tenant)
• What was called? (method, path, route, upstream service)
• When did it happen? (timestamp, latency)
• Outcome? (status code, error reason)
• Where from? (source IP, region, user agent)
• Correlation? (request ID / trace ID)

2) Log the right fields

Recommended fields:

• timestamp
• request_id / trace_id
• client_id / consumer_id
• user_id / sub claim if available
• method
• path or normalized route template
• query only if safe
• status
• latency_ms
• bytes_in / bytes_out
• upstream_service
• source_ip
• user_agent
• auth_type (JWT, OAuth2, API key, mTLS)
• decision (allowed/denied)
• reason for denies
• tenant_id if multi-tenant

3) Avoid logging sensitive data

Do not log:

• Passwords, tokens, session cookies
• Full request/response bodies by default
• PII unless required and approved
• Secrets in headers or query strings

If you must capture payloads:

• Use allowlists of safe fields
• Mask/redact sensitive values
• Store bodies only for specific routes or sampling
• Encrypt logs at rest and in transit

4) Separate access logs from audit logs

• Access logs: operational use, high volume, request summary
• Audit logs: security/compliance use, immutable, more strict, include auth decisions and admin actions

Audit logs should be:

• Append-only
• Time-synced
• Tamper-evident
• Retained per policy

5) Use structured logging

Prefer JSON logs over plain text. Example:

{
  "timestamp": "2026-10-05T12:34:56Z",
  "request_id": "abc-123",
  "client_id": "partner-42",
  "user_id": "user-987",
  "method": "GET",
  "path": "/v1/orders/123",
  "status": 200,
  "latency_ms": 47,
  "source_ip": "203.0.113.10",
  "user_agent": "Mozilla/5.0",
  "auth_type": "JWT",
  "decision": "allowed"
}

6) Correlate logs with traces and metrics

Add:

• X-Request-Id or traceparent
• Distributed tracing
• Metrics for rate, errors, latency, denied requests

This helps you connect: gateway log → app log → database query → downstream service.

7) Capture auth and policy decisions

For auditing, log:

• Authentication success/failure
• Authorization policy evaluated
• Rate-limit decisions
• WAF/security rule matches
• Schema validation failures
• Admin/config changes to gateway rules

Example deny audit entry:

{
  "timestamp": "2026-10-05T12:35:10Z",
  "request_id": "abc-124",
  "client_id": "partner-42",
  "method": "POST",
  "path": "/v1/payouts",
  "decision": "denied",
  "reason": "scope_missing:payouts.write",
  "status": 403
}

8) Centralize and protect logs

Send gateway logs to a centralized system:

• ELK / OpenSearch
• Splunk
• Datadog
• Cloud logging (CloudWatch, Stackdriver, Azure Monitor)

Security controls:

• RBAC on log access
• Immutable storage / WORM if needed
• Retention policies
• Alerting on suspicious patterns

9) Sample gateway patterns

Common gateway products support this via plugins/policies:

• Kong: http-log, file-log, syslog, OpenTelemetry
• Apigee: analytics, message logging, policies
• NGINX: access log format, Lua/OpenTelemetry
• Envoy: access logs, ext_authz, tracing
• AWS API Gateway: access logs, execution logs, CloudWatch
• Azure API Management: diagnostics, event hub, App Insights

10) A practical setup

A good baseline:

1. Log every request in JSON at the gateway.
2. Redact sensitive headers and payload fields.
3. Add request ID and trace ID.
4. Log authz decisions and rate-limit events.
5. Forward to centralized SIEM/log store.
6. Keep audit logs immutable and restricted.
7. Build alerts for anomalies:
   • excessive 401/403
   • token replay
   • unusual geographies
   • spikes in denied requests
   • access outside business hours

If you want, I can give you:

• a Kong / NGINX / Envoy / AWS API Gateway example,
• a redaction policy template, or
• a recommended audit log schema.