How do you design a distributed logging system?

Distributed logging collects and centralizes logs from all services. Architecture: log agents (Fluentd, Filebeat) on each node, message queue for buffering (Kafka), log aggregator (Logstash, Fluentd), storage (Elasticsearch), visualization (Kibana, Grafana). Design considerations: structured logging (JSON), correlation IDs (trace across services), log levels, retention policies, sampling for high-volume. ELK Stack common. Cloud options: CloudWatch, Datadog. Essential for debugging distributed systems.