Logs are a fundamental concept in computer science, used for various purposes such as storage engines, database replication, distributed consensus, and message brokering. They provide an append-only sequence of records that can be read sequentially, ensuring consistency and reliability. In this talk, we discussed four examples of practical applications of logs: storage engines (B-trees), database replication, distributed consensus (Raft algorithm), and Apache Kafka as a message broker. We also explored how to use logs for data integration by maintaining a log of writes and consuming it in sequential order to keep different datastores in sync. This approach helps avoid race conditions and partial failures, ensuring consistency across various systems.