DBLog: A Watermark Based Change-Data-Capture Framework

Abstract

It is a commonly observed pattern for applications to utilize multiple heterogeneous databases where each is used to serve a specific need such as storing the canonical form of data or providing advanced search capabilities. For applications it is hence desired to keep multiple databases in sync. We have observed a series of distinct patterns that have tried to solve this problem such as dual-writes and distributed transactions. However, these approaches have limitations with regard to feasibility, robustness, and maintenance. An alternative approach that has recently emerged is to utilize Change-Data-Capture (CDC) in order to capture changed rows from a database's transaction log and eventually deliver them downstream with low latency. In order to solve the data synchronization problem one also needs to replicate the full state of a database and transaction logs typically do not contain the full history of changes. At the same time, there are use cases that require high availability of the transaction log events so that databases stay as closely in-sync as possible. To address the above challenges, we developed a novel CDC framework for databases, namely DBLog. DBLog utilizes a watermark based approach that allows us to interleave transaction log events with rows that we directly select from tables to capture the full state. Our solution allows log events to continue progress without stalling while processing selects. Selects can be triggered at any time on all tables, a specific table, or for specific primary keys of a table. DBLog executes selects in chunks and tracks progress, allowing them to pause and resume. The watermark approach does not use locks and has minimum impact on the source. DBLog is currently used in production by tens of microservices at Netflix.