Google has open-sourced a Code Review specification, a set of general engineering practice guides for Google that covers almost all programming languages and various types of projects. This specification represents Google’s best practical experience in its long-term development. As a collection, Google expressed the hope that open source projects or other organizations will benefit from this set of specifications.
If a team is using a task branch workflow, code review is initiated before the code is merged after all the code has been written and passed the automated test. The general purpose is to find system defects, to ensure the overall quality of the software and to improve the level of the developer itself. All the tools and processes of the code review are built for this purpose.
Google Engineering Practices Documentation is hosted on Github.