We introduce Candoia, a platform and ecosystem for building Mining Software Repositories (MSR) tools. The platform is designed to support building of MSR tools by providing necessary tools and abstractions that hide the complex details of version control, bug databases, source code programming languages and forges. The ecosystem allows easy sharing and accessing of MSR apps for researchers and practitioners. We have some initial evidence about Candoia's applicability in building robust MSR tools (over two dozen prebuilt apps in the first public release of Candoia), adoptability and interoperability (apps run on widely used projects such as Apache Tomcat, Apache Hadoop etc) and easy customizability (an user study). Candoia is available for download from http://candoia.org.