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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
If you're looking only for a code reference there's https://github.com/nickjj/build-a-saas-app-with-flask which uses an app factory and Celery. It's all set up to send emails through Celery and has tests too.
NOTE:
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Hence, a higher number means a more popular project.