I built exactly the database that you are describing here. Unfortunately it is not open source. However it is no secret that we used sqlite for the storage. Inserts and queries in sqlite are fast, even for databases with billions of rows. Deletes are very, very slow, so we created a new sqlite database for each week, and simply deleted an entire database file when the retention period expired. Sqlite is ACID, supports all of the complex queries that could be imagined and is easy to embed into the overall engine code base. We use aggregate tables to more efficiently display common aggregations of the data (e.g. graph metric over the last day, or average metric over the last date). The aggregates were updated as data was inserted, so that real-time views were always available.