I have live time-series data generated by a light sensor, and presented as a rapidly changing (refreshing about every 20 milliseconds) variable in the public javascript file. How can I store them into mongo efficiently? Could anybody give me some suggestions about the best practices?
This sounds like a good case for using mongodb's Capped Collections.
Capped collections are fixed-size collections that support high-throughput operations that insert and retrieve documents based on insertion order. Capped collections work in a way similar to circular buffers: once a collection fills its allocated space, it makes room for new documents by overwriting the oldest documents in the collection.
You could insert each light sensor measurement as a new document in a Capped Collection, then you can efficiently retrieve the measurements in the same order as they were inserted, and also not have to worry about running out of storage space.
This mongodb blog post goes in-depth on various approaches to storing time-series data in mongodb: Schema Design for Time Series Data in MongoDB