We want to use Grafana to show measuring data. Now, our measuring setup creates a huge amount of data that is saved in files. We keep the files as-is and do post-processing on them directly with Spark ("Data Lake" approach).
We now want to create some visualization and I thought of setting up Cassandra on the cluster running Spark and HDFS (where the files are stored). There will be a service (or Spark-Streaming job) that dumps selected channels from the measuring data files to a Kafka topic and another job that puts them into Cassandra. I use this approach because we have other stream processing jobs that do on the fly calculations as well.
I now thought of writing a small REST service that makes Grafana's Simple JSON datasource usable to pull the data in and visualize it. So far so good, but as the amount of data we are collecting is huge (sometimes about 300MiB per minute) the Cassandra database should only hold the most recent few hours of data.
My question now is: If someone looks at the data, finds something interesting and creates a snapshot of a dashboard or panel (or a certain event occurrs and a snapshot is taken automatically), and the original data is deleted from Cassandra, can the snapshot still be viewed? Is the data saved with it? Or does the snapshot only save metadata and the data source is queried anew?