At the very beginning of the book (Installing Marvel), we encouraged you to install Marvel, a management monitoring tool for Elasticsearch, because it would enable interactive code samples throughout the book.
If you didn’t install Marvel then, we encourage you to install it now. This chapter introduces a large number of APIs that emit an even larger number of statistics. These stats track everything from heap memory usage and garbage collection counts to open file descriptors. These statistics are invaluable for debugging a misbehaving cluster.
The problem is that these APIs provide a single data point: the statistic right now. Often you’ll want to see historical data too, so you can plot a trend. Knowing memory usage at this instant is helpful, but knowing memory usage over time is much more useful.
Furthermore, the output of these APIs can get truly hairy as your cluster grows. Once you have a dozen nodes, let alone a hundred, reading through stacks of JSON becomes very tedious.
Marvel periodically polls these APIs and stores the data back in Elasticsearch. This allows Marvel to query and aggregate the metrics, and then provide interactive graphs in your browser. There are no proprietary statistics that Marvel exposes; it uses the same stats APIs that are accessible to you. But it does greatly simplify the collection and graphing of those statistics.
Marvel is free to use in development, so you should definitely try it out!