The ELK stack (Elasticsearch, Logstash, and Kibana) provides the means to make data driven decisions by analyzing event logs. Event logging has two major use cases: monitoring service health and improving the user experience.
The service health monitoring includes:
Usage information (e.g. number of active users, number of cards processed, etc.).
Performance tracking (e.g. number of requests processed per hour, average response time).
Exception tracking (e.g. error codes and messages).
The user experience related tracking measures how the user utilizes the system. This data can be analyzed to provide insights about well accepted parts of the service as well as areas that need attention and improvement.
Information can be tracked on the client and server side and in some cases on both (e.g. if user U dismissed card C). However, for optimal bandwidth utilization it is recommended to only track such events on the server.