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In GNUnet, the STATISTICS subsystem offers a central place for all subsystems to publish unsigned 64-bit integer run-time statistics. Keeping this information centrally means that there is a unified way for the user to obtain data on all subsystems, and individual subsystems do not have to always include a custom data export method for performance metrics and other statistics. For example, the TRANSPORT system uses STATISTICS to update information about the number of directly connected peers and the bandwidth that has been consumed by the various plugins. This information is valuable for diagnosing connectivity and performance issues.
Following the GNUnet service architecture, the STATISTICS subsystem is divided into an API which is exposed through the header gnunet_statistics_service.h and the STATISTICS service gnunet-service-statistics. The gnunet-statistics command-line tool can be used to obtain (and change) information about the values stored by the STATISTICS service. The STATISTICS service does not communicate with other peers.
Data is stored in the STATISTICS service in the form of tuples (subsystem, name, value, persistence). The subsystem determines to which other GNUnet's subsystem the data belongs. name is the name through which value is associated. It uniquely identifies the record from among other records belonging to the same subsystem. In some parts of the code, the pair (subsystem, name) is called a statistic as it identifies the values stored in the STATISTCS service.The persistence flag determines if the record has to be preserved across service restarts. A record is said to be persistent if this flag is set for it; if not, the record is treated as a non-persistent record and it is lost after service restart. Persistent records are written to and read from the file statistics.data before shutdown and upon startup. The file is located in the HOME directory of the peer.
An anomaly of the STATISTICS service is that it does not terminate immediately upon receiving a shutdown signal if it has any clients connected to it. It waits for all the clients that are not monitors to close their connections before terminating itself. This is to prevent the loss of data during peer shutdown -- delaying the STATISTICS service shutdown helps other services to store important data to STATISTICS during shutdown.