The web-service interface that is used for integrating with Control System Studio can also be used by other clients. The protocol used by this web-service is specified in Appendix B, JSON archive access protocol 1.0. At the moment, this protocol is limited to providing basic information (scalar and array samples of a limited set of types, including some meta-data). In the future this interface is going to be extended, so that it will be possible for each control-system support to use a custom data format.
For some applications, using the web-service protocol might not be a viable solution because the actual sample objects (as internally stored by the control-system support) are needed or high troughput for mass-processing data is required. In this case, there are two options.
The first option is writing a Java application that uses the
ArchiveAccessService (or rather its
This will directly expose the sample objects as they are provided by the
The second option is directly accessing the Cassandra database. The layout of the tables (as far as samples are concerned) is described in Appendix A, CQL table layout. Applications accessing the database directly should only read data, never insert new data or update existing data. Ensuring data consistency in a distributed system is very tricky and it is very likely that third-party applications would break the data consistency guarantees carefully protected by the Cassandra PV Archiver.