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This simple example already does something useful, it retrieves and prints event ID (copied from standard PrintEventId module). Actual modules will do more complex things but this is a simple example of obtaining something from event data.

Data Access in User Modules

As already mentioned above all event data is accessible to user module via Event object, and all non-event data is accessible through Env object. Previous example shows simple use case of extracting data from the event. This section give more detailed description of the Event and Env types and their methods.

When extracting data from event or environment it is necessary to specify at least the type of the data (EventId in the above example). If there are multiple object of the same type in the event the additional identifying information must be provided – source address and/or additional string key.

Data Source Address

Many pieces of data in the event originate from devices or processes which are parts of the LCLS DAQ. Devices in DAQ system are identified their addresses, which are special C++ data types. There are three types of addresses defined by DAQ:

• DetInfo (class name Pds::DetInfo) – this is the most frequently used type and it defines all regular devices used in DAQ such as cameras, Acqiris, etc. Complete address specification includes 4 items:
  • Detector type, one of the Pds::DetInfo::Detector enum values.
  • Detector ID, in case there is more than one detector of the same type in a system they will have different IDs.
  • Device type, one of the Pds::DetInfo::Device enum values.
  • Device ID, in case there is more than one device of the same type in a system they will have different IDs.
• BldInfo (class name Pds::BldInfo) – this address type is used for Beam Line Data sources, particular source is identified by the Pds::BldInfo::Type enum value.
• ProcInfo (class name Pds::ProcInfo) – this address type is used rarely, and only for information produced by applications constituting DAQ. Sources of this type are identified by IP address of the host where application is running.

(If you look at the C++ code you'll notice that all above classes also include process ID, but it is not used by psana and can be set to 0 if needed.)

User modules should not need to use above C++ classes directly, instead psana provides facility that simplifies specification of the addresses and does not require exact addresses to be known.

Event Data

Event data can be