Buffer Manager
The buffer manager consists of a set of library functions for event collection and distribution. A buffer is a shared memory region in RAM, which can be accessed by several processes, called applications. Processes sending events to a buffer are called producers. Processes reading events from the buffer are called consumers.
A buffer is organized as a FIFO (First-In-First-Out) memory. Consumers can specify which type of events they want to receive from a buffer. For this purpose each event contains a MIDAS header with an event ID and other pertinent information.
Buffers can be accessed locally through the shared memory or remotely via the MIDAS server acting as an interface to that same shared memory.
A common problem in DAQ systems is the possible crash of a client, such as a user analyzer. This can cause the whole system to hang up, and may require a restart of the DAQ causing a loss of both time and, eventually, precious data. In order to address this problem, a special watchdog scheme has been implemented. Each client attached to the buffer manager signals its presence periodically by storing a time-stamp in the shared memory. Every other client connected to the same buffer manager can then check if the other parties are still alive. If not, proper action is taken consisting in removing the dead client hooks from the system, leaving the system in a working condition.