Online Reconstruction Overview¶

Overview talk

Run numbers¶

Run numbers are assumed to be as follows:

• -N : Monte Carlo simulation of run N
• 0 : pure Monte Carlo simulation
• +N : run N

Transforming spills from an input stream (Input-Transform)¶

This is the algorithm used to transform spills from an input stream:

CLEAR document store
run_number = None
WHILE an input spill is available
  GET next spill
  IF spill does not have a run number
    # Assume pure MC
    spill_run_number = 0
  IF (spill_run_number != run_number)
    # We've changed run.
    IF spill is NOT a start_of_run spill
      WARN user of missing start_of_run spill
    WAIT for current Celery tasks to complete
      WRITE result spills to document store
    run_number = spill_run_number
    CONFIGURE Celery by DEATHing current transforms and BIRTHing new transforms
  TRANSFORM spill using Celery
  WRITE result spill to document store
 DEATH Celery worker transforms

If there is no initial start_of_run spill (or no spill_num in the spills) in the input stream (as can occur when using simple_histogram_example.py or simulate_mice.py) then spill_run_number will be 0, run_number will be None and a Celery configuration will be done before the first spill needs to be transformed.

Spills are inserted into the document store in the order of their return from Celery workers. This may not be in synch with the order in which they were originally read from the input stream.

Merging spills and passing results to an output stream (Merge-Output)¶

This is the algorithm used to merge spills and pass the results to an output stream:

run_number = None
end_of_run = None
is_birthed = FALSE
last_time = 01/01/1970
WHILE TRUE
  READ spills added since last time from document store
  IF spill IS "end_of_run" 
    end_of_run = spill
  IF spill_run_number != run_number
    IF is_birthed
      IF end_of_run == None
          end_of_run = {"daq_event_type":"end_of_run", "run_num":run_number}
      Send end_of_run to merger
      DEATH merger and outputter
    BIRTH merger and outputter
    run_number = spill_run_number
    end_of_run = None
    is_birthed = TRUE
  MERGE and OUTPUT spill
Send END_OF_RUN block to merger
DEATH merger and outputter

The Input-Transform policy of waiting for the processing of spills from a run to complete before starting processing spills from a new run means that all spills from run N-1 are guaranteed to have a time stamp < spills from run N.

is_birthed is used to ensure that there is no BIRTH-DEATH-BIRTH redundancy on receipt of the first spill.

Document store¶

Spills are stored in documents in a collection in the document store.

Documents are of form {"_id":ID, "date":DATE, "doc":SPILL} where:

• ID: index of this document in the chain of those successfuly transformed. It has no significance beyond being unique in an execution of Input-Transform loop below. It is not equal to the spill_num (Python string)
• DATE: date and time to the milli-second noting when the document was added (Python timestamp)
• DOC: spill document. (Python string holding a valid JSON document)

Collection names¶

For Input-Transform,

• If configuration parameter doc_collection_name is None, "", or auto then HOSTNAME_PID, where HOSTNAME is the machine name and PID the process ID, is used.
• Otherwise the value of doc_collection_name is used.
• doc_collection_name has default value spills.

For Merge-Output,

• If configuration parameter doc_collection_name is None, "", or undefined then an error is raised.
• Otherwise the value of doc_collection_name is used.

Miscellaneous¶

• Currently Celery timeouts are not used, transforming a spill takes as long as it takes.
• Celery task retries on failure option is not used. If the transformation of a spill fails first time it can't be expected to succeed on a retry.
• If memory leaks arise, e.g. from C++ code, look at Celery rate limitss, which allow the time or number of tasks before sub-process is killed and respawned, to be defined. Soft rate limits would allow death to be run on the transforms first.