VLA Dispatcher

Authors: Sarah Burke-Spolaor, Frank Schinzel, Jayce Dowell

Overview

The VLA Dispatcher monitors the Jansky Very Large Array (VLA) metadata broadcast (MCAF multicast stream) and identifies observation events matching specified project and intent criteria. When a matching scan is found, the dispatcher writes a JSON command file that is consumed by the eLWA notification server (fcn_server.py in the eLWA_triggering package) for distribution to LWA stations.

The dispatcher is one component of the eLWA coordination system:

VLA MCAF Stream --> dispatcher.py --> incoming.json --> fcn_server.py (eLWA_triggering)

Prerequisites

1. Python 2.7+ (dispatcher and support libraries use Python 2 conventions)
2. Network access to the VLA MCAF multicast group (239.192.3.2:53001)
3. The eLWA_triggering package (provides the notification server that consumes the dispatcher's output)

Files and Organization

File	Description
vla_dispatcher/dispatcher.py	Main dispatcher; monitors MCAF stream and writes commands
vla_dispatcher/mcaf_library.py	MCAF multicast client and VLA configuration parser
vla_dispatcher/obsdocxml_parser.py	Auto-generated XML parser for VLA obsdoc documents
vla_dispatcher/angles.py	Angle conversion and formatting utilities
vla_dispatcher/jdcal.py	Julian date / calendar date conversion utilities
client_tools/client_software.py	Legacy example TCP client for receiving dispatches
service/vla-dispatcher.service	systemd service file

Configuration

The dispatcher is configured entirely via command-line arguments:

Argument	Default	Description
-i, --intent	''	Trigger on scans whose intent contains this substring
-p, --project	''	Trigger on scans whose project ID contains this substring
-d, --dispatch	off	Enable dispatch mode (write command files); without this flag the dispatcher only logs matching scans
-c, --command-file	incoming.json	Path to the JSON command file consumed by fcn_server.py
-v, --verbose	off	Enable verbose (DEBUG) logging

Running

Listening Mode (Dry Run)

To monitor the MCAF stream and log matching scans without dispatching:

cd vla_dispatcher
python dispatcher.py --intent OBSERVE_PULSAR_RAW --project TSKY

Dispatch Mode

To monitor and write command files for consumption by fcn_server.py:

cd vla_dispatcher
python dispatcher.py \
    --intent OBSERVE_PULSAR_RAW \
    --dispatch \
    --command-file /home/op1/eLWA/incoming.json

The --command-file path must match the --command-file argument given to fcn_server.py in the eLWA_triggering package.

systemd Service

A systemd service file is provided in service/vla-dispatcher.service. To install:

sudo cp service/vla-dispatcher.service /etc/systemd/system/
sudo systemctl daemon-reload
sudo systemctl enable vla-dispatcher.service
sudo systemctl start vla-dispatcher.service

The service file should be edited to reflect the correct intent filter and command file path for your deployment.

Event Types

The dispatcher generates three event types:

Type	Description
ELWA_SESSION	A scan matching the intent filter is available for observation. Includes position, duration, and the VLA configuration URL.
ELWA_READY	First scan of a new scheduling block for a matching project.
ELWA_DONE	A matching project has finished (source is FINISH).

Command File Format

The dispatcher writes a JSON file with the following fields:

Field	Type	Description
notice_type	string	One of ELWA_SESSION, ELWA_READY, ELWA_DONE
event_id	int	Serial number derived from current UTC time
project_id	string	VLA project ID
scan_id	int	VLA scan number
scan_intent	string	Scan intent string
event_t	float	Event time (UNIX timestamp)
event_source	string	Source name
event_ra	float	Right Ascension in degrees (or -1 for READY/DONE)
event_dec	float	Declination in degrees (or -1 for READY/DONE)
event_duration	float	Observation duration in seconds (or -1 for READY/DONE)
config_url	string	URL to the VLA configuration XML

The file is deleted by fcn_server.py after it has been read. The dispatcher waits for the file to be consumed before writing the next command.