Messaging System

Message Data Structure

Inside the code of a worker and the nowcast manager, a nowcast message is a Python object nemo_nowcast.message.Message:

>>> from nemo_nowcast.message import Message

>>> msg = Message(
        type='success 12',
        payload={'12 forecast': True},

The value associated with the source attribute is the name of process that is sending the message; i.e. the worker name, or manager.

>>> print(msg.source)

The type attribute’s value is a key associated with the message sender in the Message Registry section of the Nowcast Configuration File. For example, the message registry entries for a worker implemented in download_weather might be:

message registry:
      success 00: 00 weather forecast ready
      failure 00: 00 weather forecast download failed
      success 06: 06 weather forecast ready
      failure 06: 06 weather forecast download failed
      success 12: 12 weather forecast ready
      failure 12: 12 weather forecast download failed
      success 18: 18 weather forecast ready
      failure 18: 18 weather forecast download failed
      crash: download_weather worker crashed

In the example message above:

>>> print(msg.type)
'success 12'

The value associated with the payload attribute can be any Python object (including None) that can be a value in a dictionary. The payload value is inserted into a checklist dictionary that the nowcast manager uses to maintain information about the state of the nowcast system. The key at which the payload value is inserted into the checklist is defined for each worker in the Message Registry section of the Nowcast Configuration File:

message registry:
          checklist key: weather forecast

Message payloads vary markedly from one worker to another depending on what information a worker needs to convey to the manager, other workers, or nowcast system users inspecting the system state.

Message Exchanges

Message exchanges are always initiated by workers. Workers send a message to the manager when they have something significant to report:

  • Successful completion of their task

  • Failure to complete their task

  • Crashing due to an unhandled exception

  • Needing information from the manager about the state of the nowcast system

  • Providing a message to be included in the nowcast system logging output (only from workers running on remote hosts)

When the manager receives a message from a worker it acknowledges the message with a return message. Those messages are also defined in the Message Registry section of the Nowcast Configuration File. An “all is good” acknowledgment message from the manager in response to a message from a worker looks like:


Message Serialization and Deserialization

Before messages can be passed among a worker, the Message Broker, and the Manager they must be transformed into strings for transmission across the network. That is a process that is known as “serialization”. It is done by calling the serialize() method to transform the message object into a YAML document:

Message(source='manager', type='ack').serialize()

The message recipient “deserializes” the YAML document to transform it back into a message Message. That is done by calling the deserialize() method with the YAML document as its argument:

message = Message.deserialize(yaml_string)

Deserialization is done using the yaml.safe_load() function. That function limits the types of Python objects that can be in a message to (more or less) the Python data (True, False, None, float, int, etc.) and data container objects (dict, list, tuple, etc.). Doing so is a security measure to prevent the possibility of injection into the system of a maliciously crafted message that could execute arbitrary code on the nowcast system server.

Network Transmission of Messages

Messages are transmitted among the workers, broker, and the manager on the TCP network layer using dedicated ports.

  • When the broker is started it binds to a workers port to listen for messages from workers, and a manager port to listen for messages from the manager. After that, the broker simply listens for messages and queues them in both directions between the workers and manager ports. It does not deserialize the YAML documents, it just passes them along.

  • When the manager is started it connects to the manager port and listens for messages. When it receives a message it deserializes it, handles it, and send the appropriate acknowledgment message back.

  • When a worker is started it connects to the workers port. When it has something to report to the manager it serializes the message, sends it, and waits for an acknowledgment from the manager.

The server on which the broker is running, and the workers and manager port numbers that the system uses are defined in the ZeroMQ Server and Ports section of the Nowcast Configuration File.


If the manager or some of the workers run on different machines than the message broker it is necessary to ensure that the appropriate firewall rules are in place to allow traffic to pass between those machines via the worker and/or manager port(s).

The nowcast messaging system is based on the ZeroMQ distributed messaging framework. You probably don’t need to delve into the details of ZeroMQ, but it is important to note that this is one of the situations where the nowcast system “stands on the shoulders of giants” rather than “re-inventing the wheel”.