MessageFilter

MessageFilter is the triage stage in front of every Frost component. It extends FrostBase and is instantiated automatically by FrostInterface — you do not create one yourself.

Its job is to take a stream of incoming messages and answer two questions for each: is this for me? and what kind of message is it?

Ports

Port	Direction	Purpose
message_type	in	The Python type messages must be an instance of. FrostInterface sets it to FrostMessage.
filter_callbacks	in	A list of predicates. A message must satisfy all of them to be accepted.
messages	in	Incoming (bank_index, message) pairs.
requests	out	Accepted messages whose header type is REQUEST.
responses	out	Accepted messages whose header type is RESPONSE.
errors	out	Accepted messages whose header type is ERROR.
discarded_messages	out	Everything else.

What gets discarded

A message goes to discarded_messages when it is malformed, when it is not an instance of message_type, or when any filter callback rejects it.

FrostInterface installs exactly one callback, which accepts a message only if its target equals this component's name:

message_filter.filter_callbacks.set(
    [lambda msg: is_target_valid(msg, self._get_reactor_name())]
)

To narrow further — accepting only certain senders, or only certain namespaces — a subclass can set a longer list of callbacks. Each is a callable taking the (bank_index, message) pair and returning a boolean.

discarded_messages is not a dead-letter port

Frost uses it for real work. Registration broadcasts are addressed to the marker "__target__" rather than to a component's own name, so the target filter rejects them and they surface here. Both FrostReactor and FrostLink react to discarded_messages to implement registration — and FrostLink additionally forwards anything else it finds there towards that message's target, which is how routing works at all.

If you react to discarded_messages in your own component, ignore anything that is not yours and leave the rest alone.

Nested payloads

If a message's payload is itself a list, the filter expands it in place and processes each element separately. This lets a component send a batch on one port and have it arrive as individual messages.