Signal

A signal is an output from a system, which could be either:

• interactive, or explicitly caused by one effect, e.g. a response to an HTTP request
• ambient, or having ambiguous causality, e.g. an application log line

In either case, every signal is stored in the simulation log and may be the target of an invariant.

Interactive signals always include an effect_id foreign key to the effect that caused it. Currently, this will only come from systems with exec-type imports.

Ambient signals come from systems with stream-type import. They may come from systems that take effects, e.g.:

format:
  type: sql
import:
  type: stream
  command: psql -q $DATABASE_URL

Or from systems that do not expect effects (and therefore do not need a format) e.g.:

import:
  type: stream
  command: tail -F logs/app.log

Both systems will produce ambient signals - i.e. without an effect FK - because they are asynchronous, so causality is generally ambiguous.