def subscribe(api, filters, on_event, poll_fallback=15):
'''
- api: Must be a newly created from arvados.api(cache=False), not shared with the caller, as it may be used by a background thread.
+ api: a client object retrieved from arvados.api(). The caller should not use this client object for anything else after calling subscribe().
filters: Initial subscription filters.
- on_event: The callback when a message is received
+ on_event: The callback when a message is received.
poll_fallback: If websockets are not available, fall back to polling every N seconds. If poll_fallback=False, this will return None if websockets are not available.
'''
ws = None