If key is missing, it'll be created, otherwise ignored or overwritten.
add_to_session(
session,
key = "rave_id",
val = paste(sample(c(letters, LETTERS, 0:9), 20), collapse = ""),
override = FALSE
)'Shiny' session
character, key to store
value to store
if key exists, whether to overwrite its value
If session is shiny session, returns current value stored in
session, otherwise returns NULL