Whether a message is of interest or not is again based on
the message content. Hence the messages which match the filter rule of interest need
to be routed further whereas the messages which didn't match the filter criterion
are ignored. All the messages which match the criterion will be routed to the output
channel of the message filter.
Illustrative Design
Acme has decided to offer discounts and gifts for all purchases exceeding a certain
amount in a single checkout. To process this, we can first use a wiretap to send a
copy of all orders to a message filter. The message filter will inspect the message
for the total dollar value of checkout and if it exceeds the threshold, the message
will be forwarded to the offer management module. Any order messages with total
dollar value less than the threshold will be ignored (dropped) at the message filter,
not forwarding them to the offer management module. Note that, we are directing
copies of orders only to the message filter hence all the original messages will be
routed to their respective inventory modules, irrespective of whether their copies are
forwarded or dropped at the message filter.
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