request is still considered idempotent
even if side effects exist.
Challenges of multimessage
Any message may arrive multiple times,
even after a long while. Think of a messaging system as containing a bunch of
Machiavellian gnomes who are watching your messages float by so they can
interject a copy of a message at precisely the worst time for your application
(see Figure 4). In most loosely coupled
systems, messages may arrive multiple
times. Furthermore, related messages
may be delivered out of order.
A typical approach to this problem
is to use request-response and then
ensure the messages processed are
idempotent. This has the benefit of
the application seeing the delivery of
the message via a positive response. If
the application gets a response from
its intended partner, then it is confident that the message has actually arrived. Because of retries, the receiving
application may receive the message
This challenge is further com-
pounded when multiple messages are
involved in making a piece of longer-
running work happen. The duration of
the work and the allowable failures for
the work must be considered, as in the
figure 6. service behavior is in the eye of the beholder.
the façade of doing
the real work.
The real application
work happens here
figure 7. transport and plumbing acknowledgments should not be visible to applications.
What Do you mean the Work is
Not Done on the Premises?
When Service A talks to Service B, you
do not know where the work is really
done. Service A believes it is doing work
with Service B, while Service B may actually subcontract all the work to Service C (see Figure 6). This is not in itself
a problem, but it can magnify the failure possibilities seen by Service A.
You cannot assume the work is actually done by the system you are chatting with. All you know is that you have
a messaging protocol, and, if things
are going well, appropriate messages
come back from the named partner
according to the protocol’s definition.
You simply do not know what goes on
behind the curtain.
You know a message has been delivered only when the answer comes back
from the partner doing the work. Tracking the intermediate waypoints does
not help to know that the work will get
done. Knowing a FedEx package has
reached Memphis will not tell you that
your grandmother will receive her box
When a messaging transport sends
an acknowledgment (ACK) to a sender, it means the message has been