4: Services slides |

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services

• Very related to a topic
• Instead of “subscribing” and “publishing”
• Services are a classic “client/server”
• Client Node “C” asks a question of Service “S” and blocks awaiting the response
• Service “S” responds immediately with a result, allowing “C” to continue
• Services are good for questions that have immediate answers
• They are synchronous whereas topics are asynchronous
• Another way to think of a service is as a “Remote Procedure Call”

The Service Node

• Declares its existence and it’s name
• Goes into a wait state for another node to request the service

A client Node

• It’s a normal node (nothing magic about it)
• It includes code to send a request to the service (ask it a question)
• The code is more, but the effect is of a function call
• Impressive because it is inter-process and even inter-host - i.e. can be over the internet

Defining a Service inputs and outputs

• Analogous to defining the “method signature” of the service
• Very reminiscent of how Topic messages were defined
• You will find this example in the samples package