System Patterns - Applied Java Patterns - page 165

Because of the amount of information that is passed, the throughput of information should be done as fast as
possible, or the system slows down dramatically. If the throughput fails or is disruptive for some reason, that
specific target is dropped. By implementing this kind of control, the quality of service can be guaranteed for the
other destinations.
Implementation
A Router class diagram is shown in Figure 4.10
Figure 4.10. Router class diagram
The Router pattern needs the following:
InputChannel - Can be of any type. It is used for mapping Input and Output in the Router ; also known as the
source.
OutputChannel - An interface that defines the method for sending the message.
Message - This class contains the information that needed to be distributed. To allow the Router to map this
message to a specific route, the message also contains a reference to the source.
Router - The Router maintains the map between InputChannel and specific OutputChannels , and
implements the OutputChannel interface. When it receives messages, it forwards them to the specified
OutputChannels .
Benefits and Drawbacks
Benefits and drawbacks include the following:
Decouples the source from the destination. The input doesn't need to know the destination; it only needs to know
the router. The router knows the mapping between the source and destination.
When trouble occurs on one specific channel, it doesn't need to affect the other channels. One channel does not
have the ability to block the Router , it continues doing its work of routing messages from an input to outputs.
Different strategies for InputChannels and OutputChannels . The Router can have each Channel in its own
Thread or combine all the InputChannels in one Thread .
Simplifies the Clients , because the Router takes over the task of message distribution.
Enhances reliability. One channel can no longer block the system. If it's not working, the Channel can be ignored or other
measures can be taken like dropping the problem channel. However, the Router still does its job.
Pattern Variants
A variation to this pattern is to have the Router keep a mapping between an arbitrary key and the destinations. A
source might have different destinations based on certain conditions. For instance, if it has two separate methods
each with its own OutputChannels . In the normal implementation of the Router pattern there is only one route