Pattern Oriented Software Architecture V1
From Mud to Structure
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The Layers pattern (31)
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helps to structure applications that can be
decomposed into groups of subtasks in which each group of
subtasks is at a particular level of abstraction.
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Example
Networking protocols are probably the best-known example of layered
architectures.
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Context
A large system that requires decomposition.
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Problem
Imagine that you are designing a system whose dominant
characteristic is a mix of low- and high-level issues, where high-level
operations rely on the lower-level ones.
Such systems often also require some horizontal structuring that is
orthogonal to their vertical subdivision. This is the case where several
operations are on the same level of abstraction but are largely independent
of each other.
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Solution
Structure your system into an appropriate number of layers and
place them on top of each other.
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Structure
The main structural characteristic of the Layers pattern is that the
services of Layer J are only used by Layer J + 1-there are no further
direct dependencies between layers. This structure can be compared
with a stack. or even an onion. Each individual layer shields all lower
layers from direct access by higher layers.
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Dynamics
Scenario I is probably the best-known one. A client Issues a request
to Layer N. Since Layer N cannot cany out the request on its own. it
calls the next Layer N - 1 for supporting subtasks. Layer N - I provides
these. In the process sending further requests to Layer N-2. and so
on until Layer I 1s reached. Here, the lowest-level servlces are finally
performed. If necessary, replies to the different requests are passed
back up from Layer 1 to Layer 2, kom Layer 2 to Layer 3, and so on
until the final reply arrives at Layer N.
A characteristic of such top-down communication Is that Layer J
often translates a slngle request from Layer J+1 Into several requests
to Layer J- 1. This is due to the fact that Layer J is on a hlgher level of
abstraction than Layer J- 1 and has to map a hlgh-level service onto
more prlmlUve ones.
Scenario II illustrates bottom-up communicaUon-a chaln of actions
starts at Layer 1, for example when a device driver detects input. The
driver translates the lnput into an Internal format and reports it to
Layer 2. which starts lnterpreting it, and so on. In thls way data
moves up through the layers until It arrives at the highest layer. While
top-down lnformauon and control flow are often described as
'requests'. bottom-up calls can be termed 'notifications'.
As mentioned in Scenario I, one top-down request often fans out to
several requests in lower layers. In contrast. several bottom-up noUfications
may either be condensed into a slngle notificauon higher In
the structure. or remain in a I : I relationship.
Scenario III descrlbes the situation where requests only travel
through a subset of the layers. A top-level request may only go to the
next lower level N- 1 lf this level can satisfy the request. An example
of this is where level N- 1 acts as a cache. and a request from level N
can be satisfied without being sent all the way down to Layer 1 and
from here to a remote server. Note that such caching layers mafntain
state information, while layers that only forward requests are often
stateless. Stateless layers usually have the advantage of being
simpler to program, particularly with respect to re-entrancy.
Scenario IV describes a situation similar to Scenario 111. An event is
detected in Layer 1, but stops at Layer 3 instead of traveling all the
way up to Layer N. In a communication protocol, for example, a resend
request may arrive from an impatient client who requested data
some time ago. In the meantime the server has already sent the
answer, and the answer and the re-send request cross. In this case,
Layer 3 of the server side may notice this and intercept the re-send
request without further action.
Scenario V involves two stacks of N layers communicating with each
other. This scenario is well-known from communication protocols
where the stacks are known as 'protocol stacks'. In the following
diagram, Layer N of the left stack issues a request. 'The request moves
down through the layers until it reaches Layer 1, is sent to Layer 1 of
the right stack, and there moves up through the layers of the right
stack. The response to the request follows the reverse path until it
arrives at Layer N of the left stack.
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Implementation p38
Not all the following steps are mandatory-it depends on your
applicauon.
1 Define the abstraction criterion for grouping tasks into layers.
2 Determine the number of abstraction levels according to your
abstraction criterion.
3 Name the layers and assign tasks to each of them
4 Specih the services.
5 Refine the layering.
6 Specify an interface for each layer.
7 Structure indivzdual layers.
8 Specth the communication between adjacent layers.
9 Decouple adjacent layers.
10 Design an error-handling strategy.
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Example Resolved
TCP/IP
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Variants
Relaxed Layered System
This is a variant of the Layers pattern that
is less restrictive about the relationship between layers.In a Relaxed
Layered System each layer may use the services of all layers below it,
not only of the next lower layer. A layer may also be partially opaquethis
means that some of its services are only visible to the next higher
layer, while others are visible to all higher layers. The gain of flexibility
and performance in a Relaxed Layered System is paid for by a loss of
maintainability.
Layering Through Inheritance
This variant can be found in some object-oriented systems. In this variant
lower layers are implemented as base classes. A higher layer requesting
services from a lower layer inherits from the lower layer's
implementation and hence can issue requests to the base class
services. An advantage of this scheme is that higher layers can modify
lower-layer services according to their needs. A drawback is that such
an inheritance relationship closely ties the higher layer to the lower
layer. If for example the data layout of a C++ base class changes, all
subclasses must be recompiled. Such unintentional dependencies
introduced by inheritance are also known as the fragile base class
problem.
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Known Uses
Virtual Machines.
APIs.
Information Systems (IS)
Presentation 、Application logic、Domain layer、Database
Windows NT
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Consequences
Reuse of layers.
Support for standardization.
Dependencies are kept local.
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See Also
Composite Message.
A Microkernel architecture
The PAC architectural pattern