Software is simply speaking applying
logic on data. For applying logic, data needs to be defined
consistently so that multiple people involved can work together over
long time without continuous training or feedback. This also helps in
consistently applying logic. A configurable system where data
definition is not coded but is defined outside in a configuration
file is much more flexible to change.
So, the pattern I am talking about is
applicable in places where you have data having some format which is
inputted through some configuration file. So, system basically have
two inputs one being data while other being format of data. System
uses the latter to apply logic on former. Logic may be hardcoded or
again can be configurable. But, that is not focus of this pattern.
This pattern only tries to focus on the two inputs specified and
their representation.
Configuration
must be known before data is available when system starts. Every
definition in configuration is represented by FieldDescriptor. The
fieldDescriptor defines the format of that field. It may contain
values like name of field, expected type, file name to expect data
from, location of field in data, etc. it is problem specific. It is
an immutable class that should not change value on run time once
created and are thus thread safe. While loading, system parses the
configuration file. Each definition in that file is converted into a
FieldDescriptor object. You can apply any pattern to create it. It
must however not hold actual data. Descriptor object should be
comparable and must use all fields into consideration while comparing
equality. Higher or lower values can be problem specific. Holding
value is job of another set called Field. Once fieldDescriptor's are
loaded and data is available, system now goes through descriptor list
and uses it to extract data from source in format specified in
fieldDescriptor. It then creates its corresponding Field object.
Field object primarily has two values descriptor and value. It can
have any other value depending on problem. If descriptor is not
available while creating field but individual description is
available then they must be passed onto field and field creates them
and stores them. Field must be able to provide its value as well as
descriptor when asked for.
Here is the basic scenario
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| Scenario in which this pattern is applied |
