Despite our many discussions regarding keys and values, we have yet to mention their
types. The MapReduce framework won’t allow them to be any arbitrary class. For example,
although we can and often do talk about certain keys and values as integers,
strings, and so on, they aren’t exactly standard Java classes, such as Integer, String,
and so forth. This is because the MapReduce framework has a certain defined way of
serializing the key/value pairs to move them across the cluster’s network, and only
classes that support this kind of serialization can function as keys or values in the
framework.
尽管我们讨论许多关于keys and values,我们还没有提到它们的类型.这个mapreduce框架不允许它们随意的定义成任何类型.例如,尽管我们常常讨论关于无疑是keys and values 是整型,或者是字符串类型,等等,他们不是明确确标准的java 类,例如integer,string,等等.这是因为mapreduce框架有一种定义序列化key/value pairs 在集群网络中传输,并且只这些类支持这种类型的序列能在做为keys or values 在这个框架起作用.
More specifically, classes that implement the Writable interface can be values, and
classes that implement the WritableComparable<T> interface can be either keys
or values. Note that the WritableComparable<T> interface is a combination of the
Writable and java.lang.Comparable<T> interfaces . We need the comparability
requirement for keys because they will be sorted at the reduce stage, whereas values
are simply passed through.
Hadoop comes with a number of predefined classes that implement
WritableComparable, including wrapper classes for all the basic data types, as seen
in table 3.1.
Table 3.1 List of frequently used types for the key/value pairs . These classes all implement the
WritableComparable interface.
列举常用的key/value pairs类型.
Class Description
BooleanWritable Wrapper for a standard Boolean variable
ByteWritable Wrapper for a single byte
DoubleWritable Wrapper for a Double
FloatWritable Wrapper for a Float
IntWritable Wrapper for a Integer
LongWritable Wrapper for a Long
Text Wrapper to store text using the UTF8 format
NullWritable Placeholder when the key or value is not needed
Keys and values can take on types beyond the basic ones which Hadoop natively supports.
You can create your own custom type as long as it implements the Writable
(or WritableComparable<T>) interface. For example, listing 3.2 shows a class
that can represent edges in a network. This may represent a flight route between
two cities.
所以最后的答案就是,key/value type是一种用于有mapreduce framework cluster进行传输的格式.
包括你在map 在reduce 进口的通讯交互格式,这机话,在不同slave里面执行时最后产生的一个格式返回汇总.
有意思,细节有待于我进一步的阅读这不好懂的英文.
这种格式