| Interface | Description | 
|---|---|
| Counter | A named counter that tracks the progress of a map/reduce job. | 
| CounterGroup | A group of  Counters that logically belong together. | 
| JobContext | A read-only view of the job that is provided to the tasks while they
 are running. | 
| MapContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> | The context that is given to the  Mapper. | 
| ReduceContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> | The context passed to the  Reducer. | 
| TaskAttemptContext | The context for task attempts. | 
| TaskInputOutputContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> | A context object that allows input and output from the task. | 
| Class | Description | 
|---|---|
| Cluster | Provides a way to access information about the map/reduce cluster. | 
| ClusterMetrics | Status information on the current state of the Map-Reduce cluster. | 
| Counters | Countersholds per job/task counters, defined either by the
 Map-Reduce framework or applications. | 
| ID | A general identifier, which internally stores the id
 as an integer. | 
| InputFormat<K,V> | InputFormatdescribes the input-specification for a 
 Map-Reduce job. | 
| InputSplit | InputSplitrepresents the data to be processed by an 
 individualMapper. | 
| Job | The job submitter's view of the Job. | 
| JobID | JobID represents the immutable and unique identifier for 
 the job. | 
| JobStatus | Describes the current status of a job. | 
| Mapper<KEYIN,VALUEIN,KEYOUT,VALUEOUT> | Maps input key/value pairs to a set of intermediate key/value pairs. | 
| MarkableIterator<VALUE> | MarkableIteratoris a wrapper iterator class that 
 implements theMarkableIteratorInterface. | 
| OutputCommitter | OutputCommitterdescribes the commit of task output for a 
 Map-Reduce job. | 
| OutputFormat<K,V> | OutputFormatdescribes the output-specification for a 
 Map-Reduce job. | 
| Partitioner<KEY,VALUE> | Partitions the key space. | 
| QueueAclsInfo | Class to encapsulate Queue ACLs for a particular
  user. | 
| QueueInfo | Class that contains the information regarding the Job Queues which are 
 maintained by the Hadoop Map/Reduce framework. | 
| RecordReader<KEYIN,VALUEIN> | The record reader breaks the data into key/value pairs for input to the
  Mapper. | 
| RecordWriter<K,V> | RecordWriterwrites the output <key, value> pairs 
 to an output file. | 
| Reducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT> | Reduces a set of intermediate values which share a key to a smaller set of
 values. | 
| TaskAttemptID | TaskAttemptID represents the immutable and unique identifier for 
 a task attempt. | 
| TaskCompletionEvent | This is used to track task completion events on 
 job tracker. | 
| TaskID | TaskID represents the immutable and unique identifier for 
 a Map or Reduce Task. | 
| TaskTrackerInfo | Information about TaskTracker. | 
| Enum | Description | 
|---|---|
| JobCounter | |
| JobPriority | Used to describe the priority of the running job. | 
| QueueState | Enum representing queue state | 
| TaskCompletionEvent.Status | |
| TaskCounter | |
| TaskType | Enum for map, reduce, job-setup, job-cleanup, task-cleanup task types. | 
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