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courses:mapreduce-tutorial:step-24 [2012/01/27 22:17] straka |
courses:mapreduce-tutorial:step-24 [2012/01/27 23:57] straka |
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We start by going through a simple Hadoop job with Mapper only. | We start by going through a simple Hadoop job with Mapper only. | ||
- | A mapper which processes (key, value) pairs of types (Kin, Vin) and produces (key, value) pairs of types (Kout, Vout) must be a subclass of [[http:// | + | A //mapper// which processes (key, value) pairs of types (Kin, Vin) and produces (key, value) pairs of types (Kout, Vout) must be a subclass of [[http:// |
The mapper must define a '' | The mapper must define a '' | ||
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</ | </ | ||
- | Remarks: | + | ==== Remarks |
* The filename //must// be the same as the name of the class -- this is enforced by Java compiler. | * The filename //must// be the same as the name of the class -- this is enforced by Java compiler. | ||
* In one class multiple jobs can be submitted, either in sequence or in parallel. | * In one class multiple jobs can be submitted, either in sequence or in parallel. | ||
* A mismatch of types is usually detected by the compiler, but sometimes it is detected only at runtime. If that happens, an exception is raised and the program crashes. For example, default key output class it '' | * A mismatch of types is usually detected by the compiler, but sometimes it is detected only at runtime. If that happens, an exception is raised and the program crashes. For example, default key output class it '' | ||
+ | * **VIM users**: The code completion plugin does not complete the '' | ||
===== Running the job ===== | ===== Running the job ===== | ||
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Mind the '' | Mind the '' | ||
- | * When using '' | + | * When using '' |
- | * When not specifying '' | + | * When not specifying '' |