Installation-free Usage

This example shows how to use the Hadoop Distributed Cache (DC) to distribute Python packages, possibly including Pydoop itself, to all cluster nodes at job launch time. This is useful in all cases where installing to each node is not feasible (e.g., lack of a shared mount point). Of course, Hadoop itself must be already installed and properly configured in all cluster nodes before you can run this.

Source code for this example is available under examples/self_contained.

Example Application: Count Vowels

The example MapReduce application, vowelcount, is rather trivial: it counts the occurrence of each vowel in the input text. Since the point here is to show how a structured package can be distributed and imported, the implementation is exceedingly verbose.

_VOWELS = set("AEIOUYaeiouy")


def is_vowel(c):
    return c in _VOWELS

class Mapper(api.Mapper):

    def map(self, context):
        for c in context.value:
            if is_vowel(c):
                context.emit(c.upper(), 1)

class Reducer(api.Reducer):

    def reduce(self, context):
        s = sum(context.values)
        context.emit(context.key, s)

How it Works

The DC supports automatic distribution of files and archives across the cluster at job launch time. This feature can be used to dispatch Python packages to all nodes, eliminating the need to install dependencies for your application, including Pydoop itself:

pydoop submit --upload-archive-to-cache vowelcount.tgz \
              --upload-archive-to-cache pydoop.tgz [...]

The pydoop.tgz and vowelcount.tgz archives will be copied to all slave nodes and unpacked; in addition, pydoop and vowelcount symlinks will be created in the current working directory of each task before it is executed. If you include in each archive the contents of the corresponding package, they will be available for import:

cd examples/self_contained/vowelcount
tar cfz ../vowelcount.tgz .

The archive must be in one of the formats supported by Hadoop: zip, tar or tgz.

Note

Pydoop submit automatically builds the name of the symlink that points to the unpacked archive by stripping the last extension. Thus, foo.tar.gz will not work as expected, since the link will be called foo.tar. Always use the .tgz extension in this case.

The example is supposed to work with Pydoop and vowelcount not installed on the slave nodes (you do need Pydoop on the client machine used to run the example, however).