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Before you can write a tool, you need a bit of background on how Penny instruments Pig scripts (called "dataflow programs" in the following diagram).
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As shown in this diagram, Penny inserts one or more "monitor agents" between steps of the pig script, which observe data flowing between the pig script steps. Monitor agents run arbitrary Java code as needed for your tool, which has access to some primitives for tagging records and communicating with other agents and with a central "coordinator" process. The coordinator also runs arbitrary code defined by your tool.
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