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<title>Core client: finite-state machine (FSM) structure</title>
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<body bgcolor=ffffff>
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<h2>Core client: finite-state machine (FSM) structure</h2>
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<p>
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The core client can perform many activities (file transfers,
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computations, RPCs to scheduling servers) in parallel.
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To manage this parallelism, the core client is structures as a number of
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<b>finite-state machines</b> (FSM).
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For example, an HTTP transaction is
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represented by an FSM whose states might include:
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</p>
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<ul>
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<li> Waiting for connection establishment.
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<li> Waiting to send request header.
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<li> Waiting to send send request body.
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<li> Waiting for reply header.
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<li> Waiting for reply body.
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<li> Finished.
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</ul>
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<p>
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FSMs of a particular type are managed by an <b>FSM container</b>.
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Each FSM container manages a set of FSMs, and provides a <b>poll()</b>
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function for detecting and performing state transitions.
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These functions are nonblocking; at the lowest level, they must
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use non-blocking network sockets, accessed using select().
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<p>
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The core client uses the following FSM types:
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<ul>
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<li>
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<b>NET_XFER</b> (container: <b>NET_XFER_SET</b>).
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Each instance represents a network connection,
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for which data is being transferred to/from memory or a disk file.
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The <b>poll()</b> function uses
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<b>select()</b> to manage the FSM without blocking.
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<li>
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<b>HTTP_OP</b> (container: <b>HTTP_OP_SET</b>).
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Each instance represents an HTTP operation (GET, PUT or POST).
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<li>
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<b>FILE_XFER</b> (container: <b>FILE_XFER_SET</b>).
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Each instance represents a file transfer (upload or download) in progress.
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<li>
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<b>PERS_FILE_XFER</b> (container: <b>PERS_FILE_XFER_SET</b>).
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Each instance represents a "persistent file transfer",
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which recovers from server failures and disconnections,
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and implements retry and give-up policies.
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<li>
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<b>SCHEDULER_OP</b>.
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There is only one instance.
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It encapsulates communication with scheduling servers,
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including backoff and retry policies.
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<li>
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<b>ACTIVE_TASK</b> (container: <b>ACTIVE_TASK_SET</b>).
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Each instance represents a running application.
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</ul>
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<p>
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An FSM may be implemented using other FSMs; for example, FILE_XFER
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is implemented using HTTP_OP, which in turn is implemented using NET_XFER.
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