Core client: finite-state machine (FSM) structure

The core client can perform many activities (file transfers, computations, RPCs to scheduling servers) in parallel. To manage this parallelism, the core client is structures as a number of finite-state machines (FSM). For example, an HTTP transaction is represented by an FSM whose states might include:

• Waiting for connection establishment.
• Waiting to send request header.
• Waiting to send send request body.
• Waiting for reply header.
• Waiting for reply body.
• Finished.

FSMs of a particular type are managed by an FSM container. Each FSM container manages a set of FSMs, and provides a poll() function for detecting and performing state transitions. These functions are nonblocking; at the lowest level, they must use non-blocking network sockets, accessed using select().

The core client uses the following FSM types:

• NET_XFER (container: NET_XFER_SET). Each instance represents a network connection, for which data is being transferred to/from memory or a disk file. The poll() function uses select() to manage the FSM without blocking.
• HTTP_OP (container: HTTP_OP_SET). Each instance represents an HTTP operation (GET, PUT or POST).
• FILE_XFER (container: FILE_XFER_SET). Each instance represents a file transfer (upload or download) in progress.
• PERS_FILE_XFER (container: PERS_FILE_XFER_SET). Each instance represents a "persistent file transfer", which recovers from server failures and disconnections, and implements retry and give-up policies.
• SCHEDULER_OP. There is only one instance. It encapsulates communication with scheduling servers, including backoff and retry policies.
• ACTIVE_TASK (container: ACTIVE_TASK_SET). Each instance represents a running application.

An FSM may be implemented using other FSMs; for example, FILE_XFER is implemented using HTTP_OP, which in turn is implemented using NET_XFER.