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Alternative Model for Structuring Server Applications

Event Driven Model

• Definition: An event driven application is characterized by the following elements:

  • Implemented in a single address space.

  • Operates with a single process and a single thread of control.

Structure of Event Driven Model

• Event Dispatcher:

  • The central component of the process.

  • Continuously operates in a loop to:

  • Look for incoming events.

  • Based on the events, invoke one or more registered handlers.

Types of Events

• Events correspond to various triggers in the system, including:

  • Receipt of a request from client browsers.

  • Message received from the network (considered an event).

  • Completion of a send operation (the send completion is treated as an event).

  • Completion of a discrete operation, which denotes other important occurrences.

• The dispatcher processes these notifications, allowing it to invoke the appropriate handler.

Functioning of the Dispatcher

• The dispatcher operates similarly to a state machine due to its handling of event notifications.

• Handler Invocation:

  • Invoking a handler involves jumping to the corresponding location in the process’s address space where the handler is implemented.

  • This allows execution of the handler to commence immediately once an event is detected.

Example of Event Handling

1. Pending Connection Request:

   • If notified of a pending connection on the network port:

     • The dispatcher passes the event to the accept connection handler.

2. Incoming Data Message:

   • If the event is the receipt of data from an existing connection:

     • The dispatcher passes this to the read request handler.

   • Once a file name is extracted from the request and verified:

     • Chunks of the file are sent in response.

   • After confirmation that a chunk has been successfully sent:

     • The process continues to invoke the handler managing the send operation.

   • If the file is not present:

     • An error message is generated and sent back to the client.

Handlers and Execution

• Handlers are sequences of code that execute upon the occurrence of events and are characterized by:

  • Running to completion: Once a handler starts executing, it will process until completion.

  • Blocking Operations: If a handler requires a blocking operation:

  • It initiates the operation and then passes control back to the event dispatcher.

  • Control is returned so that the dispatcher can manage other events or invoke other handlers, thereby maintaining system efficiency.