The scheduling problem is able to be broken into many components- listening for new work, allocating resources for new work, scheduling (maintaining the dispatcher list) as well as dispatching.
The listener is event driven. It receives messages from data communications as well as from dispatched processes. A distinguished field of the message identify a transaction name. Habitually this field has been filled in by data communications that resolved the transaction name to a reference to a transaction descriptor.
Occasionally this field is symbolic in which case the listener uses the name in a directory call to get a reference to the transaction descriptor (The directory perhaps determined by the message source.) If the name is had or if the sender isn’t authorized to invoke the transaction then the message is discarded and a negative acknowledge is sent to the source of the message. If the sender is official to invoke the named transaction then the allocator examines the transaction descriptor as well as the current system state and decides whether to put this message in a work-to-do list otherwise to allocate the transaction right away Criteria for this are-
a) The system perhaps overloaded (‘full’.)
b) There perhaps a limit on the number of transactions of this type which can run concurrently.
c) There perhaps a threshold N such that N messages of this type must arrive at which point a server is allocated as well as the messages are batched to this server.
d) The transaction may perhaps have an affinity to resources which are unavailable.
e) The transaction may perhaps run at a special time (overnight, off-shift,...)
If the transaction is able to run immediately then the allocator either allocates a new process to process the message or gives the message to a primed transaction that is waiting for input.
If a fresh process is to be created a process (domain) is allocated as well as all objects mentioned in the transaction descriptor are allocated as part of the domain, Program management puts up the address space to hold the programs, data communication allocates the necessary queues, data management will assign the cursors of the transaction for the process, the recovery component allocates a log cursor and writes a begin transaction log record and so on. The process is then put up with a pointer to the input message.
This allocated process is specified to the scheduler that eventually places it on the dispatcher queue. The dispatcher ultimately runs the process once the transaction scheduler dispatches the process the operating system scheduler is responsible for scheduling the process against the physical resources of the system.
When the transaction finished it returns to the scheduler. The scheduler may or mayn’t collapse the process structure depending on whether the transaction is batched or primed. If the transaction has free, resources necessary by waiting unscheduled transactions the scheduler will now dispatch these transactions.
Primed transactions are the optimization that dramatically reduces allocation and de-allocation overhead. Process allocation is able to be an expensive operation and so transactions that are executed frequently are often primed. A primed transaction has a huge part of the domain already built. Particularly programs are loaded cursors are allocated and the program prolog has been executed. The process (transaction) is waiting for input. The scheduler requires only pass the message to the transaction (process), habitually the system administrator or operator will prime several instances of a transaction. A banking system doing three withdrawals as well as five deposits per second might have two withdrawal transactions and four deposit transactions primed.
So far another variant has the process ask for a message after it completes. If a fresh message has arrived for that transaction type then the process processes it. If there is no job for the transaction then the process disappears. This is described batching messages as opposed to priming. It is suitable if message traffic is ‘bursty’ (not uniformly distributed in time). It evades keeping a process allocated when there is no work for it to do.
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