All real queues are limited in size, namely, they have a certain maximum possible size. If you go beyond these sizes, poorly tested systems can unexpectedly fail, so it is worthwhile to test the maximum possible queue size. If there is a queue, then there must be a queue discipline (ordering rule) specifically, a rule that determines in which order the transactions will come from the queue, to get processed. Below are some common disciplines (rules).
The FIFO rule is “first-in, first-out”, also called “First-Come-First-Served”. The transaction, which was the first to arrive in the queue, will be processed first. This very simple rule is very common. But imagine what would have happened if Vision had insisted that all the completed Forms C return to it in the same order in which they appeared to get processed.
The LIFO rule is “last-in, first-out”, also called LCFS “Last-Come-First-Served”.
The transaction, which was the last to arrive in the queue, will be processed first. A similar queue is a processing stack in which incoming transactions are placed on the top of the stack and are removed from the top.
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Package. If certain conditions occur, such as a certain number of transactions in the queue, or at the specified time all the transactions in the queue are processed in batch mode. At the same time during processing new transactions are accumulated in a new queue.
Accidental maintenance. The service is performed randomly, possibly based on a probability distribution over a certain value in the transaction check record.
Priority Queue. Each transaction has its own priority. This priority can be fixed or depend on the properties of the transaction, for instance, on its age. Each transaction group with this priority is treated as a separate queue, and the queue with the highest priority is processed first. Inside a separate transaction group with this priority, the service can be implemented on the basis of the FIFO, LIFO, etc. principle.
Multiple processing. A single server (single-server queue) or multiple servers (multi-server queue) can be used to process the queue. In addition to the rule for ordering a particular queue, there may be a rule for selecting a server. For example, in a supermarket you can choose which of the checkout lines to stand in. In the ticket office of the airport, in the bank, at the post office, you should stand in the main queue and wait for service until the employee of the institution says “Next!”