Here is an array-based queue implementation. Our choice here is to use an array of size \(n+1\). Which of these solutions to adopt is purely a matter of the One obvious solution is to keep an explicit count of the number ofĮlements in the queue, or at least a Boolean variable that indicatesĪnother solution is to make the array be of size \(n+1\),Īnd only allow \(n\) elements to be stored. We must seek some other way to distinguish full from empty queues. In similar manner, we can be sure that two of the \(n+1\) statesĪre indistinguishable by the \(n\) relative values of front The Pigeonhole Principle states that, given \(n\) pigeonholesĪnd \(n+1\) pigeons, when all of the pigeons go into the holes weĬan be sure that at least one hole contains more than one pigeon. This is an example of the Pigeonhole Principle. In the absense of a discard () method (or similar), here is what I have tried: BytesMessage msg (BytesMessage)queueReceiver.receiveNoWait () bytesRead msg.readBytes (msgBytes, 1024) // just read 1024 bytes queueReceiver.close () The above code is no quicker than retrieving the entire 4MB message from the queue (by reading into a larger. We invent a special case for, say, empty queues. However, there are only \(n\) possible values for rear unless Values for rear are needed to distinguish among the \(n+1\) You can download the tools from our website.If the value of front is fixed, then \(n+1\) different If you don't have any of these tools, but would like to try any or all of them out, please contact and a 1-month trial licence will be sent to you with no obligation to buy. You can read more about this in MQSCX: Clearly a better way. Then MQSCX will first attempt the CLEAR QLOCAL command and if that fails with "object in use" then it will switch to getting the messages off the queue instead. Instead you can issue the following command in MQSCX:- =clear qlocal(Q1) The CLEAR QLOCAL command is problematic because it can fail when someone else has the queue open, even if it is not open in a way that stops you getting messages. You can of course issue the MQSC CLEAR QLOCAL(q-name) command from MQSCX. Just fill in the holding queue name, and then press the Move All, rather than Delete All, button, or simply Drag and Drop the messages onto the new queue. MQEdit makes it simple to Move messages too. Consider that it might be safer to Move the message(s) to a separate holding queue, just in case the message proves to be important after all. Take care when deleting messages from a production queue manager. Then press the Delete All button, or select Delete All from the right-mouse button context menu and all the messages will be deleted. In MQEdit you can browse a list of messages on a queue, then in the context menu of that dialog choose Message Operations → Apply to all Messages. We would recommend using QLOAD to actually move the messages to an output file, in case you change your mind about deleting them! qload -m MQG1 -I Q1 -f c:\temp\Q1deletedmsgs.qld Here is an example which destructively gets ( -I) from Q1 and discards them ( -f null), aka sends them to the null file destination:- qload -m MQG1 -I Q1 -f null This can work even when the queue is in use (unless it is exclusively in use of course) and the CLEAR QLOCAL command can’t be used. You can use QLOAD to quickly destructively get off all the messages on a queue, to ensure it is empty before you begin using it for something else. This is simple to achieve using a command like the following: q -m MQG1 -I Q1 -o HOLDING.Q Using QLOAD Consider that it might be safer to Move the messages to a separate holding queue, just in case they prove to be important after all. Here is an example which destructively gets ( -I) from Q1 and uses a zero-length buffer and accepts that the messages will be truncated ( -=t0):- q -m MQG1 -I Q1 -=t0 You can use the Q program to quickly destructively get off all the messages on a queue, to ensure it is empty before you begin using it for something else. Remember that this command can fail if the queue is open by another application at the time. Delete all messages using the Clear command in MO71
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