alma.acs.util.stringqueue

Class EntriesQueue

java.lang.Object

alma.acs.util.stringqueue.EntriesQueue

public class EntriesQueue
extends java.lang.Object

The queue of entries.

This class has been introduced to avoid keeping in memory a never ending queue of QueueEntry and reduce the chance to face an out of memory at run-time.
This class is composed of two lists and a file. inMemoryQueue is the list of the entries to get. When this list contains few items then some more items are read from the file.
The other list, EntriesQueue, is a buffer where the entries are stored ready to be flushed on disk. This is done to write more entries at once reducing the I/O and increasing the performances.

Implementation note
QueueEntry items are read only with the get method and pushed with the put.

Adding entries:
If there is enough room in inMemoryQueue (i.e. inMemoryQueue.size()) then a new entry is stored directly in that list; otherwise it is added to cachedEntries ready to be written on file. If the size of cachedEntries is greater then PAGE_LEN, the size of a page, then a page is flushed on disk. Note that what is in the list, cacheEntries is added at the end of the file.

Getting entries:
The entry to get is always in inMemoryQueue. After getting an entry, it checks if the the size of the queue allows to get new entries from the file or from the cachedEntries. Note that the right order is first the file and then cachedEntries. In fact cachedEntries, contains the last received entries, packed to be transferred on a new page on disk while the first entries to push in the queue are on a page disk (if any).

Author:

acaproni

Field Summary

Fields

Modifier and Type	Field and Description
static int	MAX_QUEUE_LENGTH The max number of entries kept in memory.
static int	PAGE_LEN The number of QueueEntry to read/write from/to disk on each I/O
static int	THRESHOLD When in the LinkedBlockingQueue there are less entries then the THRESHOLD then the entries in the buffer are flushed in the queue

Constructor Summary

Constructors

Constructor and Description
EntriesQueue()

Method Summary

Modifier and Type	Method and Description
void	clear() Clear the queue and the file (if any)
QueueEntry	get() Get the next value from the queue.
boolean	isEmpty()
void	put(QueueEntry entry) Put an entry in Cache.
int	size() Return the number of cache entries waiting in queue

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

MAX_QUEUE_LENGTH

public static final int MAX_QUEUE_LENGTH

The max number of entries kept in memory.

See Also:

Constant Field Values

PAGE_LEN

public static final int PAGE_LEN

The number of QueueEntry to read/write from/to disk on each I/O

See Also:

Constant Field Values

THRESHOLD

public static final int THRESHOLD

When in the LinkedBlockingQueue there are less entries then the THRESHOLD then the entries in the buffer are flushed in the queue

See Also:

Constant Field Values

Constructor Detail

EntriesQueue

public EntriesQueue()

Method Detail

put

public void put(QueueEntry entry)
         throws java.io.IOException

Put an entry in Cache.

If the cache is full the entry is added to the buffer.

Parameters:

entry - The not null QueueEntry to add to the queue

Throws:

java.io.IOException - In case of I/O error while flushing the cache on disk

get

public QueueEntry get()
               throws java.io.IOException

Get the next value from the queue.

Returns:

The next item in the queue or null if the queue is empty

Throws:

java.io.IOException - In case of error during I/O

clear

public void clear()

Clear the queue and the file (if any)

size

public int size()

Return the number of cache entries waiting in queue

isEmpty

public boolean isEmpty()

Returns:

true if the queue is empty; false otherwise.