Package ch.cern.dirq
Interface Queue
- All Known Implementing Classes:
QueueNull
,QueueSimple
Queue - object oriented interface to a directory based queue.
This module allows multiple concurrent readers and writers to interact with the same queue.
Different implementations are available so readers and writers can be written in different programming languages:
There is no knowledge of priority within a queue. If multiple priorities are needed, multiple queues should be used.
A queue is a "best effort" FIFO (First In - First Out) collection of elements.
It is very hard to guarantee pure FIFO behavior with multiple writers using the same queue. Consider for instance:
For simplicity, this implementation provides only "best effort" FIFO, i.e. there is a very high probability that elements are processed in FIFO order but this is not guaranteed. This is achieved by using a high-resolution timer and having elements sorted by the time their final directory gets created.
In order to support multiple reader processes interacting with the same queue, advisory locking is used. Processes should first lock an element before working with it. In fact, the get() and remove() methods report a fatal error if they are called on unlocked elements.
If the process that created the lock dies without unlocking the element, we end up with a staled lock. The purge() method can be used to remove these staled locks.
An element can basically be in only one of two states: locked or unlocked.
A newly created element is unlocked as a writer usually does not need to do anything more with it.
Iterators return all the elements, regardless of their states.
There is no method to get an element state as this information is usually useless since it may change at any time. Instead, programs should directly try to lock elements to make sure they are indeed locked.
The elements are stored as plain files and directories. The filesystem security features (owner, group, permissions, ACLs...) should be used to adequately protect the data.
By default, the process' umask is respected. See the class constructor documentation if you want an other behavior.
If multiple readers and writers with different uids are expected, the easiest solution is to have all the files and directories inside the toplevel directory world-writable (i.e. umask=0). Then, the permissions of the toplevel directory itself (e.g. group-writable) are enough to control who can access the queue.
Description
The goal of this module is to offer a queue system using the underlying filesystem for storage, security and to prevent race * conditions via atomic operations. It focuses on simplicity, robustness and scalability.This module allows multiple concurrent readers and writers to interact with the same queue.
Different implementations are available so readers and writers can be written in different programming languages:
- A Perl implementation of the same algorithms is available at http://search.cpan.org/dist/Directory-Queue/
- A Python implementation of the same algorithms is available at https://github.com/cern-mig/python-dirq
There is no knowledge of priority within a queue. If multiple priorities are needed, multiple queues should be used.
Terminology
An element is something that contains one or more pieces of data. WithQueueSimple
queues, an element can only contain one
binary string.
A queue is a "best effort" FIFO (First In - First Out) collection of elements.
It is very hard to guarantee pure FIFO behavior with multiple writers using the same queue. Consider for instance:
- Writer1: calls the add() method
- Writer2: calls the add() method
- Writer2: the add() method returns
- Writer1: the add() method returns
For simplicity, this implementation provides only "best effort" FIFO, i.e. there is a very high probability that elements are processed in FIFO order but this is not guaranteed. This is achieved by using a high-resolution timer and having elements sorted by the time their final directory gets created.
Locking
Adding an element is not a problem because the add() method is atomic.In order to support multiple reader processes interacting with the same queue, advisory locking is used. Processes should first lock an element before working with it. In fact, the get() and remove() methods report a fatal error if they are called on unlocked elements.
If the process that created the lock dies without unlocking the element, we end up with a staled lock. The purge() method can be used to remove these staled locks.
An element can basically be in only one of two states: locked or unlocked.
A newly created element is unlocked as a writer usually does not need to do anything more with it.
Iterators return all the elements, regardless of their states.
There is no method to get an element state as this information is usually useless since it may change at any time. Instead, programs should directly try to lock elements to make sure they are indeed locked.
Security
There are no specific security mechanisms in this module.The elements are stored as plain files and directories. The filesystem security features (owner, group, permissions, ACLs...) should be used to adequately protect the data.
By default, the process' umask is respected. See the class constructor documentation if you want an other behavior.
If multiple readers and writers with different uids are expected, the easiest solution is to have all the files and directories inside the toplevel directory world-writable (i.e. umask=0). Then, the permissions of the toplevel directory itself (e.g. group-writable) are enough to control who can access the queue.
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Method Summary
Modifier and TypeMethodDescriptionadd
(byte[] data) Add byte array data to the queue.Add String data to the queue.Add the given file (identified by its path) to the queue and return the corresponding element name, the file must be on the same filesystem and will be moved to the queue.int
count()
Return the number of elements in the queue.Get the given locked element as String data.byte[]
getAsByteArray
(String name) Get the given locked element as byte array data.getId()
Return a unique identifier for the queue.Get the path of the given locked element.Return the path of the queue.boolean
Lock an element in permissive mode.boolean
Lock an element.void
purge()
Purge the queue by removing unused intermediate directories, removing too old temporary elements and unlocking too old locked elements (aka staled locks); note: this can take a long time on queues with many elements.void
purge
(int maxLock) Purge the queue by removing unused intermediate directories, removing too old temporary elements and unlocking too old locked elements (aka staled locks); note: this can take a long time on queues with many elements.void
purge
(int maxLock, int maxTemp) Purge the queue by removing unused intermediate directories, removing too old temporary elements and unlocking too old locked elements (aka staled locks); note: this can take a long time on queues with many elements.void
Remove a locked element from the queue.boolean
Unlock an element in non-permissive mode.boolean
Unlock an element.Methods inherited from interface java.lang.Iterable
forEach, iterator, spliterator
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Method Details
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getQueuePath
String getQueuePath()Return the path of the queue.- Returns:
- queue path
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getId
String getId()Return a unique identifier for the queue.- Returns:
- unique queue identifier
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add
Add String data to the queue.- Parameters:
data
- data to be added- Returns:
- element name (as directory_name/file_name)
- Throws:
IOException
- if any file operation fails
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add
Add byte array data to the queue.- Parameters:
data
- data to be added- Returns:
- element name (as directory_name/file_name)
- Throws:
IOException
- if any file operation fails
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addPath
Add the given file (identified by its path) to the queue and return the corresponding element name, the file must be on the same filesystem and will be moved to the queue.- Parameters:
path
- path of the file to be added- Returns:
- element name (as directory_name/file_name)
- Throws:
IOException
- if any file operation fails
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get
Get the given locked element as String data.- Parameters:
name
- name of the element to be retrieved- Returns:
- data associated with the given element
- Throws:
IOException
- if any file operation fails
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getAsByteArray
Get the given locked element as byte array data.- Parameters:
name
- name of the element to be retrieved- Returns:
- data associated with the given element
- Throws:
IOException
- if any file operation fails
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getPath
Get the path of the given locked element.
This pathFile can be read but not removed, you must use the remove() method for this purpose.- Parameters:
name
- name of the element- Returns:
- path of the element
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lock
Lock an element in permissive mode.- Parameters:
name
- name of the element to be locked- Returns:
true
on success,false
if the element could not be locked- Throws:
IOException
- if any file operation fails
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lock
Lock an element.- Parameters:
name
- name of the element to be lockedpermissive
- work in permissive mode- Returns:
true
on success,false
if the element could not be locked- Throws:
IOException
- if any file operation fails
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unlock
Unlock an element in non-permissive mode.- Parameters:
name
- name of the element to be unlocked- Returns:
true
on success,false
if the element could not be unlocked- Throws:
IOException
- if any file operation fails
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unlock
Unlock an element.- Parameters:
name
- name of the element to be unlockedpermissive
- work in permissive mode- Returns:
true
on success,false
if the element could not be unlocked- Throws:
IOException
- if any file operation fails
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remove
Remove a locked element from the queue.- Parameters:
name
- name of the element to be removed- Throws:
IOException
- if any file operation fails
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count
int count()Return the number of elements in the queue.
Locked elements are counted but temporary elements are not.- Returns:
- number of elements in the queue
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purge
Purge the queue by removing unused intermediate directories, removing too old temporary elements and unlocking too old locked elements (aka staled locks); note: this can take a long time on queues with many elements.
It uses default value for maxTemp and maxLock- Throws:
IOException
- if any file operation fails
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purge
Purge the queue by removing unused intermediate directories, removing too old temporary elements and unlocking too old locked elements (aka staled locks); note: this can take a long time on queues with many elements.- Parameters:
maxLock
- maximum time for a locked element (in seconds); if set to 0, locked elements will not be unlocked; if set to null, the object's default value will be used- Throws:
IOException
- if any file operation fails
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purge
Purge the queue by removing unused intermediate directories, removing too old temporary elements and unlocking too old locked elements (aka staled locks); note: this can take a long time on queues with many elements.- Parameters:
maxLock
- maximum time for a locked element (in seconds); if set to 0, locked elements will not be unlocked; if set to null, the object's default value will be usedmaxTemp
- maximum time for a temporary element (in seconds); if set to 0, temporary elements will not be removed if set to null, the object's default value will be used- Throws:
IOException
- if any file operation fails
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