Package ch.cern.dirq

Interface Queue

All Superinterfaces:
Iterable<String>
All Known Implementing Classes:
QueueNull, QueueSimple

public interface Queue extends Iterable<String>
Queue - object oriented interface to a directory based 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:
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. With QueueSimple 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
Who should be first in the queue, Writer1 or Writer2?
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.
  • Method Summary

    Modifier and Type
    Method
    Description
    add(byte[] data)
    Add byte array data to the queue.
    add(String data)
    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
    Return the number of elements in the queue.
    get(String name)
    Get the given locked element as String data.
    byte[]
    Get the given locked element as byte array data.
    Return a unique identifier for the queue.
    Get the path of the given locked element.
    Return the path of the queue.
    boolean
    lock(String name)
    Lock an element in permissive mode.
    boolean
    lock(String name, boolean permissive)
    Lock an element.
    void
    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(String name)
    Remove a locked element from the queue.
    boolean
    unlock(String name)
    Unlock an element in non-permissive mode.
    boolean
    unlock(String name, boolean permissive)
    Unlock an element.

    Methods inherited from interface java.lang.Iterable

    forEach, iterator, spliterator
  • Method Details

    • getQueuePath

      String getQueuePath()
      Return the path of the queue.
      Returns:
      queue path
    • getId

      String getId()
      Return a unique identifier for the queue.
      Returns:
      unique queue identifier
    • add

      String add(String data) throws IOException
      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
    • add

      String add(byte[] data) throws IOException
      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
    • addPath

      String addPath(String path) throws IOException
      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
    • get

      String get(String name) throws IOException
      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
    • getAsByteArray

      byte[] getAsByteArray(String name) throws IOException
      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
    • getPath

      String getPath(String name)
      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
    • lock

      boolean lock(String name) throws IOException
      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
    • lock

      boolean lock(String name, boolean permissive) throws IOException
      Lock an element.
      Parameters:
      name - name of the element to be locked
      permissive - work in permissive mode
      Returns:
      true on success, false if the element could not be locked
      Throws:
      IOException - if any file operation fails
    • unlock

      boolean unlock(String name) throws IOException
      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
    • unlock

      boolean unlock(String name, boolean permissive) throws IOException
      Unlock an element.
      Parameters:
      name - name of the element to be unlocked
      permissive - work in permissive mode
      Returns:
      true on success, false if the element could not be unlocked
      Throws:
      IOException - if any file operation fails
    • remove

      void remove(String name) throws IOException
      Remove a locked element from the queue.
      Parameters:
      name - name of the element to be removed
      Throws:
      IOException - if any file operation fails
    • 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
    • purge

      void purge() throws IOException
      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
    • purge

      void purge(int maxLock) throws IOException
      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
    • purge

      void purge(int maxLock, int maxTemp) throws IOException
      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
      maxTemp - 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