Directory based queue.
A port of Perl module Directory::Queue http://search.cpan.org/dist/Directory-Queue/ The documentation from Directory::Queue module was adapted for Python.
The goal of this module is to offer a simple 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.
Classes:
dirq.queue.Queue - directory based queue.
Usage:
from dirq.queue import Queue
# simple schema:
# - there must be a "body" which is a string
# - there can be a "header" which is a table/dictionary
schema = {"body": "string", "header": "table?"}
queuedir = "/tmp/test"
# sample producer
dirq = Queue(queuedir, schema=schema)
import os
for count in range(1,101):
name = dirq.add({"body" : "element %i"%count,
"header": dict(os.environ)})
print("# added element %i as %s" % (count, name))
# sample consumer
dirq = Queue(queuedir, schema=schema)
name = dirq.first()
while name:
if not dirq.lock(name):
name = dirq.next()
continue
print("# reading element %s" % name)
data = dirq.get(name)
# one can use data['body'] and data['header'] here...
# one could use dirq.unlock(name) to only browse the queue...
dirq.remove(name)
name = dirq.next()
An element is something that contains one or more pieces of data. A simple string may be an element but more complex schemas can also be used, see the Schema section for more information.
A queue is a “best effort FIFO” 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 time function and having elements sorted by the time the element’s final directory gets created.
Adding an element is not a problem because the add() method is atomic.
In order to support multiple 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 raise an exception if they are called on unlocked elements.
If the process that created the lock dies without unlocking the ele- ment, 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 the element once dropped in the queue.
Iterators return all the elements, regardless of their states.
There is no method to get an element state as this information is usu- ally useless since it may change at any time. Instead, programs should directly try to lock elements to make sure they are indeed locked.
For the signature of the Queue constructor see documentation to the respective __init__() method.
The schema defines how user supplied data is stored in the queue. It is only required by the add() and get() methods.
The schema must be a dictionary containing key/value pairs.
The key must contain only alphanumerical characters. It identifies the piece of data and will be used as file name when storing the data inside the element directory.
The value represents the type of the given piece of data. It can be:
- binary
- the data is a sequence of binary bytes, it will be stored directly in a plain file with no further encoding
- string
- the data is a text string (i.e. a sequence of characters), it will be UTF-8 encoded
- table
- the data is a reference to a hash of text strings, it will be seri- alized and UTF-8 encoded before being stored in a file
By default, all pieces of data are mandatory. If you append a question mark to the type, this piece of data will be marked as optional. See the comments in the Usage section for more information.
To comply with Directory::Queue implementation it is allowed to append ‘*’ (asterisk) to data type specification, which in Directory::Queue means switching to working with element references in add() and get() operations. This is irrelevant for the Python implementation.
All the directories holding the elements and all the files holding the data pieces are located under the queue toplevel directory. This direc- tory can contain:
- temporary
- the directory holding temporary elements, i.e. the elements being added
- obsolete
- the directory holding obsolete elements, i.e. the elements being removed
- NNNNNNNN
- an intermediate directory holding elements; NNNNNNNN is an 8-digits long hexadecimal number
In any of the above directories, an element is stored as a single directory with a 14-digits long hexadecimal name SSSSSSSSMMMMMR where:
- SSSSSSSS
- represents the number of seconds since the Epoch
- MMMMM
- represents the microsecond part of the time since the Epoch
R is a random digit used to reduce name collisions
Finally, inside an element directory, the different pieces of data are stored into different files, named according to the schema. A locked element contains in addition a directory named “locked”.
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.
dirq.queue.QueueSet - interface to a set of Queue objects
Usage:
from dirq.queue import Queue, QueueSet
dq1 = Queue("/tmp/q1")
dq2 = Queue("/tmp/q2")
dqset = QueueSet(dq1, dq2)
# dqs = [dq1, dq2]
# dqset = QueueSet(dqs)
(dq, elt) = dqset.first()
while dq:
# you can now process the element elt of queue dq...
(dq, elt) = dqset.next()
This class can be used to put different queues into a set and browse them as one queue. The elements from all queues are merged together and sorted independently from the queue they belong to.
For the signature of the QueueSet constructor see documentation to the respective dirq.queue.QueueSet.__init__() method.
Konstantin Skaburskas <konstantin.skaburskas@gmail.com>
ASL 2.0
Copyright (C) CERN 2011-2013
Directory based queue.
Add a new element to the queue and return its name. Arguments:
data - element as a dictionary (should conform to the schema)
Raise:
QueueError - problem with schema definition or data OSError - problem putting element on disk
Note:
the destination directory must _not_ be created beforehand as it would be seen as a valid (but empty) element directory by another process, we therefore use rename() from a temporary directory
Return the number of elements in the queue, regardless of their state.
Dequeue an element from the queue. Removes element from the queue. Performs operations: lock(name), get(name), remove(name)
Add a new element to the queue and return its name. Arguments:
data - element as a dictionary (should conform to the schema)
Raise:
QueueError - problem with schema definition or data OSError - problem putting element on disk
Note:
the destination directory must _not_ be created beforehand as it would be seen as a valid (but empty) element directory by another process, we therefore use rename() from a temporary directory
Get an element data from a locked element.
OSError - problems opening/closing file IOError - file read error
Get an element from the queue. Element will not be removed. Operations performed: lock(name), get(name), unlock(name)
Lock an element.
Return:
Note:
Purge the queue:
Remove locked element from the queue.
OSError - can’t rename/remove a file/directory
Unlock an element.
Return:
Note:
unlocking can fail:
- if the element has been unlocked by somebody else (ENOENT)
- if the element has been removed by somebody else (ENOENT)
if the optional second argument is true, it is not an error if the element cannot be unlocked (permissive mode), this is _not_ the default
Interface to elements on a set of directory based queues.
Add lists of queues to existing ones. Copies of the object instances are used.
Return the number of elements in the queue set, regardless of their state.
Return the first element in the queue set and cache information about the next ones.
Return iterator over element names on the set of queues.
Return (queue, next element) tuple from the queue set, only using cached information.
OSError - can’t list element directories
Remove a queue and its respective elements from in memory cache.
QueueError