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NAME
pyxs - pyxs Documentation
.---. .-..-..-.,-. .--.
: .; `: :; :`. .'`._-.'
: ._.'`._. ;:_,._;`.__.'
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:_; `._.' 0.4.2-dev
-- XenStore access the Python way!
It's a pure Python XenStore client implementation, which covers all of the libxs features
and adds some nice Pythonic sugar on top. Here's a shortlist:
• pyxs supports both Python 2 and 3,
• works over a Unix socket or XenBus,
• has a clean and well-documented API,
• is written in easy to understand Python,
• can be used with gevent or eventlet.
If you have pip you can do the usual:
pip install --user pyxs
Otherwise, download the source from GitHub and run:
python setup.py install
Fedora users can install the package from the system repository:
dnf install python2-pyxs
dnf install python3-pyxs
RHEL/CentOS users can install the package from the EPEL repository:
yum install python2-pyxs
yum install python34-pyxs
Head over to our brief tutorial or, if you're feeling brave, dive right into the api; pyxs
also has a couple of examples in the examples directory.
TUTORIAL
Basics
Using pyxs is easy! The only class you need to import is Client. It provides a simple
straightforward API to XenStore content with a bit of Python's syntactic sugar here and
there.
Generally, if you just need to fetch or update some XenStore items you can do:
>>> from pyxs import Client
>>> with Client() as c:
... c[b"/local/domain/0/name"] = b"Ziggy"
... c[b"/local/domain/0/name"]
b'Ziggy'
Using Client without the with statement is possible, albeit, not recommended:
>>> c = Client()
>>> c.connect()
>>> c[b"/local/domain/0/name"] = b"It works!"
>>> c.close()
The reason for preferring a context manager is simple: you don't have to DIY. The context
manager will make sure that a started transaction was either rolled back or committed and
close the underlying XenStore connection afterwards.
Connections
pyxs supports two ways of communicating with XenStore:
• over a Unix socket with UnixSocketConnection;
• over XenBus with XenBusConnection.
Connection type is determined from the arguments passed to Client constructor. For
example, the following code creates a Client instance, operating over a Unix socket:
>>> Client(unix_socket_path="/var/run/xenstored/socket_ro")
Client(UnixSocketConnection('/var/run/xenstored/socket_ro'))
>>> Client()
Client(UnixSocketConnection('/var/run/xenstored/socket'))
Use xen_bus_path argument to initialize a Client with XenBusConnection:
>>> Client(xen_bus_path="/dev/xen/xenbus")
Client(XenBusConnection('/dev/xen/xenbus'))
Transactions
Transactions allow you to operate on an isolated copy of XenStore tree and merge your
changes back atomically on commit. Keep in mind, however, that changes made within a
transaction become available to other XenStore clients only if and when committed. Here's
an example:
>>> with Client() as c:
... c.transaction()
... c[b"/foo/bar"] = b"baz"
... c.commit() # !
... print(c[b"/foo/bar"])
b'baz'
The line with an exclamation mark is a bit careless, because it ignores the fact that
committing a transaction might fail. A more robust way to commit a transaction is by using
a loop:
>>> with Client() as c:
... success = False
... while not success:
... c.transaction()
... c[b"/foo/bar"] = b"baz"
... success = c.commit()
You can also abort the current transaction by calling rollback().
Events
When a new path is created or an existing path is modified, XenStore fires an event,
notifying all watching clients that a change has been made. pyxs implements watching via
the Monitor class. To watch a path create a monitor monitor() and call watch() with a path
you want to watch and a unique token. Right after that the monitor will start to
accumulate incoming events. You can iterate over them via wait():
>>> with Client() as c:
... m = c.monitor()
... m.watch(b"/foo/bar", b"a unique token")
... next(m.wait())
Event(b"/foo/bar", b"a unique token")
XenStore has a notion of special paths, which start with @ and are reserved for special
occasions:
┌─────────────────┬──────────────────────────────────┐
│Path │ Description │
├─────────────────┼──────────────────────────────────┤
│@introduceDomain │ Fired when a new domain is │
│ │ introduced to XenStore -- you │
│ │ can also introduce domains │
│ │ yourself with a │
│ │ introduce_domain() call, but in │
│ │ most of the cases, xenstored │
│ │ will do that for you. │
├─────────────────┼──────────────────────────────────┤
│@releaseDomain │ Fired when XenStore is no longer │
│ │ communicating with a domain, see │
│ │ release_domain(). │
└─────────────────┴──────────────────────────────────┘
Events for both special and ordinary paths are simple two element tuples, where the first
element is always event target -- a path which triggered the event and second is a token
passed to watch(). A rather unfortunate consequence of this is that you can't get domid of
the domain, which triggered @introduceDomain or @releaseDomain from the received event.
Compatibility API
pyxs also provides a compatibility interface, which mimics that of xen.lowlevel.xs --- so
you don't have to change anything in the code to switch to pyxs:
>>> from pyxs import xs
>>> handle = xs()
>>> handle.read("0", b"/local/domain/0/name")
b'Domain-0'
>>> handle.close()
API REFERENCE
Client and Monitor
class pyxs.client.Client(unix_socket_path=None, xen_bus_path=None, router=None)
XenStore client.
Parameters
• unix_socket_path (str) -- path to XenStore Unix domain socket.
• xen_bus_path (str) -- path to XenBus device.
If unix_socket_path is given or Client was created with no arguments, XenStore is
accessed via UnixSocketConnection; otherwise, XenBusConnection is used.
Each client has a Router thread running in the background. The goal of the router
is to multiplex requests from different transaction through a single XenStore
connection.
Changed in version 0.4.0: The constructor no longer accepts connection argument. If
you wan't to force the use of a specific connection class, wrap it in a Router:
from pyxs import Router, Client
from pyxs.connection import XenBusConnection
router = Router(XenBusConnection())
with Client(router=router) as c:
do_something(c)
WARNING:
Always finalize the client either explicitly by calling close() or implicitly
via a context manager to prevent data loss.
SEE ALSO:
Xenstore protocol specification for a description of the protocol, implemented
by Client.
connect()
Connects to the XenStore daemon.
Raises pyxs.exceptions.ConnectionError -- if the connection could not be
opened. This could happen either because XenStore is not running on
the machine or due to the lack of permissions.
WARNING:
This method is unsafe. Please use client as a context manager to ensure
it is properly finalized.
close()
Finalizes the client.
WARNING:
This method is unsafe. Please use client as a context manager to ensure
it is properly finalized.
read(path, default=None)
Reads data from a given path.
Parameters
• path (bytes) -- a path to read from.
• default (bytes) -- default value, to be used if path doesn't exist.
write(path, value)
Writes data to a given path.
Parameters
• value (bytes) -- data to write.
• path (bytes) -- a path to write to.
mkdir(path)
Ensures that a given path exists, by creating it and any missing parents
with empty values. If path or any parent already exist, its value is left
unchanged.
Parameters
path (bytes) -- path to directory to create.
delete(path)
Ensures that a given does not exist, by deleting it and all of its children.
It is not an error if path doesn't exist, but it is an error if path's
immediate parent does not exist either.
Parameters
path (bytes) -- path to directory to remove.
list(path)
Returns a list of names of the immediate children of path.
Parameters
path (bytes) -- path to list.
exists(path)
Checks if a given path exists.
Parameters
path (bytes) -- path to check.
get_perms(path)
Returns a list of permissions for a given path, see InvalidPermission for
details on permission format.
Parameters
path (bytes) -- path to get permissions for.
set_perms(path, perms)
Sets a access permissions for a given path, see InvalidPermission for
details on permission format.
Parameters
• path (bytes) -- path to set permissions for.
• perms (list) -- a list of permissions to set.
walk(top, topdown=True)
Walk XenStore, yielding 3-tuples (path, value, children) for each node in
the tree, rooted at node top.
Parameters
• top (bytes) -- node to start from.
• topdown (bool) -- see os.walk() for details.
get_domain_path(domid)
Returns the domain's base path, as used for relative requests: e.g.
b"/local/domain/<domid>". If a given domid doesn't exists the answer is
undefined.
Parameters
domid (int) -- domain to get base path for.
is_domain_introduced(domid)
Returns True if xenstored is in communication with the domain; that is when
INTRODUCE for the domain has not yet been followed by domain destruction or
explicit RELEASE; and False otherwise.
Parameters
domid (int) -- domain to check status for.
introduce_domain(domid, mfn, eventchn)
Tells xenstored to communicate with this domain.
Parameters
• domid (int) -- a real domain id, (0 is forbidden).
• mfn (int) -- address of xenstore page in domid.
• eventchn (int) -- an unbound event chanel in domid.
release_domain(domid)
Manually requests xenstored to disconnect from the domain.
Parameters
domid (int) -- domain to disconnect.
NOTE:
xenstored will in any case detect domain destruction and disconnect by
itself.
resume_domain(domid)
Tells xenstored to clear its shutdown flag for a domain. This ensures that a
subsequent shutdown will fire the appropriate watches.
Parameters
domid (int) -- domain to resume.
set_target(domid, target)
Tells xenstored that a domain is targetting another one, so it should let it
tinker with it. This grants domain domid full access to paths owned by
target. Domain domid also inherits all permissions granted to target on all
other paths.
Parameters
• domid (int) -- domain to set target for.
• target (int) -- target domain (yours truly, Captain).
transaction()
Starts a new transaction.
Returns int
transaction handle.
Raises pyxs.exceptions.PyXSError -- with errno.EALREADY if this client is
already in a transaction.
WARNING:
Currently xenstored has a bug that after 2**32 transactions it will
allocate id 0 for an actual transaction.
rollback()
Rolls back a transaction currently in progress.
commit()
Commits a transaction currently in progress.
Returns bool
False if commit failed because of the intervening writes and True
otherwise. In any case transaction is invalidated. The caller is
responsible for starting a new transaction, repeating all of the
operations a re-committing.
monitor()
Returns a new Monitor instance, which is currently the only way of doing
PUBSUB.
The monitor shares the router with its parent client. Thus closing the
client invalidates the monitor. Closing the monitor, on the other hand, had
no effect on the router state.
NOTE:
Using monitor() over XenBusConnection is currently unsupported, because
XenBus does not obey XenStore protocol specification. See xen-devel
discussion for details.
class pyxs.client.Monitor(client)
Monitor implements minimal PUBSUB functionality on top of XenStore.
>>> with Client() as c:
... m = c.monitor():
... m.watch("foo/bar")
... print(next(c.wait()))
Event(...)
Parameters
client (Client) -- a reference to the parent client.
NOTE:
When used as a context manager the monitor will try to unwatch all watched
paths.
property watched
A set of paths currently watched by the monitor.
close()
Finalizes the monitor by unwatching all watched paths.
watch(wpath, token)
Adds a watch.
Any alteration to the watched path generates an event. This includes path
creation, removal, contents change or permission change. An event can also
be triggered spuriously.
Changes made in transactions cause an event only if and when committed.
Parameters
• wpath (bytes) -- path to watch.
• token (bytes) -- watch token, returned in watch notification.
unwatch(wpath, token)
Removes a previously added watch.
Parameters
• wpath (bytes) -- path to unwatch.
• token (bytes) -- watch token, passed to watch().
wait(unwatched=False)
Yields events for all of the watched paths.
An event is a (path, token) pair, where the first element is event path,
i.e. the actual path that was modified, and the second -- a token, passed to
watch().
Parameters
unwatched (bool) -- if True wait() might yield spurious unwatched
packets, otherwise these are dropped. Defaults to False.
pyxs.monitor(*args, **kwargs)
A simple shortcut for creating Monitor instances. All arguments are forwared to
Client constructor.
Exceptions
class pyxs.exceptions.PyXSError
Base class for all pyxs exceptions.
class pyxs.exceptions.InvalidOperation
Exception raised when Packet is passed an operation, which isn't listed in Op.
Parameters
operation (int) -- invalid operation value.
class pyxs.exceptions.InvalidPayload
Exception raised when Packet is initialized with payload, which exceeds 4096 bytes
restriction or contains a trailing NULL.
Parameters
operation (bytes) -- invalid payload value.
class pyxs.exceptions.InvalidPath
Exception raised when a path proccessed by a command doesn't match the following
constraints:
• its length should not exceed 3072 or 2048 for absolute and relative path
respectively.
• it should only consist of ASCII alphanumerics and the four punctuation characters
-/_@ -- hyphen, slash, underscore and atsign.
• it shouldn't have a trailing /, except for the root path.
Parameters
path (bytes) -- invalid path value.
class pyxs.exceptions.InvalidPermission
Exception raised for permission which don't match the following format:
w<domid> write only
r<domid> read only
b<domid> both read and write
n<domid> no access
Parameters
perm (bytes) -- invalid permission value.
class pyxs.exceptions.ConnectionError
Exception raised for failures during socket operations.
class pyxs.exceptions.UnexpectedPacket
Exception raised when received packet header doesn't match the header of the packet
sent, for example if outgoing packet has op = Op.READ the incoming packet is
expected to have op = Op.READ as well.
Internals
class pyxs.client.Router(connection)
Router.
The goal of the router is to multiplex XenStore connection between multiple clients
and monitors.
Parameters
FileDescriptorConnection (connection) -- owned by the router. The connection
is open when the router is started and remains open until the router is
terminated.
NOTE:
Python lacks API for interrupting a thread from another thread. This means that
when a router cannot be stopped when it is blocked in select.select() or wait().
The following two "hacks" are used to ensure prompt termination.
1. A router is equipped with a socket.socketpair(). The reader-end of the pair
is selected in the mainloop alongside the XenStore connection, while the
writer-end is used in terminate() to force-stop the mainloop.
2. All operations with threading.Condition variables user a 1 second timeout.
This "hack" is only relevant for Python prior to 3.2 which didn't allow one
to interrupt lock acquisitions. See issue8844 on CPython issue tracker for
details. On Python 3.2 and later no timeout is used.
property is_connected
Checks if the underlying connection is active.
subscribe(token, monitor)
Subscribes a monitor from events with a given token.
unsubscribe(token, monitor)
Unsubscribes a monitor to events with a given token.
send(packet)
Sends a packet to XenStore.
Returns RVar
a reference to the XenStore response.
start()
Starts the router thread.
Does nothing if the router is already started.
terminate()
Terminates the router.
After termination the router can no longer send or receive packets. Does
nothing if the router was already terminated.
class pyxs.connection.XenBusConnection(path=None)
XenStore connection through XenBus.
Parameters
path (str) -- path to XenBus. A predefined OS-specific constant is used, if
a value isn't provided explicitly.
class pyxs.connection.UnixSocketConnection(path=None)
XenStore connection through Unix domain socket.
Parameters
path (str) -- path to XenStore unix domain socket, if not provided
explicitly is restored from process environment -- similar to what libxs
does.
class pyxs._internal.Packet(op, payload, rq_id, tx_id=None)
A message to or from XenStore.
Parameters
• op (int) -- an item from Op, representing operation, performed by this
packet.
• payload (bytes) -- packet payload, should be a valid ASCII-string with
characters between [0x20; 0x7f].
• rq_id (int) -- request id -- hopefully a unique identifier for this
packet, XenStore simply echoes this value back in response.
• tx_id (int) -- transaction id, defaults to 0 , which means no transaction
is running.
Changed in version 0.4.0: rq_id no longer defaults to 0 and should be provided
explicitly.
pyxs._internal.Op = Operations(DEBUG=0, DIRECTORY=1, READ=2, GET_PERMS=3, WATCH=4,
UNWATCH=5, TRANSACTION_START=6, TRANSACTION_END=7, INTRODUCE=8, RELEASE=9,
GET_DOMAIN_PATH=10, WRITE=11, MKDIR=12, RM=13, SET_PERMS=14, WATCH_EVENT=15, ERROR=16,
IS_DOMAIN_INTRODUCED=17, RESUME=18, SET_TARGET=19, RESTRICT=128)
Operations supported by XenStore.
CONTRIBUTING
Submitting a bug report
In case you experience issues using pyxs, do not hesitate to report it to the Bug Tracker
on GitHub.
Setting up development environment
Writing a XenStore client library without having access to a running XenStore instance can
be troublesome. Luckily, there is a way to setup a development using VirtualBox.
1. Create a VM running Ubuntu 14.04 or later.
2. Install Xen hypervisor: sudo apt-get install xen-hypervisor-4.4-amd64.
3. Configure VM for SSH access.
4. Done! You can now rsync your changes to the VM and run the tests.
Running the tests
Only root is allowed to access XenStore, so the tests require sudo:
$ sudo python setup.py test
pyxs strives to work across a range of Python versions. Use tox to run the tests on all
supported versions:
$ cat tox.ini
[tox]
envlist = py26,py27,py34,py35,pypy
[testenv]
commands = python setup.py test
$ sudo tox
Style guide
pyxs follows Pocoo style guide. Please read it before you start implementing your changes.
PYXS CHANGELOG
Here you can see the full list of changes between each pyxs release.
Version 0.4.2-dev
• Allowed values to be empty b"". Thanks to Stephen Czetty. See PR #13 on GitHub.
Version 0.4.1
Bugfix release, released on May 11th, 2016
• Fixed a bug in XenBusConnection.create_transport which failed on attribute lookup. See
PR #7 on GitHub.
Version 0.4.0
Released on March 6th, 2016
• Fixed a bug in Client.set_permissions which coerced permission lists (e.g. ["b0"]) to
repr-strings prior to validation.
• The API is now based around bytes, which means that all methods which used to accept str
(or text) now require bytes. XenStore paths and values are specified to be 7-bit ASCII,
thus it makes little sense to allow any Unicode string as input and then validate if it
matches the spec.
• Removed transaction argument from Client constructor. The user is advised to use the
corresponding methods explicitly.
• Removed connection argument from Client constructor. The user should now wrap it in a
Router.
• Renamed some of the Client methods to more human-readable names:
┌──────────────────┬─────────────────────┐
│Old name │ New name │
├──────────────────┼─────────────────────┤
│ls │ list │
├──────────────────┼─────────────────────┤
│rm │ delete │
├──────────────────┼─────────────────────┤
│get_permissions │ get_perms │
├──────────────────┼─────────────────────┤
│set_permissions │ set_perms │
├──────────────────┼─────────────────────┤
│transaction_start │ transaction │
├──────────────────┼─────────────────────┤
│transaction_end │ commit and rollback │
└──────────────────┴─────────────────────┘
• Removed Client.transaction context manager, because it didn't provide a way to handle
possible commit failure.
• Added Client.exists for one-line path existence checks. See PR #6 on GitHub. Thanks to
Sandeep Murthy.
• Removed implicit reconnection logic from FileDescriptorConnection. The user is now
expected to connect manually.
• Changed XenBusConnection to prefer /dev/xen/xenbus on Linux due to a possible deadlock
in XenBus backend.
• Changed UnixSocketConnection to use socket.socket instead of the corresponding file
descriptor.
• Disallowed calling Client.monitor over XenBusConnection. See
http://lists.xen.org/archives/html/xen-devel/2016-02/msg03816 for details.
Version 0.3.1
Released on November 29th 2012
• Added default argument to Client.read(), which acts similar to dict.get().
• Fixed a lot of minor quirks so pyxs can be Debianized.
Version 0.3
Released on September 12th 2011
• Moved all PUBSUB functionality into a separate Monitor class, which uses a separate
connection. That way, we'll never have to worry about mixing incoming XenStore events
and command replies.
• Fixed a couple of nasty bugs in concurrent use of Client.wait() with other commands (see
above).
Version 0.2
Released on August 18th 2011
• Completely refactored validation -- no more @spec magic, everything is checked
explicitly inside Client.execute_command().
• Added a compatibility interface, which mimics xen.lowlevel.xs behaviour, using pyxs as a
backend, see pyxs/_compat.py.
• Restricted SET_TARGET, INTRODUCE and RELEASE operations to Dom0 only --
/proc/xen/capabilities is used to check domain role.
• Fixed a bug in Client.wait() -- queued watch events weren't wrapped in
pyxs._internal.Event class, unlike the received ones.
• Added Client.walk() method for walking XenStore tree -- similar to os.walk()
Version 0.1
Initial release, released on July 16th 2011
• Added a complete implementation of XenStore protocol, including transactions and path
watching, see pyxs.Client for details.
• Added generic validation helper -- @spec, which forces arguments to match the scheme
from the wire protocol specification.
• Added two connection backends -- XenBusConnection for connecting from DomU through a
block device and UnixSocketConnection, communicating with XenStore via a Unix domain
socket.
AUTHOR
Sergei Lebedev, Fedor Gogolev
COPYRIGHT
2011-2022, Selectel