zc.zk 0.2.0

Installation

You can install this as you would any other distribution. Note, however, that you must also install the Python ZooKeeper binding provided with ZooKeeper. Because this binding is packaged a number of different ways, it isn’t listed as a distribution requirement.

An easy way to get the Python zookeeper binding is by installing zc-zookeeper-static, which is a self-contained statically built distribution.

Instantiating a ZooKeeper helper

To use the helper API, create a ZooKeeper instance:

>>> import zc.zk
>>> zk = zc.zk.ZooKeeper('zookeeper.example.com:2181')

The ZooKeeper constructor takes a ZooKeeper connection string, which is a comma-separated list of addresses of the form HOST:PORT. It defaults to '127.0.0.1:2181', which is convenient during development.

Register a server providing a service.

To register a server, use the register_server method, which takes a service path and the address a server is listing on:

>>> zk.register_server('/fooservice/providers', ('192.168.0.42', 8080))

register_server creates a read-only ephemeral ZooKeeper node as a child of the given service path. The name of the new node is (a string representation of) the given address. This allows clients to get the list of addresses by just getting the list of the names of children of the service path.

Ephemeral nodes have the useful property that they’re automatically removed when a ZooKeeper session is closed or when the process containing it dies. De-registration is automatic.

When registering a server, you can optionally provide server (node) data as additional keyword arguments to register_server. By default, the process id is set as the pid property. This is useful to tracking down the server process.

Get the addresses of service providers.

Getting the addresses providing a service is accomplished by getting the children of a service node:

>>> addresses = zk.children('/fooservice/providers')
>>> sorted(addresses)
['192.168.0.42:8080']

The children method returns an iterable of names of child nodes of the node specified by the given path. The iterable is automatically updated when new providers are registered:

>>> zk.register_server('/fooservice/providers', ('192.168.0.42', 8081))
>>> sorted(addresses)
['192.168.0.42:8080', '192.168.0.42:8081']

You can call the iterable with a callback function that is called whenever the list of children changes:

>>> @zk.children('/fooservice/providers')
... def addresses_updated(addresses):
...     print 'addresses changed'
...     print sorted(addresses)
addresses changed
['192.168.0.42:8080', '192.168.0.42:8081']

The callback is called immediately with the children. When we add another child, it’ll be called again:

>>> zk.register_server('/fooservice/providers', ('192.168.0.42', 8082))
addresses changed
['192.168.0.42:8080', '192.168.0.42:8081', '192.168.0.42:8082']

Get service configuration data.

You get service configuration data by getting properties associated with a ZooKeeper node. The interface for getting properties is similar to the interface for getting children:

>>> data = zk.properties('/fooservice')
>>> data['database']
u'/databases/foomain'
>>> data['threads']
1

The properties method returns a mapping object that provides access to node data. (ZooKeeper only stores string data for nodes. zc.zk provides a higher-level data interface by storing JSON strings.)

The properties objects can be called with callback functions and used as function decorators to get update notification:

>>> @zk.properties('/fooservice')
... def data_updated(data):
...     print 'data updated'
...     for item in sorted(data.items()):
...         print '%s: %r' % item
data updated
database: u'/databases/foomain'
favorite_color: u'red'
threads: 1

The callback is called immediately. It’ll also be called when data are updated.

Updating node properties

You can’t set properties, but you can update properties by calling the update method:

>>> thread_info = {'threads': 2}
>>> data.update(thread_info, secret='123')
data updated
database: u'/databases/foomain'
favorite_color: u'red'
secret: u'123'
threads: 2

or by calling the set method, which removes keys not listed:

>>> data.set(threads= 3, secret='1234')
data updated
secret: u'1234'
threads: 3

Both update and set can take data from a positional data argument, or from keyword parameters. Keyword parameters take precedent over the positional data argument.

ZooKeeper Session Management

zc.zk takes care of ZooKeeper session management for you. It establishes and, if necessary, reestablishes sessions for you. In particular, it takes care of reestablishing ZooKeeper watches when a session is reestablished.

ZooKeeper logging

zc.zk bridges the low-level ZooKeeper logging API and the Python logging API. ZooKeeper log messages are forwarded to the Python 'ZooKeeper' logger.

Tree-definition format, import, and export

You can describe a ZooKeeper tree using a textual tree representation. You can then populate the tree by importing the representation. Heres an example:

/lb : ipvs
  /pools
    /cms
      # The address is fixed because it's
      # exposed externally
      address = '1.2.3.4:80'
      providers -> /cms/providers
    /retail
      address = '1.2.3.5:80'
      providers -> /cms/providers

/cms : z4m cms
  threads = 3
  /providers
  /databases
    /main
      /providers

/retail : z4m retail
  threads = 1
  /providers
  /databases
    main -> /cms/databases/main
    /ugc
      /providers

This example defines a tree with 3 top nodes, lb and cms, and retail. The retail node has two sub-nodes, providers and databases and a property threads.

The /retail/databases node has symbolic link, main and a ugc sub-node. The symbolic link is implemented as a property named `` We’ll say more about symbolic links in a later section.

The lb, cms and retail nodes have types. A type is indicated by following a node name with a colon and a string value. The string value is used to populate a type property. Types are useful to document the kinds of services provided at a node and can be used by deployment tools to deploy service providers.

You can import a tree definition with the import_tree method:

>>> zk.import_tree(tree_text)

This imports the tree at the top of the ZooKeeper tree.

We can also export a ZooKeeper tree:

>>> print zk.export_tree(),
/cms : z4m cms
  threads = 3
  /databases
    /main
      /providers
  /providers
/fooservice
  secret = u'1234'
  threads = 3
  /providers
/lb : ipvs
  /pools
    /cms
      address = u'1.2.3.4:80'
      providers -> /cms/providers
    /retail
      address = u'1.2.3.5:80'
      providers -> /cms/providers
/retail : z4m retail
  threads = 1
  /databases
    main -> /cms/databases/main
    /ugc
      /providers
  /providers

Note that when we export a tree:

• The special reserved top-level zookeeper node is omitted.

• Ephemeral nodes are omitted.

• Each node’s information is sorted by type (properties, then links,

• then sub-nodes) and then by name,

You can export just a portion of a tree:

>>> print zk.export_tree('/fooservice'),
/fooservice
  secret = u'1234'
  threads = 3
  /providers

You can optionally see ephemeral nodes:

>>> print zk.export_tree('/fooservice', ephemeral=True),
/fooservice
  secret = u'1234'
  threads = 3
  /providers
    /192.168.0.42:8080
      pid = 81176
    /192.168.0.42:8081
      pid = 81176
    /192.168.0.42:8082
      pid = 81176

We can import a tree over an existing tree and changes will be applied. Let’s update our textual description:

/lb : ipvs
  /pools
    /cms
      # The address is fixed because it's
      # exposed externally
      address = '1.2.3.4:80'
      providers -> /cms/providers

/cms : z4m cms
  threads = 4
  /providers
  /databases
    /main
      /providers

and re-import:

>>> zk.import_tree(tree_text)
extra path not trimmed: /lb/pools/retail

We got a warning about nodes left over from the old tree. We can see this if we export the tree:

>>> print zk.export_tree(),
/cms : z4m cms
  threads = 4
  /databases
    /main
      /providers
  /providers
/fooservice
  secret = u'1234'
  threads = 3
  /providers
/lb : ipvs
  /pools
    /cms
      address = u'1.2.3.4:80'
      providers -> /cms/providers
    /retail
      address = u'1.2.3.5:80'
      providers -> /cms/providers
/retail : z4m retail
  threads = 1
  /databases
    main -> /cms/databases/main
    /ugc
      /providers
  /providers

If we want to trim these, we can add a trim option. This is a little scary, so we’ll use the dry-run option to see what it’s going to do:

>>> zk.import_tree(tree_text, trim=True, dry_run=True)
would delete /lb/pools/retail.

That’s what we’d expect, so we go ahead:

>>> zk.import_tree(tree_text, trim=True)
>>> print zk.export_tree(),
/cms : z4m cms
  threads = 4
  /databases
    /main
      /providers
  /providers
/fooservice
  secret = u'1234'
  threads = 3
  /providers
/lb : ipvs
  /pools
    /cms
      address = u'1.2.3.4:80'
      providers -> /cms/providers
/retail : z4m retail
  threads = 1
  /databases
    main -> /cms/databases/main
    /ugc
      /providers
  /providers

Note that nodes containing (directly or recursively) ephemeral nodes will never be trimmed. Also node that top-level nodes are never automatically trimmed. So we weren’t warned about the unreferenced top-level nodes in the import.

Recursive Deletion

ZooKeeper only allows deletion of nodes without children. The delete_recursive method automates removing a node and all of it’s children.

If we want to remove the retail top-level node, we can use delete_recursive:

>>> zk.delete_recursive('/retail')
>>> print zk.export_tree(),
/cms : z4m cms
  threads = 4
  /databases
    /main
      /providers
  /providers
/fooservice
  secret = u'1234'
  threads = 3
  /providers
/lb : ipvs
  /pools
    /cms
      address = u'1.2.3.4:80'
      providers -> /cms/providers

You can’t delete nodes ephemeral nodes, or nodes that contain them:

>>> zk.delete_recursive('/fooservice')
Not deleting /fooservice/providers/192.168.0.42:8080 because it's ephemeral.
Not deleting /fooservice/providers/192.168.0.42:8081 because it's ephemeral.
Not deleting /fooservice/providers/192.168.0.42:8082 because it's ephemeral.
/fooservice/providers not deleted due to ephemeral descendent.
/fooservice not deleted due to ephemeral descendent.

Symbolic links

ZooKeeper doesn’t have a concept of symbolic links, but zc.zk provides a convention for dealing with symbolic links. When trying to resolve a path, if a node lacks a child, but have a property with a name ending in ' ->', the child will be found by following the path in the property value.

The resolve method is used to resolve a path to a real path:

>>> zk.resolve('/lb/pools/cms/providers')
u'/cms/providers'

In this example, the link was at the endpoint of the virtual path, but it could be anywhere:

>>> zk.register_server('/cms/providers', '1.2.3.4:5')
>>> zk.resolve('/lb/pools/cms/providers/1.2.3.4:5')
u'/cms/providers/1.2.3.4:5'

Note a limitation of symbolic links is that they can be hidden by children. For example, if we added a real node, at /lb/pools/cms/provioders, it would shadow the link.

children, properties, and register_server will automatically use resolve to resolve paths.

When the children and properties are used for a node, the paths they use will be adjusted dynamically when paths are removed. To illustrate this, let’s get children of /cms/databases/main:

>>> main_children = zk.children('/cms/databases/main')
>>> main_children.path
'/cms/databases/main'
>>> main_children.real_path
'/cms/databases/main'

Children and Properties objects have a path attribute that has the value passed to the children or properties methods. They have a real_path attribute that contains the path after resolving symbolic links. Let’s suppose we want to move the database node to ‘/databases/cms’. First we’ll export it:

>>> export = zk.export_tree('/cms/databases/main', name='cms')
>>> print export,
/cms
  /providers

Note that we used the export name option to specify a new name for the exported tree.

Now, we’ll create a databases node:

>>> zk.create('/databases', '', zc.zk.OPEN_ACL_UNSAFE)
'/databases'

And import the export:

>>> zk.import_tree(export, '/databases')
>>> print zk.export_tree('/databases'),
/databases
  /cms
    /providers

Next, we’ll create a symbolic link at the old location. We can use the ln convenience method:

>>> zk.ln('/databases/cms', '/cms/databases/main')
>>> zk.get_properties('/cms/databases')
{u'main ->': u'/databases/cms'}

Now, we can remove /cms/databases/main and main_children will be updated:

>>> zk.delete_recursive('/cms/databases/main')
>>> main_children.path
'/cms/databases/main'
>>> main_children.real_path
u'/databases/cms'

If we update /databases/cms, main_children will see the updates:

>>> sorted(main_children)
['providers']
>>> zk.delete('/databases/cms/providers')
>>> sorted(main_children)
[]

Node deletion

If a node is deleted and Children or Properties instances have been created for it, and the paths they were created with can’t be resolved using symbolic links, then the instances’ data will be cleared. Attempts to update properties will fail. If callbacks have been registered, they will be called without arguments, if possible. It would be bad, in practice, to remove a node that processes are watching.

zc.zk.ZooKeeper

zc.zk.ZooKeeper(connection_string)

Return a new instance given a ZooKeeper connection string.

children(path)

Return a zc.zk.Children for the path.

Note that there is a fair bit of machinery in zc.zk.Children objects to support keeping them up to date, callbacks, and cleaning them up when they are no-longer used. If you only want to get the list of children once, use get_children.

properties(path)

Return a zc.zk.Properties for the path.

Note that there is a fair bit of machinery in zc.zk.Properties objects to support keeping them up to date, callbacks, and cleaning them up when they are no-longer used. If you only want to get the properties once, use get_properties.

handle

The ZooKeeper session handle

This attribute can be used to call the lower-level API provided by the zookeeper extension.

import_tree(text[, path='/'[, trim[, acl[, dry_run]]]])

Create tree nodes by importing a textual tree representation.

text

A textual representation of the tree.

path

The path at which to create the top-level nodes.

trim

Boolean, defaulting to false, indicating whether nodes not in the textual representation should be removed.

acl

An access control-list to use for imported nodes. If not specified, then full access is allowed to everyone.

dry_run

Boolean, defaulting to false, indicating whether to do a dry run of the import, without applying any changes.

export_tree(path[, ephemeral[, name]])

Export a tree to a text representation.

path

The path to export.

ephemeral

Boolean, defaulting to false, indicating whether to include ephemeral nodes in the export. Including ephemeral nodes is mainly useful for visualizing the tree state.

name

The name to use for the top-level node.

This is useful when using export and import to copy a tree to a different location and name in the hierarchy.

Normally, when exporting the root node, /, the root isn’t included, but it is included if a name is given.

delete_recursive(path[, dry_run])

Delete a node and all of it’s sub-nodes.

Ephemeral nodes or nodes containing them are not deleted.

The dry_run option causes a summary of what would be deleted to be printed without actually deleting anything.

get_children(path)

Get a list of the names of the children the node at the given path.

This is more efficient than children when all you need is to read the list once, as it doesn’t create a zc.zk.Children object.

get_properties(path)

Get the properties for the node at the given path as a dictionary.

This is more efficient than properties when all you need is to read the properties once, as it doesn’t create a zc.zk.Properties object.

ln(source, destination)

Create a symbolic link at the destination path pointing to the source path.

If the destination path ends with '/', then the source name is appended to the destination.

resolve(path)

Find the real path for the given path.

register_server(path, address, **data)

Register a server at a path with the address.

An ephemeral child node of path will be created with name equal to the string representation (HOST:PORT) of the given address.

address must be a host and port tuple.

Optional node properties can be provided as keyword arguments.

close()

Close the ZooKeeper session.

This should be called when cleanly shutting down servers to more quickly remove ephemeral nodes.

In addition, ZooKeeper instances provide access to the following ZooKeeper functions as methods: acreate, add_auth, adelete, aexists, aget, aget_acl, aget_children, aset, aset_acl, async, client_id, create, delete, exists, get, get_acl, is_unrecoverable, recv_timeout, set, set2, set_acl, set_debug_level, set_log_stream, set_watcher, and zerror. When calling these as methods on ZooKeeper instances, it isn’t necessary to pass a handle, as that is provided automatically.

zc.zk.Children

__iter__()

Return an iterator over the child names.

__call__(callable)

Register a callback to be called whenever a child node is added or removed.

The callback is passed the children instance when a child node is added or removed.

The Children instance is returned.

zc.zk.Properties

Properties objects provide the usual read-only mapping methods, __getitem__, __len__, etc..

set(data=None, **properties)

Set the properties for the node, replacing existing data.

The data argument, if given, must be a dictionary or something that can be passed to the dict constructor. Items supplied as keywords take precedence over items supplied in the data argument.

update(data=None, **properties)

Update the properties for the node.

The data argument, if given, must be a dictionary or something that can be passed to a dictionary’s update method. Items supplied as keywords take precedence over items supplied in the data argument.

__call__(callable)

Register a callback to be called whenever a node’s properties are changed.

The callback is passed the properties instance when properties are changed.

The Properties instance is returned.

Changes