ZooKeeper - Perl bindings for Apache ZooKeeper
my $zk = ZooKeeper->new(hosts => 'localhost:2181'); my $cv = AE::cv; my @children = $zk->get_children('/', watcher => sub { my $event = shift; $cv->send($event) }); my $child_event = $cv->recv;
ZooKeeper is a perl interface to the Apache ZooKeeper C client library.
To support asynchronous programs, watchers were implemented as code refs, which a ZooKeeper::Dispatcher asynchronously invokes with ZooKeeper event data. Conversely, Net::ZooKeeper used Net::ZooKeeper::Watch classes, which users must interact with using the wait method(which blocks).
ZooKeeper event and stat data are simply hashrefs and arrayrefs. Net::ZooKeeper instead provides specific perl classes for interacting with this data.
Instead of returning the C error codes, as Net::ZooKeeper does, ZooKeeper throws ZooKeeper::Error exceptions for unexpected return codes.
Acls are represented as an arrayrefs of hashrefs, where each hashref includes an id, scheme, and permissions. Permissions flags can be imported from the ZooKeeper::Constants package.
For instance, ZOO_READ_ACL_UNSAFE would be represented as:
[{id => 'anyone', scheme => 'world', perms => ZOO_PERM_READ}]
A hashref of attributes for a watcher event. Includes the type of event(a ZooKeeper::Constants event), connection state(a ZooKeeper::Constants state) and the path of the node triggering the event.
{ path => '/child', state => ZOO_CONNECTED_STATE, type => ZOO_CHILD_EVENT, }
A hashref of fields from a C Stat struct.
{ aversion => 0, ctime => 0, cversion => 0, czxid => 0, ephemeralOwner => 0, dataLength => 0, mtime => 0, mzxid => 0, numChildren => 2, pzxid => 2334, version => 0, }
ZooKeeper uses ZooKeeper::Dispatchers for communicating with callbacks registered by the C library. These callbacks are executed in separate POSIX threads, which write event data to a ZooKeeper::Channel and notify the dispatcher that an event is ready to be processed. How this notification occurs, and how perl callbacks are invoked, is what differentiates the types of dispatchers.
ZooKeeper writes to a Unix pipe with an attached AnyEvent I/O watcher. This means that perl callbacks for watchers will be executed by the AnyEvent event loop.
ZooKeeper uses Async::Interrupt callbacks. This means the perl interpreter will be safely interrupted(waits for the current op to finish) in order to execute the corresponding perl callback. See Async::Interrupt for more details on how callbacks are executed. Be aware that this does not interrupt system calls(such as select) and XS code. This means if your code is blocking on a select(such as during an AnyEvent recv), the interrupt callback will not execute until the call has finished.
A comma separated list of ZooKeeper server hostnames and ports.
'localhost:2181' 'zoo1.domain:2181,zoo2.domain:2181'
The session timout used for the ZooKeeper connection.
A subroutine reference to be called by a default watcher for the ZooKeeper session.
An arrayref used for authenticating with ZooKeeper. This will be passed as an array to add_auth.
[$scheme, $credentials, %extra]
The default length of the buffer used for retrieving ZooKeeper data and paths. Defaults to 2048 bytes.
The implementation of ZooKeeper::Dispatcher to be used. Defaults to AnyEvent.
Valid types include:
The client_id for a ZooKeeper session. Can be set during construction to resume a previous session.
Instantiate a new ZooKeeper connection.
my $zk = ZooKeeper->new(%args) %args REQUIRED hosts OPTIONAL authentication OPTIONAL buffer_length OPTIONAL dispatcher OPTIONAL timeout OPTIONAL watcher
Get the state of the ZooKeeper connection. Returns a state enum from ZooKeeper::Constants.
Calls wait on the underlying ZooKeeper::Dispatcher.
Synchronously dispatch one event. Returns the event hashref the watcher was called with. Can optionally be passed a timeout(specified in seconds), which will cause wait to return undef if it does not complete in the specified time.
my $event = $zk->wait($seconds) OPTIONAL $seconds
Create a new node with the given path and data. Returns the path for the newly created node on succes. Otherwise a ZooKeeper::Error is thrown.
my $created_path = $zk->create($requested_path, %extra); REQUIRED $requested_path OPTIONAL %extra acl buffer_length persistent sequential value
Add authentication credentials for the session. Will automatically be invoked if the authentication attribute was set during construction. If the digest scheme is used, and encoded is not set, then credentials will be automatically hashed with Digest::SHA::sha1_base64.
A ZooKeeper::Error will be thrown if the request could not be made. To determine success or failure authenticating, a watcher must be passed.
$zk->add_auth($scheme, $credentials, %extra) REQUIRED $scheme REQUIRED $credentials OPTIONAL %extra watcher encoded
Delete a node at the given path. Throws a ZooKeeper::Error if the delete was unsuccessful.
$zk->delete($path, %extra) REQUIRED $path OPTIONAL %extra version
Check whether a node exists at the given path, and optionally set a watcher for when the node is created or deleted. On success, returns a stat hashref for the node. Otherwise returns undef.
my $stat = $zk->exists($path, %extra) REQUIRED $path OPTIONAL %extra watcher
Get the children stored directly under the given path. Optionally set a watcher for when a child is created or deleted. Returns an array of child path names.
my @child_paths = $zk->get_children($path, %extra) REQUIRED $path OPTIONAL %extra watcher
Retrieve data stored at the given path. Optionally set a watcher for when the data is changed. In list context, the data and stat hashref of the node is returned. Otherwise just the data is returned.
my $data = $zk->get($path, %extra) my ($data, $stat) = $zk->get($path, %extra) REQUIRED $path OPTIONAL %extra watcher buffer_length
Set data at the given path. On succes, returns a stat hashref of the node. Otherwise a ZooKeeper::Error is thrown.
my $stat = $zk->set($path => $value, %extra) REQUIRED $path REQUIRED $value OPTIONAL %extra version
Get ACLs for the given node. Returns an ACLs arrayref on success, otherwise throws a ZooKeeper::Error
my $acl = $zk->get_acl($path) REQUIRED $path
Set ACls for a node at the given path. Throws a ZooKeeper::Error on failure.
$zk->set_acl($path => $acl, %extra) REQUIRED $path REQUIRED $acl OPTIONAL %extra version
The underlying C library uses POSIX threads. This means that ZooKeeper is not fork safe. Only exec and POSIX::_exit can be guaranteed to work safely in the child process.
Many ZooKeeper recipes(such as in the examples directory), rely on clients properly shutting down to delete ephemeral nodes. Otherwise the ZooKeeper server will wait for the entire duration of the timeout specified by the session, before cleaning up. If you are expecting your program to handle signals(such as SIGINT), make sure the program is properly catching them and exiting. See the examples for more information.
The Apache ZooKeeper project's home page at http://zookeeper.apache.org/ provides a wealth of detail on how to develop applications using ZooKeeper.
Mark Flickinger <maf@cpan.org>
This software is licensed under the same terms as Perl itself.
To install ZooKeeper, copy and paste the appropriate command in to your terminal.
cpanm
cpanm ZooKeeper
CPAN shell
perl -MCPAN -e shell install ZooKeeper
For more information on module installation, please visit the detailed CPAN module installation guide.