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Tom van der Woerdt

NAME

Cassandra::Client - Perl interface to Cassandra's native protocol

EXAMPLE

    use Cassandra::Client;
    my $client= Cassandra::Client->new(
        contact_points => [ '127.0.0.1', '192.168.0.1' ],
        username => "my_user",
        password => "my_password",
        keyspace => "my_keyspace",
    );
    $client->connect;

    $client->each_page("SELECT id, column FROM my_table WHERE id=?", [ 5 ], undef, sub {
        for my $row (@{shift->rows}) {
            my ($id, $column)= @$row;
            say "$id: $column";
        }
    });

DESCRIPTION

Cassandra::Client is a Perl library giving its users access to the Cassandra database, through the native protocol. Both synchronous and asynchronous querying is supported, through various common calling styles.

METHODS

Cassandra::Client->new(%options)

Create a new Cassandra::Client instance, with the given options.

contact_points

Required. Arrayref of seed hosts to use when connecting. Specify more than one for increased reliability. This array is shuffled before use, so that random hosts are picked from the array.

keyspace

Default keyspace to use on all underlying connections. Can be overridden by querying for specific keyspaces, eg SELECT * FROM system.peers.

anyevent

Should our internal event loop be based on AnyEvent, or should we just use our own? A true value means enable AnyEvent. Needed for promises to work.

port

Port number to use. Defaults to 9042.

cql_version

CQL version to use. Defaults to the version the server is running. Override only if your client has specific CQL requirements.

compression

Compression method to use. Defaults to the best available version, based on server and client support. Possible values are snappy, lz4, and none.

default_consistency

Default consistency level to use. Defaults to one. Can be overridden on a query basis as well, by passing a consistency attribute.

max_page_size

Default max page size to pass to the server. This defaults to 5000. Note that large values can cause trouble on Cassandra. Can be overridden by passing page_size in query attributes.

max_connections

Maximum amount of connections to keep open in the Cassandra connection pool. Defaults to 2 for historical reasons, raise this if appropriate.

timer_granularity

Timer granularity used for timeouts. Defaults to 0.1 (100ms). Change this if you're setting timeouts to values lower than a second.

request_timeout

Maximum time to wait for a query, in seconds. Defaults to 11.

warmup

Whether to connect to the full cluster in connect(), or delay that until queries come in.

$client->batch($queries[, $attributes])

Run one or more queries, in a batch, on Cassandra. Queries must be specified as an arrayref of [$query, \@bind] pairs.

Defaults to a logged batch, which can be overridden by passing logged, unlogged or counter as the batch_type attribute.

    $client->batch([
        [ "INSERT INTO my_table (a, b) VALUES (?, ?)", [ $row1_a, $row1_b ] ],
        [ "INSERT INTO my_table (a, b) VALUES (?, ?)", [ $row2_a, $row2_b ] ],
    ], { batch_type => "unlogged" });
$client->execute($query[, $bound_parameters[, $attributes]])

Executes a single query on Cassandra, and fetch the results (if any).

For queries that have large amounts of result rows and end up spanning multiple pages, each_page is the function you need. execute does not handle pagination, and may end up missing rows unless pagination is implemented by its user through the page attribute.

    $client->execute(
        "UPDATE my_table SET column=:new_column WHERE id=:id",
        { new_column => 2, id => 5 },
        { consistency => "quorum" },
    );
$client->each_page($query, $bound_parameters, $attributes, $page_callback)

Executes a query and invokes $page_callback with each page of the results, represented as Cassandra::Client::ResultSet objects.

    # Downloads the entire table from the database, even if it's terabytes in size
    $client->each_page( "SELECT id, column FROM my_table", undef, undef, sub {
        my $page= shift;
        for my $row (@{$page->rows}) {
            say $row->[0];
        }
    });
$client->prepare($query)

Prepares a query on the server. execute and each_page already do this internally, so this method is only useful for preloading purposes (and to check whether queries even compile, I guess).

$client->shutdown()

Disconnect all connections and abort all current queries. After this, the Cassandra::Client object considers itself shut down and must be reconstructed with new().

$client->wait_for_schema_agreement()

Wait until all nodes agree on the schema version. Useful after changing table or keyspace definitions.

(A?)SYNCHRONOUS

It's up to the user to choose which calling style to use: synchronous, asynchronous with promises, or through returned coderefs.

Synchronous

All Cassandra::Client methods are available as synchronous methods by using their normal names. For example, $client->connect(); will block until the client has connected. Similarly, $client->execute($query) will wait for the query response. These are arguably not the fastest variants (there's no parallelism in queries) but certainly the most convenient.

    my $client= Cassandra::Client->new( ... );
    $client->connect;
    $client->execute("INSERT INTO my_table (id, value) VALUES (?, ?) USING TTL ?",
        [ 1, "test", 86400 ],
        { consistency => "quorum" });

Promises

Cassandra::Client methods are also available as promises (see perldoc Promises). This integrates well with other libraries that deal with promises or asynchronous callbacks. Note that for promises to work, AnyEvent is required, and needs to be enabled by passing anyevent => 1 to Cassandra::Client->new().

Promise variants are available by prefixing method names with async_, eg. async_connect, async_execute, etc. The usual result of the method is passed to the promise's success handler, or to the failure handler if there was an error.

    # Asynchronously pages through the result set, processing data as it comes in.
    my $promise= $client->async_each_page("SELECT id, column FROM my_table WHERE id=?", [ 5 ], undef, sub {
        for my $row (@{shift->rows}) {
            my ($id, $column)= @$row;
            say "Row: $id $column";
        }
    })->then(sub {
        say "We finished paging through all the rows";
    }, sub {
        my $error= shift;
    });

Promises normally get resolved from event loops, so for this to work you need one. Normally you would deal with that by collecting all your promises and then waiting for that :

    use Promises qw/collect/;
    use AnyEvent;

    my @promises= ( ... ); # See other examples
    my $condvar= AnyEvent->condvar;

    collect(@promises)->then(sub {
        $condvar->send;
    }, sub {
        my $error= shift;
        warn "Unhandled error! $error";
        $condvar->send;
    });
    $condvar->recv; # Wait for the promsie to resolve or fail

How you integrate this into your infrastructure is of course up to you, and beyond the scope of the Cassandra::Client documentation.

Coderefs

These are the simplest form of asynchronous querying in Cassandra::Client. Instead of dealing with complex callback resolution, the client simply returns a coderef that, once invoked, returns what the original method would have retruned.

The variants are available by prefixing method names with future_, eg. future_connect, future_execute, etc. These methods return a coderef.

    my $coderef= $client->future_execute("INSERT INTO table (id, value) VALUES (?, ?), [ $id, $value ]);

    # Do other things
    ...

    # Wait for the query to finish
    $coderef->();

Upon errors, the coderef will die, just like the synchronous methods would. Because of this, invoking the coderef immediately after getting it is equal to using the synchronous methods :

    # This :
    $client->connect;

    # Is the same as :
    $client->future_connect->();

When used properly, coderefs can give a modest performance boost, but their real value is in the ease of use compared to promises.

CAVEATS, BUGS, TODO

  • Thread support is untested. Use at your own risk.

  • The timestamp format is implemented naively by returning milliseconds since the UNIX epoch. In Perl you get this number through time() * 1000. Trying to save times as DateTime objects or strings will not work, and will likely result in warnings and unexpected behavior.

LICENSE

Copyright (C) 2016 Tom van der Woerdt

This program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

See http://dev.perl.org/licenses/ for more information.