oracular (3) DBD::Cassandra.3pm.gz
NAME
DBD::Cassandra - DBI database backend for Cassandra
VERSION
version 0.57
DESCRIPTION
DBD::Cassandra is a Perl5 Database Interface driver for Cassandra, using the CQL3 query language.
EXAMPLE
use DBI;
my $dbh = DBI->connect("dbi:Cassandra:host=localhost;keyspace=test", $user, $password, { RaiseError => 1 });
my $rows = $dbh->selectall_arrayref("SELECT id, field_one, field_two FROM some_table");
for my $row (@$rows) {
# Do something with your row
}
$dbh->do("INSERT INTO some_table (id, field_one, field_two) VALUES (?, ?, ?)",
{ Consistency => "quorum" },
1, "String value", 38962986
);
$dbh->disconnect;
Configuration
Database handles
use DBI;
$dsn = "dbi:Cassandra:database=$database";
$dsn = "dbi:Cassandra:keyspace=$keyspace;host=$hostname;port=$port";
$dsn = "dbi:Cassandra:keyspace=$keyspace;consistency=local_quorum";
my $dbh = DBI->connect($dsn, $username, $password);
keyspace
database
db Optionally, a keyspace to use by default. If this is not specified, all queries must include the
keyspace name.
hostname
hosts
Hostname to initially connect to. Defaults to "localhost". Can be comma-separated to specify
multiple hosts.
port
Port number to connect to. Defaults to 9042
compression
The compression method we should use for the connection. Currently Cassandra allows "lz4" and
"snappy". Defaults to the algorithm with the best compression ratio, if the server supports it.
Compression can be disabled by setting "compression=none".
Only used for data frames longer than 512 bytes, smaller frames get sent uncompressed.
cql_version
There are several versions of the CQL language and this option lets you pick one. Defaults to the
highest available version. Consult your Cassandra manual to see which versions your database
supports.
consistency
See the chapter on consistency levels
request_timeout
Maximum amount of time (in seconds) to wait for a Cassandra network operation to finish.
read_timeout
write_timeout
Deprecated. These two are summed and used as "request_timeout".
tls Boolean (1|0); whether to use TLS. Defaults to off.
Statement handles
my $sth= $dbh->prepare('SELECT "id", "field1", "field2" FROM table_name WHERE id=?', { Consistency => 'one' });
async
See "asynchronous queries".
consistency
See "consistency levels".
per_page
Cassandra supports pagination through result sets, to avoid having the entire result set in
memory.
my $sth = $dbh->prepare('SELECT id FROM tablename', { PerPage => 1000 });
$sth->execute;
while (my $row = $sth->fetchrow_arrayref()) {
print "$row->[0]\n";
}
It is important to keep in mind that this mode can cause errors while fetching rows, as extra
queries may be executed by the driver internally.
COLLECTION TYPES
Cassandra supports collection types natively, eg. "list" and "map". DBD::Cassandra translates them to
native Perl types, eg. hashes and arrays.
When doing queries, placeholders can be substituted by these collections. For example, inserting a map
into a table is done by passing a Perl hash.
my $sth= $dbh->prepare('INSERT INTO some_table (id, value) VALUES (?,?);');
$sth->execute(5, { days => 15 });
This will also work for "IN" queries, which accept an array.
my $sth= $dbh->prepare('SELECT id, value FROM some_table WHERE id IN ?');
$sth->execute([1, 2, 3]);
my $rows= $sth->fetchall_arrayref();
ASYNCHRONOUS QUERIES
my $sth= $dbh->prepare("SELECT id FROM some_table WHERE x=?",
{ async => 1 });
$sth->execute(5);
some_other_function();
while (my $row = $sth->fetchrow_arrayref()) {
print "$row->[0]\n";
}
DBD::Cassandra supports asynchronous queries in an easy to use form. When "async =" 1> is passed to
"prepare()", any subsequent executes on the handle are not read back immediately. Instead, these are
delayed until the result is actually needed.
For inserts and other writes, a convenience method "x_finish_async" is provided, which returns an
approximation to what "execute()" would have returned in an non-asynchronous context. This method also
raises errors, if needed.
my $sth= $dbh->prepare("INSERT INTO table (a, b) VALUES (?, ?)",
{ async => 1 });
$sth->execute(5, 6);
some_other_function_that_takes_a_while();
$sth->x_finish_async;
Performance considerations
When using asynchronous queries, some previously premature optimizations become relevant. For example, it
is very helpful to re-use statement handles in large volumes of inserts :
my @dataset_to_insert= ([1, 2, 3, 4], [5, 6, 7, 8]);
my (@pending, @reusable);
while (my $row= shift @dataset_to_insert) {
my $sth= (shift @reusable) || $dbh->prepare(
"INSERT INTO some_table (a, b, c, d) VALUES (?, ?, ?, ?)"
);
$sth->execute(@$row);
push @pending, $sth;
if (@pending > 500) { # Tune this number!
my $pending_sth= shift @pending;
$pending_sth->x_finish_async;
push @reusable, $pending_sth;
}
}
$_->x_finish_async for @pending;
CONSISTENCY LEVELS
$dbh->do("INSERT INTO some_table (id, field_name) VALUES (?, ?)",
{ Consistency => "quorum" },
@values
);
DBD::Cassandra accepts a Consistency attribute for statements. Supported consistency levels are "any",
"one", "two", "three", "quorum", "all", "local_quorum", "each_quorum", "serial", "local_serial" and
"local_one".
This attribute is ignored on statements that do not support it, such as "CREATE".
A global consistency level can be defined as part of the DSN.
CAVEATS, BUGS, TODO
• There is currently no support for transactions. "begin_work" will die if you try to use it.
• Thread support is untested. Use at your own risk.
• The "timestamp" format is implemented naively by returning milliseconds since the UNIX epoch. In Perl
you get this number through "time() * 1000". Trying to save times as "DateTime" objects or strings
will not work, and will likely result in warnings and unexpected behavior.
• When using asynchronous queries, more functions than just execute() may throw errors. It is
recommended that you enable RaiseError. If this is not possible, it should also suffice to call
"$sth-"x_finish_async> and check its return value before reading any data from the handle.
• Cassandra/CQL3 is strict about the queries you write. When switching from other databases, such as
MySQL, this may come as a surprise. This module supports "quote(..)", but try to use prepared
statements instead. They will save you a lot of trouble.
UPGRADING
From versions 0.25 and lower
As of "DBD::Cassandra" 0.51, this module uses "Cassandra::Client" internally. The unit tests from the
previous release all still pass, but there are subtle changes :
read_timeout/write_timeout are deprecated, use request_timeout instead
the driver now manages a pool of connections internally
Instead of only connecting to the one specified host, multiple hosts can be passed as seed-hosts.
These are then used to bootstrap the actual internal pool of connections.
From versions 0.24 and lower
Prior to version 0.25 there was a bug corrupting float and double values as they were stored in the
database. The endianness on these values was wrong, which only shows when reading stored data back in an
application written using a different driver.
If you were writing float or double values using a DBD::Cassandra prior to 0.25, please be careful with
this upgrade. A way to rewrite your values between the two formats is :
my $good_float = unpack('f>', pack('f', $bad_float));
my $good_double= unpack('d>', pack('d', $bad_double));
If you never used a DBD::Cassandra version prior to 0.25, or do not use floats or doubles, this bug does
not affect you and upgrading to 0.25 is safe.
AUTHOR
Tom van der Woerdt <tvdw@cpan.org>
COPYRIGHT AND LICENSE
This software is copyright (c) 2017 by Tom van der Woerdt.
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5
programming language system itself.