Puppet advanced default params - if-statement

Question

Basically I have a puppet class cassandra_manager::params. I have a ton of params set like so:
# etc...
$cluster_name = undef,
$num_tokens = 256,
$initial_token = undef,
$hinted_handoff_enabled = true,
# etc...
Now most of these ones set undef, I just handle in my template just commenting it out if it's undef.
But there are a few that I need to use some previous params, and facts to figure out the defaults. Here is one main example:
$memtable_flush_writers = undef,
I then try to set it later like this:
$min_from_cores_and_disks = min([ $::processorcount, size(split($::hard_drives, ',')) ])
if !$memtable_flush_writers {
if 0 + (0 + $min_from_cores_and_disks[0]) > 8 or (0 + $min_from_cores_and_disks[0]) < 2 {
if (0 + $min_from_cores_and_disks[0]) < 2 {
$memtable_flush_writers = 2
}
elsif (0 + $min_from_cores_and_disks[0]) > 8 {
$memtable_flush_writers = 8
}
} else {
$memtable_flush_writers = (0 + $min_from_cores_and_disks[0])
}
}
Then puppet tells me I can't set $memtable_flush_writers because all variables are immutable in puppet.
So then I changed checking for whether the variable was "false" and didn't set the variable up above, but that just told me the obvious, that $memtable_flush_writers wasn't set.
What's the best way that I can still get the functionality from the if statements above without having these errors pop up. Thanks in advance.

Answer 1

You have to create a new variable which you use only for declaration and setting any default value in the class definition.
class cassandra_manager::params (
$memtable_flush_writers_count = undef
){
if !$memtable_flush_writers_count {
# Whatever you want to do
$memtable_flush_writers = 'your_value'
} else {
$memtable_flush_writers = $memtable_flus_writers_count
}
}

Answer 2

The variables in cassandra_manager::params are typically used as default values for parameters of class cassandra_manager.
class cassandra_manager(
$cluster_name = $cassandra_manager::params::cluster_name,
$num_tokens = $cassandra_manager::params::num_tokens,
...) inherits cassandra_manager::params {
}
You override the defaults by passing such parameters.
class {
'cassandra_manager':
memtable_flush_writers => <value for this node>
}