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=over

=item expire_if [CODEREF]

If I<$key> exists and has not expired, call code reference with the
L<CHI::CacheObject|CHI::CacheObject> and L<CHI::Driver|CHI::Driver> as the
parameters. If code returns a true value, C<get> returns undef as if the item
were expired. For example, to treat the cache as expired if I<$file> has
changed since the value was computed:

    $cache->get('foo', expire_if => sub { $_[0]->created_at < (stat($file))[9] });

=item busy_lock [DURATION]

If the value has expired, the get will still return undef, but the expiration
time of the cache entry will be set to the current time plus the specified
L<duration|/DURATION EXPRESSIONS>.  This is used to prevent multiple processes
from recomputing the same expensive value simultaneously. The problem with this
technique is that it doubles the number of writes performed - see
L</expires_variance> for another technique.