2:12 p.m.
On Wed, 2014-03-12 at 18:45 +0100, Galder Zamarreño wrote:
On 05 Mar 2014, at 18:16, Mircea Markus <mmarkus(a)redhat.com>
wrote:
> Sanne came with a good follow up to this email, just some small clarifications:
>
> On Mar 4, 2014, at 6:02 PM, Emmanuel Bernard <emmanuel(a)hibernate.org> wrote:
>
>>>> If you have to do a map reduce for tasks so simple as age > 18, I
think you system better have to be prepared to run gazillions of M/R jobs.
>>>
>>> I want to run a simple M/R job in the evening to determine who turns 18
tomorrow, to congratulate them. Once a day, not gazzilions of times, and I don't need
to index the age filed just for that. Also when it comes to Map/Reduce, the drawback of
holding all the data in a single cache is two-folded:
>>> - performance: you iterate over the data that is not related to your query.
>>
>> If the data are never related (query wise), then we are in the database split
category. Which is fine. But if some of your queries are related, what do you do? Deny the
user the ability to do them?
>
> Here's where cross-site query would have been used. As Sanne suggested (next
post) these limitations overcome the advantages.
>
>>
>>> - programming model: the Map/Reduce implementation has a dependency on both
Dog and Person. If I add Cats to the cache, I'll need to update the M/R code to be
aware of that as well. Same if I rename/remove Dog. Not nice.
>>
>> Well it’s called type safety, some people find it good ;)
>
> If anything, this model reduces type safety and reusability. E.g. say you want a M/R
task to see how many Persons speak French. With the single cache model(both Dog and Person
int he Cache<String, Mammal>) it would look something like:
>
> a)
> //pseudocode
> map (String k, Mammal value) {
> if (value instanceof Person)) { //this is the ugly part
> if (((Person)value).speaks("French")) ...
> } else {
> //ignore it, it's an Dog
> }
> }
>
> Same thing written for a Cache<String, Person>:
>
> b)
> map (String k, Person value) {
> if (value.speaks("French")) ...
> }
>
> I don't think people would prefer writing a) instead of b) ;)
It’s a pity that having discussed in Mallorca and having referenced a fair few times
already, you could not think of an option based on what Paul suggsted in
https://issues.jboss.org/browse/ISPN-3640
Here’s my attempt:
Cache<?, ?> cache = …;
ValueFilter filter = … // filter would check those values that are persons...
Cache<String, Person> view = cache.filter(filter);
view.map(String k, Person value) {
...
}
Paul referred to per key type filters, but maybe per value type filters, such as the one
I did here, might be useful.
We could also generalize KeyFilter to something like:
public interface Filter {
boolean accepts(Object key, Object value);
}
Infinispan could ship some simple default implementations that filter
based on a specific key type, value type, or both.
e.g.
Cache<?, ?> cache = ...;
Cache<String, Integer> view = cache.filter(new KeyValueTypeFilter(String.class,
Integer.class));
view.addListener(...); // Listens only to events accepted by filter
for (Map.Entry<String, Integer> entry: view.entrySet()) {
// No casting necessary!
}
where KeyValueTypeFilter looks like:
public class KeyValueTypeFilter implements Filter {
private Class<?> keyType;
private Class<?> valueType;
public KeyValueTypeFilter(Class<?> keyType, Class<?> valueType) {
this.keyType = keyType;
this.valueType = valueType;
}
@Override
public boolean accepts(Object key, Object value) {
return this.keyType.isInstance(key) && this.valueType.isInstance(value);
}
}
Cheers,
>
>
> Cheers,
> --
> Mircea Markus
> Infinispan lead (www.infinispan.org)
>
>
>
>
>
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--
Galder Zamarreño
galder(a)redhat.com
twitter.com/galderz
Project Lead, Escalante
http://escalante.io
Engineer, Infinispan
http://infinispan.org