[Scala 2.2.3] Can I store a Future[T] in Cache

[John Langley]

I believe the answer is yes, but I'd love a confirmation. 

The motivation is that I want to cache the result from making a WebService (WS) call, but of course, I don't want to block while I'm waiting for the result. So it seems the obvious solution is to use a Future. 

Thoughts? Thanks in advance. 

[Filippo De Luca]

You can store the result of a future. You can store also the future, but it is not portable you cannot serialize a Future.

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[Mariot Chauvin]

If it's in memory caching, have a look to spray-cache 

Cheers,

Mariot

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[John Langley]

This http://www.playframework.com/documentation/2.2.x/api/scala/index.html#play.api.cache.Cache$ 

says

set(key: String, value: Any, expiration: Duration)(implicit app: Application): Unit

so it looks like it could store a Future because Future is an Any, right? 

However, I believe your point is still valid, that if I store something that is not Serializable into it, then it won't be portable across "application servers" / instances of the Play server, so using the memcached or ehcache implementation would fail. 

Sound right? (THANKS for your pointer!) 

[John Langley]

Thanks, I hadn't seen the spray-caching capability before. I've been pretty focused on using Play, but of course as the two merge... ;^) 

Thanks again. 

[Adam Evans]

I also recommend spray cache. 

Play will cache your future even if it's a failure so if your api call fails the future in the cache would be the exception. Spray-Cache only caches successful futures http://spray.io/documentation/1.2.1/spray-caching/ . 

[John Langley]

To ask a "lazy" question... do you know if spray-caching can be used in Play 2.2.3 application w/out problems? Looks like I might have to check the versions of scala, play, akka and spray with pointers from here: http://spray.io/project-info/current-versions/#current-versions  (Play includes akka, so may pin me to something that doesn't work with a particular version of spray-caching w/out doing a local build of things) 

[Foo Lim]

Is there a way to use Cache.getOrElse with Futures in Play 2.3.x?

Using an Await on the Future seems a bit like brute force.

Thanks!

[Naftoli Gugenheim]

Can you be more specific? Some context seems to be missing.

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[Will Sargent]

You can use spray-cache, which works with Future directly:

http://spray.io/documentation/1.2.3/spray-caching/

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[Foo Lim]

Is there any more details on how to use spray-caching as a plugin? The
documentation just says:

https://www.playframework.com/documentation/2.3.x/ScalaCache

You can also provide your own implementation via a plug-in.
...
Note: The API is intentionally minimal to allow several implementation
to be plugged. If you need a more specific API, use the one provided
by your Cache plugin.


Thanks!

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[Foo Lim]

Well, others on this thread have said you shouldn't a Future in the
cache but the value, but the only way to get the value is to Await the
Future to end or am I missing something here?

</psuedocode>
val maybeUser: Option[User] = Cache.getAs[User]("user.key")

// if maybeUser is None,
val futureUser: Future[User] = Db.getAFutureUser("user.key")
val user: User = Await.result(futureUser, 5 seconds)
Cache.set("user.key", user)
</psuedocode>

Rather than:

val user: User = Cache.getOrElse("user.key") {
???
}


Thanks

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[James Ward]

You don't need to use a blocking Await.  Just do:

futureUser.foreach { user => Cache.set("user.key", user) }

That will set the user in the cache once the future completes.

[Naftoli Gugenheim]

What's wrong with storing the Future in the cache?

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[Adam Evans]

You will suffer from Thundering herds doing this? Each request while the future isn't resolved ie Cache.set hasn't yet been called so it's a cache miss will cause `futureUser.foreach { user => Cache.set("user.key", user) }` to be evaluated again if it's in the getOrElse block.

Re nafg "What's wrong with storing the Future in the cache?":
If the future fails to resolve ie has a failure you have a failed future in the cache, if your cache ttl is 1 hour your serving up an error for 1 hour unless the item is purged. If it's a short ttl (seconds/minutes) you may be able to live with it, I'd think for most use cases only caching a successful future is what most people want to do.

Spray-Cache only caches successful futures and resolves thundering heards. If you only want a local cache not a shared distributed cache it works well and easy to setup. Don't try hooking it in to the play cache framework, use as advertised on the Spracy-Cache docs.

[James Ward]

Futures aren't serializable. And even though it might work to put a Future in an in-memory cache, that limits your options if you later want to move that cache somewhere else.

Likely Spray-Cache uses an actor which is the best way to deal with the Thundering herds. I handle this exact situation on webjars.org with just a raw Actor:
https://github.com/webjars/webjars/blob/master/app/utils/MavenCentral.scala#L118-L140