Async everything will always save you instance hours. It doesn't matter if
you are threadsafe or not, the async operations allow you to perform a ds /
memcahce / url fetch in a background thread, which you can check on
whenever you want.
Async is extremely useful if you use it to perform "datastore streaming";
if you have to operate on thousands of entities, you can cut your runtime
from "all of the time to create and persist all of the entities" to "all of
the time to create all of the entities, and little to no time waiting for
ds operations to complete". I use a helper object to stream all my writes,
deletes and reads in async batches, with custom page sizing. The helper
holds the entities in memory, async puts() them in batches when the page
size is reached, and holds the entities in memory until their put()
succeeds, so it can retry anything that fails. Another easy way to retry a
future, at least in Java, is to create a subclass of Future<Entity> that
takes the Entity as a param, wraps the appengine Future<Key>, and
automatically retries in transient errors. For retry, I use synchronous
rather than async again, but you can do whatever you like.
Using async everywhere, with threadsafe, got our app down from $30/day for
frontend instance hours to ~$0/day for frontend instance hours. We
generally have 4-6 live instances, but only use approximately 1 instance
hour / hour.
The trick is that all your api operations can happen in the background, so
your total processing time is near or equal to your "userland" processing
only.
PS - Anyone that doesn't actually call .get() on their futures to finalize
your async operations could lose data, especially if the last operation in
your method is an async put. Async operations started by your current
processing thread die when the thread returns {in production}, so make sure
you call .get() on the returned future.
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