High Performance Multithreaded Access to Amazon SimpleDB is a great follow up to the idea in How SimpleDB Differs from a RDBMS that more programming is the price paid for performance in SimpleDB. It shows how much work and infrastructure is required to batter better performance out of SimpleDB.
Remember, in SimpleDB you get keys to records from queries so if you want to get all the fields for records you need to make separate requests. Since SimpleDB isn't exactly a speed daemon the obvious strategy is to parallelize. Even if a job takes a 100 msecs you can get a lot done in a little time if you can execute enough jobs in parallel.
Parallelization is the approach taken by Haakon@AWS in his Java code example of how to get the most out of SimpleDB. You can find the code at Indexing and Querying Amazon S3 Metadata with Amazon SimpleDB. We'll also consider how a back-end service architecture built on Erlang may be a better fit with cloud computing.
Two general mechanisms of parallelism are available: threads and boxes. To get the most bang out of a single machine you need threads (events, etc). To scale beyond the load handled by a single machine you need multiple boxes. The example code uses the Executor Thread Pool for parallelism within a program. Thread pools are a pretty common idiom by now. Amazon's queue service SQS was used to distribute work amongst boxes.
Work was queued to SQS in batches of 1000 work items. The items were pulled by the thread pool and processed. Why 1000? The idea is to balance processing overhead with work overhead. You don't want popping items off SQS to dominate your processing time so you have to do enough work in each pass to make it worth the investment.
The architecture uses two thread pools: one to run queries and one to get record values. Applications must carefully tune the number of threads in each pool so the queries to overwhelm the gets. Using a query thread pool with 2 threads and a get thread pool with 32 threads it was possible to perform 300 TPS on a small EC2 instances.
Theoretically the advantage of this architecture is that it will scale to any size you need. SQS is your work distribution backbone and you just spin up the number of thread pool instances you need. The disadvantage is that this is a lot of programmer effort. But let's consider that you had to do some serious processing on each record, you would need something like this approach anyway to scale out the processing. But to perform simple aggregation operations it's total overkill which is why more time needs to be spent on the write site of the equation in SimpleDB/BigTable than the read side as we are used to with a RDBMS.
What's the best way to go parallel? On the front-end life is simple. Go shared nothing and compose your pages from scalable back-end services. This is how Amazon does it and it's how Google AppEngine does it. GAE completely punts on the back-end service layer architecture. Unfortunately we still need to create a back-end architecture for more complex applications.
Thread pools and SQS is one parallelization approach. Instead of thread pools something like Java's fork/join framework could be used. Initially I thought piling on more low level primitive threading facilities into Java was the wrong way to go. Yes, it is a "'multicore-friendly lightweight parallel framework' that supports a style of parallel programming where problems are recursively split into smaller fragments, solved in parallel and recombined," but it's also a style of programming that is very difficult to program correctly. If cloud architectures will rely on these primitives for efficiency then I think we have regressed.
Erlang style architectures described by Luke Hoersten in Scalable Web Apps: Erlang + Python is a simpler more reliable to programming model. An event driven actor based approach is much harder to screw up than closely cooperating threads in a shared memory space. Erlang originally ran in embedded systems where the requirement was to reliably squeeze the most work possible out of limited CPU and other compute resources. Oddly enough the embedded node of old closely parallels your basic cloud VM. Start your work horse Erlang (or other similar system) instances and let them efficiently chew up your work loads. Erlang's scheduling model fits perfectly with a service centric job engine cloud instance. It will get more work done then your typical thread based system ever would.