spike report:

Given a render(url) function that is expensive, and the following template:

{template .MyTemplate}
    {let $a: render('/feed/A.json') /}
    {let $b: render('/feed/B.json') /}
    {let $c: render('/feed/C.json') /}
    {$b}{$a}{$c}
{/template}

Is it possible to have invocations of render() execute in parallel? It would seem that the type SoyValueProvider would allow for this, in that the resolve() method could be lazily evaluated.

I've created RenderFunction.java file to emulate an expensive operation:2

public SoyValue computeForJava(final List<SoyValue> list) {
    final CompletableFuture<String> future = getFuture(list.get(0).coerceToString());
    final SoyValueProvider provider = valueConverter.convert(future);
    return new SoyProviderFacadeValue(provider);
}

private CompletableFuture<String> getFuture(final String url) {
    return CompletableFuture.supplyAsync(() -> {
        logger.info("starting future async of: " + url);
        try {
            Thread.sleep(5000);
            logger.info("done calculating: " + url);
            return "VALUE[" + url + "]";
        } catch (InterruptedException e) {
            logger.error("error", e);
            return "ERROR[" + url + "]";
        }
    }, executor);
}

see: SoyProviderFacadeValue

Running the template above yields the following output:

------------------------------------
printing template - foo.MyTemplate
[pool-1-thread-1] INFO com.geniussportsmedia.spikes.soyasync.RenderFunction - starting future async of: /feed/B.json
[pool-1-thread-1] INFO com.geniussportsmedia.spikes.soyasync.RenderFunction - done calculating: /feed/B.json
[pool-1-thread-2] INFO com.geniussportsmedia.spikes.soyasync.RenderFunction - starting future async of: /feed/A.json
[pool-1-thread-2] INFO com.geniussportsmedia.spikes.soyasync.RenderFunction - done calculating: /feed/A.json
[pool-1-thread-3] INFO com.geniussportsmedia.spikes.soyasync.RenderFunction - starting future async of: /feed/C.json
[pool-1-thread-3] INFO com.geniussportsmedia.spikes.soyasync.RenderFunction - done calculating: /feed/C.json
took: 15074ms
------------------------------------

All the operations are done synchronously. Ideally all the render(url) calls would happen in parallel as the reduce the total wait time.

A workaround we found to achieve this was to use to store the operation results in a map literal:

{template .MyTemplate2}
    {let $data: [
      'd': render('/feed/C.json'),
      'e': render('/feed/E.json'),
      'f': render('/feed/F.json')
    ] /}
    {$data.d}
    {$data.e}
    {$data.f}
{/template}

------------------------------------
printing template - foo.MyTemplate2
[pool-1-thread-4] INFO com.geniussportsmedia.spikes.soyasync.RenderFunction - starting future async of: /feed/C.json
[pool-1-thread-5] INFO com.geniussportsmedia.spikes.soyasync.RenderFunction - starting future async of: /feed/E.json
[pool-1-thread-1] INFO com.geniussportsmedia.spikes.soyasync.RenderFunction - starting future async of: /feed/F.json
[pool-1-thread-4] INFO com.geniussportsmedia.spikes.soyasync.RenderFunction - done calculating: /feed/C.json
[pool-1-thread-5] INFO com.geniussportsmedia.spikes.soyasync.RenderFunction - done calculating: /feed/E.json
[pool-1-thread-1] INFO com.geniussportsmedia.spikes.soyasync.RenderFunction - done calculating: /feed/F.json
took: 5003ms
------------------------------------