Bundling vs sending serialized dependency graph

Ian Hickson ian at hixie.ch
Wed Aug 20 16:53:31 PDT 2014


On Wed, Aug 20, 2014 at 4:06 PM, John Barton <johnjbarton at google.com> wrote:
> On Mon, Aug 18, 2014 at 10:43 AM, Ian Hickson <ian at hixie.ch> wrote:
>>
>> This just doens't work.
>>
>> Suppose the dependency graph looks like this:
>>
>>      Feature A --> Dependency A1 \__\ Dependency    \
>>      Feature B --> Dependency B1 /  /    AB          >--> Dependency D
>>      Feature C --> Dependency C1 ---> Dependency C2 /
>>
>> All of A, B, and C are to be fetched on-demand-only, to avoid using up
>> too much bandwidth. All the files here are non-trivial in size.
>>
>> How do you package this?
>>
>> If you make a package for A, a package for B, and a package for C, then
>> you'll have redundant content in the packages, and when the client asks
>> for B after already having asked for A, the amount of content sent back
>> will be greater than necessary and therefore it'll be slower than
>> necessary. If you create multiple packages such that you group as much as
>> possible into each package as possible without overlap, then you still end
>> up with multiple resources to download when you need any of A, B, or C.
>> Basically, it boils down to:
>>
>>      Package A \__\ Package    \
>>      Package B /  /    AB       >--> Package D
>>      Package C ------------->  /
>>
>> ...and then you're back to the problem I asked about. If you don't have
>> server-side support, then to avoid round-trips the client needs to know
>> about the dependencies before it makes the first request. It can't wait
>> til it receives the packages to discover the dependencies because if you
>> do that then you're serialising your RTTs instead of pipelining them.
>
> I assume you are imagining a densely connected graph with random access to
> any of the roots.

I'm not quite sure what that means.

I mean a world where different otherwise unrelated leaf modules or
resources depend on common shared dependencies.


> I expect that real life pages have well partitioned graphs
> (widgets) that share some dense parts (utilities) and simple access patterns
> -- main page, utilities, a set of widgets.

I doubt that the Web is that convenient. On some well-designed sites
it might work out that way.

Consider a site like Google+, though. However well-designed it is, it
fundamentally has a lot of common files used by lots of intermediate
shared files used by lots of leaf modules (and a lot more depth while
we're at it). What exactly is needed depends on what posts are
displayed, the user's preferences with respect to features like
Hangouts, etc. It's a complicated graph.


> But sure, it would be great to have a complete solution if it's not a lot
> more complex.

I don't think it should be particularly complex. It only requires some
minor changes. One is that we need to be able to declare dependencies
ahead of the "instantiate" hook. Another (a subset, really) is that we
need to be able to declare dependencies for ES6 modules as well as
letting the ES6 infrastructure discover them automatically. Finally,
it would be ideal if we could also adjust those dependencies on the
fly, since if we're reflecting dependencies described in the mutable
DOM structure, it might be mutated.


> I guess you're proposing the send the dependency graph to the browser, then
> when a new root is needed, the stored graph is compared with the
> currently-loaded modules. The additional modules needed are then requested
> as a group. Up to this point we can just use build tools and browsers.

Actually, modulo the changes described above, the ES6 loader already
does all this. It just doesn't quite handle it at the level of
pre-emptive declaration of dependencies. But suppose you had two
modules A, B, and C. A and B depend on C. With ES6 today, when A is
loaded, it loads C. If late you load B, B doesn't reload C; it just
links into it. So this is all already supported. All that's needed is
a way to tell the ES6 system to get C before it has even received A.


> How will we tell the server "send me this list of files all in one response?"

HTTP pipelining does this today, that's a solved problem.

-- 
Ian Hickson


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