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[whatwg] MessagePorts in Web Workers: implementation feedback

From: Drew Wilson <atwilson@google.com>
Date: Thu, 7 May 2009 13:39:26 -0700
Message-ID: <f965ae410905071339w282160dehd0f345902644795e@mail.gmail.com>
Hi all,

I've been hashing through a bunch of the design issues around using
MessagePorts within Workers with IanH and the Chrome/WebKit teams and I
wanted to follow up with the list with my progress.

The problems we've encountered are all solveable, but I've been surprised at
the amount of work involved in implementing worker MessagePorts (and the
resulting implications that MessagePorts have on worker
lifecycles/reachability). My concern is that the amount of work to implement
MessagePorts within Worker context may be so high that it will prevent
vendors from implementing the SharedWorker API. Have other implementers
started working on this part of the spec yet?

Let me quickly run down some of the implementation issues I've run into -
some of these may be WebKit/Chrome specific, but other browsers may run into
some of them as well:

1) MessagePort reachability is challenging in the context of separate Worker
heaps

In WebKit, each worker has its own heap (in Chrome, they will have their own
process as well). The spec reads:

User agents must act as if
MessagePort<http://www.w3.org/TR/html5/comms.html#messageport>objects
have a strong reference to their entangled
MessagePort <http://www.w3.org/TR/html5/comms.html#messageport> object.

Thus, a message port can be received, given an event listener, and then
forgotten, and so long as that event listener could receive a message, the
channel will be maintained.

Of course, if this was to occur on both sides of the channel, then both
ports would be garbage collected, since they would not be reachable from
live code, despite having a strong reference to each other.

Furthermore, a MessagePort<http://www.w3.org/TR/html5/comms.html#messageport>object
must not be garbage collected while there exists a message in a task
queue <http://www.w3.org/TR/html5/browsers.html#task-queue> that is to be
dispatched on that
MessagePort<http://www.w3.org/TR/html5/comms.html#messageport>object,
or while the
MessagePort <http://www.w3.org/TR/html5/comms.html#messageport> object's port
message queue <http://www.w3.org/TR/html5/comms.html#port-message-queue> is
open and there exists a
message<http://www.w3.org/TR/html5/comms.html#event-message>event in
that queue.

The end result of this is the need to track some common state across an
entangled MessagePort pair such as: number of outstanding messages, open
state of each end, and number of active references to each port (zero or
non-zero). Turns out this last bit will require adding new hooks to the
JavaScriptCore garbage collector to detect transitioning between 1 and 0
references without actually freeing the object - not that difficult, but
possibly something that other implementers should keep in mind.

2) MessagePorts dramatically change the worker lifecycle

Having MessagePorts in worker context means that Workers can outlive their
parent window(s) - I can create a worker, pass off an entangled MessagePort
to another window (say, to a different domain), then close the original
window, and the worker should stay alive. In the case of WebKit, this causes
some problems for things like worker-initiated network requests - if workers
can continue to run even though there are no open windows for that origin,
then it becomes problematic to perform network requests (part of this is due
to the architecture of WebKit which requires proxying network requests to
window context, but part of this is just a general problem of "how do you
handle things like HTTP Auth when there are no open windows for that
origin?")

Finally, the spec defines a fairly broad definition of what makes a worker
reachable - here's an excerpt from my WebKit Shared Worker design doc, where
I summarize the spec (possibly incorrectly - feel free to correct any
misconceptions):

> PermissibleThe spec specifies that a worker is *permissible* based on
> whether it has a reachable MessagePort that has been entangled *at some
> point in the past* with an active window (or with a worker who is itself
> permissible). Basically, if a worker has *ever* been entangled with an
> active window, or if it's ever been entangled with a worker who is itself
> permissible (i.e. it's associated with an active window via a chain of
> workers that have been entangled at some point in the past) then it's
> permissible.
>
> The reason why the "at some point in the past" language is present is to
> allow a page to create a fire-and-forget worker (for example, a worker that
> does a set of long network operations) without having to keep a reference to
> that worker around.
>
> Once the referent windows close, the worker should also close, as being
> permissible is a necessary (but not sufficient) criteria for being
> runnable.
> Active neededA permissible worker is *active needed* if:
>
>    1. it has pending timers/network requests/DB activity, or
>    2. it is currently entangled with an active window, or another active
>    needed worker.
>
>
> The intent behind #1 is to enable fire-and-forget workers that don't exit
> until they are idle. The intent behind #2 is that an idle worker shouldn't
> exit as long as it's reachable by an active window (possibly chained through
> other workers).
>
The end result is that for each worker we need to keep track of a big list
of every window it's ever been entangled with. As workers become entangled
with other workers, they each inherit the list of entangled windows from the
other worker. As windows become inactive, we then walk the lists of every
worker to remove references to the window and properly shutdown the worker
as appropriate. All of this with the appropriate cross-thread
synchronization, of course :)

Likewise, determining when a worker is active needed requires tracking a
graph of entangled message ports, and walking that graph to determine
whether a given worker is reachable by any active window. Typically this is
only needed when either a window closes, or when a worker goes idle.

Again, none of these issues individually are insurmountable, but in total
they add up to a significant amount of work for what should be a fairly
incremental improvement (going from dedicated workers to shared workers).
Have other vendors started investigating what it takes to implement
SharedWorkers (and therefore MessagePorts in workers)?

Another approach for SharedWorkers would be to give them an implicit
MessagePort-esque API like dedicated Workers and not allow passing in
MessagePorts to postMessage(). This would mean that references to workers
can't really be passed around to other windows/workers, but rather are kept
per-origin. Dedicated workers could work as they do now in Firefox/WebKit
(with no MessagePorts). The SharedWorker lifecycle could be significantly
simplified such that a SharedWorker is permissible as long as there's an
active window under the same origin (no more walking some distributed
cross-thread dependency graph).

The thing we'd give up is the capabilities-based API that MessagePorts
provide, but I'd argue that the workaround is simple: the creating window
can just act as a proxy for the worker. IMO, the implementation burden far
outstrips the benefit of allowing direct foreign access to workers.
Literally 90% of the work on my plate for SharedWorkers seems to derive from
MessagePorts in one form or another, which seems completely wrong.

I'd like to hear your thoughts on this - are people open to removing
MessagePort support from Workers?

-atw
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