[e-lang] VatTP and Waterken protocols compared

[Bill Frantz]

tyler.close at gmail.com (Tyler Close) on Sunday, September 21, 2008 wrote:

>On Sun, Sep 21, 2008 at 2:20 PM, Ben Laurie <benl at google.com> wrote:
>> On Sun, Sep 21, 2008 at 2:17 PM, Tyler Close <tyler.close at gmail.com> wrote:
>>> On Sun, Sep 21, 2008 at 12:29 PM, Ben Laurie <benl at google.com> wrote:
>>>> Also, an evil recipient can stall the sender by never processing messages.
>>>
>>> The sender has an independent message queue for each recipient host,
>>> so an evil recipient can only stall its own reception of messages.
>>> Other recipients are not affected.
>>
>> So what happens when the sender tries to queue a message to the full queue?
>
>There's no notion of the sender's queue being full. If an application
>is worried about this kind of resource exhaustion, it can keep track
>of its outstanding messages and stop sending them when appropriate.

When I think about these issues, I think about a program which
wishes to invoke a remote operation on each member of a large
collection class. The naive approach is to enumerate the collection
class and send a message, with a promise for the result, for each
member.

In E on Java, if the remote processing is slower than the
communication system, the remote vat queue will fill up and
generate an out-of-memory exception.

In Waterken, the sending vat will be blocked on the send via TCP
backpressure. If the only function of the sending vat is to get
answers for each member, then this result is fine. If the remote
end continues to process messages, then eventually the local end
will get all its messages sent and get the answers.

The other case is Ben's evil recipient. In that case, the local vat
will never get the answers, and how it fails to not get them is
less interesting.

There are two cases where the local vat needs to be aware of the
problem:

  (1) It has other functions it must perform, and must remain
      responsive for them.

  (2) It has other remote vats that also could provide the answers
      it needs.

Cheers - Bill

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