[ZODB-Dev] [Danger] "ZEOClientStorage" does not detect lost ZEOconnection

[Dieter Maurer]

Tim Peters wrote at 2005-6-27 15:38 -0400:
> ...
[Dieter]
>> Probably, "ZEOClientStorage" (and the ZEO server) should use
>> "SO_KEEPALIVE" to enable TCP keepalive messages. However, the default
>> TCP timeouts are probably too high (2 hours) for many ZODB applications
>> (like ours).

[Tim]
>Section 4.2.3.6 of RFC 1122 doesn't read like it thinks keep-alives are
>either portable or appropriate for this.  In particular,
>
>    It is extremely important to remember that ACK segments that
>    contain no data are not reliably transmitted by TCP.
>    Consequently, if a keep-alive mechanism is implemented it
>    MUST NOT interpret failure to respond to any specific probe
>    as a dead connection.

[Dieter]
Then hopefully, TCP knows that and observes it in the
implementation of its "keep-alive" mechanism.

At least, it is quite careful (with Linux default parameters):

   Only after 9 keep alive messages are lost, the connection is
   considered broken.

>...
[Dieter]
>> I will implement an application specific keep alive mechanism.

[Tim]
>How will you do that?

I attach the script (wrapped up as a Zope "product").

> Would it be appropriate for ZEO to do something
>similar itself?

Yes. It would be very nice.

> If ZEO did, would you still need to bother?

No.

And I would be happy, because I am using internal
undocumented implementation details that already once
changed between ZEO releases.

-- 
Dieter

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