[omniORB] Issues with persistent objects

[Martin B.]

On 10.02.2011 18:31, Duncan Grisby wrote:
> On Mon, 2011-02-07 at 10:26 +0100, Martin B. wrote:
>
>>> That is intentional. What happens is that the client does not notice
>>> that the connection has broken until after it has sent its request. You
>>> snipped the bit of the omniORB log that shows the exception details but
>>> you'll see that it has a completion status of COMPLETION_MAYBE. i.e. the
>>> (...)
>
> [...]
>> WHY does the client not notice that the connection is broken?
>>
>>   From what I understand, shouldn't the server properly close it's TCP
>> connection to the client when it's shutting down. Shouldn't the client
>> notice this disconnect?
>
> The way TCP sockets work means that the client doesn't learn about the
> disconnect until too late.
>
> The sequence of steps a client uses to do a call is:
>
> 1. find an existing connection to the server, or open a new one
> 2. perform one or more send() calls to send the request
> 3. block in recv() waiting for a reply
> 4. potentially perform more recv() calls to receive all the data
>
> If the server has closed the connection, it is common for the OS to
> report success for the send() calls. It is generally only if a large
> amount of data must be sent that send() returns an error.
>
> It is thus only when the client has sent the request and blocks in
> recv() that recv() returns an error and the client knows the connection
> has broken. By that stage it doesn't know whether the server received
> the request or not, hence the COMPLETED_MAYBE status and the lack of
> automatic retry by default.
>
> In some circumstances, if omniORB did a poll() or select() on the socket
> before its send(), it would be able to see the error state. However,
> that would still be vulnerable to exactly the same situation some of the
> time, and it would slow down all calls, so omniORB doesn't do that.
>

Oh my! Thanks a lot for explaining that!

(One day I should do some serious socket programming myself -- but who 
ever needs that if one's got C-omniORB-A to do the details :-)

cheers,
Martin