Re: BUG #7534: walreceiver takes long time to detect n/w breakdown

On 21.09.2012 14:18, Amit kapila wrote:
> On Tuesday, September 18, 2012 6:02 PM Fujii Masao wrote:
> On Mon, Sep 17, 2012 at 4:03 PM, Amit Kapila<amit(dot)kapila(at)huawei(dot)com> wrote:
>
>>> Approach-2 :
>>> Provide a variable wal_send_status_interval, such that if this is 0, then
>>> the current behavior would prevail and if its non-zero then KeepAlive
>>> message would be send maximum after that time.
>>> The modified code of WALSendLoop will be as follows:
>
> <snip>
>>> Which way you think is better or you have any other idea to handle.
>
>> I think #2 is better because it's more intuitive to a user.
>
> Please find a patch attached for implementation of Approach-2.

Hmm, I think we need to step back a bit. I've never liked the way
replication_timeout works, where it's the user's responsibility to set
wal_receiver_status_interval < replication_timeout. It's not very
user-friendly. I'd rather not copy that same design to this walreceiver
timeout. If there's two different timeouts like that, it's even worse,
because it's easy to confuse the two.

So let's think how this should ideally work from a user's point of view.
I think there should be just two settings: walsender_timeout and
walreceiver_timeout. walsender_timeout specifies how long a walsender
will keep a connection open if it doesn't hear from the walreceiver, and
walreceiver_timeout is the same for walreceiver. The system should
figure out itself how often to send keepalive messages so that those
timeouts are not reached.

In walsender, after half of walsender_timeout has elapsed and we haven't
received anything from the client, the walsender process should send a
"ping" message to the client. Whenever the client receives a Ping, it
replies. The walreceiver does the same; when half of walreceiver_timeout
has elapsed, send a Ping message to the server. Each Ping-Pong roundtrip
resets the timer in both ends, regardless of which side initiated it, so
if e.g walsender_timeout < walreceiver_timeout, the client will never
have to initiate a Ping message, because walsender will always reach the
walsender_timeout/2 point first and initiate the heartbeat message.

The Ping/Pong messages don't necessarily need to be new message types,
we can use the message types we currently have, perhaps with an
additional flag attached to them, to request the other side to reply
immediately.

- Heikki