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Torsten Schlabach schrieb:
Hi Volker!
> If you give your domain e.g. yourdomain.com more than one IP e.g.
> Datacenter1 : IP = 123.123.123.1
> Datacenter2 : IP = 146.234.12.2
> the DNS performes a round robin loadbalancing on DNS -> IP resolving.
I have had that idea as well. But just help me understand what problem
this solved and which it doesn't:
> The first time a webbrowser accesses yourdomain.com it will get the
> first IP
> and your customer lands in DC1. If the next webbrowser accesses
> yourdomain.com
> it will land in DC2. Next in DC1 and so on.
That will for sure distribute load. But if one of the DCs (or one of
the servers) goes down, doesn't that mean that only every 2nd customer
will see our site while every other customer will end up with a timeout?
Thats right. You will have to set the TTL of the domain record as short
enought that you can
switch the DNS to only serve the DC that is still up.
We have made some testing on how long webbrowsers will cache the DNS
information. It seems as that the DNS information
is hold longer in the webbroser than the given TTL. So this gives hope
to reduce the TTL to say 10 Seconds.
Now you will need a third instance that monitors your two DCs. If one of
the DCs went down the monitoring instance have to modify the DNS entry.
We had also been asking ourselves if we could have a mechanism where
the DNS servers will receive a heartbeat from the actual servers and
just switch off A records for servers which are currently unavailable.
But the problem with that is caching nameservers. AFAIK even if we
would set the TTL on our nameservers to 10 minutes to make sure that a
cached record for a server that died will expired after no more than
10 minutes, I understand for caching nameservers at any ISP they
decide how long they will cache the record or not.
It would be a violation of the DNS if a ISP caches the domain entries
longer as the TTL.
Have you tested this? Our two big ISPs in germany respect the TTL set by
the domain provider.
mira2:~# dig @195.50.140.250 inqbus.de
;; ANSWER SECTION:
inqbus.de. 300 IN A 193.239.28.142
mira2:~# dig @195.50.140.250 inqbus.de
; <<>> DiG 9.2.4 <<>> @195.50.140.250 inqbus.de
;; ANSWER SECTION:
inqbus.de. 297 IN A 193.239.28.142
As you see the TTL decreases on consequtive queries, as expected. The
DNS server queried is a DNS server of the second largest ISP in germany.
Try it out. We have had to set the "minimum TTL" limit at our Domain to
get this to work.
But the naming of this parameter may vary between the domain providers.
One idea: Setup an arbitrary domain with two IPs. I will test your setup
from germany. Maybe others on this list will support this testing
and we may discover if this IP RR thingy works worldwide over a broad
variety of ISPs or not.
Best regards
Volker
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Dr. Volker Jaenisch http://www.inqbus.de
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