Helping Kristian to answer questions (see below). He can elaborate if
he wish to.
On Tue, May 7, 2013 at 12:52 PM, Alex Yurchenko
<alexey.yurchenko@xxxxxxxxxxxxx> wrote:
On 2013-05-07 17:13, Kristian Nielsen wrote:
Alex Yurchenko <alexey.yurchenko@xxxxxxxxxxxxx> writes:
From the documentation the purpose of domain ID in GTID is quite
clear. But what is the role of server ID?
The role is mainly to ensure uniqueness of GTID when domain_id is
not
configured correctly.
Since both are configured manually, and domain ID can simply default
to 0 in
simple setups, I'd imagine that the possibility of having server ID
configured incorrectly (just missing to configure it) is way more
probable
than having domain ID incorrect. Actually, being an arbitrary node
group ID
what is "incorrect" here? ANY value just makes the node a member of
the
corresponding domain, so ANY domain ID value is legal. Whereas server
ID can
certainly be incorrect (a duplicate within the domain).
Incorrect in this case would be having multi-master replication with
independent replication streams having the same domain_id.
Replication already requires server ID to be unique, so (server_id,
sequence
number) will be globally unique as long as sequence number is
increased
locally on each server.
Domain id is not required to be unique, in fact it will typically be
shared by
master and slave. It is a common mistake to do a manual transaction
on the
slave while transactions are also being done at the same time on the
master. Having server_id in the GTID prevents that two different
transactions
end up with the same GTID.
So suppose we have nodes N0, N1 and N2 with IDs 0-0, 0-1, 0-2
respectively.
Initially N1 and N2 both replicate from N0 and have identical DB
contents.
At 0-0-10 N2 goes to maintenance.
After 0-0-100 someone executes local transaction on N1 and it gets
logged as
0-1-101. Right?
Right.
So if now N0 executes another transaction, what will be its GTID?
a) on N0 - 0-0-101?
Correct.
b) on N1 - 0-0-102 or 0-0-101?
(as your documentation states
On N1 it will be the same -- 0-0-101. The purpose of GTID is that the
same transaction has the same GTID on each server.
The server ID is set to the server ID of the server where the
event
group is first logged into the binlog. The sequence number is
increased on a
server for every event group logged.
So it is actually another question, sequence number is not set on the
master
server but always computed locally?)
For every transaction replicated from master sequence number is set on
the master. For each transaction executed on the slave locally
sequence number is generated locally.
Or, does N1 detect a problem at this point? If yes, how exactly?
N1 doesn't detect a problem currently, but Kristian plans to implement
a "strict gtid mode" when N1 will detect a problem.
How server ID is involved there?
Server ID is involved in a sense that there's no confusion when you
talk about transaction with sequence number 101 in your example above.
There are two transactions with sequence number 101 -- 0-0-101 and
0-1-101. So server ID gives distinct GTID to these transactions
despite the same sequence number.
Now if we can get past this point without an error at N1, and start
N2 to
replicate from N1, I take it will receive and commit 0-1-101. But
will it
ever record it somewhere or its state will be simply 0-0-XXX?
Yes, N2 will receive transaction 0-1-101 and it will put it in binlogs
as 0-1-101, the same way as it was on N1.
If at 0-0-110
we failover N0 to replicate from N2, will it receive 0-1-101?
No, if you failover when both N0 and N2 have already 0-0-110 then N0
won't receive 0-1-101 because it will start replication from the first
transaction after 0-0-110.
OTOH if at 0-0-110 you take N0 out, restore it to the pre- 0-1-101
state and connect to replicate from N2 then N0 will receive 0-1-101.
Pavel
