On 02/01/14 19:39, Matan Shukry wrote:
(Steven Dake)
I have to say I'm not afraid (and even prefer a bit) API, and I am
interested in as much availability as I can get, so corosync is what I'm
looking for. Good to know though :)
(Christine Caulfield)
As far as I can tell, pacemaker (and products alike) are designed to
deliver high availability to services. Although my application will
become a service at the end,
I also need a way for the services, running on different machines (or
not; might be multiple processes on the same, probably in debug/test
environments),
to talk with each other.
That is, I can understand how pacemaker can replace corosync 'sam'
(leaving aside the efficiency of each one), however, I don't see how
pacemaker will be able
to replace CPG.
I will say this though:
1. The use of CPG between the services, is not directly related to
availability. They need to talk when ever a process goes up/down, so
that one process can
'take over' the data owned by the failed process(should be as fast as
possible), or 'share' more data to a new process(this can have a bit of
delay, if needed).
In order to balance the cluster load, when a new process 'goes up',
it should take smaller pieces of data from each node, rather than a big
piece
from one node, Which is why the new process require talking to all
other processes.
When failing the requirement is the same, although this may happen
over time to decrease the downtime of an 'unowned data'.
2. I am not 100% sure the CPG protocol (Totem Single Ring Ordering and
Membership Protocol, as far as I know) is the best
fit for such case. Then again, I am not 100% sure how the protocol
works. from a brief overview of the protocol,
it seems it match the need requirement in (1).
However I would love to hear someone else's opinion on the matter.
3. Lately I have been messing around with hadoop, and it seems my 'data
sharing' requirement also exists in mapredce/hdfs(although with less time
requirement, I think), and seems to be achieved using ZooKeeper.
I was wondering what are the main differences between the
ZooKeeper/Corosync, specifically about performance.
I did try to 'google' the subject a bit, although most answers were
just in the style of 'they look the same', which
as far as I can tell even the goals of each project are different
(key value store vs high availability? ...)
4. I did eventually found the man pages for the cpg api (after seeing
your comment). I just had to search
for cpg rather than corosync.
5. I looked at the source tree for examples, however all I could find
were tests. Even though the tests may do the basic
connection/messages/etc (Not completely sure about it by the way),
it is not explained nor easy to read what is there.
I could not find any example, even with simple good comments.
Is there any chance you can link me to an existing example in the
source tree?
6. You also said there is 'low' user-level documentation. By any chance
you know a simple tutorial on setting up CPG,
hopefully including sending/receiving messages and anything
related, and may link it?
There are no tutorials for coding services that I know of, they've never
been asked for before, and none of us on the team are tech writers. The
man pages, sources and examples are probably all you will find on the
subject I'm afraid.
For data sharing you might find the openais Ckpt service useful, but be
wary of the other openais services, few are complete or well tested.
Without knowing more about what you are planning to do it's hard to be
more specific(and I wouldn't have the time anyway!). Pretty much all of
the documentation you#ll find is about managing existing services using
pacemaker and rgmanager which is what the vast majority of people seem
to want to do.
Chrissie
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