all,
i got the failover to work.. listing the steps which might be useful for
other beginners like me

node1 = master, node2=slave

node1 goes down (crash or power failure). while node1 is being recovered, if
your apps have to continue writing to DB, only way i found is that we have
to do the failover

on node 2, run the following slonik commands
     1) failover (id = 1, backup node = 2); (this works only if you
subscribed your set originally with forward=yes)
     2) ensure DB can be written into (not readonly anymore)
     3) drop node (id = 1, event node = 2);
on node 1, run the following commands once it comes back in service
      1) dropdb <dbname>
       2) createdb -O <dbuser> <dbname>
       3) psql -U user -d DB < backup.sql
      //Note : this backup.sql should not have slony tables, so its backup
taken before slony was setup using pg_dump -c -s -U <user> <db> > backup.sql
        4) Run slonik commands to store node (1) and store path
              cluster name = clus_name;
              node 1 admin conninfo = 'dbname=yourdb host=yournode1host
user=dbuser password=pass';
              node 2 admin conninfo = 'dbname=yourdb
host=yournode2host user=dbuser
password=pass';
              store node (id = 1, comment = 'Node 1 slave', event node=2);
              store path (server = 1, client = 2,
                  conninfo = 'dbname=yourdb host=node1host user=repusr
password=pass');
              store path (server = 2, client = 1,
                 conninfo = 'dbname=yourdb host=node2host user=dbuser
password=pass');
         5)Start slony daemon on node 1
         6) subscribe to your sets
             subscribe set (id = 1, provider = 2, receiver = 1, forward =
yes);
             Note above, i am showing the provider is node 2 and receiver is
node 1 (opposite of my initial subscribe)
        7) your replication should be working..


On Tue, Feb 15, 2011 at 9:52 PM, Tech Madhu <technimadhu at gmail.com> wrote:

> Thanks for your reply.
>
> if the orig master is down say due to some hardware issue (for few
> hours/days say), we have to get the system on the slave up (we accept the
> loss of some N txn)
>
> In this case, Is the following pseudcode correct. Assuming before crash of
> master, my master was node:1 and slave node:2
>    On the slave (node 2),
>           a) i run failover command (failover (id=1, backup node = 2)
>           b) Run drop node command of node (1)
>    When the orig master is ready to be brought back in service can i re-use
> the node (1) for it?
>    if so , is it enough to run just the following 2 commands on the
> original master
>        store node (id = 1, event node = 2);
>        store path (server=2, client=1, conninfo='connection info to node2')
>
>
> On Tue, Feb 15, 2011 at 2:52 PM, Jan Wieck <JanWieck at yahoo.com> wrote:
>
>> On 2/15/2011 2:44 PM, Jan Wieck wrote:
>>
>>> This is NOT possible given the Slony-I design.
>>>
>>> Slony-I is an asynchronous replication system. That means that changes
>>> to the origin are replicated some time AFTER they have been committed.
>>> That means that if the origin goes down unexpectedly, you have no chance
>>> of knowing what changes did not propagate to the replica before it
>>> crashed.
>>>
>>> The only way to solve this situation is to actually do a hard FAILOVER,
>>> abandoning the old origin and rebuilding it from scratch.
>>>
>>> To illustrate, think about a simple foreign key constraint, t2.fk
>>> references t1.pk. There currently are no rows in t2 referencing a
>>> certain t1.pk, so node:1 will allow to DELETE it. Node:1 crashes before
>>> the DELETE can propagate to node:2. You failover to node:2 and since it
>>> still has the t1 row, it will happily allow you to INSERT references to
>>> it into t1. Now you bring back node:1 and ... how exactly do you get the
>>>
>>
>> into t2, of course.
>>
>>
>>  two to agree what is right? Will you forcefully remove the rows, node:2
>>> inserted into t2 or will you recreate the t1 row in node:1 so that the
>>> INSERT's can propagate from node:2 to node:1?
>>>
>>>
>>> Jan
>>>
>>>
>>
>> --
>> Anyone who trades liberty for security deserves neither
>> liberty nor security. -- Benjamin Franklin
>>
>
>
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