Dave is correct - if you have N nodes you need (N/2) + 1 votes (i.e. a majority) in the standard case to get a vote to pass.
Adding a fourth voting node to a three node cluster will cause the size of a majority to jump from 2 to 3. The number of nodes that need to fail before you can no longer get a majority is 2 in both cases - so you don't get any reliability for adding a new voting node to a odd-numbered cluster. The new node will always act as a voter unless you explicitly configure it as an observer. Henry On 25 August 2010 15:11, Dave Wright <wrig...@gmail.com> wrote: > I'm not an expert on voting, so there may be a better answer, but from my > understanding all 4 nodes participate in the voting and you need a majority > of 3 to elect a leader. > > -Dave > > On Wed, Aug 25, 2010 at 6:09 PM, Todd Nine <t...@spidertracks.co.nz> > wrote: > > > Thanks for that Dave. If I do not configure it as an observer just a > > normal member, what will the last even node to join do? > > > > > > 1. Will it participate as a voter on startup? (I'm assuming not, just > read > > only) > > > > 2. If one of the voter nodes 1 through 3 dies, does it become a voter? > > > > > > todd > > SENIOR SOFTWARE ENGINEER > > > > todd nine| spidertracks ltd | 117a the square > > po box 5203 | palmerston north 4441 | new zealand > > P: +64 6 353 3395 | M: +64 210 255 8576 > > E: t...@spidertracks.co.nz W: www.spidertracks.com > > > > > > > > > > > > On Wed, 2010-08-25 at 17:57 -0400, Dave Wright wrote: > > > > > > > > 1. When the 4th ZK node joins the cluster, does it take on the observer > > > role since a quorum cannot be reached with the new node? Can I still > > > connect my clients to it and create/remove nodes and receive events? > > > > No, it joins as a normal member unless you've configured it as an > > observer. Note that with 4 nodes you now need 3 running to get a > > majority, which is why even numbers aren't recommended. > > > > > > > > > > > 2. In the event 1 of the 3 voting nodes fails, will this 4th node > become > > > a voting member of the ensemble? > > > > If configured as an observer it remains an observer. > > > > > > > > 3. When a new node comes online, it may have a different ip than the > > > previous node. Do I need to update all node configurations and perform > > > a rolling restart, or will simply connecting the new node to the > > > existing ensemble make all nodes aware it is running? > > > > Unfortunately ZK doesn't have any kind of dynamic configuration like > > that currently. You need to update all the config files and restart > > the ensemble. > > > > -Dave Wright > > > > > -- Henry Robinson Software Engineer Cloudera 415-994-6679