IMO latency is the primary issue you will face, but also keep in mind reliability w/in a colo.

Say you have 3 colos (obv can't be 2), if you only have 3 servers, one in each colo, you will be reliable but clients w/in each colo will have to connect to a remote colo if the local fails. You will want to prioritize the local colo given that reads can be serviced entirely local that way. If you have 7 servers (2-2-3) that would be better - if a local server fails you have a redundant, if both fail then you go remote.

You want to keep your writes as few as possible and as small as possible? Why? Say you have 100ms latency btw colos, let's go through a scenario for a client in a colo where the local servers are not the leader (zk cluster leader).

1) client reads a znode from local server
2) local server (usually < 1ms if "in colo" comm) responds in 1ms

1) client writes a znode to local server A
2) A proposes change to the ZK Leader (L) in remote colo
3) L gets the proposal in 100ms
4) L proposes the change to all followers
5) all followers (not exactly, but hopefully) get the proposal in 100ms
6) followers ack the change
7) L gets the acks in 100ms
8) L commits the change (message to all followers)
9) A gets the commit in 100ms
10) A responds to client (< 1ms)

write latency: 100 + 100 + 100 + 100 = 400ms

Obviously keeping these writes small is also critical.


Martin Waite wrote:
Hi Ted,

If the links do not work for us for zk, then they are unlikely to work with
any other solution - such as trying to stretch Pacemaker or Red Hat Cluster
with their multicast protocols across the links.

If the links are not good enough, we might have to spend some more money to
fix this.


On 8 March 2010 02:14, Ted Dunning <> wrote:

If you can stand the latency for updates then zk should work well for you.
It is unlikely that you will be able to better than zk does and still
maintain correctness.

Do note that you can, probalbly bias client to use a local server. That
should make things more efficient.

Sent from my iPhone

On Mar 7, 2010, at 3:00 PM, Mahadev Konar <> wrote:

 The inter-site links are a nuisance.  We have two data-centres with 100Mb
links which I hope would be good enough for most uses, but we need a 3rd
site - and currently that only has 2Mb links to the other sites.  This
be a problem.

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