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Zookeeper >> mail # dev >> Performing no downtime hardware changes to a live zookeeper cluster


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Re: Performing no downtime hardware changes to a live zookeeper cluster
I was reading through the client code and saw that zookeeper client
caches the server IPs during startup and maintains it for the rest of
its lifetime. If we go with the DNS RR approach or a load balancer
approach, and later swap out a server with a new one ( with a new IP
), all clients would have to be restarted to be able to "forget" the
old IP and see the new one. That doesn't look like a clean approach to
such upgrades. One way of getting around this problem, is adding the
resolution of host names to IPs in the "reconnect" logic in addition
to the constructor. So when such upgrades happen and the client
reconnects, it will see the new list of IPs, and wouldn't require to
be restarted.

Does this approach sound good or am I missing something here ?

Thanks,
Neha

On Wed, Dec 21, 2011 at 7:21 PM, Camille Fournier <[EMAIL PROTECTED]> wrote:
> DNS RR is good. I had good experiences using that for my client
> configs for exactly the reasons you are listing.
>
> On Wed, Dec 21, 2011 at 8:43 PM, Neha Narkhede <[EMAIL PROTECTED]> wrote:
>> Thanks for the responses!
>>
>>>> How are your clients configured to find the zks now?
>>
>> Our clients currently use the list of hostnames and ports that
>> comprise the zookeeper cluster. For example,
>> zoo1:port1,zoo2:port2,zoo3:port3
>>
>>>> > - switch DNS,
>>> - wait for caches to die,
>>
>> This is something we thought about however, if I understand it
>> correctly, doesn't JVM cache DNS entries forever until it is restarted
>> ? We haven't specifically turned DNS caching off on our clients. So
>> this solution would require us to restart the clients to see the new
>> list of zookeeper hosts.
>>
>> Another thought is to use DNS RR and have the client zk url have one
>> name that resolves to and returns a list of IPs to the zookeeper
>> client. This has the advantage of being able to perform hardware
>> migration without changing the client connection url, in the future.
>> Do people have thoughts about using a DNS RR ?
>>
>> Thanks,
>> Neha
>>
>> On Tue, Dec 20, 2011 at 1:06 PM, Ted Dunning <[EMAIL PROTECTED]> wrote:
>>> In particular, aren't you using DNS names?  If you are, then you can
>>>
>>> - expand the quorum with the new hardware on new IP addresses,
>>> - switch DNS,
>>> - wait for caches to die,
>>> - restart applications without reconfig or otherwise force new connections,
>>> - decrease quorum size again
>>>
>>> On Tue, Dec 20, 2011 at 12:26 PM, Camille Fournier <[EMAIL PROTECTED]>wrote:
>>>
>>>> How are your clients configured to find the zks now? How many clients do
>>>> you have?
>>>>
>>>> From my phone
>>>> On Dec 20, 2011 3:14 PM, "Neha Narkhede" <[EMAIL PROTECTED]> wrote:
>>>>
>>>> > Hi,
>>>> >
>>>> > As part of upgrading to Zookeeper 3.3.4, we also have to migrate our
>>>> > zookeeper cluster to new hardware. I'm trying to figure out the best
>>>> > strategy to achieve that with no downtime.
>>>> > Here are some possible solutions I see at the moment, I could have
>>>> > missed a few though -
>>>> >
>>>> > 1. Swap each machine out with a new machine, but with the same host/IP.
>>>> >
>>>> > Pros: No client side config needs to be changed.
>>>> > Cons: Relatively tedious task for Operations
>>>> >
>>>> > 2. Add new machines, with different host/IPs to the existing cluster,
>>>> > and remove the older machines, taking care to maintain the quorum at
>>>> > all times
>>>> >
>>>> > Pros: Easier for Operations
>>>> > Cons: Client side configs need to be changed and clients need to be
>>>> > restarted/bounced. Another problem is having a large quorum for
>>>> > sometime (potentially 9 nodes).
>>>> >
>>>> > 3. Hide the new cluster behind either a Hardware load balancer or a
>>>> > DNS server resolving to all host ips.
>>>> >
>>>> > Pros: Makes it easier to move hardware around in the future
>>>> > Cons: Possible timeout issues with load balancers messing with
>>>> > zookeeper functionality or performance
>>>> >
>>>> > Read this and found it helpful -
>>>> >