Continuous HTTP POSTs to Riak

Qiang Cao caoqiang.cs at gmail.com
Sun Mar 6 11:43:45 EST 2016


Thanks for sending this explanation, Vitaly! This helps for sure. Thank you
all, guys!

-Qiang

On Sun, Mar 6, 2016 at 3:24 AM, Vitaly E <13vitamins at gmail.com> wrote:

> Hi Qiang,
>
> You mentioned that my suggestion to synchronize the clocks of your Riak
> nodes had solved the problem. Let me explain why -- it may clear things up
> for you.
>
> Riak uses vector clocks / version vectors
> <http://docs.basho.com/riak/latest/theory/concepts/context/#Vector-Clocks>
> to keep track of the sequence of updates to a particular key. These are
> "logical clocks" as opposed to physical clocks that usually can't be relied
> upon in a distributed system like Riak. Fetching the vector clock of a key
> and passing it back on write (along with a new value), helps Riak decide
> which modification is the "latest" one. Keep in mind that "concurrent
> modification" does not necessary means simultaneous, but can happen due to
> a network partition, for example.
>
> Normally, when a concurrent update is detected (based on vector clocks),
> Riak returns siblings and lets a client decide which version should be
> used. Since you're running with allow_mult = false, you've actually told
> Riak to resolve conflicts for you. In this case Riak still uses vector
> clocks when possible, and then time-stamps if there's still a conflict.
> Obviously, if you don't pass vector clocks, only time-stamps will be used.
> Moreover, without vector clocks any update is considered concurrent, Riak
> just doesn't expose it with allow_mult = false (remember, you opted for
> automatic conflict resolution?).
>
> Keep in mind that a time-stamp is assigned by the first Riak node a
> request hits. The node then forwards the request based on the key this
> request is trying to write
> <http://docs.basho.com/riak/latest/theory/concepts/Clusters/>.
>
> Now, imagine that a write to a key passes through node X, and within 30
> sec another update to the same key passes through node Y, which is 2 min
> behind node X. Since time-stamps are being used to resolve conflicts, the
> second update will be considered older than the first one, and eventually
> discarded.
>
> So, if this kind of conflict resolution is good enough for you, make sure
> your Riak's clocks are in sync (NTP?). I would also suggest to pass vector
> clocks anyway.
>
> Hope this helps. Everyone is welcome to correct/clarify.
>
> Vitaly
>
>
> On Mar 5, 2016 11:07 PM, "Qiang Cao" <caoqiang.cs at gmail.com> wrote:
>
>> Thanks, Russell. I'm just curious. My application handles that. Maybe
>> that's just because of the vclock.
>>
>> -Qiang
>>
>> On Sat, Mar 5, 2016 at 2:09 PM, Russell Brown <russell.brown at me.com>
>> wrote:
>>
>>> And you pass the vclock back after a GET with the next POST? Unless we
>>> get a look at the vclocks then it's hard to say why or where you have
>>> concurrency. But since concurrency is ultimately unavoidable using riak,
>>> why the concern? Can your application/data model handle it is the main
>>> question?
>>>
>>>
>>> On 5 Mar 2016, at 19:04, Qiang Cao <caoqiang.cs at gmail.com> wrote:
>>>
>>> Thanks, Russell! I do a GET immediately after a POST is done. I use
>>> apache httpclient, which handles requests synchronously. On the client,
>>> POSTs and GETs are sent out sequentially.
>>>
>>> On Sat, Mar 5, 2016 at 1:57 PM, Russell Brown <russell.brown at me.com>
>>> wrote:
>>>
>>>>
>>>> On 5 Mar 2016, at 18:43, Qiang Cao <caoqiang.cs at gmail.com> wrote:
>>>>
>>>> > Just curious. The POSTs are sent out sequentially and a quorum is set
>>>> up on Riak. I wonder how would it happen that Riak still considers the POST
>>>> requests concurrent?
>>>> Did you read the result of POST 1 before sending POST 2? If not, and
>>>> you don’t send the causal context, Riak has to view them as concurrent.
>>>>
>>>> How does the quorum work here?
>>>
>>> Thanks,
>>> Qiang
>>>
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>>>
>>>
>>
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