On Dec 12, 2012, at 4:01 PM, "Vergara, Jeaneth Aguilar" <[email protected]> wrote:
> Thanks Reid, we have that implementation for Read Your Writes already. We'd > like to implement Monotonic Read Consistency as well. For Monotonic Read > Consistency, we were hoping to do Vector Clock comparison but we don't know > how to convert the byte[] into different client's timestamps. Can you go into more explanation of exactly what you mean by 'Monotonic Read Consistency'? Vectors clocks in Riak are meant to be opaque. It's not impossible to use/decode them, but it's not trivial (particularly outside of Erlang). We've not run into a use-case (afaik) where someone _needed_ to decode them. > > Here's a work around we've thought of for now to implement Monotonic Read > Consistency: > > 1. We'll just add another field in our object to track which client wrote it > and a version number (so sort of implementing our own vector clock) > 2. Then we'll just do last write wins (no siblings) > 3. We'll cache our writes and reads where is this cache? is it local to each actor? If so, sounds like you've just implemented read-your-writes > 4. If we do a read and it has a vector clock less than what's in our cache, > we'll use the one in our cache > 5. If we do a read and it has a vector clock greater than what's in our > cache, we'll update our cache and use that latest version > > If you know of a way how a client can interpret the vector clock returned by > Riak API, then we don't have to implement our own poor man's vector clock. > > Also, what's the timestamp returned as part of the Riak API? Is that the > timestamp when the response was created? Or is that the timestamp of the > record inside each instance of Riak DB where that data came from -- is that > timestamp consistent across all replicas? The last-modified timestamp should be when the write for that version of the value was recorded. > > From: Reid Draper [[email protected]] > Sent: Wednesday, December 12, 2012 11:21 AM > To: Vergara, Jeaneth Aguilar > Cc: [email protected] > Subject: Re: Monotonic Read Consistency > > If your writes and reads are written with PR+PW > N (note PR/PW and not R/W), > then you should get read-your-writes consistency. There is unfortunately a > bug with PR/PW [1] that means this is not a 100% guarantee at the moment. I'm > hoping for it to be fixed by Riak 1.4, if not earlier. I don't have JavaDoc > handy, but setting PR and PW should be the same as setting any other > per-request parameter. For an N value of 3 (the default), I'd likely > recommend setting PR and PW to 2. You can get around the aforementioned bug > by setting PR=PW=N, which will lower your availability, but gaurantee > read-your-writes consistency. > > Note that read-your-writes is slightly different than just monotonic read > consistency, but I'm guessing it's closer to what you had in mind. > > [1] https://github.com/basho/riak_kv/issues/388 > > Reid > > > On Dec 5, 2012, at 11:13 PM, "Vergara, Jeaneth Aguilar" > <[email protected]> wrote: > >> Hi there, >> >> We're trying to implement monotonic read consistency at the client side >> using Riak's Java client. We're kind of stumped on how to do vector clock >> comparison so we can ensure that we're always getting the latest version. >> >> Any tips would be appreciated. >> >> Thanks, >> Jing >> _______________________________________________ >> riak-users mailing list >> [email protected] >> http://lists.basho.com/mailman/listinfo/riak-users_lists.basho.com
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