Related: For raft I would use a local data store for each of the node's
logs, as the comitting and agreeing is something the protocol is supposed
to do - storing everyone's logs in the same db would miss the point of
having raft running I believe.
On Wed, Jul 16, 2014 at 12:57 PM, Konrad Malawski <kt...@typesafe.com>
wrote:
> Hello again, I seem to have missed this email somehow - sorry!
>
> // Thanks for noticing my akka-raft project! :-)
>
> About RAFT... The problem with RAFTs log is that in there a delete can
> happen from the opposite side as you'd normally do in event sourcing.
> So in event sourcing you only delete "until sequence number N", where as
> in raft when a leader changes and decides "my log is the right one", it
> will slowly send older and older log entries to followers,
> until the follower takes the write (they get a match), and from then on
> the leader will replay *his* log to the follower. The follower will take
> all writes, and in it should delete all of it's previous log entries "from
> the one that matches up with the current leader, until
> max(ThatFollowersIndex)". This is exacly the opposite side from where we
> run the deletion ("from N, to most recent", where as event sourcing would
> only ever do "from most distant past, to N").
>
> There is a clear mismatch in the design of these two "logs" (event-log /
> raft-log). We can however implement a raft log on top of an event sourcing
> based journal (akka persistence),
> by storing tombstones instead of issuing deletes - as in "the leader told
> my my index=12 is invalid" and store such events.
> I have not yet implemented this in akka-raft, but it certainly would work
> - and writes are fast in append-only logs, so it could turn out to be
> pretty good anyway!
> On the other hand, perhaps akka-persistence is an overkill for storing a
> raft-log, and explicitly managing the log would be better.
> As I said, this is still an open question and remains to be implemented in
> akka-raft (I guess I'll try both ways and see what works better).
>
> I hope this shines some light on the difference of what raft calls a log
> (can delete going from most-recent) and what an event sourced log
> is (deletes from least-recent to given id).
>
>
>
> On Sun, Jul 13, 2014 at 12:10 PM, Roland Kuhn <goo...@rkuhn.info> wrote:
>
>> Hi Moiz,
>>
>> it seems that you are trying to use Akka Persistence as a log
>> implementation instead of for making an Actor persistent. In this case I
>> would rather recommend to use the log implementation (that underlies the
>> Journal) directly. A solution with PersistentActor would tend to keep all
>> the log in memory and persist only the events which modify it, making the
>> original question moot (since the item to be shipped is not a persisted
>> event).
>>
>> Regards,
>>
>> Roland
>>
>> 7 jul 2014 kl. 19:32 skrev Moiz Raja <moizr...@gmail.com>:
>>
>> Hi Konrad,
>>
>> So I want to create a distributed state machine based on RAFT which I
>> know you're pretty familiar with. I want to use this facility when
>> reconciling logs. My understanding is that when an AppendEntries fails the
>> Leader tries to send the Follower a previous entry from the log till we
>> reach the point where the previous term and sequence match in the two logs.
>> I want to use the read facility in that scenario.
>>
>> I'm not sure reading until a certain sequence number will be performant.
>>
>> I know you must have done more thinking on this since you have worked
>> both on akka-persistence and akka-raft :) so I would appreciate your input.
>>
>> Thanks,
>> -Moiz
>>
>> On Monday, 7 July 2014 10:18:31 UTC-7, Konrad Malawski wrote:
>>>
>>> Hello Moiz,
>>> an event detached from it’s lineage does not make much sense for
>>> building up state - which is how persistent actors are designed to be used.
>>> An persistent actor’s state, and your “domain state”, should be built up
>>> from such series of events (and snapshots).
>>>
>>> We do support playback “until” a given sequence number, you can achieve
>>> this via:
>>>
>>> class Peter extends PersistentActor {
>>> override def preStart() {
>>> self ! Recover(toSequenceNr = 20)
>>> }
>>>
>>> def receiveCommand = { }
>>> def receiveRecover = {
>>> case m if lastSequenceNr == 15 => // ...
>>>
>>> case _ => // ...
>>> }
>>>
>>> If you only want one of these items, you can just ignore the others like
>>> shown in the example above.
>>> But reading out one specific event is not something we will want to
>>> support (we are actively removing methods that target “one message” in
>>> favour of ranged operations (see deprecated deleteMessage vs.
>>> deleteMessages(toSequenceNr)).
>>>
>>>
>>> May I ask why you need to read exactly one event?
>>> It would help to understand your use case, and then advice. :-)
>>>
>>>
>>>
>>> On Mon, Jul 7, 2014 at 7:00 PM, Moiz Raja <moiz...@gmail.com> wrote:
>>>
>>>> I am using akka-persistence and I have a need to read a specific
>>>> journal entry but I do not see a way in akka-persistence to do that.
>>>>
>>>> Is there any reason why an API has not been provided for this? Are
>>>> there any plans to provide this in an upcoming release?
>>>>
>>>> Thanks,
>>>> -Moiz
>>>>
>>>> --
>>>> >>>>>>>>>> Read the docs: http://akka.io/docs/
>>>> >>>>>>>>>> Check the FAQ: http://doc.akka.io/docs/akka/
>>>> current/additional/faq.html
>>>> >>>>>>>>>> Search the archives: https://groups.google.com/
>>>> group/akka-user
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>>>
>>>
>>>
>>> --
>>> Cheers,
>>> Konrad 'ktoso' Malawski
>>> hAkker @ Typesafe
>>>
>>> <http://typesafe.com/>
>>>
>>
>> --
>> >>>>>>>>>> Read the docs: http://akka.io/docs/
>> >>>>>>>>>> Check the FAQ:
>> http://doc.akka.io/docs/akka/current/additional/faq.html
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>>
>>
>>
>> *Dr. Roland Kuhn*
>> *Akka Tech Lead*
>> Typesafe <http://typesafe.com/> – Reactive apps on the JVM.
>> twitter: @rolandkuhn
>> <http://twitter.com/#!/rolandkuhn>
>>
>> --
>> >>>>>>>>>> Read the docs: http://akka.io/docs/
>> >>>>>>>>>> Check the FAQ:
>> http://doc.akka.io/docs/akka/current/additional/faq.html
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>
>
>
> --
> Cheers,
> Konrad 'ktoso' Malawski
> hAkker @ Typesafe
>
> <http://typesafe.com>
>
--
Cheers,
Konrad 'ktoso' Malawski
hAkker @ Typesafe
<http://typesafe.com>
--
>>>>>>>>>> Read the docs: http://akka.io/docs/
>>>>>>>>>> Check the FAQ:
>>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html
>>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user
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