[jira] [Commented] (OAK-7932) A distributed segment store for the cloud

["Axel Hanikel (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/OAK-7932?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16962800#comment-16962800 ] 

Axel Hanikel commented on OAK-7932:
-----------------------------------

Added reference to second implementation to description.

> A distributed segment store for the cloud
> -----------------------------------------
>
>                 Key: OAK-7932
>                 URL: https://issues.apache.org/jira/browse/OAK-7932
>             Project: Jackrabbit Oak
>          Issue Type: Wish
>          Components: segment-tar
>            Reporter: Axel Hanikel
>            Assignee: Axel Hanikel
>            Priority: Minor
>
> h1. Outline
> This issue documents some proof-of-concept work for adapting the segment tar nodestore to a
>  distributed environment. The main idea is to adopt an actor-like model, meaning:
> -   Communication between actors (services) is done exclusively via messages.
>  -   An actor (which could also be a thread) processes one message at a time, avoiding sharing
>      state with other actors as far as possible.
>  -   Segments are kept in RAM and are written to external storage lazily only for disaster recovery.
>  -   As RAM is a very limited resource, different actors own their share of the total segment space.
>  -   An actor can also cache a few segments which it does not own but which it uses often (such as
>      the one containing the root node)
>  -   The granularity of operating on whole segments may be too coarse, so perhaps reducing the segment
>      size would improve performance.
>  -   We could even use the segment solely as an addressing component and operate at the record level.
>      That would avoid copying data around when collecting garbage: garbage records would just be
>      evicted from RAM.
> h1. Implementation
> The first idea was to use ZeroMQ for communication because it seems to be a high-quality and
>  easy to use implementation. A major drawback is that the library is written in C and the Java
>  library which does the JNI stuff seems hard to set up and did not work for me. There is a native
>  Java implementation of the ZeroMQ protocol, aptly called jeromq, which seems to work well so far,
>  but I don't know about its performance yet.
> There is an attempt to use jeromq in the segment store in a very very very early stage at
>  [https://github.com/ahanikel/jackrabbit-oak/tree/zeromq] . It is based on the memory segment store
>  and currently just replaces direct function calls for reading and writing segments with messages being
>  sent and received.
> A second implementation, at [https://github.com/ahanikel/jackrabbit-oak/tree/zeromq-nodestore] is a simple
> nodestore implementation which is kind of a dual to the segment store in the sense that it is on the other end
> of the compactness spectrum. The segment store is very dense and avoids duplication whereever possible.
> The nodestore in this implementation, however, is quite redundant: Every nodestate gets its own UUID and is saved together
> with its properties, similar to the document node store. This redundancy wastes space, but on the other hand garbage
> collection (yet unimplemented) is easier because there is no segment that needs to be rewritten to get rid of data that is no
> longer referenced; unreferenced nodes can just be deleted. This implementation still has bugs, but being much simpler
> than the segment store, it can eventually be used to experiment with different configurations and examine their
> performance.
>  



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