[jira] [Updated] (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:all-tabpanel ]

Axel Hanikel updated OAK-7932:
------------------------------
    Description: 
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.

 

  was:
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 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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