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From "Alexander Klimetschek (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (JCR-3534) Add JackrabbitSession.getValueByContentId method
Date Wed, 24 Apr 2013 14:01:18 GMT

    [ https://issues.apache.org/jira/browse/JCR-3534?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13640457#comment-13640457
] 

Alexander Klimetschek commented on JCR-3534:
--------------------------------------------

I agree with Jukka - we are not talking about a protocol here that needs to be protected,
but an API access. A replication protocol that would make use of this on top of Jackrabbit
would need its own protection (e.g. SSL) to protect the message contents, but that should
be clearly separated.

BTW, yesterday I came across another use case that the solution should include: not only a
shared DataStore between sender and receiver of that application-level replication mechanism,
but also the case of multiple receivers (horizontally scaled boxes) that share one datastore.
In this scenario the sender sends N replication messages to N receivers. Ideally you want
to avoid sending large binaries N times, so one could imagine the receivers sharing a common
DataStore (but not with the sender, as their is usually a strict firewall separation between
sender and receiver and possibly a higher latency as they might reside in different data centers).
In that case I imagine that after the first successful transfer of the binary to receiver
1 and storage in the DataStore, the other replications to the other receivers see that the
binary is present already and don't need to send it over again (although one has to avoid
that all replications happen at the same time to benefit from that).

This means that one should be able to configure a shared secret in both the sender and receiver
DataStores, so they could "trust" each other, but wouldn't necessarily have shared content.

IMHO this case is even more applicable to real-world scenarios and performance benefits -
because a shared DataStore between sender and receiver is definitely less possible than a
shared DataStore among multiple receivers.
                
> Add JackrabbitSession.getValueByContentId method
> ------------------------------------------------
>
>                 Key: JCR-3534
>                 URL: https://issues.apache.org/jira/browse/JCR-3534
>             Project: Jackrabbit Content Repository
>          Issue Type: New Feature
>          Components: jackrabbit-api, jackrabbit-core
>    Affects Versions: 2.6
>            Reporter: Felix Meschberger
>         Attachments: JCR-3534.patch
>
>
> we have a couple of use cases, where we would like to leverage the global data store
to prevent sending around and copying around large binary data unnecessarily: We have two
separate Jackrabbit instances configured to use the same DataStore (for the sake of this discussion
assume we have the problems of concurrent access and garbage collection under control). When
sending content from one instance to the other instance we don't want to send potentially
large binary data (e.g. video files) if not needed.
> The idea is for the sender to just send the content identity from JackrabbitValue.getContentIdentity().
The receiver would then check whether the such content already exists and would reuse if so:
> String ci = contentIdentity_from_sender;
> try {
>     Value v = session.getValueByContentIdentity(ci);
>     Property p = targetNode.setProperty(propName, v);
> } catch (ItemNotFoundException ie) {
>     // unknown or invalid content Identity
> } catch (RepositoryException re) {
>     // some other exception
> }
> Thus the proposed JackrabbitSession.getValueByContentIdentity(String) method would allow
for round tripping the JackrabbitValue.getContentIdentity() preventing superfluous binary
data copying and moving. 
> See also the dev@ thread http://jackrabbit.markmail.org/thread/gedk5jsrp6offkhi

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