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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5071) Hadoop 1.0 Compatibility Requirements
Date Fri, 16 Jan 2009 22:39:59 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5071?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12664717#action_12664717

Doug Cutting commented on HADOOP-5071:

Job persistence would be a great feature to add at some point, but I don't see why it is essential
for 1.0.  Hadoop 1.0 is a batch-oriented system, not a high-availability system.  We'll force
clients to stop accessing HDFS while it is upgraded, and I think a 1.0.1 release that forces
folks to empty their job queues before they upgrade would be acceptable.  Why is this critical
to 1.0, and what does it have to do with compatibility?

> Hadoop 1.0 Compatibility Requirements
> -------------------------------------
>                 Key: HADOOP-5071
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5071
>             Project: Hadoop Core
>          Issue Type: Sub-task
>            Reporter: Sanjay Radia
>            Assignee: Sanjay Radia
> The purpose of this Jira is to decide on  Hadoop 1.0 Compatibility requirements
> A proposal is described below that was discussed on email alias core-dev@hadoop.apache.org
> Release terminology used below:
> *Standard release numbering: major, minor, dot releases*
> * Only bug fixes in dot releases: m.x.y
> ** no changes to API, disk format, protocols or config etc. in a dot release
> * new features in major (m.0) and minor (m.x.0) releases
> *Hadoop Compatibility Proposal*
> - *1 API Compatibility*
> No need for client recompilation when upgrading across minor releases (ie. from m.x to
m.y, where x <= y)
> Classes or methods deprecated in m.x can be removed in (m+1).0
> Note that this is stronger than what we have been doing in Hadoop 0.x releases.
> 	This is fairly standard compatibility rules for major and minor releases.
> - *2 Data Compatibility*
> -- Motivation: Users expect File systems preserve data transparently across releases.
> -- 2.a HDFS metadata and data can change across minor or major releases , but such changes
are transparent to user application. That is release upgrade must automatically convert the
metadata and data as needed. Further, a release upgrade must allow a cluster to roll back
to the older version and its older disk format. (rollback needs to restore the orignal data
not any updated data).
> -- 2.a-WeakerAutomaticConversion:
> Automatic conversion is support across a small number of releases. If a user wants to
jump across multiple releases he may be forced to go through a few intermediate release to
get to the final desired release.
> - *3 Wire Protocol Compatibility*
> We offer no wire compatibility in our 0.x release today.
> -- Motivation: The motivation *isn't* to make the hadoop protocols public. Applications
will not call the protocol directly but through a library (in our case FileSystem class and
its implementations). Instead the motivation is that customers run multiple clusters and have
apps that access data across clusters. Customers cannot be expected to update all clusters
> -- 3.a Old m.x clients can connect to new m.y servers, where x <= y but the old clients
might get reduced functionality or performance. m.x clients might not be able to connect to
(m+1).z servers
> -- 3.b. New m.y clients must be able to connect to old m.x server, where x< y but
only for old m.x functionality.
> Comment: Generally old API methods continue to use old rpc methods. However, it is legal
to have new implementations of old API methods call new
> rpcs methods, as long as the library transparently handles the fallback case for old
> -- 3.c. At any major release transition [ ie from a release m.x to a release (m+1).0],
a user should be able to read data from the cluster running the old version.
> --- Motivation: data copying across clusters is a common operation for many customers.
For example this is routinely at done at Yahoo; another use case is HADOOP-4058. Today, http
(or hftp) provides a guaranteed compatible way of copying data across versions. Clearly one
cannot force a customer to simultaneously update all its Hadoop clusters on to a new major
release.  We can satisfy this requirement via the http/hftp mechanism or some other mechanism.
> -- 3.c-Stronger
> Shall we add a stronger requirement for 1. 0 : wire compatibility across major versions?
That is not just for reading but for all operations. This can be supported by class loading
or other games.
> Note we can wait to provide this when 2. 0 happens. If Hadoop provided this guarantee
then it would allow customers to partition their data across clusters without risking apps
breaking across major releases due to wire incompatibility issues.
> --- Motivation: Data copying is a compromise. Customers really want to run apps across
clusters running different versions.

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