[jira] [Commented] (SLIDER-600) RoleHistory to support reloads with different # of roles from current set

["Steve Loughran (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SLIDER-600?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14383849#comment-14383849 ] 

Steve Loughran commented on SLIDER-600:
---------------------------------------

Requirements

# All existing role histories load without problems; location retained. Old files will be used for regression testing here.
# backwards compatibility: new histories reloadable by old code. This is hard to test if the same generated avro classes are used by any copy of the original code.
# reload histories where a named role is not currently defined. Options: (a) discard details or (b) save to role history structures, even though there is not yet any data on them.
# if there are now more roles than in the history, just generate empty history for them.
# detect changes in name => priority mappings. If the name of a priority has changed, treat that as a deleted role followed by a new role. Discard prior history & treat new role as "new".
# treat two roles swapping prorities as a variant of the one above: both roles have been replaced.

Key point: changing the priority of a role will lose all historical data, as will changing the name of a role. But added and removed roles, including reuse of existing role numbers, will be supported by assuming no placement history.

> RoleHistory to support reloads with different # of roles from current set
> -------------------------------------------------------------------------
>
>                 Key: SLIDER-600
>                 URL: https://issues.apache.org/jira/browse/SLIDER-600
>             Project: Slider
>          Issue Type: Sub-task
>          Components: appmaster
>    Affects Versions: Slider 0.60
>            Reporter: Steve Loughran
>             Fix For: Slider 2.0.0
>
>
> The role history won't reload historical placement data if the no. of roles doesn't match what it saved. It only persists numeric values and can't rebuild the information. 
> This means if you add a new role to a cluster: no recovery of any previous placement
> It could persist the names -> priorities of each role, and reload the positions of those roles about which there are currently records, ignoring the rest.



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