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https://issues.apache.org/jira/browse/TS-2501?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13883431#comment-13883431
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Leif Hedstrom commented on TS-2501:
-----------------------------------

Cool, thanks for the review! One question: This doesn't do the optimizations 
done in the ToDo: (above), do you guys feel that's worth pursuing? Should we 
maybe close this one for now, and open a new one for such optimizations? I 
haven't looked into the usage of that, so don't know if it's even possible 
(feels like it could be though).

> VariableExpander in header_rewrite is unnecessary expensive
> -----------------------------------------------------------
>
>                 Key: TS-2501
>                 URL: https://issues.apache.org/jira/browse/TS-2501
>             Project: Traffic Server
>          Issue Type: Bug
>          Components: Plugins
>            Reporter: Leif Hedstrom
>            Assignee: Leif Hedstrom
>             Fix For: 4.2.0
>
>         Attachments: TS-2501.diff
>
>
> It seems that VariableExpander gets instantiated and executed on every 
> add-header operator. However, this seems rather suboptimal for a number of 
> reasons:
> 1. It might not even be necessary (i.e. there might not be any %<> strings in 
> the "static" string.
> 2. Perhaps even more important, we look for these strings on every request, 
> even though if they are there, they would always be in the same position on 
> every request.
> One suggestion would be to incorporate the VariableExpander "state" as part 
> of parsing the configuration on startup / reload. Maybe it gets complicated 
> when there are other expansions, but it still feels we can pre-parse these 
> strings and get some ideas of what needs to be expanded once, and not on 
> every request.
> This is similar to how e.g. the regex_remap plugin works, it recalculates the 
> positions and expansion once only.
> Thoughts?



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