davoustp commented on pull request #504: URL: https://github.com/apache/struts/pull/504#issuecomment-968036028
> Thanks for producing this PR in response to the JIRA issue. 👍 My pleasure, that's normal to contribute back when we find something useful for the community (OSS rocks). > Looking at the history of OgnlUtil (as far back as I can see in the repository), it looks like the logic considered an expression "valid" to put in the expression cache if-and-only-if it could be successfully executed. It seems like that may have been an intentional decision based on the original code comment ? Yep, that's my belief as well, the logic/behaviour is not documented, neither as a comment nor in commit history, so we're stuck in a guess game. > There may be a trade-off in execution performance (by allowing more expressions to be considered "valid" and cached) and memory utilization (due to the larger number of potential expressions now being cached). Very true, as with any caching strategy: you trade off CPU or latency against memory. > Having expressions considered "valid" if-and-only-if they can be executed successfully may have provided a limiting factor to keep the cache size more manageable in the past, but that is just my speculation. Definitely. However, I do think that this is a weird way to implement memory usage control. The problem I see is that the fact that an expression evaluates successfully or not does *not* depend on the expression itself, but on the context it's evaluated against. Such an expression may fail to execute 1000 times, and succeeds on the next attempt (because using a different context which allows the expression to evaluate successfully). So all in a sudden, the expression gets cached... All in all, the condition to cache a valid expression entirely depends onto the context we throw at it. Weird, isn't it? If we care about memory control, I would then rather go using a real cache with a proper eviction policy, such as https://github.com/ben-manes/caffeine (I use it a lot, it's rock stable, highly efficient and extremely flexible). This would allow to evict least frequently used expressions, which can even be balanced against its cost to build if needed. This could be bounded or not, time-based... (we'd need to check the minimum JDK version supported by Struts2, though, since caffeine requires JDK 8 minimum if I remember correctly) This way, any application generating a very large set of expressions would still get the benefit of caching for the more frequently used expressions without growing the cache too large. And expressions get cached irrespective of any evaluation result. A small set of configuration knobs might be required as well (to allow user control on cache retention duration, bounded/unbounded and upper bound if needed...). And going this way could only go into the 2.6.x branch of course. > This change may be safer to attempt for the 2.6.x branch (as opposed to PR #503 for 2.5.x), since it introduces a new behaviour for the expression cache. If we prefer keeping the changes low, yes, of course, we could have that change in 2.6.x only. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
