[jira] [Resolved] (SOLR-15630) MDC "prefix labels" should be handled by Layout - not hardcoded in solr code

["Chris M. Hostetter (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SOLR-15630?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Chris M. Hostetter resolved SOLR-15630.
---------------------------------------
    Fix Version/s: main (9.0)
       Resolution: Fixed

> MDC "prefix labels" should be handled by Layout - not hardcoded in solr code
> ----------------------------------------------------------------------------
>
>                 Key: SOLR-15630
>                 URL: https://issues.apache.org/jira/browse/SOLR-15630
>             Project: Solr
>          Issue Type: Improvement
>      Security Level: Public(Default Security Level. Issues are Public) 
>            Reporter: Chris M. Hostetter
>            Assignee: Chris M. Hostetter
>            Priority: Major
>             Fix For: main (9.0)
>
>         Attachments: SOLR-15630.patch, SOLR-15630.patch, SOLR-15630.patch
>
>
> Prior to Solr 9, Solr code included hardcoded, single character, prefixes on all MDC values passed to the logging framework.
> Starting with Solr 9, these hardcoded prefixes have been removed, and the Logging "appends" are expected to provide any labeling/prefixing of what the values are.  The default Solr log4j configuration has been updated to include the same single character prefixes (that were previously hardcoded).
> Users who have a custom log4j configuration that wish to ensure Solr 9.x logs are consistently formatted after upgrading will need to make similar changes to their logging configuration files, by using {{%notEmpty}} macro patterns that include the prefixes, and wrapping those  around the existing {{%X}} MDC patterns:
> {code:title=8.x}
> [%X{collection} %X{shard} %X{replica} %X{core}]
> {code} 
> {code:title=9.x}
> [%notEmpty{c:%X{collection}}%notEmpty{ s:%X{shard}}%notEmpty{ r:%X{replica}}%notEmpty{ x:%X{core}}]
> {code}
> ----
> {panel:title=Original Description}
> Currently, when Solr puts values in the MDC for logging, it always prepends them with a "CHAR" + "COLON" prefix that uniquely identifies what the value is – even though the MDC is a map and every value has a "key" associated with it that identifies what it is, ex...
> {code:java}
>   public static void setCollection(String collection) {
>     if (collection != null) {
>       MDC.put(COLLECTION_PROP, "c:" + collection);
>     } else {
>       MDC.remove(COLLECTION_PROP);
>     }
>   }
> {code}
> This choice evidently traces back to when MDC values where initially added SOLR-6673/SOLR-7590 (when Solr was still using slf4j->JUL IIUC) because that was best (only?) way folks found to ensure that log messages that did _not_ have values for specific MDC keys weren't cluttered with valueless prefixes  even when a log message didn't correspond to a specific collection (or replica, or shard, etc...)
> But with modern log4j, we can use use things like the {{%notEmpty}} syntax to prevent excessive verbosity when MDC values aren't set – allowing us to move the prefixes out of the Solr code and into Solr default log4j configuration – so that users who customize their log4j configuration with things like {{%mdc}} (or JSON logging) can get the "bare" values w/o hardcoded prefixes. (while keeping the exact same default log format ... or even improving it by removing some of the wasted whitespace)
> {panel}
>  



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