[GitHub] [tomcat] ChristopherSchultz commented on pull request #596: Use SELECT...FOR UPDATE to update session data in the database instead of DELETE, INSERT.

["ChristopherSchultz (via GitHub)" <gi...@apache.org>]

ChristopherSchultz commented on PR #596:
URL: https://github.com/apache/tomcat/pull/596#issuecomment-1462052065

   > @ChristopherSchultz Is there a list of supported database systems with which the DataSourceStore is compatible? Are you sure that they all support "SELECT FOR UPDATE"? I tried to look that up and the "best" thing I found was https://www.sql-workbench.eu/dbms_comparison.html
   
   Good question. I was originally going to use `MERGE` but the database I have at-hand (MariaDB) doesn't support it, so I went with `SELECT...FOR UPDATE`. It's not clear to me if the `SELECT...FOR UPDATE NOWAIT` listed in that table is specifically talking about the `NOWAIT` part of the query. For example, I know that both MariaDB and MySQL have supported `SELECT...FOR UPDATE` for a long time, but the entry for MySQL states "_Since 8.0_" and MariaDB says "_No_". A quick check of the [HSQLDB Documentation](http://hsqldb.org/doc/2.0/guide/dataaccess-chapt.html#dac_jdbc_cursor_updatability) shows that `SELECT...FOR UPDATE` is available while that list claims it is not supported. Same thing with [IBM DB2](https://www.ibm.com/docs/en/informix-servers/12.10?topic=statement-update-clause) and [SQL Server](https://learn.microsoft.com/en-us/sql/odbc/reference/appendixes/processing-select-for-update-statements?view=sql-server-ver16). But it's clear that not all RDMBSs support that syntax.
   
   Maybe it's not possible to write a single code-path to satisfy all major RDBMS systems. I'd be happy to change the PR to offer a selection between the two via configuration.


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