Also minor typo in "From MariaDB 10.4.5, the above is included the default." on https://mariadb.com/kb/en/spider-storage-engine-overview/

This is correct. "Default value" means that if you don't specify --log-tc, the transaction coordinator log name will be "tc.log"

serg Thanks. At what point is tc.log created? Starting with and without the option (--log-tc=tc.log) does not create tc.log.

https://mariadb.com/kb/en/xa-transactions/#transaction-coordinator-log

If you have two or more XA-capable storage engines enabled, then a transaction coordinator log must be available.
...
If the binary log is enabled on a server, then the server will use the binary log-based transaction coordinator log. Otherwise, it will use the memory-mapped file-based transaction coordinator log.

so, you need "two or more XA-capable storage engines enabled" and you need to disable a binary log

serg Thank you! I had seen this before but did not make the connection between this and the file being created or not. Got it now and it works.
Using --plugin-load=ha_spider.so w/o binlog creates tc.log.

Further question; starting server with --plugin-load=ha_spider.so and --log-bin (with or without --log-tc=tc.log) does not create tc.log (as expected), however XA is still enabled for both InnoDB and Spider. Should this be impossible?

Also it looks like using --plugin-load=ha_spider.so sporadically hangs mysqld. Will log a separate bug for this.

I just wrote it, If the binary log is enabled on a server, then the server will use the binary log-based transaction coordinator log

XA works either way, binlog or not. logging file name can differ.

Note to self; don't skim read. All clear now. Thank you