Experimental: Mysql Protocol support for VStream

[rohit-nayak-ps]

Note: This is a quick experimental POC and NOT for production use

We add a new vstream command in vtgate that uses the VStream API to stream data
from a table.

To try it out:

• run local_example upto (and not including) the 401_cleanup.sh
• start the mysql client pointing to vtgate
  mysql -u root --host=127.0.0.1 -P 15306 --quick

mysql> set workload=olap;
Query OK, 0 rows affected (0.00 sec)

mysql> vstream * from product \G
*************************** 1. row ***************************
         op: +
        sku: SKU-1001
description: Monitor
      price: 100
*************************** 2. row ***************************
         op: +
        sku: SKU-1002
description: Keyboard
      price: 30

/* In another mysql client run "update product set price=200 where sku = 'SKU-1001';", gives: */
*************************** 3. row ***************************
         op: *
        sku: SKU-1001
description: Monitor
      price: 200

/* In another mysql client run "delete from product where sku = 'SKU-1001';", gives: */
*************************** 4. row ***************************
         op: -
        sku: SKU-1001
description: Monitor
      price: 200

Notes

• The first column contains + for inserts, - for deletes and * for updates
• Without any where clause all binlog events for that table are streamed
• We can get just new event using "vstream * from product where pos > 'current';"
• The pos in the where clause can be any gtid to start streaming from
• There is a limit clause to only stream a few rows "vstream * from product limit 10;"
• Currently data is streamed from all shards in the keyspace
• Only one table can be streamed at a time
• This is separate from the "stream * from " support which uses the message subsystem

[rohit-nayak-ps]

The functionality detailed above has been implemented in #6670.