Топ-100
 
RDBMS Type: MySQL
Topic

Syntax:
XA {START|BEGIN} xid [JOIN|RESUME]

XA END xid [SUSPEND [FOR MIGRATE]]

XA PREPARE xid

XA COMMIT xid [ONE PHASE]

XA ROLLBACK xid

XA RECOVER [CONVERT XID]

For XA START, the JOIN and RESUME clauses are recognized but have no
effect.

For XA END the SUSPEND [FOR MIGRATE] clause is recognized but has no
effect.

Each XA statement begins with the XA keyword, and most of them require
an xid value. An xid is an XA transaction identifier. It indicates
which transaction the statement applies to. xid values are supplied by
the client, or generated by the MySQL server. An xid value has from one
to three parts:

xid: gtrid [, bqual [, formatID ]]

gtrid is a global transaction identifier, bqual is a branch qualifier,
and formatID is a number that identifies the format used by the gtrid
and bqual values. As indicated by the syntax, bqual and formatID are
optional. The default bqual value is '' if not given. The default
formatID value is 1 if not given.

gtrid and bqual must be string literals, each up to 64 bytes (not
characters) long. gtrid and bqual can be specified in several ways. You
can use a quoted string ('ab'), hex string (X'6162', 0x6162), or bit
value (b'nnnn').

formatID is an unsigned integer.

The gtrid and bqual values are interpreted in bytes by the MySQL
server's underlying XA support routines. However, while an SQL
statement containing an XA statement is being parsed, the server works
with some specific character set. To be safe, write gtrid and bqual as
hex strings.

xid values typically are generated by the Transaction Manager. Values
generated by one TM must be different from values generated by other
TMs. A given TM must be able to recognize its own xid values in a list
of values returned by the XA RECOVER statement.

XA START xid starts an XA transaction with the given xid value. Each XA
transaction must have a unique xid value, so the value must not
currently be used by another XA transaction. Uniqueness is assessed
using the gtrid and bqual values. All following XA statements for the
XA transaction must be specified using the same xid value as that given
in the XA START statement. If you use any of those statements but
specify an xid value that does not correspond to some existing XA
transaction, an error occurs.

One or more XA transactions can be part of the same global transaction.
All XA transactions within a given global transaction must use the same
gtrid value in the xid value. For this reason, gtrid values must be
globally unique so that there is no ambiguity about which global
transaction a given XA transaction is part of. The bqual part of the
xid value must be different for each XA transaction within a global
transaction. (The requirement that bqual values be different is a
limitation of the current MySQL XA implementation. It is not part of
the XA specification.)

The XA RECOVER statement returns information for those XA transactions
on the MySQL server that are in the PREPARED state. (See
https://dev.mysql.com/doc/refman/8.0/en/xa-states.html.) The output
includes a row for each such XA transaction on the server, regardless
of which client started it.

XA RECOVER requires the XA_RECOVER_ADMIN privilege. This privilege
requirement prevents users from discovering the XID values for
outstanding prepared XA transactions other than their own. It does not
affect normal commit or rollback of an XA transaction because the user
who started it knows its XID.

XA RECOVER output rows look like this (for an example xid value
consisting of the parts 'abc', 'def', and 7):

mysql> XA RECOVER;
+----------+--------------+--------------+--------+
| formatID | gtrid_length | bqual_length | data |
+----------+--------------+--------------+--------+
| 7 | 3 | 3 | abcdef |
+----------+--------------+--------------+--------+

The output columns have the following meanings:

o formatID is the formatID part of the transaction xid

o gtrid_length is the length in bytes of the gtrid part of the xid

o bqual_length is the length in bytes of the bqual part of the xid

o data is the concatenation of the gtrid and bqual parts of the xid

XID values may contain nonprintable characters. XA RECOVER permits an
optional CONVERT XID clause so that clients can request XID values in
hexadecimal.

URL: https://dev.mysql.com/doc/refman/8.0/en/xa-statements.html

Example