XA related functions have been introduced in
PECL/mysqlnd_ms version 1.6.0-alpha.
The feature is currently under development. There may be issues and/or feature limitations. Do not use in production environments, although early lab tests indicate reasonable quality.
Please, contact the development team if you are interested in this feature. We are looking for real life feedback to complement the feature.
Below is a list of some feature restrictions.
The feature is not yet compatible with the MySQL Fabric support . This limitation is soon to be lifted.
XA transaction identifier are currently restricted to numbers. This limitation will be lifted upon request, it is a simplification used during the initial implementation.
The XA support by the MySQL server has some restrictions. Most noteably, the servers binary log may lack changes made by XA transactions in case of certain errors. Please, see the MySQL manual for details.
XA/Distributed transactions can spawn multiple MySQL servers.
Thus, they may seem like a perfect tool for sharded MySQL
clusters, for example, clusters managed with MySQL Fabric.
PECL/mysqlnd_ms hides most of the SQL
commands to control XA transactions and performs automatic
administrative tasks in cases of errors, to provide the user
with a comprehensive API. Users should setup the plugin
carefully and be well aware of server restrictions prior to
using the feature.
Example 7.257 General pattern for XA transactions
<?php $mysqli = new mysqli("myapp", "username", "password", "database"); /* BEGIN */ mysqlnd_ms_xa_begin($mysqli, 1 /* xa id */); /* run queries on various servers */ $mysqli->query("UPDATE some_table SET col_a = 1"); ... /* COMMIT */ mysqlnd_ms_xa_commit($link, 1); ?>
XA transactions use the two-phase commit protocol. The two-phase commit protocol is a blocking protocol. During the first phase participating servers begin a transaction and the client carries out its work. This phase is followed by a second voting phase. During voting, the servers first make a firm promise that they are ready to commit the work even in case of their possible unexpected failure. Should a server crash in this phase, it will still recall the aborted transaction after recover and wait for the client to decide on whether it shall be committed or rolled back.
Should a client that has initiated a global transaction crash after all the participating servers gave their promise to be ready to commit, then the servers must wait for a decision. The servers are not allowed to unilaterally decide on the transaction.
A client crash or disconnect from a participant, a server crash
or server error during the fist phase of the protocol is
uncritical. In most cases, the server will forget about the XA
transaction and its work is rolled back. Additionally, the
plugin tries to reach out to as many participants as it can to
instruct the server to roll back the work immediately. It is not
possible to disable this implicit rollback carried out by
PECL/mysqlnd_ms in case of errors during the
first phase of the protocol. This design decision has been made
to keep the implementation simple.
An error during the second phase of the commit protocol can
develop into a more severe situation. The servers will not
forget about prepared but unfinished transactions in all cases.
The plugin will not attempt to solve these cases immediately but
waits for optional background garbage collection to ensure
progress of the commit protocol. It is assumed that a solution
will take significant time as it may include waiting for a
participating server to recover from a crash. This time span may
be longer than a developer and end user expects when trying to
commit a global transaction with
Thus, the function returns with the unfinished global
transaction still requiring attention. Please, be warned that at
this point, it is not yet clear whether the global transaction
will be committed or rolled back later on.
Errors during the second phase can be ignored, handled by yourself or solved by the build-int garbage collection logic. Ignoring them is not recommended as you may experience unfinished global transactions on your servers that block resources virtually indefinitely. Handling the errors requires knowing the participants, checking their state and issuing appropriate SQL commands on them. There are no user API calls to expose this very information. You will have to configure a state store and make the plugin record its actions in it to receive the desired facts.
Please, see the quickstart and related plugin configuration file settings for an example how to configure a state. In addition to configuring a state store, you have to setup some SQL tables. The table definitions are given in the description of the plugin configuration settings.
Setting up and configuring a state store is also a precondition for using the built-in garbage collection for XA transactions that fail during the second commit phase. Recording information about ongoing XA transactions is an unavoidable extra task. The extra task consists of updating the state store after each and every operation that changes the state of the global transaction itself (started, committed, rolled back, errors and aborts), the addition of participants (host, optionally user and password required to connect) and any changes to a participants state. Please note, depending on configuration and your security policies, these recordings may be considered sensitive. It is therefore recommended to restrict access to the state store. Unless the state store itself becomes overloaded, writing the state information may contribute noteworthy to the runtime but should overall be only a minor factor.
It is possible that the effort it takes to implement your own
routines for handling XA transactions that failed during the
second commit phase exceeds the benefits of using the XA feature
PECL/mysqlnd_ms in the first place. Thus,
the manual focussed on using the built-on garbage collection
Garbage collection can be triggered manually or automatically in
the background. You may want to call
immediately after a commit failure to attempt to solve any
failed but still open global transactions as soon as possible.
You may also decide to disable the automatic background garbage
collection, implement your own rule set for invoking the
built-in garbage collection and trigger it when desired.
By default the plugin will start the garbage collection with a
certain probability in the extensions internal
RSHUTDOWN method. The request shutdown is
called after your script finished. Whether the garbage
collection will be triggered is determined by computing a random
1...1000 and comparing it with
the configuration setting
(default: 5). If the setting is greater or equal to the random
value, the garbage collection will be triggered.
Once started, the garbage collection acts upon up to
max_transactions_per_run (default: 100)
global transactions recorded. Records include successfully
finished but also unfinished XA transactions. Records for
successful transactions are removed and unfinished transactions
are attempted to be solved. There are no statistics that help
you finding the right balance between keeping garbage collection
runs short by limiting the number of transactions considered per
run and preventing the garbage collection to fall behind,
resulting in many records.
For each failed XA transaction the garbage collection makes
max_retries (default: 5) attempts to finish
it. After that
PECL/mysqlnd_ms gives up.
There are two possible reasons for this. Either a participating
server crashed and has not become accessible again within
max_retries invocations of the garbage
collection, or there is a situation that the built-in garbage
collection cannot cope with. Likely, the latter would be
considered a bug. However, you can manually force more garbage
collection runs calling
with the appropriate parameter set. Should even those function
runs fail to solve the situation, then the problem must be
solved by an operator.
provides some statistics on how many XA transactions have been
started, committed, failed or rolled back.