mysqlnd-ms-php-concept_xa_trx-6

  • Concepts
  • XA/Distributed transactions

  • XA/Distributed transactions
  • XA/Distributed transactions

    XA/Distributed transactions

    Note: Version
    requirement

    XA related functions have been introduced in
    PECL/mysqlnd_ms version 1.6.0-alpha.

    Note: Early adaptors
    wanted

    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.

    Note: MySQL server
    restrictions

    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 #1 General pattern for XA
    transactions

    <?php
    $mysqli 
    = new mysqli("myapp""username""password""database");

    /* BEGIN */
    mysqlnd_ms_xa_begin($mysqli/* xa id */);

    /* run queries on various servers */
    $mysqli->query("UPDATE some_table SET col_a = 1");
    ...

    /* COMMIT */
    mysqlnd_ms_xa_commit($link1);
    ?>

    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 mysqlnd_ms_xa_commit(). 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 of PECL/mysqlnd_ms in the first place.
    Thus, the manual focussed on using the built-on garbage collection
    only.

    Garbage collection can be triggered manually or
    automatically in the background. You may want to call mysqlnd_ms_xa_gc() 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 value between
    1…1000 and comparing it with the configuration setting
    probability (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
    mysqlnd_ms_xa_gc() with the appropriate
    parameter set. Should even those function runs fail to solve the
    situation, then the problem must be solved by an operator.

    The function mysqlnd_ms_get_stats() provides some
    statistics on how many XA transactions have been started,
    committed, failed or rolled back.