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Re: [extremeperl] refactoring databases

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  • Rob Nagler
    ... Yes, we try our best to automate everything. If we think there is going to be multiple upgrades, one of which is critical, we ll organize these in
    Message 1 of 11 , Feb 19, 2005
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      Johan Lindstrom writes:
      > > > Sometimes one can prepare the migration and just do the most critical part
      > > > during the downtime.
      > >
      > >We try to do this, too.
      >
      > Do you automate these steps also?

      Yes, we try our best to automate everything. If we think there is
      going to be multiple upgrades, one of which is critical, we'll
      organize these in separate releases. If this isn't possible, we'll
      make it a two-part upgrade. That script I included in a previous
      message doesn't update the db_upgrade_t if you just call one of the
      methods.

      > Does that mean you don't share identities between tables at all? Just one
      > global sequence (or whatever)?

      Not exactly. The following describes it in detail:

      http://petshop.bivio.biz/src?s=Bivio::Type::PrimaryId

      This the base class of all serial keys. A PrimaryId is a NUMERIC(18)
      (but could be larger) which looks like:

      18 5 3 2 0
      +------------------+-------------+----------+-------+
      | Monotonically | Site Id or | Mistake | Type |
      | Increasing Seq | Other info | Bit | |
      +------------------+-------------+----------+-------+

      The numbers above are digit index. Type is a number that uniquely
      identifies the key type. This divides the name space. The Site Id
      allows you to divide the name space across independent databases. The
      Mistake Bit allows you to make mistakes. :-) We used it in one case
      to transform one key into another without changing the type It saved
      a lookup, and was related to finding the start of a tree for Oracle's
      CONNECT BY. Now they we are dumping CONNECT BY, we have no need for
      it. But that's another discussion.

      Here's how we initialize the sequences:

      CREATE sequence user_s
      MINVALUE 100001
      CACHE 1 INCREMENT BY 100000
      /

      CREATE SEQUENCE club_s
      MINVALUE 100002
      CACHE 1 INCREMENT BY 100000
      /

      CREATE SEQUENCE ec_payment_s
      MINVALUE 100015
      CACHE 1 INCREMENT BY 100000
      /

      CREATE sequence bulletin_s
      MINVALUE 100016
      CACHE 1 INCREMENT BY 100000
      /

      As you can see the spaces are unique, but overlapping. Some people
      might complain it junks up the indexes, but we haven't seen that to be
      a problem. Once the tables get big enough, it really doesn't matter.

      Rob
    • Rob Nagler
      ... Perhaps you should pay them to upgrade. :-) Yes, you ve got a logistical problem, and it sounds like you have a good solution for it. ... Interesting you
      Message 2 of 11 , Feb 19, 2005
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        Matthew Albright writes:
        > We are actually shipping a software product, and we make the new releases
        > available to our customers, but they don't have to upgrade every time we
        > release. We're an XP shop, so we release often, which means we have a wide
        > variety of versions in the field.

        Perhaps you should pay them to upgrade. :-) Yes, you've got a
        logistical problem, and it sounds like you have a good solution for
        it.

        > been in an organization that had a DBA. We have people who are stronger and
        > weaker in certain areas, but really, anyone can modify anything... that's what
        > pair programming is for.

        Interesting you should mention that. I've been a bit turned off to
        "general pair programming" lately. We pair for the same reason. One
        of the things about pair programming that I don't like is that when
        people are alone, they learn from their mistakes at their own speed.
        If they are with another programmer, they not only get the problem
        solved more quickly, but they also don't necessarily assimilate the
        thought process and tools the other programmer brins to the table.
        Experience caused by thousands of wrong paths is a great guide.
        Finding the path right away isn't always the best way. It's certainly
        my experience with kids that they learn much better when they burn
        their hands than when you tell them the iron is hot.

        > Like most relational database-backed applications, we've coded up a
        > object-relational mapping layer that follows the "Active Record" pattern
        > (http://www.martinfowler.com/eaaCatalog/activeRecord.html). The problems come
        > in when you call too much of that mapping code from your update_db.pl script.
        > At the time the update_db.pl code is run, it's running against the newest
        > production code. If your migration code calls methods in the "real" code, that
        > code may change, and your migration code might cease to work in the future.

        Excellent point. We see this sometimes when trying to do a current
        upgrade. I would hate to solve this problem for N versions in the
        past.

        > To avoid this rather subtle problem (which I suspect I've not explained very
        > lucidly), we do 2 things:
        >
        > - Avoid the use of "real" code in the migration code, even if it means
        > duplication
        > - Test to make sure it is possible to upgrade from the oldest customer
        > version to the new version
        >
        > Sometimes, the use of real code is unavoidable, and we've had to reorder things
        > in the update_db.pl file... that's something to be avoided.

        Great solution. Seems like you have to do a lot of feature-based
        tests. For example, you might add a feature, fix it, then remove it
        (because it wasn't the right solution in the first place). Those
        three upgrades are a no-op after the "remove it" upgrade.

        One issue I don't see how you get around is the collection of new
        types of data. You can never assume that a new data type of data
        exists in the database. This means you probably can't retire
        "deprecated" code that works around the non-existence of the new type
        of data. This may create lots and lots of paths to test. Ugh...

        Rob
      • Rob Nagler
        ... Yikes. Remember there s a clear separation of concerns between the customer and the programmer. Be sure to yell at your customer. ;-) ... Yup. I wonder
        Message 3 of 11 , Feb 19, 2005
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          Adrian Howard writes:
          > Just to clarify - I was talking about a /client/ DBA who tweaked the
          > schema of software that we shipped to them - not the DBA of the
          > development team.

          Yikes. Remember there's a clear separation of concerns between the
          customer and the programmer. Be sure to yell at your customer. ;-)

          > Yes this was (from one point of view) the clients "fault" - but if
          > you're going to ship them automated scripts to update there database
          > having an automated double-check that you're really updating the right
          > schema is a nice thing.

          Yup. I wonder though if you don't also have to protect the data?
          Usually apps trust the data that comes from the database. In your
          case, that wouldn't be the case. Do you have extra validation to make
          sure the data hasn't been tampered with?

          Rob
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