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Thomas Klausner > DBIx-SchemaChecksum > DBIx::SchemaChecksum

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Module Version: 1.102   Source  

NAME ^

DBIx::SchemaChecksum - Manage your datebase schema via checksums

VERSION ^

version 1.102

SYNOPSIS ^

    my $sc = DBIx::SchemaChecksum->new( dbh => $dbh );
    print $sc->checksum;

DESCRIPTION ^

When you're dealing with several instances of the same database (eg. developer, testing, stage, production), it is crucial to make sure that all databases use the same schema. This can be quite an hair-pulling experience, and this module should help you keep your hair (if you're already bald, it won't make your hair grow back, sorry...)

DBIx::SchemaChecksum gets schema information (tables, columns, primary keys, foreign keys and some more depending on your DB) and generates a SHA1 digest. This digest can then be used to easily verify schema consistency across different databases, and to build an update graph of changes. Therefor, DBIx::SchemaChecksum does not requires you to add a meta-table to your database to keep track of which changes have already been deployed.

Caveat: The same schema might produce different checksums on different database versions.

Caveat: DBIx::SchemaChecksum only works with database engines that support changes to the schema inside a transaction. We know this works with PostgreSQL and SQLite. We know it does not work with MySQL and Oracle. We don't know how other database engines behave, but would be happy to hear about your experiences.

RUNNING DBIx::SchemaChecksum

Please take a look at the dbchecksum script included in this distribution. It provides a nice and powerful commandline interface to make working with your schema a breeze.

EXAMPLE WORKFLOW

So you have this genious idea for a new startup that will make you incredibly rich and famous...

Collect underpants

Usually such ideas involve a database. So you grab your favourite database engine and start a new database:

  ~/Gnomes$ createdb gnomes    # createdb is a postgres tool

Of course this new DB is rather empty:

  gnomes=# \d
  No relations found.

So you think long and hard about your database schema and write it down

  ~/Gnomes$ cat sql/handcrafted_schema.sql
  create table underpants (
    id serial primary key,
    type text,
    size text,
    color text
  );

But instead of going down the rabbit hole of manually keeping the dev-DB on your laptop, the one on the workstation in the office, the staging and the production one in sync (and don't forget all the databases running on the laptops of the countless coding monkeys you're going to hire after all the VC money starts flowing), you grab a (free!) copy of DBIx::SchemaChecksum

  ~/Gnomes$ cpanm DBIx::SchemaChecksum
  .. wait a bit while the giant, on which shoulders we are standing, is being assembled
  Successfully installed DBIx-SchemaChecksum
  42 distribution installed

Now you can create a new changes file:

  ~/Gnomes$ dbchecksum new_changes_file --sqlsnippetdir sql --dsn dbi:Pg:dbname=gnomes --change_name "initial schema"
  New change-file ready at sql/inital_schema.sql

Let's take a look:

  ~/Gnomes$ cat sql/inital_schema.sql
  -- preSHA1sum:  54aa14e7b7e54cce8ae07c441f6bda316aa8458c
  -- postSHA1sum: xxx-New-Checksum-xxx
  -- inital schema

Each changes file contains two very import "header" lines masked as a SQL comment:

preSHA1sum is the checksum of the DB schema before the changes in this file have been applied. postSHA1sum is (you probably guessed it) the checksum we expect after the changes have been applied. Currently the postSHA1sum is "xxx-New-Checksum-xxx" because we have neither defined nor run the changes yet.

So let's append the handcrafted schema from earlier to the change file:

  ~/Gnomes$ cat sql/handcrafted_schema.sql >> sql/inital_schema.sql

The changes file now looks like this:

  ~/Gnomes$ cat sql/inital_schema.sql
  -- preSHA1sum:  54aa14e7b7e54cce8ae07c441f6bda316aa8458c
  -- postSHA1sum: xxx-New-Checksum-xxx
  -- inital schema

  create table underpants (
    id serial primary key,
    type text,
    size text,
    color text
  );

Let's apply this schema change, so we can finally start coding (you just can't wait to get rich, can you?)

  ~/Gnomes$ dbchecksum apply_changes --sqlsnippetdir sql --dsn dbi:Pg:dbname=gnomes
  Apply inital_schema.sql? [y/n] [y]
  post checksum mismatch!
    expected 
    got      611481f7599cc286fa539dbeb7ea27f049744dc7
  ABORTING!

Woops! What happend here? Why couldn't the change be applied? Well, we haven't yet defined the postSHA1sum, so we cannot be sure that the database is in the state we expect it to be.

When you author a sql change, you will always have to first apply the change to figure out the new postSHA1sum. As soon as DBIx::SchemaChecksum tells you the checksum the DB will have after the change is applied, you have to add the new checksum to your changes file:

  ~/Gnomes$ vim sql/inital_schema.sql
  # replace xxx-New-Checksum-xxx with 611481f7599cc286fa539dbeb7ea27f049744dc7

  ~/Gnomes$ head -2 sql/inital_schema.sql 
  -- preSHA1sum:  54aa14e7b7e54cce8ae07c441f6bda316aa8458c
  -- postSHA1sum: 611481f7599cc286fa539dbeb7ea27f049744dc7

Now we can try again:

  ~/Gnomes$ dbchecksum apply_changes --sqlsnippetdir sql --dsn dbi:Pg:dbname=gnomes
  Apply inital_schema.sql? [y/n] [y]
  post checksum OK
  No more changes

Yay, this looks much better!

Now you can finally start to collect underpants!

Teamwork

Some weeks later (you have now convinced a friend to join you in your quest for fortune) a git pull drops a new file into your sql directory. It seems that your colleague needs some tweaks to the database:

  ~/Gnomes$ cat sql/underpants_need_washing.sql
  -- preSHA1sum:  611481f7599cc286fa539dbeb7ea27f049744dc7
  -- postSHA1sum: 094ef4321e60b50c1d34529c312ecc2fcbbdfb51
  -- underpants need washing
  
  ALTER TABLE underpants ADD COLUMN needs_washing BOOLEAN NOT NULL DEFAULT false;

Seems reasonable, so you apply it:

  ~/Gnomes$ dbchecksum apply_changes --sqlsnippetdir sql --dsn dbi:Pg:dbname=gnomes
  Apply underpants_need_washing.sql? [y/n] [y]
  post checksum OK
  No more changes

Now that was easy!

Making things even easier: Config file

DBIx::SchemaChecksum uses MooseX::App to power the commandline interface. We use the Config and ConfigHome plugins, so you can pack some of the flags into a config file, for even less typing (and typos):

  ~/Gnomes$ cat dbchecksum.yml
  global:
    sqlsnippetdir: sql
    dsn: dbi:Pg:dbname=gnomes

Now run:

  ~/Gnomes$ dbchecksum apply_changes --config dbchecksum.yml
  db checksum 094ef4321e60b50c1d34529c312ecc2fcbbdfb51 matching sql/underpants_need_washing.sql

Or you can store the config file into your ~/.dbchecksum/config.yml:

  ~/Gnomes$ cat ~/.dbchecksum/config.yml
  global:
    sqlsnippetdir: sql
    dsn: dbi:Pg:dbname=gnomes

And it magically works:

  ~/Gnomes$ dbchecksum apply_changes
  db checksum 094ef4321e60b50c1d34529c312ecc2fcbbdfb51 matching sql/underpants_need_washing.sql

Profit!

This section is left empty as an exercise for the reader!

Anatomy of a changes-file

sqlsnippetdir points to a directory containing so-called changes files. For a file to be picked up by dbchecksum it needs to use the extension .sql.

The file itself has to contain a header formated as sql comments, i.e. starting with --. The header has to contain the preSHA1sum and should include the postSHA1sum.

If the postSHA1sum is missing, we assume that you don't know it yet and try to apply the change. As the new checksum will not match the empty postSHA1sum the change will fail. But we will report the new checksum, which you can now insert into the changes file.

After the header, the changes file should list all sql commands you want to apply to change the schema, seperated by a semicolon ;, just as you would type them into your sql prompt.

  -- preSHA1sum:  b1387d808800a5969f0aa9bcae2d89a0d0b4620b
  -- postSHA1sum: 55df89fd956a03d637b52d13281bc252896f602f
  
  CREATE TABLE nochntest (foo TEXT);

Not all commands need to actually alter the schema, you can also include sql that just updates some data. In fact, some schmema changes even require that: for example, if you want to add a NOT NULL constraint to a column, you first have to make sure that the column in fact does not contain a NULL.

  -- preSHA1sum:  c50519c54300ec2670618371a06f9140fa552965
  -- postSHA1sum: 48dd6b3710a716fb85b005077dc534a8f9c11cba
  
  UPDATE foo SET some_field = 42 WHERE some_field IS NULL;
  ALTER TABLE foo ALTER some_filed SET NOT NULL;

Creating functions / stored procedures

Functions usually contain semicolons inside the function definition, so we cannot split the file on semicolon. Luckily, you can specifiy a different splitter using -- split-at. We usually use ---- (again, the SQL comment marker) so the changes file is still valid SQL.

  -- preSHA1sum  c50519c54300ec2670618371a06f9140fa552965
  -- postSHA1sum 48dd6b3710a716fb85b005077dc534a8f9c11cba
  -- split-at ------

  ALTER TABLE underpants
        ADD COLUMN modified timestamp with time zone DEFAULT now() NOT NULL;
  ------
  CREATE FUNCTION update_modified() RETURNS trigger
      LANGUAGE plpgsql
      AS $$
  BEGIN
      if NEW <> OLD THEN
        NEW.modified = now();
      END IF;
      RETURN NEW;
  END;
  $$;
  ------
  CREATE TRIGGER underpants_modified
         BEFORE UPDATE ON underpants
         FOR EACH ROW EXECUTE PROCEDURE update_modified();

TIPS & TRICKS

We have been using DBIx::SchemaChecksum since 2008 and encountered a few issues. Here are our solutions:

Using 'checksum --show_dump' to find inconsistencies between databases

Sometimes two databases will produce different checksums. This can be caused by a number of things. A good method to figure out what's causing the problem is running <dbchecksum checksum --show_dump some_name>> on the databases causing the problem. Then you can use diff or vim -d to inspect the raw dump.

Some problems we have encountered, and how to fix them:

Use show_update_path if DBIx::SchemaChecksum cannot run on the database server

Sometimes it's impossible to get DBIx::SchemaChecksum installed on the database server (or on some other machine, I have horrible recollections about a colleague using Windows..). And the sysadmin won't let you access the database over the network...

Fix: Prepare all changes on your local machine, and run them manually on the target machine.

  ~/Gnomes$ dbchecksum show_update_path --from_checksum 54aa14e7b7e54cce8ae07c441f6bda316aa8458c
  inital_schema.sql (611481f7599cc286fa539dbeb7ea27f049744dc7)
  underpants_need_washing.sql (094ef4321e60b50c1d34529c312ecc2fcbbdfb51)
  No update found that's based on 094ef4321e60b50c1d34529c312ecc2fcbbdfb51.

Now you could import the changes manually on the server. But it's even easier using the --output flag:

  ~/Gnomes$ dbchecksum show_update_path --output psql --dbname gnomes --from_checksum 54aa14e7b7e54cce8ae07c441f6bda316aa8458c
  psql gnomes -1 -f inital_schema.sql
  psql gnomes -1 -f underpants_need_washing.sql
  # No update found that's based on 094ef4321e60b50c1d34529c312ecc2fcbbdfb51.

You could pipe this into changes.sh and then run that.

Or use --output concat:

  ~/Gnomes$ dbchecksum show_update_path --output concat --from_checksum 54aa14e7b7e54cce8ae07c441f6bda316aa8458c > changes.sql
  ~/Gnomes$ cat changes.sql
  -- file: inital_schema.sql
  -- preSHA1sum:  54aa14e7b7e54cce8ae07c441f6bda316aa8458c
  -- postSHA1sum: 611481f7599cc286fa539dbeb7ea27f049744dc7
  -- inital schema
  
  create table underpants (
    id serial primary key,
    type text,
    size text,
    color text
  );
  
  -- file: underpants_need_washing.sql
  -- preSHA1sum:  611481f7599cc286fa539dbeb7ea27f049744dc7
  -- postSHA1sum: 094ef4321e60b50c1d34529c312ecc2fcbbdfb51
  -- underpants need washing
  
  ALTER TABLE underpants ADD COLUMN needs_washing BOOLEAN NOT NULL DEFAULT false;
  
  -- No update found that's based on 094ef4321e60b50c1d34529c312ecc2fcbbdfb51.

Happyness!

METHODS ^

You will only need those methods if you want to use the library itself instead of using the dbchecksum wrapper script.

checksum

    my $sha1_hex = $self->checksum();

Gets the schemadump and runs it through Digest::SHA1, returning the current checksum.

schemadump

    my $schemadump = $self->schemadump;

Returns a string representation of the whole schema (as a Data::Dumper Dump).

Lazy Moose attribute.

_build_schemadump_schema

    my $hashref = $self->_build_schemadump_schema( $schema );

This is the main entry point for checksum calculations per schema. Method-modifiy it if you need to alter the complete schema data structure before/after checksumming.

Returns a HashRef like:

    {
        tables => $hash_ref
    }

_build_schemadump_tables

    my $hashref = $self->_build_schemadump_tables( $schema );

Iterate through all tables in a schema, calling _build_schemadump_table for each table and collecting the results in a HashRef

_build_schemadump_table

    my $hashref = $self->_build_schemadump_table( $schema, $table );

Get metadata on a table (columns, primary keys & foreign keys) via DBI introspection.

This is a good place to method-modify if you need some special processing for your database

Returns a hashref like

    {
        columns      => $data,
        primary_keys => $data,
        foreign_keys => $data,
    }

_build_schemadump_column

    my $hashref = $self->_build_schemadump_column( $schema, $table, $column, $raw_dbi_data );

Does some cleanup on the data returned by DBI.

update_path

    my $update_info = $self->update_path

Lazy Moose attribute that returns the data structure needed by apply_sql_update.

_build_update_path

_build_update_path reads in all files ending in ".sql" in $self->sqlsnippetdir. It builds something like a linked list of files, which are chained by their preSHA1sum and postSHA1sum.

get_checksums_from_snippet

    my ($pre, $post) = $self->get_checksums_from_snippet( $filename );

Returns a list of the preSHA1sum and postSHA1sum for the given file in sqlnippetdir.

The file has to contain this info in SQL comments, eg:

  -- preSHA1sum: 89049e457886a86886a4fdf1f905b69250a8236c
  -- postSHA1sum: d9a02517255045167053ea92dace728e1389f8ca

  alter table foo add column bar;

dbh

Database handle (DBH::db). Moose attribute

catalog

The database catalog searched for data. Not implemented by all DBs. See DBI::table_info

Default %.

Moose attribute

schemata

An Arrayref containing names of schematas to include in checksum calculation. See DBI::table_info

Default %.

Moose attribute

sqlsnippetdir

Path to the directory where the sql change files are stored.

Moose attribute

verbose

Be verbose or not. Default: 0

driveropts

Additional options for the specific database driver.

GLOBAL OPTIONS ^

Connecting to the database

These options define how to connect to your database.

dsn

Required. The Data Source Name (DSN) as used by DBI to connect to your database.

Some examples: dbi:SQLite:dbname=sqlite.db, dbi:Pg:dbname=my_project;host=db.example.com;port=5433, dbi:Pg:service=my_project_dbadmin

user

Username to use to connect to your database.

password

Password to use to connect to your database.

Defining the schema dump

These options define which parts of the schema are relevant to the checksum

catalog

Default: %

Needed during DBI introspection. Pg does not need it.

schemata

Default: % (all schemata)

If you have several schemata in your database, but only want to consider some for the checksum, use --schemata to list the ones you care about. Can be specified more than once to list several schemata:

  dbchecksum apply --schemata foo --schemata bar

driveropts

Some database drivers might implement further options only relevant for the specific driver. As of now, this only applies to DBIx::SchemaChecksum::Driver::Pg, which defines the driveropts triggers, sequences and functions

SEE ALSO ^

"dbchecksum" in bin for a command line frontend powered by MooseX::App

There are quite a lot of other database schema management tools out there, but nearly all of them need to store meta-info in some magic table in your database.

Talks

You can find more information on the rational, usage & implementation in the slides for my talk at the Austrian Perl Workshop 2012, available here: http://domm.plix.at/talks/dbix_schemachecksum.html

ACKNOWLEDGMENTS ^

Thanks to

AUTHORS ^

COPYRIGHT AND LICENSE ^

This software is copyright (c) 2012 by Thomas Klausner, Maroš Kollár, Klaus Ita.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

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