DbMover\Core

PHP-based database versioning tool, core package.

Installation

The recommended way of installing DbMover is via Composer. Currently DbMover supports PostgreSQL and MySQL via dbmover/pgsql and dbmover/mysql respectively. E.g.:

composer require dbmover/pgsql

Design goals

Web applications often work with SQL databases. Programmers will layout such a database in a "schema file", which is essentially just SQL statements. This is fine when a new programmer starts working on a project, since she can simply create the database and run the schema against it to get up and running. The problem arises when, during the course of development or an application's lifetime, changes to this schema are required. This involves manually applying the changes to all developers' test databases, perhaps a staging database and eventually the production database(s).

Doing this manually is tedious and error-prone. Remembering to write migrations for each change is also tedious, and keeping track of which migrations have already been applied (or not) is error-prone too (real life use case: importing older versions of a particular database to resolve a particular issue, and the migration "registry" itself being out of date).

DbMover automates this task for you by just looking at the central, leading, version controlled schema files and applying any changes required. This allows you to blindly run vendor/bin/dbmover e.g. in a post-receive hook.

The dbmover.json file

DbMover is configured using a dbmover.json file. This should be at the root of your project (i.e., two folders up from vendor/bin). The format is as follows:

{
    "your dsn": {
        "user": "yourUserName",
        "pass": "something secret",
        "schema": ["path/to/file1.sql", "path/to/file2.sql"[, ...]],
        "plugins": []
    }
}

Whenever you run DbMover, it will loop through all entries and apply whatever you asked for. Many projects will use a single database, but as you can see DbMover fully supports multiple databases in one configuration file.

DSN

This is the "DSN" connection string for a database. The exact format will vary per vendor, but is usually of the type "vendor:dname=NAME;host=HOST;port=PORT", where port can usually be omitted to use the default. If in doubt, consult your system administrator. Currently supported vendors are pgsql and mysql; if you'd like to contribute another vendor, see the "contributing" section below.

schema

This is an array of schema files, relative to the root of your repository. DbMover will process them in order. Note that the option to split your schema into multiple files is supplied for convenience/maintainability - internally, DbMover cats them all together before starting work.

plugins

An array of plugins DbMover will load to perform the migration. By using plugins, we make DbMover very configurable to your exact needs. To just use sane defaults, simply specify the database vendor specific plugin (e.g. Dbmover\Pgsql\Plugin). More on plugins below.

Running DbMover

Simply execute vendor/bin/dbmover. For each database specified, it will perform the requested operations against your defined schemas. If you've been filling in your dbmover.json following the above tutorial and run it now... nothing happens. This is because all actual functionality is in plugins. You need to specify them in your dbmover.json config.

Plugins

As of version 0.6, DbMover uses plugins to specify actions. It is important to note that a plugin by itself should not change anything in the database; they are used to gather commands to execute when performing the migration. Hence, since your plugins array is at this point empty, DbMover doesn't know what to do yet. See above for the syntax.

Plugins are processed in the order specified and may be specified more than once. In that case, they'll simply be run multiple times (this is actually useful).

Each plugin is actually run twice; once to modify the SQL, and once on __destruct for cleanup. The destruction calls are made in the same order as the invoke calls (see "Writing custom plugins" below).

Metapackages

Plugins can also load other plugins; in fact, there's a number of metaplugins officially provided. Generally, they'll do what you need for your database type and design. But, you can also mix and match, write your own or combine these.

As an example, say you have a MySQL database and just want DbMover to migrate everything it can. In that case, you should install the following plugin:

composer require dbmover/mysql

...and register this single plugin:

{
    ...
    plugins: ["Dbmover\\Mysql\\Plugin"]
}

Writing custom plugins

Each plugin must implement Dbmover\Core\PluginInterface. Usually you'll want to extend the abstract Dbmover\Core\Plugin, but there are cases thinkable where this is undesired (hence the interface).

Plugins get constructed with a single argument: the instance of Dbmover\Core\Loader currently running a migration. Via this object you may access the underlying PDO instance using the getPdo() method. It also exposes the name of the current database via getDatabase().

The main task of a plugin is to receive the currently available SQL, transform the parts relevant to that into operations for the migration loader, and return the (usually modified) SQL as a string. Ideally, after all plugins have run there is no SQL left to inspect.

Above main task is accomplished via the magic __invoke method. It takes the current SQL as a string parameter, and must return the (optionally modified) new current SQL.

Plugins can also optionally implement a __destroy method. Plugins are destroyed in the same order they are run, after all plugins have been run.

Metaplugins will override the __construct method and manually load their own "subplugins". Do not add new plugins in the __invoke or __destruct implementations - by the time these are run DbMover is done assembling plugins and behaviour is unspecified (and likely erratic).

Database vendors tend to add very specific behaviour. We've implemented the most common use cases we could think of (i.e., they work for our own rather complex databases) but improvements can always be made. If you wrote a useful plugin of your own you'd like to share, please see "Contributing" below.

Writing your schema

You should write your schema as if it were to be run against a completely empty database - afterwards you should have something you can work with, possibly including default data.

Adding tables

Just add the new table definition to the schema and re-run.

Adding columns

Forgot a column in a database? No problem, just add it in your schema and re-run DbMover.

Note that new columns will always be appended to the end of the table. Some database drivers (like MySQL) support the BEFORE keyword, but e.g. PostgreSQL doesn't and DbMover is as database-agnostic as possible.

Altering columns

Just change the column definition in the schema file and DbMover will alter it for you. This assumes the column retains the same name and whatever data it contains is compatible with the new type (or can be discarded); for more complex alterations, see below.

Dropping columns

Just remove them from the schema and re-run. Note: they'll be really, really gone aftwards, databases don't support undo.

Dropping tables, views etc.

Just remove them from the schema and re-run. Again: they'll be really gone.

Indexes and foreign key constraints

Depending on your database vendor, it may be allowed to specify these during table creation. Support for this is still very experimental and most definitely not complete. So don't do that if possible. Instead, create these constraints after table creation using CREATE INDEX or ALTER TABLE statements.

Primary keys may already be speficied in the CREATE TABLE block. Other constraints are still a work in progress.

Loose ALTER statements

Sometimes you need to ALTER a table after creation specifically, e.g. when it has a foreign key referring to a table you need to create later on. For example, a blog_posts table might refer to a lastcomment, while blog_post_comments in turn refers to a blog_id on blog_posts. Here you would first create the posts table, then the comments table (with its foreign key constraint), and finally add the constraint to the posts table.

Each ALTER TABLE statement is run in isolation by DbMover in the order specified in the schema file, so just (re)add the foreign key where you would logically add it if running against an empty database. The statement will either fail silently (if the column doesn't exist or is of the wrong type pending a migration) or will succeed eventually.

More complex schema changes

Some things are hard(er) to automatically determine, like a table or column rename. You should wrap these changes in IF blocks with a condition that will pass when the migration needs to be done, and will otherwise fail.

Depending on your database vendor, it might be required to wrap these in a "throwaway" procedure. E.g. MySQL only supports IF inside a procedure. The vendor-specific classes in DbMover handle this for you. Throwaway procedures are prefixed with tmp_.

Note that the exact syntax of conditionals (ELSE IF, ELSIF) is also vendor-dependent. The exact way to determine whether a table needs renaming is also vendor-dependent (though in the current version DbMover only supports ANSI-compatible databases anyway, so you can use INFORMATION_SCHEMA for this purpose).

Conditionals

DbMover supports, via the dbmover/conditionals plugin, the inclusion of IF blocks in your schema. This is an extension on SQL in that these blocks are generally only allowed inside procedures. DbMover will wrap them for you.

Inserting default data

To prevent duplicate inserts, these should be wrapped in an IF NOT EXISTS () condition like so:

IF NOT EXISTS (SELECT 1 FROM mytable WHERE id = 1) THEN
    INSERT INTO mytable (id, value1, value2, valueN)
        VALUES (1, 2, 3, 4);
END IF;

This will usually require the dbmover/VENDOR-conditionals plugin (which isn't bundled in the meta-packages). See dbmover/mysql-conditionals and dbmover/pgsql-conditionals for more information.

Transferring data from one table to another

This is sometimes necessary. In these cases, you should use IF blocks and query e.g. INFORMATION_SCHEMA (depending on your vendor) to determine if the migration has already run.

Important: the IF should evaluate to false if the migration has already run to avoid running it twice. Take care here.

A simplified and abstract pseudo example:

IF NOT EXISTS (SELECT 1 FROM INFORMATION_SCHEMA.TABLES WHERE ...) THEN
    INSERT INTO target SELECT * FROM original;
END IF;

Also, see the note in the previous section about conditionals.

Caveats

Be neat

DbMover assumes well-formed SQL, where keywords are written in ALL CAPS. It does not specifically validate your SQL, though any errors will cause the statement to fail and the script to halt (so theoretically they can't do much harm...). DbMover will tell you what error it got.

By "be neat", we mean write CREATE TABLE instead of create Table etc.

DbMover also doesn't recognise e.g. MySQL's escaping of reserved words using backticks. Just don't do that, it's evil.

For the ignore regexes, you can perfectly use "strange" object names if you need to since these are regexed verbatim.

For hoisting, it is assumed that statements-to-be-hoisted are at the beginning of lines (i.e., e.g. /^IF / in regular expression terms).

Databases may or may not be case-sensitive; keep in mind that DbMover is case-sensitive, so just be consistent in your spelling.

Storage engines and collations

Currently DbMover ignores these. Support for MySQL is planned; for PostgreSQL, changing the collation is a database-wide operation which cannot be handled by DbMover (it requires recreation of the entire database).

Test your schema first

Always run DbMover against a test database for an updated schema. Everybody makes typos, you don't want those to mangle a production database. Preferably you'd test against a copy of the actual production database.

Bring down your application during migration

Depending on what you're requesting and how big your dataset is, migrations might take a few minutes. You don't want users editing any data while the schema isn't in a stable state yet!

How your application handles its down state is not up to DbMover. A simple way would be to wrap write your own plugins for this:

{
    "dsn": {
        "plugins": ['Myplugins\\Down', ..., 'Myplugins\\Up']
    }
}

A simple way to handle down/up states would be to write an empty file (e.g. called simply down) in the root of your application, check for it in a front controller, and remove it when bringing the application up again. A very basic example:

<?php

namespace Myplugins;

use Dbmover\Core\PluginInterface;

class Down implements PluginInterface
{
    public $description = 'Bringing application down...';

    public function __invoke(string $sql) : string
    {
        $cwd = getcwd();
        `touch $cwd/down`;
        return $sql;
    }
}

class Up implements PluginInterface
{
    public $description = 'Briging application back up...';

    public function __destruct()
    {
        $cwd = getcwd();
        `rm $cwd/down`;
        parent::__destruct();
    }
}

...and in your front controller (in this example, simply index.php):

<?php

if (file_exists('/path/to/down')) {
    die("Application is down for maintainance.");
}
// ...other code...

Backup your database before migration

If you tested against an actual copy and it worked fine this shouldn't be necessary, but better safe than sorry. You might suffer a power outage during the migration!

Besides, the simple fact that the script runs correctly doesn't necessarily mean it did what you intended. Always verify your data after a migration.

Using the Up and Down custom plugins from the previous section, you could handle this automatically. Use the Loader's getErrors() method to see if a rollback is required or you can simply remove the backup (or keep it just in case manual inspection happens to turn up something fishy).

Contributing

SQLite support is sort-of planned for the near future, but is not extremely high on my priority list (clients use it occasionally, but it's really not a database suited very well for web development).

MSSQL and Oracle are valid choices, but we don't have access to them. If you do and you fancy porting the database-specific parts of DbMover, by all means fork the repository and send us a pull request!

There's no formal style guide, but look at the existing code and please try to keep your coding style consitent with it. If you work on vendors I can't/won't support, please also make sure you add unit tests for those.

Contributions are also very welcome in the form of bug reports (please file against the affected package!) and feature requests (vendor support is by no means exhaustive yet, just the most-used options).

Plugin packages sometimes also contain a TODO-list in their README.md. If your request is already listed there, there's no need to report it since it's already on the roadmap.

Debugging and development

Run dbmover with the --dry-run flag to just assemble a list of operations to perform, without actually making any changes.