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dbMover

A PHP-based database versioning tool.

Installation

Composer (recommended)

Manual

1. Download or clone the repository;
2. There is an executable dbmover in the root.

Vendor support

dbMover currently supports the MySQL and PostgreSQL database engines. Support for SQLite is sort-of planned for the near future. If you have access to MSSQL or Oracle (or yet another database) and would like to contribute, you're more than welcome! See the end of this readme.

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. 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older versions of a particular database to resolve a particular issue, and the migration "registry" itself being out of date).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.

Usage

In the root of your project, place a dbmover.json file. This will contain the settings for dbMover like connections, databasename(s) and the location of your schema file(s).

The format is as follows:

The contents of "dsn" are a bit driver-specific, but will usually be along the lines of "engine:host=host,dbname=name,port=1234", with one or more being optional (defaulting to the engine defaults). This is the exact same string that PHP's PDO constructor expects.

The file names of the schemas must be either relative to the directory you're running from (recommended, since typically you want to keep those in version control alongside your project's code) or absolute.

Best practice: leave the config file out of source control, e.g. by adding it to .gitignore. The database connection credentials will change seldom (if ever) so setting this up should mostly be a one-time manual operation. This way your development database can use a throwaway password, and the production database can use something much stronger, use a host different than localhost etc.

The optional "ignore" entry can contain an array of regexes of objects to ignore during the migration (e.g. when your application automatically creates tables for cached data or something). The regular expressions are injected into PHP's preg_match verbatim so should also contain delimiters and can optionally specify pattern modifiers like "/i". They are checked for all objects (tables, views, procedures etc.).

After defining the config file, run the executable from that same location:

That's it - your database should now be up to date with your schema(s).

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.

If your vendor supports it and you really need to add a column at a certain position, use an IF block as described below.

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.

Dropping tables

Just remove them from the schema and re-run.

Foreign key constraints and indexes

Depending on your database vendor, it may be allowed to specify these during table creation. That would mean dbMover never sees them if the table already exists! So don't do that. Instead, create these constraints after table creation using ALTER TABLE statements. The exception is a primary key on an AUTOINCREMENT/SERIAL field since table creation would fail when the constraint is missing. However, these columns rarely if ever change, and even if they do you can manually apply the change using an IF block with the correct check on pre/post state.

An example in MySQL:

In PostgreSQL:

The reason is that during the migration, dbMover will DROP all existing constraints and recreate them during the migration to ensure only the constraints intended by your schema exist on the database. This is the most surefire way to accomplish this, as opposed to analysing your SQL statements, accounting for vendor-specific syntax etc.etc.etc.

For extremely large tables, recreating indexes might considerably slow down the moving process. C'est la vie.

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).

Inserting default data

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

The order of things

While your schema file should run perfectly when called against an empty database, if the database already contains objects dbMover will reorder the statements as best it can. In particular:

1. All statements beginning with IF are hoisted.
2. All ALTER TABLE statements are also hoisted.
3. The above two are run in isolation.
4. Indexes and foreign key constraints are dropped.
5. All CREATE TABLE statements are hoisted and analysed.
   1. If the table should be created, issue that statement verbatim.
   2. If the table needs updating, issue the required update statements.
6. Drop existing views, routines and triggers.
7. Run all other statements (CREATE PROCEDURE, CREATE VIEW etc.).
8. Re-run step 3. Note that ALTER TABLE statements will silently fail, and presumably some or all conditions that were false will now evaluate to true and vice versa.
9. Attempt to drop tables that were not or not longer present in the schema.

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:

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

dbMover ignores these. The assumption is that modifying these are a risky and very rare operation that you want to do manually and/or monitor more closely anyway.

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 the dbMover call in a script of your own, e.g.:

...and in your application something like:

This is of course an extremely simple example but should point you in the right direction.

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.

Note on PostgreSQL

PostgreSQL's INFORMATION_SCHEMA aliases contain more data than you would define in a schema file, especially for routines (its native functions are also exposed there). Since these native functions are normally owned by the postgresql user, dbMover will try to drop them and just silently fail. So always run dbMover as an actual database user, not as a master user (this goes for MySQL as well, although the above problem isn't applicable there).

"Master" or "root" users have privileges normal users don't, so theoretically you could include something like in the schema file. You'd only have yourself to blame of course, but best to avoid the risk.

Sequences in PostgreSQL

PostgreSQL stores "auto_increment" unique ID's in so called "sequences" (to all intents and purposes, a "special" separate table). To allow migrations to run correctly, you should write these in the following manner:

One can also use a shorthand SERIAL or BIGSERIAL type that automates this process, but that doesn't play well with DbMover's analysis and table modification statements.

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.

A todo for 0.4 is also to issue warnings if certain statements produced errors both before and after table migration, since that means they probably didn't do what you intended.