Migrations

PG2 provides its own migration engine through the pg2-migration package (see Installation). Like Migratus or Ragtime, it allows to grow the database schema continuously, track changes and apply them with care.

Concepts

Migrations are SQL files that are applied to the database in certain order. A migration has an id and a direction: next/up or prev/down. Usually it's split on two files called <id>.up.sql and <id>.down.sql holding SQL commands. Say, the -up file creates a table with an index, and the -down one drops the index first, and then the table.

Migrations might have a slug: a short human-friendly text describing changes. For example, in a file called 002.create-users-table.up.sql, the slug is "Create users table".

Naming

In PG2, the migration framework looks for files matching the following pattern:

<id>.<slug>.<direction>.sql

where:

• id is a Long number, for example 12345 (a counter), or 20240311 (date precision), or 20240311235959 (date & time precision);

• slug is an optional word or group of words joined with - or _, for example create-users-table-and-index or remove_some_view. When rendered, both - and _ are replaced with spaces, and the phrase is capitalized.

• direction is either prev/down or next/up. Internally, down and up are transformed to prev and next because these two have the same amount of characters and files look better.

Examples:

• 001.create-users.next.sql
• 012.next-only-migration.up.sql
• 153.add-some-table.next.sql

Above, the leading zeroes in ids are used for better alignment only. Infernally they are transferred into 1, 12 and 153 Long numbers. Thus, 001, 01 and 1 become the same id 1 after parsing.

Each id has at most two directions: prev/down and next/up. On bootstrap, the engine checks it to prevent weird behaviour. The table below shows there are two rows which, after parsing, have the same (id, direction) pair. The bootstrap step will end up with an exception saying which files duplicate each other.

Filename	Parsed
001.some-trivial-slug.next.sql	(1, next)
001.some-simple-slug.next.sql	(1, next)

A migration might have only one direction, e.g. next/up or prev/down file only.

When parsing, the registry is ignored meaning that both 001-Create-Users.NEXT.sql and 001-CREATE-USERS.next.SQL files produce the same map.

SQL

The files hold SQL expressions to be evaluated by the engine. Here is the content of the 001.create-users.next.sql file:

create table IF NOT EXISTS test_users (
  id serial primary key,
  name text not null
);

BEGIN;

insert into test_users (name) values ('Ivan');
insert into test_users (name) values ('Huan');
insert into test_users (name) values ('Juan');

COMMIT;

Pay attention to the following points.

• A single file might have as many SQL expressions as you want. There is no need to separate them with magic comments like --;; as Migratus requires. The whole file is executed in a single query. Use the standard semicolon at the end of each expression.

• There is no a hidden transaction management. Transactions are up to you: they are explicit! Above, we wrap tree INSERT queries into a single transaction. You can use save-points, rollbacks, or whatever you want. Note that not all expressions can be in a transaction. Say, the CREATE TABLE one cannot and thus is out from the transaction scope.

For granular transaction control, split your complex changes on two or three files named like this:

# direct parts
001.huge-update-step-1.next.sql
002.huge-update-step-2.next.sql
003.huge-update-step-3.next.sql

# backward counterparts
003.huge-update-step-3.prev.sql
002.huge-update-step-2.prev.sql
001.huge-update-step-1.prev.sql

No Code-Driven Migrations

At the moment, neither .edn nor .clj migrations are supported. This is by design because personally I'm highly against mixing SQL and Clojure. Every time I see an EDN transaction, I get angry. Mixing these two for database management is the worst idea one can come up with. If you're thinking about migrating a database with Clojure, please close you laptop and have a walk to the nearest park.

Migration Resources

Migration files are stored in project resources. The default search path is migrations. Thus, their physical location is resources/migrations. The engine scans the migrations resource for children files. Files from nested directories are also taken into account. The engine supports Jar resources when running the code from an uberjar.

The resource path can be overridden with settings.

Migration Table

All the applied migrations are tracked in a database table called migrations by default. The engine saves the id and the slug or a migration applied as well as the current timestamp of the event. The timestamp field has a time zone. Here is the structure of the table:

CREATE TABLE IF NOT EXISTS migrations (
  id BIGINT PRIMARY KEY,
  slug TEXT,
  created_at timestamp with time zone not null default current_timestamp
)

Every time you apply a migration, a new record is inserted into the table. On rollback, a corresponding migration is deleted.

You can override the name of the table in settings (see below).

CLI Interface

The migration engine is controlled with both API and CLI interface. Let's review CLI first.

The pg.migration.cli namespaces acts like the main entry point. It accepts general options, a command, and command-specific options:

<global options> <command> <command options>

General options are:

-c, --config CONNFIG     migration.config.edn      Path to the .edn config file
-p, --port PORT          5432                      Port number
-h, --host HOST          localhost                 Host name
-u, --user USER          The current USER env var  User
-w, --password PASSWORD  <empty string>            Password
-d, --database DATABASE  The current USER env var  Database
    --table TABLE        :migrations               Migrations table
    --path PATH          migrations                Migrations path

Most of the options have default values. Both user and database names come from the USER environment variable. The password is an empty string by default. For local trusted connections, the password might not be required.

The list of the commands:

Name	Meaning
create	Create a pair of blank up & down migration files
help	Print a help message
list	Show all the migrations and their status (applied or not)
migrate	Migrate forward (everything, next only, or up to a certain ID)
rollback	Rollback (the current one, everything, or down to a certain ID)

Each command has its own sub-options which we will describe below.

Here is how you review the migrations:

<lein or deps preamble> \
    -h 127.0.0.1 \
    -p 10150 \
    -u test \
    -w test \
    -d test \
    --table migrations_test \
    --path migrations \
    list

|    ID | Applied? | Slug
| ----- | -------- | --------
|     1 | true     | create users
|     2 | false    | create profiles
|     3 | false    | next only migration
|     4 | false    | prev only migration
|     5 | false    | add some table

Every command has its own arguments and help message. For example, to review the create command, run:

lein with-profile +migrations run -m pg.migration.cli -c config.example.edn create --help

Syntax:
      --id ID             The id of the migration (auto-generated if not set)
      --slug SLUG         Optional slug (e.g. 'create-users-table')
      --help       false  Show help message

Config

Passing -u, -h, and other arguments all the time is inconvenient. The engine can read them at once from a config file. The default config location is migration.config.edn. Override the path to the config using the -c parameter:

<lein/deps> -c config.edn list

The config file has the following structure:

{:host "127.0.0.1"
 :port 10150
 :user "test"
 :password #env PG_PASSWORD
 :database "test"
 :migrations-table :migrations_test
 :migrations-path "migrations"}

The :migrations-table field must be a keyword because it takes place in a HoneySQL map.

The :migrations-path field is a string referencing a resource with migrations.

Pay attention to the #env tag. The engine uses custom readers when loading a config. The tag reads the actual value from an environment variable. Thus, the database password won't be exposed to everyone. When the variable is not set, an exception is thrown.

Commands

Create

The create command makes a pair of two blank migration files. If not set, the id is generated automatically using the YYYYmmddHHMMSS pattern.

lein with-profile +migration run -m pg.migration.cli \
  -c config.example.edn \
  create

ls -l migrations

20240312074156.next.sql
20240312074156.prev.sql

You can also provide a custom id and a slug as well:

lein with-profile +migration run -m pg.migration.cli \
  -c config.example.edn \
  create \
  --id 100500 \
  --slug 'some huge changes in tables'

ll migrations

100500.some-huge-changes-in-tables.next.sql
100500.some-huge-changes-in-tables.prev.sql
20240312074156.next.sql
20240312074156.prev.sql

List

The list command renders all the migrations and their status: whether they are applied or not.

lein with-profile +migration run -m pg.migration.cli -c config.example.edn list

|    ID | Applied? | Slug
| ----- | -------- | --------
|     1 | true     | create users
|     2 | true     | create profiles
|     3 | true     | next only migration
|     4 | false    | prev only migration
|     5 | false    | add some table

Migrate

The migrate command applies migrations to the database. By default, all the pending migrations are processed. You can change this behaviour using these flags:

... migrate --help

Syntax:
      --all           Migrate all the pending migrations
      --one           Migrate next a single pending migration
      --to ID         Migrate next to certain migration
      --help   false  Show help message

With the --one flag set, only one next migration will be applied. If --to parameter is set, only migrations up to this given ID are processed. Examples:

... migrate           # all migrations
... migrate --all     # all migrations
... migrate --one     # next only
... migrate --to 123  # all that <= 123

Rollback

The rollback command reverses changes in the database and removes corresponding records from the migration table. By default, only the current migration is rolled back. Syntax:

... rollback --help

Syntax:
      --all           Rollback all the previous migrations
      --one           Rollback to the previous migration
      --to ID         Rollback to certain migration
      --help   false  Show help message

The --one argument is the default behaviour. When --all is passed, all the backward migrations are processed. To rollback to a certain migration, pass --to ID. Examples:

... rollback               # current only
... rollback --one         # current only
... rollback --to 20240515 # down to 20240515
... rollback --all         # down to the very beginning

Lein examples

Lein preamble looks usually something like this:

> lein run -m pg.migration.cli <ARGS>

The pg2-migration library must be in dependencies. Since migrations are managed aside from the main application, they're put into a separate profile, for example:

:profiles
{:migrations
 {:main pg.migration.cli
  :resource-paths ["path/to/resources"]
  :dependencies
  [[com.github.igrishaev/pg2-core ...]]}}

Above, the migrations profile has the dependency and the :main attribute. Now run lein run with migration arguments:

> lein with-profile +migrations run -c migration.config.edn migrate --to 100500

Deps.edn examples

Here is an example of an alias in deps.edn that prints pending migrations:

{:aliases
 {:migrations-list
  {:extra-deps
   {com.github.igrishaev/pg2-migration {:mvn/version "..."}}
   :extra-paths
   ["test/resources"]
   :main-opts
   ["-m" "pg.migration.cli"
    "-h" "127.0.0.1"
    "-p" "10150"
    "-u" "test"
    "-w" "test"
    "-d" "test"
    "--table" "migrations_test"
    "--path" "migrations"
    "list"]}}}

Run it as follows:

> clj -M:migrations-list

You can shorten it by using the config file. Move all the parameters into the migration.config.edn file, and keep only a command with its sub-arguments in the :main-opts vector:

{:aliases
 {:migrations-migrate
  {:extra-deps
   {com.github.igrishaev/pg2-migration {:mvn/version "..."}}
   :extra-paths
   ["test/resources"]
   :main-opts ["migrate" "--all"]}}}

To migrate:

> clj -M:migrations-migrate

API Interface

There is a way to manage migrations through code. The pg.migration.core namespace provides basic functions to list, create, migrate, and rollback migrations.

To migrate, call one of the following functions: migrate-to, migrate-all, and migrate-one. All of them accept a config map:

(ns demo
  (:require
   [pg.migration.core :as mig]))

(def CONFIG
  {:host "127.0.0.1"
   :port 5432
   :user "test"
   :password "secret"
   :database "test"
   :migrations-table :test_migrations
   :migrations-path "migrations"})

;; migrate all pinding migrations
(mig/migrate-all CONFIG)

;; migrate only one next migration
(mig/migrate-one CONFIG)

;; migrate to a certain migration
(mig/migrate-to CONFIG 20240313)

The same applies to rollback:

;; rollback all previously applied migrations
(mig/rollback-all CONFIG)

;; rollback the current migration
(mig/rollback-one CONFIG)

;; rollback to the given migration
(mig/rollback-to CONFIG 20230228)

The read-disk-migrations function reads migrations from disk. It returns a sorted map without information about whether migrations have been applied:

(mig/read-disk-migrations "migrations")

{1
 {:id 1
  :slug "create users"
  :url-prev #object[java.net.URL "file:/.../migrations/001.create-users.prev.sql"]
  :url-next #object[java.net.URL "file:/.../migrations/001.create-users.next.sql"]}
 2
 {:id 2
  :slug "create profiles"
  :url-prev #object[java.net.URL "file:/.../migrations/foobar/002.create-profiles.prev.sql"]
  :url-next #object[java.net.URL "file:/.../migrations/foobar/002.create-profiles.next.sql"]}
 ...}

The make-scope function accepts a config map and returns a scope map. The scope map knows everything about the state of migrations, namely: which of them have been applied, what is the current migration, the table name, the resource path, and more.

The function create-migration-files creates and returns a pair of empty SQL files. By default, the id is generated from the current date & time, and the slug is missing:

(create-migration-files "migrations")

[#object[java.io.File "migrations/20240313120122.prev.sql"]
 #object[java.io.File "migrations/20240313120122.next.sql"]]

Pass id and slug in options if needed:

(create-migration-files "migrations" {:id 12345 :slug "Hello migration"})

[#object[java.io.File "migrations/12345.hello-migration.prev.sql"]
 #object[java.io.File "migrations/12345.hello-migration.next.sql"]]

Conflicts

On bootstrap, the engine checks migrations for conflicts. A conflict is a situation when a migration with less id has been applied before a migration with greater id. Usually it happens when two developers create migrations in parallel and merge them in a wrong order. For example:

• the latest migration id is 20240312;
• developer A makes a new branch and creates a migration 20240315;
• the next day, developer B opens a new branch with a migration 20240316;
• dev B merges the branch, now we have 20240312, then 20240316;
• dev A merges the branch, and we have 20240312, 20240316, 20240315.

When you try to apply migration 20240315, the engine will check if 20240316 has already been applied. If yes, an exception pops up saying which migration cause the problem (in our case, these are 20240316 and 20240315). To recover from the conflict, rename 20240315 to 20240317.

In other words: this is a conflict:

id        applied?
20240312  true
20240315  false
20240316  true  ;; applied before 20240315

And this is a solution:

id        applied?
20240312  true
20240316  true
20240317  false ;; 20240315 renamed to 20240317