Erlang ❤ pure database migrations

PostgreSQL | MySQL version control engine. Effects-free.

Migrate your PostgreSQL or MySQL database from Erlang code with no effort. This amazing toolkit has one and only purpose - consistently upgrade database schema, using Erlang stack and plain SQL. Feel free to run it with any PostgreSQL/MySQL Erlang library (and see several ready-to-use examples below). As an extra - do this in “no side-effects” mode.

Table of content

Current limitations
Quick start
- Compatibility table
- Live integrations
“No-effects” approach and tools used to achieve it
- Tool #1: effects externalization
- Tool #2: make effects explicit
Functional programming abstractions used

Current limitations

up transactional migration available only. No downgrade calls available. Either whole up migration completes OK or failed and rolled back to the state before migration.
Validated MySQL implementation obviously featured with implicit commit behavior, which means that truly transactional MySQL upgrades limited in scope. At the same time you may adjust MySQL transaction callback, as it is proposed by API.
migrations engine deliberately isolated from any specific database library. This way engine user is free to choose from variety of frameworks (see tested combinations here) and so on.

Quick start

Just call pure_migrations:migrate/3 (see specification here), providing:

Path to migration scripts folder (strictly and incrementally enumerated).
FTx transaction handler
FQuery database queries execution handler

Migration logic is idempotent and could be executed multiple times against the same database with the same migration scripts set. Moreover, it is safe to migrate your database concurrently (as a part of nodes startup in scalable environments and if you providing proper transaction handler). Please see verified integrations and live code snippets below.

Compatibility table

All integrations validated against PostgreSQL 9.4/9.6

Database dialect	Library	Example

Live integrations

PostgreSQL and epgsql/epgsql

Onboarding comments

most popular out of onboarded postgres integrations
transactions with proper stack trace available out of the box
reasonably structured query responses, provided with data and its schema

although binary strings could be very reasonable, sometimes code too verbose because of this

Code sample

Click to expand

```erlang Conn = ?config(conn, Opts), MigrationCall = pure_migrations:migrate( "scripts/folder/path", fun(F) -> epgsql:with_transaction(Conn, fun(_) -> F() end) end, fun(Q) -> case epgsql:squery(Conn, Q) of {ok, [ {column, <<"version">>, _, _, _, _, _}, {column, <<"filename">>, _, _, _, _, _}], Data} -> [{list_to_integer(binary_to_list(BinV)), binary_to_list(BinF)} || {BinV, BinF} <- Data]; {ok, [{column, <<"max">>, _, _, _, _, _}], [{null}]} -> -1; {ok, [{column, <<"max">>, _, _, _, _, _}], [{N}]} -> list_to_integer(binary_to_list(N)); [{ok, _, _}, {ok, _}] -> ok; {ok, _, _} -> ok; {ok, _} -> ok; Default -> Default end end), ... %% more preparation steps if needed ... %% migration call ok = MigrationCall(), ``` Also see examples from live epgsql integration tests [here](test/epgsql_migrations_SUITE.erl)

PostgreSQL and semiocast/pgsql

Onboarding comments

no need for extra parsing (strings, numbers, …)

queries results structure has no metadata, like column types or names, which could be sub-optimal sometimes
no transactions out of the box

Code sample

Click to expand

```erlang Conn = ?config(conn, Opts), MigrationCall = pure_migrations:migrate( "scripts/folder/path", fun(F) -> pgsql_connection:simple_query("BEGIN", Conn), try F() of Res -> pgsql_connection:simple_query("COMMIT", Conn), Res catch _:Problem -> pgsql_connection:simple_query("ROLLBACK", Conn), {rollback, Problem} end end, fun(Q) -> case pgsql_connection:simple_query(Q, Conn) of {{select, 0}, []} -> []; {{select, 1}, Data = [{_V, _F}|_]} -> [{V, binary_to_list(BinF)} || {V, BinF} <- Data]; {{select, 1}, [{null}]} -> -1; {{select, 1}, [{N}]} -> N; {{insert, 0, 1}, []} -> ok; {{create, table},[]} -> ok; {error, Details} -> {error, Details}; _ -> ok end end), ... %% more preparation steps if needed ... %% migration call ok = MigrationCall(), ``` Also see examples from live semiocast/pgsql integration tests [here](test/spgsql_migrations_SUITE.erl)

PostgreSQL and processone/p1_pgsql

Onboarding comments

least popular lib,but at the same time - most succinct in terms of integration code (see below)
decent types balance gives opportunity to keep code clean

no transactions out of the box
error reporting different for postgres 9.4/9.6

Code sample

Click to expand

```erlang Conn = ?config(conn, Opts), MigrationCall = pure_migrations:migrate( "scripts/folder/path", fun(F) -> pgsql:squery(Conn, "BEGIN"), try F() of Res -> pgsql:squery(Conn, "COMMIT"), Res catch _:Problem -> pgsql:squery(Conn, "ROLLBACK"), {rollback, Problem} end end, fun(Q) -> case pgsql:squery(Conn, Q) of {ok, [{error, Details}]} -> {error, Details}; {ok, [{_, [ {"version", text, _, _, _, _, _}, {"filename", text, _, _, _, _, _}], Data}]} -> [{list_to_integer(V), F} || [V, F] <- Data]; {ok, [{"SELECT 1", [{"max", text, _, _, _, _, _}], [[null]]}]} -> -1; {ok, [{"SELECT 1", [{"max", text, _, _, _, _, _}], [[N]]}]} -> list_to_integer(N); {ok, _} -> ok end end), ... %% more preparation steps if needed ... %% migration call ok = MigrationCall(), ``` Also see examples from live epgsql integration tests [here](test/p1pgsql_migrations_SUITE.erl)

MySQL and mysql-otp/mysql-otp

Onboarding comments

almost no result-set parsing required

implicit commit specifics a kind an obstacle for simple and safe migration
mysql docker tooling should be operated carefully and ensured for proper startup before any use

Code sample

Click to expand

```erlang Conn = ?config(conn, Opts), MigrationCall = pure_migrations:migrate( "scripts/folder/path", fun(F) -> %% no full-scope tx API available here %% alternatively use mysql:transaction/2, but please be aware about %% mysql implicit transactions commit behavior try F() of Res -> Res catch _:Problem -> {rollback_unavailable, Problem} end end, fun(Q) -> case mysql:query(Conn, Q) of {error, Details} -> {error, Details}; {ok,[<<"version">>,<<"filename">>],[]} -> []; {ok,[<<"version">>,<<"filename">>], Data} -> [{V, binary_to_list(F)} || [V, F] <- Data]; {ok,[<<"max(version)">>],[[null]]} -> -1; {ok,[<<"max(version)">>],[[V]]} -> V; {ok, _} -> ok; ok -> ok end end), ... %% more preparation steps if needed ... %% migration call ok = MigrationCall(), ``` Also see examples from live epgsql integration tests [here](test/otp_mysql_migrations_SUITE.erl)

“No-effects” approach and tools used to achieve it

Oh, there is more! Library implemented in the way, that all side-effects either externalized or deferred explicitly. Reasons are quite common:

bring side-effects as close to program edges as possible. Which may mean enhanced code reasoning, better bugs reproduceability, etc…
simplify module contracts testing
library users empowered to re-run idempotent code safely. Well, if tx/query handlers are real ones - execution is still idempotent (at application level) and formally pure. But purity maintained inside library code only. One call is to be issued anyway - migrations table creation, if this one does not exists.

Tool #1: effects externalization

There are 2 externalized kind of effects:

transaction management handler
database queries handler Although, those two can`t be pure in real application, it is fairly simple to replace them with their pure versions if we would like to (for debug purposes, or testing, or something else). ## Tool #2: make effects explicit Other effects (like file operations) are deferred in bulk with outcome like: pure referentially-transparent program actions composed only. Impact or any communication with external world postponed until later stages library users decide when they ready to apply migration changes. Maybe for some reason they would like to prepare execution -> prepare migrations folder content -> run migrations. # Functional programming abstractions used ## Functions composition This trick is quite useful if someone would like to compose two functions without their actual execution (or without their application, alternatively speaking). This pretty standard routine may look like below (Scala or Kotlin+Arrow): scala val divideByTwo = (number : Int) => number / 2; val addThree = (number: Int) => number + 3; val composed = addThree compose divideByTwo Simplistic Erlang version: erlang compose(F1, F2) -> fun() -> F2(F1()) end. ## Functor applications There area few places in library with clear need to compose function A and another function B inside deferred execution context. Specifics is that A supplies list of objects, and B should be applied to each of them. Sounds like some functor B to be applied to A output, when this output is being wrapped into future execution context. Two cases of this appeared in library: have functor running and produce nested list of contexts: erlang %% Map/1 call here produces new context (defferred function call) map(Generate, Map) -> fun() -> [Map(R) || R <- Generate()] end. flatten (or fold) contexts (or function calls) list to a single one: erlang flatten(Generate) -> fun() -> [ok = R() || R <- Generate()], ok end. ## Partial function applications Partial application is very useful, in case if not all function arguments known yet. Or maybe there is deliberate decision to pass some of arguments later on. Again, in Scala it may look like: scala val add = (a: Int, b: Int) => a + b val partiallyApplied = add(3, _) Library code has very simplistic partial application, done for exact arguments number (although it is easy to generalize it for arguments, represented as list): erlang Partial = fun(V_F) -> do_migration(Path, FQuery, V_F) end,