Question: How to start using migrations on existing database?

[gexterio]

I want to start using neo4j-migrations. For now i have a working neo4j database with thousands of nodes and relations, a lot of constraints and indexes. Is that okay if I will write V1 migration with only new changes on existing database? For example, some refactoring.
P.S. I feel a little shame because of naive question, but migrations in graph database confusing me a bit.

[michael-simons]

Hello @gexterio naive maybe, but for sure not dumb!! No worries. Welcome.

So this library does not do any assumption about the structure of the graph, it does not do any "cleanup" on structures it didn't create.

Here you see what it does when "cleaning (everything)":

neo4j-migrations/neo4j-migrations-core/src/main/java/ac/simons/neo4j/migrations/core/Migrations.java

Lines 391 to 427 in a6f5398

	private DeletedChainsWithCounters clean0(
	@SuppressWarnings("OptionalUsedAsFieldOrParameterType") Optional<String> migrationTarget,
	boolean all
	) {
	String query = """
	MATCH (n:__Neo4jMigration)
	WITH n, coalesce(n.migrationTarget, '<default>') as migrationTarget
	WHERE (migrationTarget = coalesce($migrationTarget,'<default>') OR $all)
	DETACH DELETE n
	RETURN DISTINCT migrationTarget
	ORDER BY migrationTarget ASC
	""";
	try (Session session = context.getSchemaSession()) {
	DeletedChainsWithCounters deletedChainsWithCounters = session.executeWrite(tx -> {
	Result result = tx.run(query, Values.parameters(PROPERTY_MIGRATION_TARGET, migrationTarget.orElse(null), "all", all));
	return new DeletedChainsWithCounters(
	result.list(r -> r.get(PROPERTY_MIGRATION_TARGET).asString()),
	result.consume().counters()
	);
	});
	ConnectionDetails cd = context.getConnectionDetails();
	if (all && HBD.is44OrHigher(cd)) {
	Renderer<Constraint> renderer = Renderer.get(Renderer.Format.CYPHER, Constraint.class);
	RenderConfig dropConfig = RenderConfig.drop()
	.ifExists()
	.forVersionAndEdition(cd.getServerVersion(), cd.getServerEdition());
	return new DeletedChainsWithCounters(deletedChainsWithCounters,
	session.run(renderer.render(UNIQUE_VERSION, dropConfig)).consume().counters().constraintsRemoved());
	}
	return deletedChainsWithCounters;
	}
	}

It only deletes it's own subgraph, optional for the database it runs on only or for all databases (you can use a dedicated database for managing others).

So when you start writing a V1 that is when you start maintaining evolution of indexes, constraints and even data in a controlled and traceable fashion. Neo4j Migrations will keep a sub graph what has been applied and what not and you can read it directly or with the tooling, whatever you prefer.

Hope that help.s

[gexterio]

Oh, thank you for an immediately and specific response. Yes, that helps. I'll try it out.

[michael-simons]

My pleasure!