gnomon for Java

// sessions that start on a branch (pgjdbc reads options from the URL)
String url = Gnomon.jdbcUrl("jdbc:postgresql://db/app", "feature", false); // true with the C module

// or ask the database what a branch needs (schema versions, masks)
String opts = Gnomon.connectionOptions(conn, "feature", false);
String url2 = Gnomon.withOptions("jdbc:postgresql://db/app", opts);

// every connection of a Hikari pool on a branch
hikari.setConnectionInitSql(Gnomon.connectionInitSql("feature"));

// a request's branch, chosen per thread (gnomon.use on each connection handed out)
DataSource ds = new BranchDataSource(hikari);
BranchDataSource.on("feature", () -> repo.save(order));

// Spring behind a transaction pooler (pgbouncer pool_mode=transaction): gnomon.begin before the
// first statement of each transaction, no session state; @Transactional works unchanged
@Bean DataSource dataSource(HikariDataSource hikari) {
  return new BranchDataSource(hikari, BranchDataSource.Mode.TRANSACTION);
}
// a servlet filter or HandlerInterceptor wraps the request: BranchDataSource.on(branch, chain::doFilter)

// one transaction on a branch, by hand
Gnomon.withBranch(ds, "feature", c -> { /* ... */ return null; });

// errors: the stable slug, and what the database says about it
catch (SQLException e) { if ("ddl_on_branch".equals(Gnomon.code(e))) ... Gnomon.explain(conn, e); }

@ExtendWith(GnomonBranchExtension.class)
class SignupTest {
  @Test void signup(Connection c) throws Exception {
    c.createStatement().execute("insert into users(email) values ('a@example.com')");
  }
}