package appview

import (
	"fmt"

	"gorm.io/driver/postgres"
	"gorm.io/gorm"
	"gorm.io/plugin/dbresolver"
)

// registerReadReplica wires the dbresolver plugin to route reads to
// cfg.DatabaseReaderURL when that is non-empty. Writes always stay on the
// primary opened by the caller. When the config is empty the function is
// a no-op so single-DB deployments keep their existing behavior.
//
// Transactions and locked reads (FOR UPDATE) route to the primary via
// dbresolver's default policy. Replica lag is a real concern — callers
// that must read-their-own-write should wrap the sequence in a
// transaction, which pins the whole block to the primary.
func registerReadReplica(db *gorm.DB, cfg Config) error {
	if cfg.DatabaseReaderURL == "" {
		return nil
	}

	resolver := dbresolver.Register(dbresolver.Config{
		Replicas: []gorm.Dialector{postgres.Open(cfg.DatabaseReaderURL)},
		Policy:   dbresolver.RandomPolicy{},
	})

	// Apply the same pool shape to the replica so saturation behavior
	// stays predictable across both databases.
	resolver = resolver.
		SetMaxOpenConns(cfg.DBMaxOpenConns).
		SetMaxIdleConns(cfg.DBMaxIdleConns).
		SetConnMaxLifetime(cfg.DBConnMaxLifetime).
		SetConnMaxIdleTime(cfg.DBConnMaxIdleTime)

	if err := db.Use(resolver); err != nil {
		return fmt.Errorf("install dbresolver: %w", err)
	}
	return nil
}