chore: initial commit

internal/svc/payment/api/gateway.go

@@ -0,0 +1,39 @@
+// Copyright (C) 2024 Declan Teevan
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package api
+
+import (
+	"context"
+
+	"github.com/grpc-ecosystem/grpc-gateway/v2/runtime"
+	"github.com/rs/zerolog/log"
+
+	pb "github.com/hexolan/stocklet/internal/pkg/protogen/payment/v1"
+	"github.com/hexolan/stocklet/internal/pkg/serve"
+	"github.com/hexolan/stocklet/internal/svc/payment"
+)
+
+func PrepareGateway(cfg *payment.ServiceConfig) *runtime.ServeMux {
+	mux, clientOpts := serve.NewGatewayServeBase(&cfg.Shared)
+
+	ctx := context.Background()
+	err := pb.RegisterPaymentServiceHandlerFromEndpoint(ctx, mux, serve.GetAddrToGrpc("localhost"), clientOpts)
+	if err != nil {
+		log.Panic().Err(err).Msg("failed to register endpoint for gateway")
+	}
+
+	return mux
+}

internal/svc/payment/api/grpc.go

@@ -0,0 +1,30 @@
+// Copyright (C) 2024 Declan Teevan
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package api
+
+import (
+	"google.golang.org/grpc"
+
+	pb "github.com/hexolan/stocklet/internal/pkg/protogen/payment/v1"
+	"github.com/hexolan/stocklet/internal/pkg/serve"
+	"github.com/hexolan/stocklet/internal/svc/payment"
+)
+
+func PrepareGrpc(cfg *payment.ServiceConfig, svc *payment.PaymentService) *grpc.Server {
+	svr := serve.NewGrpcServeBase(&cfg.Shared)
+	pb.RegisterPaymentServiceServer(svr, svc)
+	return svr
+}

internal/svc/payment/config.go

@@ -0,0 +1,42 @@
+// Copyright (C) 2024 Declan Teevan
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package payment
+
+import (
+	"github.com/hexolan/stocklet/internal/pkg/config"
+)
+
+// Order Service Configuration
+type ServiceConfig struct {
+	// Core Configuration
+	Shared config.SharedConfig
+
+	// Dynamically loaded configuration
+	Postgres config.PostgresConfig
+	Kafka    config.KafkaConfig
+}
+
+// load the base service configuration
+func NewServiceConfig() (*ServiceConfig, error) {
+	cfg := ServiceConfig{}
+
+	// Load the core configuration
+	if err := cfg.Shared.Load(); err != nil {
+		return nil, err
+	}
+
+	return &cfg, nil
+}

internal/svc/payment/controller/kafka.go

@@ -0,0 +1,140 @@
+// Copyright (C) 2024 Declan Teevan
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package controller
+
+import (
+	"context"
+
+	"github.com/rs/zerolog/log"
+	"github.com/twmb/franz-go/pkg/kgo"
+	"google.golang.org/protobuf/proto"
+
+	"github.com/hexolan/stocklet/internal/pkg/messaging"
+	eventpb "github.com/hexolan/stocklet/internal/pkg/protogen/events/v1"
+	pb "github.com/hexolan/stocklet/internal/pkg/protogen/payment/v1"
+	"github.com/hexolan/stocklet/internal/svc/payment"
+)
+
+type kafkaController struct {
+	cl *kgo.Client
+
+	svc pb.PaymentServiceServer
+
+	ctx       context.Context
+	ctxCancel context.CancelFunc
+}
+
+func NewKafkaController(cl *kgo.Client) payment.ConsumerController {
+	// Create a cancellable context for the consumer
+	ctx, ctxCancel := context.WithCancel(context.Background())
+
+	// Ensure the required Kafka topics exist
+	err := messaging.EnsureKafkaTopics(
+		cl,
+
+		messaging.Payment_Balance_Created_Topic,
+		messaging.Payment_Balance_Credited_Topic,
+		messaging.Payment_Balance_Debited_Topic,
+		messaging.Payment_Balance_Closed_Topic,
+		messaging.Payment_Transaction_Created_Topic,
+		messaging.Payment_Transaction_Reversed_Topic,
+		messaging.Payment_Processing_Topic,
+
+		messaging.User_State_Created_Topic,
+
+		messaging.Shipping_Shipment_Allocation_Topic,
+	)
+	if err != nil {
+		log.Warn().Err(err).Msg("kafka: raised attempting to ensure svc topics")
+	}
+
+	// Add the consumption topics
+	cl.AddConsumeTopics(
+		messaging.User_State_Created_Topic,
+		messaging.Shipping_Shipment_Allocation_Topic,
+	)
+
+	return &kafkaController{cl: cl, ctx: ctx, ctxCancel: ctxCancel}
+}
+
+func (c *kafkaController) Attach(svc pb.PaymentServiceServer) {
+	c.svc = svc
+}
+
+func (c *kafkaController) Start() {
+	if c.svc == nil {
+		log.Panic().Msg("consumer: no service interface attached")
+	}
+
+	for {
+		fetches := c.cl.PollFetches(c.ctx)
+		if errs := fetches.Errors(); len(errs) > 0 {
+			log.Panic().Any("kafka-errs", errs).Msg("consumer: unrecoverable kafka errors")
+		}
+
+		fetches.EachTopic(func(ft kgo.FetchTopic) {
+			switch ft.Topic {
+			case messaging.User_State_Created_Topic:
+				c.consumeUserCreatedEventTopic(ft)
+			case messaging.Shipping_Shipment_Allocation_Topic:
+				c.consumeShipmentAllocationEventTopic(ft)
+			default:
+				log.Warn().Str("topic", ft.Topic).Msg("consumer: recieved records from unexpected topic")
+			}
+		})
+	}
+}
+
+func (c *kafkaController) Stop() {
+	// Cancel the consumer context
+	c.ctxCancel()
+}
+
+func (c *kafkaController) consumeUserCreatedEventTopic(ft kgo.FetchTopic) {
+	log.Info().Str("topic", ft.Topic).Msg("consumer: recieved records from topic")
+
+	// Process each message from the topic
+	ft.EachRecord(func(record *kgo.Record) {
+		// Unmarshal the event
+		var event eventpb.UserCreatedEvent
+		err := proto.Unmarshal(record.Value, &event)
+		if err != nil {
+			log.Panic().Err(err).Msg("consumer: failed to unmarshal event")
+		}
+
+		// Process the event
+		ctx := context.Background()
+		c.svc.ProcessUserCreatedEvent(ctx, &event)
+	})
+}
+
+func (c *kafkaController) consumeShipmentAllocationEventTopic(ft kgo.FetchTopic) {
+	log.Info().Str("topic", ft.Topic).Msg("consumer: recieved records from topic")
+
+	// Process each message from the topic
+	ft.EachRecord(func(record *kgo.Record) {
+		// Unmarshal the event
+		var event eventpb.ShipmentAllocationEvent
+		err := proto.Unmarshal(record.Value, &event)
+		if err != nil {
+			log.Panic().Err(err).Msg("consumer: failed to unmarshal event")
+		}
+
+		// Process the event
+		ctx := context.Background()
+		c.svc.ProcessShipmentAllocationEvent(ctx, &event)
+	})
+}

internal/svc/payment/controller/postgres.go

@@ -0,0 +1,458 @@
+// Copyright (C) 2024 Declan Teevan
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package controller
+
+import (
+	"context"
+	"strings"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgtype"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"github.com/hexolan/stocklet/internal/pkg/errors"
+	pb "github.com/hexolan/stocklet/internal/pkg/protogen/payment/v1"
+	"github.com/hexolan/stocklet/internal/svc/payment"
+)
+
+const (
+	pgTransactionBaseQuery     string = "SELECT id, order_id, customer_id, amount, reversed_at, processed_at FROM transactions"
+	pgCustomerBalanceBaseQuery string = "SELECT customer_id, balance FROM customer_balances"
+)
+
+type postgresController struct {
+	cl *pgxpool.Pool
+}
+
+func NewPostgresController(cl *pgxpool.Pool) payment.StorageController {
+	return postgresController{cl: cl}
+}
+
+func (c postgresController) GetBalance(ctx context.Context, customerId string) (*pb.CustomerBalance, error) {
+	return c.getBalance(ctx, nil, customerId)
+}
+
+func (c postgresController) getBalance(ctx context.Context, tx *pgx.Tx, customerId string) (*pb.CustomerBalance, error) {
+	// Determine if a db transaction is being used
+	var row pgx.Row
+	const query = pgCustomerBalanceBaseQuery + " WHERE customer_id=$1"
+	if tx == nil {
+		row = c.cl.QueryRow(ctx, query, customerId)
+	} else {
+		row = (*tx).QueryRow(ctx, query, customerId)
+	}
+
+	// Scan row to protobuf obj
+	balance, err := scanRowToCustomerBalance(row)
+	if err != nil {
+		return nil, err
+	}
+
+	return balance, nil
+}
+
+func (c postgresController) GetTransaction(ctx context.Context, transactionId string) (*pb.Transaction, error) {
+	return c.getTransaction(ctx, nil, transactionId)
+}
+
+func (c postgresController) getTransaction(ctx context.Context, tx *pgx.Tx, transactionId string) (*pb.Transaction, error) {
+	// Determine if a db transaction is being used
+	var row pgx.Row
+	const query = pgTransactionBaseQuery + " WHERE id=$1"
+	if tx == nil {
+		row = c.cl.QueryRow(ctx, query, transactionId)
+	} else {
+		row = (*tx).QueryRow(ctx, query, transactionId)
+	}
+
+	// Scan row to protobuf obj
+	transaction, err := scanRowToTransaction(row)
+	if err != nil {
+		return nil, err
+	}
+
+	return transaction, nil
+}
+
+func (c postgresController) CreateBalance(ctx context.Context, customerId string) error {
+	// Begin a DB transaction
+	tx, err := c.cl.Begin(ctx)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to begin transaction", err)
+	}
+	defer tx.Rollback(ctx)
+
+	_, err = tx.Exec(
+		ctx,
+		"INSERT INTO customer_balances (customer_id, balance) VALUES ($1, $2)",
+		customerId,
+		0.00,
+	)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to create balance", err)
+	}
+
+	// Add the event to the outbox table with the transaction
+	evt, evtTopic, err := payment.PrepareBalanceCreatedEvent(&pb.CustomerBalance{CustomerId: customerId, Balance: 0.00})
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeService, "failed to create event", err)
+	}
+
+	_, err = tx.Exec(ctx, "INSERT INTO event_outbox (aggregateid, aggregatetype, payload) VALUES ($1, $2, $3)", customerId, evtTopic, evt)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to insert event", err)
+	}
+
+	// Commit the transaction
+	err = tx.Commit(ctx)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to commit transaction", err)
+	}
+
+	return nil
+}
+
+func (c postgresController) CreditBalance(ctx context.Context, customerId string, amount float32) error {
+	// Begin a DB transaction
+	tx, err := c.cl.Begin(ctx)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to begin transaction", err)
+	}
+	defer tx.Rollback(ctx)
+
+	// Add to balance
+	_, err = tx.Exec(
+		ctx,
+		"UPDATE customer_balances SET balance = balance + $1 WHERE customer_id=$2",
+		amount,
+		customerId,
+	)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to update balance", err)
+	}
+
+	// Get updated balance
+	balance, err := c.getBalance(ctx, &tx, customerId)
+	if err != nil {
+		return err
+	}
+
+	// Add the event to the outbox table with the transaction
+	evt, evtTopic, err := payment.PrepareBalanceCreditedEvent(
+		balance.CustomerId,
+		amount,
+		balance.Balance,
+	)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeService, "failed to create event", err)
+	}
+
+	_, err = tx.Exec(ctx, "INSERT INTO event_outbox (aggregateid, aggregatetype, payload) VALUES ($1, $2, $3)", balance.CustomerId, evtTopic, evt)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to insert event", err)
+	}
+
+	// Commit the transaction
+	err = tx.Commit(ctx)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to commit transaction", err)
+	}
+
+	return nil
+}
+
+func (c postgresController) DebitBalance(ctx context.Context, customerId string, amount float32, orderId *string) (*pb.Transaction, error) {
+	return c.debitBalance(ctx, nil, customerId, amount, orderId)
+}
+
+func (c postgresController) debitBalance(ctx context.Context, tx *pgx.Tx, customerId string, amount float32, orderId *string) (*pb.Transaction, error) {
+	// Determine if a transaction has already been provided
+	var (
+		funcTx pgx.Tx
+		err    error
+	)
+	if tx != nil {
+		funcTx = *tx
+	} else {
+		funcTx, err = c.cl.Begin(ctx)
+		if err != nil {
+			return nil, errors.WrapServiceError(errors.ErrCodeExtService, "failed to begin transaction", err)
+		}
+		defer funcTx.Rollback(ctx)
+	}
+
+	// Subtract from balance
+	_, err = funcTx.Exec(
+		ctx,
+		"UPDATE customer_balances SET balance = balance - $1 WHERE customer_id=$2",
+		amount,
+		customerId,
+	)
+	if err != nil {
+		return nil, errors.WrapServiceError(errors.ErrCodeExtService, "failed to update balance", err)
+	}
+
+	// Get updated balance
+	balance, err := c.getBalance(ctx, &funcTx, customerId)
+	if err != nil {
+		return nil, err
+	}
+
+	// Ensure that the balance is not negative
+	if balance.Balance < 0.00 {
+		return nil, errors.NewServiceError(errors.ErrCodeInvalidArgument, "insufficient balance")
+	}
+
+	// Add the balance event to the outbox table with the transaction
+	evt, evtTopic, err := payment.PrepareBalanceDebitedEvent(
+		balance.CustomerId,
+		amount,
+		balance.Balance,
+	)
+	if err != nil {
+		return nil, errors.WrapServiceError(errors.ErrCodeService, "failed to create event", err)
+	}
+
+	_, err = funcTx.Exec(ctx, "INSERT INTO event_outbox (aggregateid, aggregatetype, payload) VALUES ($1, $2, $3)", balance.CustomerId, evtTopic, evt)
+	if err != nil {
+		return nil, errors.WrapServiceError(errors.ErrCodeExtService, "failed to insert event", err)
+	}
+
+	// Create a payment transaction record
+	transaction, err := c.createTransaction(ctx, &funcTx, orderId, customerId, amount)
+	if err != nil {
+		return nil, err
+	}
+
+	// Commit the transaction (if created in this func)
+	if tx == nil {
+		err = funcTx.Commit(ctx)
+		if err != nil {
+			return nil, errors.WrapServiceError(errors.ErrCodeExtService, "failed to commit transaction", err)
+		}
+	}
+
+	return transaction, nil
+}
+
+func (c postgresController) CloseBalance(ctx context.Context, customerId string) error {
+	// Begin a DB transaction
+	tx, err := c.cl.Begin(ctx)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to begin transaction", err)
+	}
+	defer tx.Rollback(ctx)
+
+	// Get current balance
+	balance, err := c.getBalance(ctx, &tx, customerId)
+	if err != nil {
+		return err
+	}
+
+	// Delete balance
+	_, err = tx.Exec(ctx, "DELETE FROM customer_balances WHERE customer_id=$1", customerId)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to delete balance", err)
+	}
+
+	// Add the event to the outbox table with the transaction
+	evt, evtTopic, err := payment.PrepareBalanceClosedEvent(balance)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeService, "failed to create event", err)
+	}
+
+	_, err = tx.Exec(ctx, "INSERT INTO event_outbox (aggregateid, aggregatetype, payload) VALUES ($1, $2, $3)", balance.CustomerId, evtTopic, evt)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to insert event", err)
+	}
+
+	// Commit the transaction
+	err = tx.Commit(ctx)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to commit transaction", err)
+	}
+
+	return nil
+}
+
+func (c postgresController) PaymentForOrder(ctx context.Context, orderId string, customerId string, amount float32) error {
+	// Begin a DB transaction
+	tx, err := c.cl.Begin(ctx)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to begin transaction", err)
+	}
+	defer tx.Rollback(ctx)
+
+	// Attempt to debit balance for the order
+	transaction, err := c.debitBalance(ctx, &tx, customerId, amount, &orderId)
+	if err != nil {
+		// check that error is not a result of insufficient balance
+		// - or the customer not having a balance
+		errText := err.Error()
+		if errText != "insufficient balance" && !strings.HasPrefix(errText, "failed to update balance") {
+			return err
+		}
+	}
+
+	// Prepare response event
+	var (
+		evt      []byte
+		evtTopic string
+	)
+	if transaction != nil {
+		// Succesful
+		evt, evtTopic, err = payment.PreparePaymentProcessedEvent_Success(transaction)
+	} else {
+		// Failure
+		// - result of insufficient/non-existent balance
+		evt, evtTopic, err = payment.PreparePaymentProcessedEvent_Failure(orderId, customerId, amount)
+	}
+
+	// Ensure the event was prepared succesfully
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeService, "failed to create event", err)
+	}
+
+	// Add the event to the outbox table
+	_, err = tx.Exec(ctx, "INSERT INTO event_outbox (aggregateid, aggregatetype, payload) VALUES ($1, $2, $3)", orderId, evtTopic, evt)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to insert event", err)
+	}
+
+	// Commit the transaction
+	err = tx.Commit(ctx)
+	if err != nil {
+		return errors.WrapServiceError(errors.ErrCodeExtService, "failed to commit transaction", err)
+	}
+
+	return nil
+}
+
+func (c postgresController) createTransaction(ctx context.Context, tx *pgx.Tx, orderId *string, customerId string, amount float32) (*pb.Transaction, error) {
+	// Determine if a transaction has already been provided
+	var (
+		funcTx pgx.Tx
+		err    error
+	)
+	if tx != nil {
+		funcTx = *tx
+	} else {
+		funcTx, err = c.cl.Begin(ctx)
+		if err != nil {
+			return nil, errors.WrapServiceError(errors.ErrCodeExtService, "failed to begin transaction", err)
+		}
+		defer funcTx.Rollback(ctx)
+	}
+
+	// Insert the transaction
+	var transactionId string
+	err = funcTx.QueryRow(
+		ctx,
+		"INSERT INTO transactions (order_id, customer_id, amount) VALUES ($1, $2, $3) RETURNING id",
+		orderId,
+		customerId,
+		amount,
+	).Scan(&transactionId)
+	if err != nil {
+		return nil, errors.WrapServiceError(errors.ErrCodeExtService, "failed to insert transaction", err)
+	}
+
+	// Get the transaction obj
+	transaction, err := c.getTransaction(ctx, &funcTx, transactionId)
+	if err != nil {
+		return nil, err
+	}
+
+	// Add the event to the outbox table with the transaction
+	evt, evtTopic, err := payment.PrepareTransactionLoggedEvent(transaction)
+	if err != nil {
+		return nil, errors.WrapServiceError(errors.ErrCodeService, "failed to create event", err)
+	}
+
+	_, err = funcTx.Exec(ctx, "INSERT INTO event_outbox (aggregateid, aggregatetype, payload) VALUES ($1, $2, $3)", transaction.Id, evtTopic, evt)
+	if err != nil {
+		return nil, errors.WrapServiceError(errors.ErrCodeExtService, "failed to insert event", err)
+	}
+
+	// Commit the transaction (if created in this func)
+	if tx == nil {
+		err = funcTx.Commit(ctx)
+		if err != nil {
+			return nil, errors.WrapServiceError(errors.ErrCodeExtService, "failed to commit transaction", err)
+		}
+	}
+
+	return transaction, nil
+}
+
+// Scan a postgres row to a protobuf object
+func scanRowToTransaction(row pgx.Row) (*pb.Transaction, error) {
+	var transaction pb.Transaction
+
+	// Temporary variables that require conversion
+	var tmpProcessedAt pgtype.Timestamp
+	var tmpReversedAt pgtype.Timestamp
+
+	err := row.Scan(
+		&transaction.Id,
+		&transaction.OrderId,
+		&transaction.CustomerId,
+		&transaction.Amount,
+		&tmpReversedAt,
+		&tmpProcessedAt,
+	)
+	if err != nil {
+		if err == pgx.ErrNoRows {
+			return nil, errors.WrapServiceError(errors.ErrCodeNotFound, "transaction not found", err)
+		} else {
+			return nil, errors.WrapServiceError(errors.ErrCodeExtService, "failed to scan object from database", err)
+		}
+	}
+
+	// Convert the temporary variables
+	// - converting postgres timestamps to unix format
+	if tmpProcessedAt.Valid {
+		transaction.ProcessedAt = tmpProcessedAt.Time.Unix()
+	} else {
+		return nil, errors.NewServiceError(errors.ErrCodeUnknown, "failed to scan object from database (timestamp conversion)")
+	}
+
+	if tmpReversedAt.Valid {
+		unixReversed := tmpReversedAt.Time.Unix()
+		transaction.ReversedAt = &unixReversed
+	}
+
+	return &transaction, nil
+}
+
+// Scan a postgres row to a protobuf object
+func scanRowToCustomerBalance(row pgx.Row) (*pb.CustomerBalance, error) {
+	var balance pb.CustomerBalance
+
+	err := row.Scan(
+		&balance.CustomerId,
+		&balance.Balance,
+	)
+	if err != nil {
+		if err == pgx.ErrNoRows {
+			return nil, errors.WrapServiceError(errors.ErrCodeNotFound, "balance not found", err)
+		} else {
+			return nil, errors.WrapServiceError(errors.ErrCodeExtService, "failed to scan object from database", err)
+		}
+	}
+
+	return &balance, nil
+}

internal/svc/payment/event.go

@@ -0,0 +1,129 @@
+// Copyright (C) 2024 Declan Teevan
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package payment
+
+import (
+	"github.com/hexolan/stocklet/internal/pkg/messaging"
+	eventspb "github.com/hexolan/stocklet/internal/pkg/protogen/events/v1"
+	pb "github.com/hexolan/stocklet/internal/pkg/protogen/payment/v1"
+)
+
+func PrepareBalanceCreatedEvent(bal *pb.CustomerBalance) ([]byte, string, error) {
+	topic := messaging.Payment_Balance_Created_Topic
+	event := &eventspb.BalanceCreatedEvent{
+		Revision: 1,
+
+		CustomerId: bal.CustomerId,
+		Balance:    bal.Balance,
+	}
+
+	return messaging.MarshalEvent(event, topic)
+}
+
+func PrepareBalanceCreditedEvent(customerId string, amount float32, newBalance float32) ([]byte, string, error) {
+	topic := messaging.Payment_Balance_Credited_Topic
+	event := &eventspb.BalanceCreditedEvent{
+		Revision: 1,
+
+		CustomerId: customerId,
+		Amount:     amount,
+		NewBalance: newBalance,
+	}
+
+	return messaging.MarshalEvent(event, topic)
+}
+
+func PrepareBalanceDebitedEvent(customerId string, amount float32, newBalance float32) ([]byte, string, error) {
+	topic := messaging.Payment_Balance_Debited_Topic
+	event := &eventspb.BalanceDebitedEvent{
+		Revision: 1,
+
+		CustomerId: customerId,
+		Amount:     amount,
+		NewBalance: newBalance,
+	}
+
+	return messaging.MarshalEvent(event, topic)
+}
+
+func PrepareBalanceClosedEvent(bal *pb.CustomerBalance) ([]byte, string, error) {
+	topic := messaging.Payment_Balance_Closed_Topic
+	event := &eventspb.BalanceClosedEvent{
+		Revision: 1,
+
+		CustomerId: bal.CustomerId,
+		Balance:    bal.Balance,
+	}
+
+	return messaging.MarshalEvent(event, topic)
+}
+
+func PrepareTransactionLoggedEvent(transaction *pb.Transaction) ([]byte, string, error) {
+	topic := messaging.Payment_Transaction_Created_Topic
+	event := &eventspb.TransactionLoggedEvent{
+		Revision: 1,
+
+		TransactionId: transaction.Id,
+		Amount:        transaction.Amount,
+		OrderId:       transaction.OrderId,
+		CustomerId:    transaction.CustomerId,
+	}
+
+	return messaging.MarshalEvent(event, topic)
+}
+
+func PrepareTransactionReversedEvent(transaction *pb.Transaction) ([]byte, string, error) {
+	topic := messaging.Payment_Transaction_Reversed_Topic
+	event := &eventspb.TransactionReversedEvent{
+		Revision: 1,
+
+		TransactionId: transaction.Id,
+		Amount:        transaction.Amount,
+		OrderId:       transaction.OrderId,
+		CustomerId:    transaction.CustomerId,
+	}
+
+	return messaging.MarshalEvent(event, topic)
+}
+
+func PreparePaymentProcessedEvent_Success(transaction *pb.Transaction) ([]byte, string, error) {
+	topic := messaging.Payment_Processing_Topic
+	event := &eventspb.PaymentProcessedEvent{
+		Revision: 1,
+
+		Type:          eventspb.PaymentProcessedEvent_TYPE_SUCCESS,
+		OrderId:       transaction.OrderId,
+		CustomerId:    transaction.CustomerId,
+		Amount:        transaction.Amount,
+		TransactionId: &transaction.Id,
+	}
+
+	return messaging.MarshalEvent(event, topic)
+}
+
+func PreparePaymentProcessedEvent_Failure(orderId string, customerId string, amount float32) ([]byte, string, error) {
+	topic := messaging.Payment_Processing_Topic
+	event := &eventspb.PaymentProcessedEvent{
+		Revision: 1,
+
+		Type:       eventspb.PaymentProcessedEvent_TYPE_FAILED,
+		OrderId:    orderId,
+		CustomerId: customerId,
+		Amount:     amount,
+	}
+
+	return messaging.MarshalEvent(event, topic)
+}

internal/svc/payment/payment.go

@@ -0,0 +1,132 @@
+// Copyright (C) 2024 Declan Teevan
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package payment
+
+import (
+	"context"
+
+	"github.com/bufbuild/protovalidate-go"
+	"github.com/rs/zerolog/log"
+	"google.golang.org/protobuf/types/known/emptypb"
+
+	"github.com/hexolan/stocklet/internal/pkg/errors"
+	"github.com/hexolan/stocklet/internal/pkg/messaging"
+	commonpb "github.com/hexolan/stocklet/internal/pkg/protogen/common/v1"
+	eventpb "github.com/hexolan/stocklet/internal/pkg/protogen/events/v1"
+	pb "github.com/hexolan/stocklet/internal/pkg/protogen/payment/v1"
+)
+
+// Interface for the service
+type PaymentService struct {
+	pb.UnimplementedPaymentServiceServer
+
+	store StorageController
+	pbVal *protovalidate.Validator
+}
+
+// Interface for database methods
+// Flexibility for implementing seperate controllers for different databases (e.g. Postgres, MongoDB, etc)
+type StorageController interface {
+	GetBalance(ctx context.Context, customerId string) (*pb.CustomerBalance, error)
+	GetTransaction(ctx context.Context, transactionId string) (*pb.Transaction, error)
+
+	CreateBalance(ctx context.Context, customerId string) error
+	CreditBalance(ctx context.Context, customerId string, amount float32) error
+	DebitBalance(ctx context.Context, customerId string, amount float32, orderId *string) (*pb.Transaction, error)
+	CloseBalance(ctx context.Context, customerId string) error
+
+	PaymentForOrder(ctx context.Context, orderId string, customerId string, amount float32) error
+}
+
+// Interface for event consumption
+// Flexibility for seperate controllers for different messaging systems (e.g. Kafka, NATS, etc)
+type ConsumerController interface {
+	messaging.ConsumerController
+
+	Attach(svc pb.PaymentServiceServer)
+}
+
+// Create the payment service
+func NewPaymentService(cfg *ServiceConfig, store StorageController) *PaymentService {
+	// Initialise the protobuf validator
+	pbVal, err := protovalidate.New()
+	if err != nil {
+		log.Panic().Err(err).Msg("failed to initialise protobuf validator")
+	}
+
+	// Initialise the service
+	return &PaymentService{
+		store: store,
+		pbVal: pbVal,
+	}
+}
+
+func (svc PaymentService) ServiceInfo(ctx context.Context, req *commonpb.ServiceInfoRequest) (*commonpb.ServiceInfoResponse, error) {
+	return &commonpb.ServiceInfoResponse{
+		Name:          "payment",
+		Source:        "https://github.com/hexolan/stocklet",
+		SourceLicense: "AGPL-3.0",
+	}, nil
+}
+
+func (svc PaymentService) ViewTransaction(ctx context.Context, req *pb.ViewTransactionRequest) (*pb.ViewTransactionResponse, error) {
+	// Attempt to get the transaction from the db
+	transaction, err := svc.store.GetTransaction(ctx, req.TransactionId)
+	if err != nil {
+		return nil, err
+	}
+
+	return &pb.ViewTransactionResponse{Transaction: transaction}, nil
+}
+
+func (svc PaymentService) ViewBalance(ctx context.Context, req *pb.ViewBalanceRequest) (*pb.ViewBalanceResponse, error) {
+	// todo: permission checking
+
+	// Attempt to get the balance from the db
+	balance, err := svc.store.GetBalance(ctx, req.CustomerId)
+	if err != nil {
+		return nil, err
+	}
+
+	return &pb.ViewBalanceResponse{Balance: balance}, nil
+}
+
+func (svc PaymentService) ProcessUserCreatedEvent(ctx context.Context, req *eventpb.UserCreatedEvent) (*emptypb.Empty, error) {
+	err := svc.store.CreateBalance(ctx, req.UserId)
+	if err != nil {
+		return nil, errors.WrapServiceError(errors.ErrCodeExtService, "error processing event", err)
+	}
+
+	return &emptypb.Empty{}, nil
+}
+
+func (svc PaymentService) ProcessUserDeletedEvent(ctx context.Context, req *eventpb.UserDeletedEvent) (*emptypb.Empty, error) {
+	err := svc.store.CloseBalance(ctx, req.UserId)
+	if err != nil {
+		return nil, errors.WrapServiceError(errors.ErrCodeExtService, "error processing event", err)
+	}
+
+	return &emptypb.Empty{}, nil
+}
+
+func (svc PaymentService) ProcessShipmentAllocationEvent(ctx context.Context, req *eventpb.ShipmentAllocationEvent) (*emptypb.Empty, error) {
+	err := svc.store.PaymentForOrder(ctx, req.OrderId, req.OrderMetadata.CustomerId, req.OrderMetadata.TotalPrice)
+	if err != nil {
+		return nil, errors.WrapServiceError(errors.ErrCodeExtService, "error processing event", err)
+	}
+
+	return &emptypb.Empty{}, nil
+}