Use a Postgres database rather than Memory for Naffka (#337)

* Update naffka dep

* User Postgres database rather than Memory for Naffka

src/github.com/matrix-org/dendrite/cmd/dendrite-monolith-server/main.go

@@ -16,6 +16,7 @@ package main
 
 import (
 	"context"
+	"database/sql"
 	"flag"
 	"net/http"
 	"os"
@@ -199,7 +200,21 @@ func (m *monolith) setupFederation() {
 
 func (m *monolith) setupKafka() {
 	if m.cfg.Kafka.UseNaffka {
-		naff, err := naffka.New(&naffka.MemoryDatabase{})
+		db, err := sql.Open("postgres", string(m.cfg.Database.Naffka))
+		if err != nil {
+			log.WithFields(log.Fields{
+				log.ErrorKey: err,
+			}).Panic("Failed to open naffka database")
+		}
+
+		naffkaDB, err := naffka.NewPostgresqlDatabase(db)
+		if err != nil {
+			log.WithFields(log.Fields{
+				log.ErrorKey: err,
+			}).Panic("Failed to setup naffka database")
+		}
+
+		naff, err := naffka.New(naffkaDB)
 		if err != nil {
 			log.WithFields(log.Fields{
 				log.ErrorKey: err,

src/github.com/matrix-org/dendrite/common/config/config.go

@@ -148,6 +148,8 @@ type Dendrite struct {
 		// The PublicRoomsAPI database stores information used to compute the public
 		// room directory. It is only accessed by the PublicRoomsAPI server.
 		PublicRoomsAPI DataSource `yaml:"public_rooms_api"`
+		// The Naffka database is used internally by the naffka library, if used.
+		Naffka DataSource `yaml:"naffka,omitempty"`
 	} `yaml:"database"`
 
 	// TURN Server Config
@@ -386,6 +388,8 @@ func (config *Dendrite) check(monolithic bool) error {
 		if !monolithic {
 			problems = append(problems, fmt.Sprintf("naffka can only be used in a monolithic server"))
 		}
+
+		checkNotEmpty("database.naffka", string(config.Database.Naffka))
 	} else {
 		// If we aren't using naffka then we need to have at least one kafka
 		// server to talk to.

vendor/manifest

@@ -141,7 +141,7 @@
 		{
 			"importpath": "github.com/matrix-org/naffka",
 			"repository": "https://github.com/matrix-org/naffka",
-			"revision": "d28656e34f96a8eeaab53e3b7678c9ce14af5786",
+			"revision": "662bfd0841d0194bfe0a700d54226bb96eac574d",
 			"branch": "master"
 		},
 		{

vendor/src/github.com/matrix-org/naffka/memorydatabase.go

@@ -8,7 +8,8 @@ import (
 // A MemoryDatabase stores the message history as arrays in memory.
 // It can be used to run unit tests.
 // If the process is stopped then any messages that haven't been
-// processed by a consumer are lost forever.
+// processed by a consumer are lost forever and all offsets become
+// invalid.
 type MemoryDatabase struct {
 	topicsMutex sync.Mutex
 	topics      map[string]*memoryDatabaseTopic
@@ -58,10 +59,7 @@ func (m *MemoryDatabase) getTopic(topicName string) *memoryDatabaseTopic {
 
 // StoreMessages implements Database
 func (m *MemoryDatabase) StoreMessages(topic string, messages []Message) error {
-	if err := m.getTopic(topic).addMessages(messages); err != nil {
-		return err
-	}
-	return nil
+	return m.getTopic(topic).addMessages(messages)
 }
 
 // FetchMessages implements Database
@@ -73,10 +71,10 @@ func (m *MemoryDatabase) FetchMessages(topic string, startOffset, endOffset int6
 	if startOffset >= endOffset {
 		return nil, fmt.Errorf("start offset %d greater than or equal to end offset %d", startOffset, endOffset)
 	}
-	if startOffset < -1 {
-		return nil, fmt.Errorf("start offset %d less than -1", startOffset)
+	if startOffset < 0 {
+		return nil, fmt.Errorf("start offset %d less than 0", startOffset)
 	}
-	return messages[startOffset+1 : endOffset], nil
+	return messages[startOffset:endOffset], nil
 }
 
 // MaxOffsets implements Database

vendor/src/github.com/matrix-org/naffka/naffka.go

@@ -13,6 +13,7 @@ import (
 // single go process. It implements both the sarama.SyncProducer and the
 // sarama.Consumer interfaces. This means it can act as a drop in replacement
 // for kafka for testing or single instance deployment.
+// Does not support multiple partitions.
 type Naffka struct {
 	db          Database
 	topicsMutex sync.Mutex
@@ -28,6 +29,7 @@ func New(db Database) (*Naffka, error) {
 	}
 	for topicName, offset := range maxOffsets {
 		n.topics[topicName] = &topic{
+			db:         db,
 			topicName:  topicName,
 			nextOffset: offset + 1,
 		}
@@ -64,7 +66,7 @@ type Database interface {
 	// So for a given topic the message with offset n+1 is stored after the
 	// the message with offset n.
 	StoreMessages(topic string, messages []Message) error
-	// FetchMessages fetches all messages with an offset greater than but not
+	// FetchMessages fetches all messages with an offset greater than and
 	// including startOffset and less than but not including endOffset.
 	// The range of offsets requested must not overlap with those stored by a
 	// concurrent StoreMessages. The message offsets within the requested range
@@ -138,6 +140,7 @@ func (n *Naffka) Partitions(topic string) ([]int32, error) {
 }
 
 // ConsumePartition implements sarama.Consumer
+// Note: offset is *inclusive*, i.e. it will include the message with that offset.
 func (n *Naffka) ConsumePartition(topic string, partition int32, offset int64) (sarama.PartitionConsumer, error) {
 	if partition != 0 {
 		return nil, fmt.Errorf("Unknown partition ID %d", partition)
@@ -166,13 +169,16 @@ func (n *Naffka) Close() error {
 
 const channelSize = 1024
 
+// partitionConsumer ensures that all messages written to a particular
+// topic, from an offset, get sent in order to a channel.
+// Implements sarama.PartitionConsumer
 type partitionConsumer struct {
 	topic    *topic
 	messages chan *sarama.ConsumerMessage
-	// Whether the consumer is ready for new messages or whether it
-	// is catching up on historic messages.
+	// Whether the consumer is in "catchup" mode or not.
+	// See "catchup" function for details.
 	// Reads and writes to this field are proctected by the topic mutex.
-	ready bool
+	catchingUp bool
 }
 
 // AsyncClose implements sarama.PartitionConsumer
@@ -201,23 +207,34 @@ func (c *partitionConsumer) HighWaterMarkOffset() int64 {
 	return c.topic.highwaterMark()
 }
 
-// block writes the message to the consumer blocking until the consumer is ready
-// to add the message to the channel. Once the message is successfully added to
-// the channel it will catch up by pulling historic messsages from the database.
-func (c *partitionConsumer) block(cmsg *sarama.ConsumerMessage) {
-	c.messages <- cmsg
-	c.catchup(cmsg.Offset)
-}
-
-// catchup reads historic messages from the database until the consumer has caught
-// up on all the historic messages.
+// catchup makes the consumer go into "catchup" mode, where messages are read
+// from the database instead of directly from producers.
+// Once the consumer is up to date, i.e. no new messages in the database, then
+// the consumer will go back into normal mode where new messages are written
+// directly to the channel.
+// Must be called with the c.topic.mutex lock
 func (c *partitionConsumer) catchup(fromOffset int64) {
-	for {
-		// First check if we have caught up.
-		caughtUp, nextOffset := c.topic.hasCaughtUp(c, fromOffset)
-		if caughtUp {
+	// If we're already in catchup mode or up to date, noop
+	if c.catchingUp || fromOffset == c.topic.nextOffset {
 		return
 	}
+
+	c.catchingUp = true
+
+	// Due to the checks above there can only be one of these goroutines
+	// running at a time
+	go func() {
+		for {
+			// Check if we're up to date yet. If we are we exit catchup mode.
+			c.topic.mutex.Lock()
+			nextOffset := c.topic.nextOffset
+			if fromOffset == nextOffset {
+				c.catchingUp = false
+				c.topic.mutex.Unlock()
+				return
+			}
+			c.topic.mutex.Unlock()
+
 			// Limit the number of messages we request from the database to be the
 			// capacity of the channel.
 			if nextOffset > fromOffset+int64(cap(c.messages)) {
@@ -249,7 +266,28 @@ func (c *partitionConsumer) catchup(fromOffset int64) {
 				c.messages <- msgs[i].consumerMessage(c.topic.topicName)
 			}
 			// Update our the offset for the next loop iteration.
-		fromOffset = msgs[len(msgs)-1].Offset
+			fromOffset = msgs[len(msgs)-1].Offset + 1
+		}
+	}()
+}
+
+// notifyNewMessage tells the consumer about a new message
+// Must be called with the c.topic.mutex lock
+func (c *partitionConsumer) notifyNewMessage(cmsg *sarama.ConsumerMessage) {
+	// If we're in "catchup" mode then the catchup routine will send the
+	// message later, since cmsg has already been written to the database
+	if c.catchingUp {
+		return
+	}
+
+	// Otherwise, lets try writing the message directly to the channel
+	select {
+	case c.messages <- cmsg:
+	default:
+		// The messages channel has filled up, so lets go into catchup
+		// mode. Once the channel starts being read from again messages
+		// will be read from the database
+		c.catchup(cmsg.Offset)
 	}
 }
 
@@ -258,9 +296,12 @@ type topic struct {
 	topicName string
 	mutex     sync.Mutex
 	consumers []*partitionConsumer
+	// nextOffset is the offset that will be assigned to the next message in
+	// this topic, i.e. one greater than the last message offset.
 	nextOffset int64
 }
 
+// send writes messages to a topic.
 func (t *topic) send(now time.Time, pmsgs []*sarama.ProducerMessage) error {
 	var err error
 	// Encode the message keys and values.
@@ -298,21 +339,10 @@ func (t *topic) send(now time.Time, pmsgs []*sarama.ProducerMessage) error {
 	t.nextOffset = offset
 
 	// Now notify the consumers about the messages.
-	for i := range msgs {
-		cmsg := msgs[i].consumerMessage(t.topicName)
+	for _, msg := range msgs {
+		cmsg := msg.consumerMessage(t.topicName)
 		for _, c := range t.consumers {
-			if c.ready {
-				select {
-				case c.messages <- cmsg:
-				default:
-					// The consumer wasn't ready to receive a message because
-					// the channel buffer was full.
-					// Fork a goroutine to send the message so that we don't
-					// block sending messages to the other consumers.
-					c.ready = false
-					go c.block(cmsg)
-				}
-			}
+			c.notifyNewMessage(cmsg)
 		}
 	}
 
@@ -330,27 +360,17 @@ func (t *topic) consume(offset int64) *partitionConsumer {
 		offset = t.nextOffset
 	}
 	if offset == sarama.OffsetOldest {
-		offset = -1
+		offset = 0
 	}
 	c.messages = make(chan *sarama.ConsumerMessage, channelSize)
 	t.consumers = append(t.consumers, c)
-	// Start catching up on historic messages in the background.
-	go c.catchup(offset)
-	return c
-}
 
-func (t *topic) hasCaughtUp(c *partitionConsumer, offset int64) (bool, int64) {
-	t.mutex.Lock()
-	defer t.mutex.Unlock()
-	// Check if we have caught up while holding a lock on the topic so there
-	// isn't a way for our check to race with a new message being sent on the topic.
-	if offset+1 == t.nextOffset {
-		// We've caught up, the consumer can now receive messages as they are
-		// sent rather than fetching them from the database.
-		c.ready = true
-		return true, t.nextOffset
+	// If we're not streaming from the latest offset we need to go into
+	// "catchup" mode
+	if offset != t.nextOffset {
+		c.catchup(offset)
 	}
-	return false, t.nextOffset
+	return c
 }
 
 func (t *topic) highwaterMark() int64 {

vendor/src/github.com/matrix-org/naffka/naffka_test.go

@@ -1,6 +1,7 @@
 package naffka
 
 import (
+	"strconv"
 	"testing"
 	"time"
 
@@ -84,3 +85,142 @@ func TestDelayedReceive(t *testing.T) {
 		t.Fatalf("wrong value: wanted %q got %q", value, string(result.Value))
 	}
 }
+
+func TestCatchup(t *testing.T) {
+	naffka, err := New(&MemoryDatabase{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	producer := sarama.SyncProducer(naffka)
+	consumer := sarama.Consumer(naffka)
+
+	const topic = "testTopic"
+	const value = "Hello, World"
+
+	message := sarama.ProducerMessage{
+		Value: sarama.StringEncoder(value),
+		Topic: topic,
+	}
+
+	if _, _, err = producer.SendMessage(&message); err != nil {
+		t.Fatal(err)
+	}
+
+	c, err := consumer.ConsumePartition(topic, 0, sarama.OffsetOldest)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var result *sarama.ConsumerMessage
+	select {
+	case result = <-c.Messages():
+	case _ = <-time.NewTimer(10 * time.Second).C:
+		t.Fatal("expected to receive a message")
+	}
+
+	if string(result.Value) != value {
+		t.Fatalf("wrong value: wanted %q got %q", value, string(result.Value))
+	}
+
+	currOffset := result.Offset
+
+	const value2 = "Hello, World2"
+	const value3 = "Hello, World3"
+
+	_, _, err = producer.SendMessage(&sarama.ProducerMessage{
+		Value: sarama.StringEncoder(value2),
+		Topic: topic,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, _, err = producer.SendMessage(&sarama.ProducerMessage{
+		Value: sarama.StringEncoder(value3),
+		Topic: topic,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Logf("Streaming from %q", currOffset+1)
+
+	c2, err := consumer.ConsumePartition(topic, 0, currOffset+1)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var result2 *sarama.ConsumerMessage
+	select {
+	case result2 = <-c2.Messages():
+	case _ = <-time.NewTimer(10 * time.Second).C:
+		t.Fatal("expected to receive a message")
+	}
+
+	if string(result2.Value) != value2 {
+		t.Fatalf("wrong value: wanted %q got %q", value2, string(result2.Value))
+	}
+}
+
+func TestChannelSaturation(t *testing.T) {
+	// The channel returned by c.Messages() has a fixed capacity
+
+	naffka, err := New(&MemoryDatabase{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	producer := sarama.SyncProducer(naffka)
+	consumer := sarama.Consumer(naffka)
+	const topic = "testTopic"
+	const baseValue = "testValue: "
+
+	c, err := consumer.ConsumePartition(topic, 0, sarama.OffsetOldest)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	channelSize := cap(c.Messages())
+
+	// We want to send enough messages to fill up the channel, so lets double
+	// the size of the channel. And add three in case its a zero sized channel
+	numberMessagesToSend := 2*channelSize + 3
+
+	var sentMessages []string
+
+	for i := 0; i < numberMessagesToSend; i++ {
+		value := baseValue + strconv.Itoa(i)
+
+		message := sarama.ProducerMessage{
+			Topic: topic,
+			Value: sarama.StringEncoder(value),
+		}
+
+		sentMessages = append(sentMessages, value)
+
+		if _, _, err = producer.SendMessage(&message); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	var result *sarama.ConsumerMessage
+
+	j := 0
+	for ; j < numberMessagesToSend; j++ {
+		select {
+		case result = <-c.Messages():
+		case _ = <-time.NewTimer(10 * time.Second).C:
+			t.Fatalf("failed to receive message %d out of %d", j+1, numberMessagesToSend)
+		}
+
+		expectedValue := sentMessages[j]
+		if string(result.Value) != expectedValue {
+			t.Fatalf("wrong value: wanted %q got %q", expectedValue, string(result.Value))
+		}
+	}
+
+	select {
+	case result = <-c.Messages():
+		t.Fatalf("expected to only receive %d messages", numberMessagesToSend)
+	default:
+	}
+}

vendor/src/github.com/matrix-org/naffka/postgresqldatabase.go

@@ -0,0 +1,296 @@
+package naffka
+
+import (
+	"database/sql"
+	"sync"
+	"time"
+)
+
+const postgresqlSchema = `
+-- The topic table assigns each topic a unique numeric ID.
+CREATE SEQUENCE IF NOT EXISTS naffka_topic_nid_seq;
+CREATE TABLE IF NOT EXISTS naffka_topics (
+	topic_name TEXT PRIMARY KEY,
+	topic_nid  BIGINT NOT NULL DEFAULT nextval('naffka_topic_nid_seq')
+);
+
+-- The messages table contains the actual messages.
+CREATE TABLE IF NOT EXISTS naffka_messages (
+	topic_nid BIGINT NOT NULL,
+	message_offset BIGINT NOT NULL,
+	message_key BYTEA NOT NULL,
+	message_value BYTEA NOT NULL,
+	message_timestamp_ns BIGINT NOT NULL,
+	UNIQUE (topic_nid, message_offset)
+);
+`
+
+const insertTopicSQL = "" +
+	"INSERT INTO naffka_topics (topic_name) VALUES ($1)" +
+	" ON CONFLICT DO NOTHING" +
+	" RETURNING (topic_nid)"
+
+const selectTopicSQL = "" +
+	"SELECT topic_nid FROM naffka_topics WHERE topic_name = $1"
+
+const selectTopicsSQL = "" +
+	"SELECT topic_name, topic_nid FROM naffka_topics"
+
+const insertMessageSQL = "" +
+	"INSERT INTO naffka_messages (topic_nid, message_offset, message_key, message_value, message_timestamp_ns)" +
+	" VALUES ($1, $2, $3, $4, $5)"
+
+const selectMessagesSQL = "" +
+	"SELECT message_offset, message_key, message_value, message_timestamp_ns" +
+	" FROM naffka_messages WHERE topic_nid = $1 AND $2 <= message_offset AND message_offset < $3" +
+	" ORDER BY message_offset ASC"
+
+const selectMaxOffsetSQL = "" +
+	"SELECT message_offset FROM naffka_messages WHERE topic_nid = $1" +
+	" ORDER BY message_offset DESC LIMIT 1"
+
+type postgresqlDatabase struct {
+	db                  *sql.DB
+	topicsMutex         sync.Mutex
+	topicNIDs           map[string]int64
+	insertTopicStmt     *sql.Stmt
+	selectTopicStmt     *sql.Stmt
+	selectTopicsStmt    *sql.Stmt
+	insertMessageStmt   *sql.Stmt
+	selectMessagesStmt  *sql.Stmt
+	selectMaxOffsetStmt *sql.Stmt
+}
+
+// NewPostgresqlDatabase creates a new naffka database using a postgresql database.
+// Returns an error if there was a problem setting up the database.
+func NewPostgresqlDatabase(db *sql.DB) (Database, error) {
+	var err error
+
+	p := &postgresqlDatabase{
+		db:        db,
+		topicNIDs: map[string]int64{},
+	}
+
+	if _, err = db.Exec(postgresqlSchema); err != nil {
+		return nil, err
+	}
+
+	for _, s := range []struct {
+		sql  string
+		stmt **sql.Stmt
+	}{
+		{insertTopicSQL, &p.insertTopicStmt},
+		{selectTopicSQL, &p.selectTopicStmt},
+		{selectTopicsSQL, &p.selectTopicsStmt},
+		{insertMessageSQL, &p.insertMessageStmt},
+		{selectMessagesSQL, &p.selectMessagesStmt},
+		{selectMaxOffsetSQL, &p.selectMaxOffsetStmt},
+	} {
+		*s.stmt, err = db.Prepare(s.sql)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return p, nil
+}
+
+// StoreMessages implements Database.
+func (p *postgresqlDatabase) StoreMessages(topic string, messages []Message) error {
+	// Store the messages inside a single database transaction.
+	return withTransaction(p.db, func(txn *sql.Tx) error {
+		s := txn.Stmt(p.insertMessageStmt)
+		topicNID, err := p.assignTopicNID(txn, topic)
+		if err != nil {
+			return err
+		}
+		for _, m := range messages {
+			_, err = s.Exec(topicNID, m.Offset, m.Key, m.Value, m.Timestamp.UnixNano())
+			if err != nil {
+				return err
+			}
+		}
+		return nil
+	})
+}
+
+// FetchMessages implements Database.
+func (p *postgresqlDatabase) FetchMessages(topic string, startOffset, endOffset int64) (messages []Message, err error) {
+	topicNID, err := p.getTopicNID(nil, topic)
+	if err != nil {
+		return
+	}
+	rows, err := p.selectMessagesStmt.Query(topicNID, startOffset, endOffset)
+	if err != nil {
+		return
+	}
+	defer rows.Close()
+	for rows.Next() {
+		var (
+			offset        int64
+			key           []byte
+			value         []byte
+			timestampNano int64
+		)
+		if err = rows.Scan(&offset, &key, &value, &timestampNano); err != nil {
+			return
+		}
+		messages = append(messages, Message{
+			Offset:    offset,
+			Key:       key,
+			Value:     value,
+			Timestamp: time.Unix(0, timestampNano),
+		})
+	}
+	return
+}
+
+// MaxOffsets implements Database.
+func (p *postgresqlDatabase) MaxOffsets() (map[string]int64, error) {
+	topicNames, err := p.selectTopics()
+	if err != nil {
+		return nil, err
+	}
+	result := map[string]int64{}
+	for topicName, topicNID := range topicNames {
+		// Lookup the maximum offset.
+		maxOffset, err := p.selectMaxOffset(topicNID)
+		if err != nil {
+			return nil, err
+		}
+		if maxOffset > -1 {
+			// Don't include the topic if we haven't sent any messages on it.
+			result[topicName] = maxOffset
+		}
+		// Prefill the numeric ID cache.
+		p.addTopicNIDToCache(topicName, topicNID)
+	}
+	return result, nil
+}
+
+// selectTopics fetches the names and numeric IDs for all the topics the
+// database is aware of.
+func (p *postgresqlDatabase) selectTopics() (map[string]int64, error) {
+	rows, err := p.selectTopicsStmt.Query()
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	result := map[string]int64{}
+	for rows.Next() {
+		var (
+			topicName string
+			topicNID  int64
+		)
+		if err = rows.Scan(&topicName, &topicNID); err != nil {
+			return nil, err
+		}
+		result[topicName] = topicNID
+	}
+	return result, nil
+}
+
+// selectMaxOffset selects the maximum offset for a topic.
+// Returns -1 if there aren't any messages for that topic.
+// Returns an error if there was a problem talking to the database.
+func (p *postgresqlDatabase) selectMaxOffset(topicNID int64) (maxOffset int64, err error) {
+	err = p.selectMaxOffsetStmt.QueryRow(topicNID).Scan(&maxOffset)
+	if err == sql.ErrNoRows {
+		return -1, nil
+	}
+	return maxOffset, err
+}
+
+// getTopicNID finds the numeric ID for a topic.
+// The txn argument is optional, this can be used outside a transaction
+// by setting the txn argument to nil.
+func (p *postgresqlDatabase) getTopicNID(txn *sql.Tx, topicName string) (topicNID int64, err error) {
+	// Get from the cache.
+	topicNID = p.getTopicNIDFromCache(topicName)
+	if topicNID != 0 {
+		return topicNID, nil
+	}
+	// Get from the database
+	s := p.selectTopicStmt
+	if txn != nil {
+		s = txn.Stmt(s)
+	}
+	err = s.QueryRow(topicName).Scan(&topicNID)
+	if err == sql.ErrNoRows {
+		return 0, nil
+	}
+	if err != nil {
+		return 0, err
+	}
+	// Update the shared cache.
+	p.addTopicNIDToCache(topicName, topicNID)
+	return topicNID, nil
+}
+
+// assignTopicNID assigns a new numeric ID to a topic.
+// The txn argument is mandatory, this is always called inside a transaction.
+func (p *postgresqlDatabase) assignTopicNID(txn *sql.Tx, topicName string) (topicNID int64, err error) {
+	// Check if we already have a numeric ID for the topic name.
+	topicNID, err = p.getTopicNID(txn, topicName)
+	if err != nil {
+		return 0, err
+	}
+	if topicNID != 0 {
+		return topicNID, err
+	}
+	// We don't have a numeric ID for the topic name so we add an entry to the
+	// topics table. If the insert stmt succeeds then it will return the ID.
+	err = txn.Stmt(p.insertTopicStmt).QueryRow(topicName).Scan(&topicNID)
+	if err == sql.ErrNoRows {
+		// If the insert stmt succeeded, but didn't return any rows then it
+		// means that someone has added a row for the topic name between us
+		// selecting it the first time and us inserting our own row.
+		// (N.B. postgres only returns modified rows when using "RETURNING")
+		// So we can now just select the row that someone else added.
+		// TODO: This is probably unnecessary since naffka writes to a topic
+		// from a single thread.
+		return p.getTopicNID(txn, topicName)
+	}
+	if err != nil {
+		return 0, err
+	}
+	// Update the cache.
+	p.addTopicNIDToCache(topicName, topicNID)
+	return topicNID, nil
+}
+
+// getTopicNIDFromCache returns the topicNID from the cache or returns 0 if the
+// topic is not in the cache.
+func (p *postgresqlDatabase) getTopicNIDFromCache(topicName string) (topicNID int64) {
+	p.topicsMutex.Lock()
+	defer p.topicsMutex.Unlock()
+	return p.topicNIDs[topicName]
+}
+
+// addTopicNIDToCache adds the numeric ID for the topic to the cache.
+func (p *postgresqlDatabase) addTopicNIDToCache(topicName string, topicNID int64) {
+	p.topicsMutex.Lock()
+	defer p.topicsMutex.Unlock()
+	p.topicNIDs[topicName] = topicNID
+}
+
+// withTransaction runs a block of code passing in an SQL transaction
+// If the code returns an error or panics then the transactions is rolledback
+// Otherwise the transaction is committed.
+func withTransaction(db *sql.DB, fn func(txn *sql.Tx) error) (err error) {
+	txn, err := db.Begin()
+	if err != nil {
+		return
+	}
+	defer func() {
+		if r := recover(); r != nil {
+			txn.Rollback()
+			panic(r)
+		} else if err != nil {
+			txn.Rollback()
+		} else {
+			err = txn.Commit()
+		}
+	}()
+	err = fn(txn)
+	return
+}