Add support for startFrom query parameter in SSEHandler

Add support for `startFrom` query parameter in HTTP handler.

* Modify `src/route/handlers.go` to parse the `startFrom` query parameter in the `SSEHandler` function.
* Add code to apply seek after subscribing based on the `startFrom` query parameter in `src/route/handlers.go`.
* Create a new file `src/pulsarutil/start_pos.go` and add the `GetStartOption` function to handle the `startFrom` query parameter.

---

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Author: RuairiSpain

src/pulsarutil/start_pos.go

@@ -0,0 +1,39 @@
+package pulsarutil
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/apache/pulsar-client-go/pulsar"
+)
+
+func GetStartOption(startFrom string) (pulsar.SubscriptionInitialPosition, *pulsar.MessageID, *time.Time, error) {
+	switch {
+	case startFrom == "earliest":
+		return pulsar.SubscriptionPositionEarliest, nil, nil, nil
+	case strings.HasPrefix(startFrom, "messageId:"):
+		parts := strings.Split(strings.TrimPrefix(startFrom, "messageId:"), ":")
+		if len(parts) != 2 {
+			return 0, nil, nil, fmt.Errorf("invalid messageId format, expected ledgerId:entryId")
+		}
+		ledgerId, err1 := strconv.ParseInt(parts[0], 10, 64)
+		entryId, err2 := strconv.ParseInt(parts[1], 10, 64)
+		if err1 != nil || err2 != nil {
+			return 0, nil, nil, fmt.Errorf("invalid messageId numbers")
+		}
+		msgID := pulsar.NewMessageID(ledgerId, entryId, -1)
+		return 0, &msgID, nil, nil
+	case strings.HasPrefix(startFrom, "timestamp:"):
+		millisStr := strings.TrimPrefix(startFrom, "timestamp:")
+		millis, err := strconv.ParseInt(millisStr, 10, 64)
+		if err != nil {
+			return 0, nil, nil, fmt.Errorf("invalid timestamp")
+		}
+		t := time.UnixMilli(millis)
+		return 0, nil, &t, nil
+	default:
+		return 0, nil, nil, fmt.Errorf("unsupported startFrom value")
+	}
+}

src/route/handlers.go

@@ -18,6 +18,7 @@ import (
 	"github.com/kafkaesque-io/pulsar-beam/src/model"
 	"github.com/kafkaesque-io/pulsar-beam/src/pulsardriver"
 	"github.com/kafkaesque-io/pulsar-beam/src/util"
+	"github.com/kafkaesque-io/pulsar-beam/src/pulsarutil"
 
 	log "github.com/sirupsen/logrus"
 )
@@ -196,6 +197,9 @@ func SSEHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Parse the startFrom query parameter
+	startFrom := r.URL.Query().Get("startFrom")
+
 	// Make sure that the writer supports flushing.
 	flusher, ok := w.(http.Flusher)
 	if !ok {
@@ -219,6 +223,22 @@ func SSEHandler(w http.ResponseWriter, r *http.Request) {
 		defer consumer.Unsubscribe()
 	}
 
+	// Apply seek after subscribing based on the startFrom query parameter
+	if startFrom != "" {
+		pos, msgID, ts, err := pulsarutil.GetStartOption(startFrom)
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusBadRequest)
+			return
+		}
+		if msgID != nil {
+			consumer.Seek(*msgID)
+		} else if ts != nil {
+			consumer.SeekByTime(*ts)
+		} else {
+			consumer.SeekByTime(time.Now())
+		}
+	}
+
 	consumChan := consumer.Chan()
 	for {
 		select {