refactoring

debug.log

@@ -0,0 +1 @@
+no Go files in /Users/stephaniegredell/projects/systemdesigngame

internal/level/levels_test.go

@@ -1,8 +1,6 @@
 package level
 
 import (
-	"fmt"
-	"os"
 	"path/filepath"
 	"testing"
 )
@@ -10,10 +8,6 @@ import (
 func TestLoadLevels(t *testing.T) {
 	path := filepath.Join("..", "..", "data", "levels.json")
 
-	cwd, _ := os.Getwd()
-	fmt.Println("Current working directory: ", cwd)
-	fmt.Println("loading path: ", path)
-
 	levels, err := LoadLevels(path)
 	if err != nil {
 		t.Fatalf("failed to load levels.json: %v", err)

internal/simulation/cachenode.go

@@ -1,115 +0,0 @@
-package simulation
-
-import (
-	"math/rand"
-	"sort"
-)
-
-type CacheEntry struct {
-	Value       interface{}
-	Timestamp   int
-	ExpireAt    int
-	AccessCount int
-}
-
-type CacheNode struct {
-	ID             string
-	Label          string
-	CacheTTL       int
-	MaxEntries     int
-	EvictionPolicy string
-	CurrentLoad    int
-	Queue          []*Request
-	Cache          map[string]CacheEntry
-	Alive          bool
-	Targets        []string
-	Output         []*Request
-}
-
-func (c *CacheNode) GetID() string {
-	return c.ID
-}
-
-func (c *CacheNode) Type() string {
-	return "cache"
-}
-
-func (c *CacheNode) IsAlive() bool {
-	return c.Alive
-}
-
-func (c *CacheNode) Tick(tick int, currentTimeMs int) {
-	for key, entry := range c.Cache {
-		if currentTimeMs > entry.ExpireAt {
-			delete(c.Cache, key)
-		}
-	}
-
-	if len(c.Cache) > c.MaxEntries {
-		evictCount := len(c.Cache) - c.MaxEntries
-		keys := make([]string, 0, len(c.Cache))
-		for k := range c.Cache {
-			keys = append(keys, k)
-		}
-
-		switch c.EvictionPolicy {
-		case "Random":
-			rand.Shuffle(len(keys), func(i, j int) { keys[i], keys[j] = keys[j], keys[i] })
-		case "LRU":
-			sort.Slice(keys, func(i, j int) bool {
-				return c.Cache[keys[i]].Timestamp < c.Cache[keys[j]].Timestamp
-			})
-		case "LFU":
-			sort.Slice(keys, func(i, j int) bool {
-				return c.Cache[keys[i]].AccessCount < c.Cache[keys[j]].AccessCount
-			})
-		}
-
-		for i := 0; i < evictCount && i < len(keys); i++ {
-			delete(c.Cache, keys[i])
-		}
-	}
-
-	toProcess := min(len(c.Queue), 10)
-	for i := 0; i < toProcess; i++ {
-		req := c.Queue[i]
-		if entry, found := c.Cache[req.ID]; found && currentTimeMs <= entry.ExpireAt {
-			// Cache hit
-			req.LatencyMS += 2
-			req.Path = append(req.Path, c.ID+"(hit)")
-		} else {
-			// Cache miss
-			req.LatencyMS += 5
-			req.Path = append(req.Path, c.ID+"(miss)")
-			c.Cache[req.ID] = CacheEntry{
-				Value:     req,
-				Timestamp: currentTimeMs,
-				ExpireAt:  currentTimeMs + c.CacheTTL*1000,
-			}
-			c.Output = append(c.Output, req)
-		}
-	}
-	c.Queue = c.Queue[toProcess:]
-}
-
-func (c *CacheNode) Receive(req *Request) {
-	c.Queue = append(c.Queue, req)
-}
-
-func (c *CacheNode) Emit() []*Request {
-	out := append([]*Request(nil), c.Output...)
-	c.Output = c.Output[:0]
-	return out
-}
-
-func (c *CacheNode) AddTarget(targetID string) {
-	c.Targets = append(c.Targets, targetID)
-}
-
-func (c *CacheNode) GetTargets() []string {
-	return c.Targets
-}
-
-func (n *CacheNode) GetQueue() []*Request {
-	return n.Queue
-}

internal/simulation/cdn.go

@@ -0,0 +1,22 @@
+package simulation
+
+import "math/rand"
+
+type CDNLogic struct{}
+
+func (c CDNLogic) Tick(props map[string]any, queue []*Request, tick int) ([]*Request, bool) {
+	hitRate := AsFloat64("hitRate")
+	var output []*Request
+
+	for _, req := range queue {
+		if rand.Float64() < hitRate {
+			continue
+		}
+
+		reqCopy := *req
+		reqCopy.Path = append(reqCopy.Path, "target-0")
+		output = append(output, &reqCopy)
+	}
+
+	return output, true
+}

internal/simulation/cdnnode.go

@@ -1,93 +0,0 @@
-package simulation
-
-import (
-	"math/rand"
-)
-
-type CDNNode struct {
-	ID              string
-	Label           string
-	TTL             int
-	GeoReplication  string
-	CachingStrategy string
-	Compression     string
-	HTTP2           string
-	CacheHitRate    float64
-	CurrentLoad     int
-	Queue           []*Request
-	EdgeNodes       map[string]*CDNNode
-	Alive           bool
-	Targets         []string
-	output          []*Request
-	missQueue       []*Request
-}
-
-func (n *CDNNode) GetID() string          { return n.ID }
-func (n *CDNNode) Type() string           { return "cdn" }
-func (n *CDNNode) IsAlive() bool          { return n.Alive }
-func (n *CDNNode) QueueState() []*Request { return n.Queue }
-
-func (n *CDNNode) Tick(tick int, currentTimeMs int) {
-	if len(n.Queue) == 0 {
-		return
-	}
-
-	maxProcessPerTick := 10
-	processCount := min(len(n.Queue), maxProcessPerTick)
-
-	queue := n.Queue
-	n.Queue = n.Queue[:0]
-
-	for i := 0; i < processCount; i++ {
-		req := queue[i]
-
-		hitRate := n.CacheHitRate
-		if hitRate == 0 {
-			hitRate = 0.8
-		}
-
-		if rand.Float64() < hitRate {
-			// Cache HIT
-			req.LatencyMS += 5
-			req.Path = append(req.Path, n.ID)
-			n.output = append(n.output, req)
-		} else {
-			// Cache MISS
-			req.LatencyMS += 10
-			req.Path = append(req.Path, n.ID+"(miss)")
-			n.missQueue = append(n.missQueue, req)
-		}
-	}
-
-	if len(queue) > processCount {
-		n.Queue = append(n.Queue, queue[processCount:]...)
-	}
-}
-
-func (n *CDNNode) Receive(req *Request) {
-	if req == nil {
-		return
-	}
-
-	geoLatency := rand.Intn(20) + 5 // 5–25ms routing delay
-	req.LatencyMS += geoLatency
-
-	n.Queue = append(n.Queue, req)
-}
-
-func (n *CDNNode) Emit() []*Request {
-	out := append(n.output, n.missQueue...)
-
-	n.output = n.output[:0]
-	n.missQueue = n.missQueue[:0]
-
-	return out
-}
-
-func (n *CDNNode) GetTargets() []string {
-	return n.Targets
-}
-
-func (n *CDNNode) GetQueue() []*Request {
-	return n.Queue
-}

internal/simulation/databasenode.go

@@ -1,83 +0,0 @@
-package simulation
-
-type DatabaseNode struct {
-	ID               string
-	Label            string
-	Replication      int
-	CurrentLoad      int
-	Queue            []*Request
-	Replicas         []*DatabaseNode
-	Alive            bool
-	Targets          []string
-	Output           []*Request
-	ReplicationQueue []*Request
-}
-
-func (n *DatabaseNode) GetID() string        { return n.ID }
-func (n *DatabaseNode) Type() string         { return "database" }
-func (n *DatabaseNode) IsAlive() bool        { return n.Alive }
-func (n *DatabaseNode) GetQueue() []*Request { return n.Queue }
-
-func (n *DatabaseNode) Tick(tick int, currentTimeMs int) {
-	if len(n.Queue) == 0 {
-		return
-	}
-
-	maxProcessPerTick := 3
-	processCount := min(len(n.Queue), maxProcessPerTick)
-
-	queue := n.Queue
-	n.Queue = n.Queue[:0]
-
-	for i := 0; i < processCount; i++ {
-		req := queue[i]
-
-		if req.Type == "READ" {
-			req.LatencyMS += 20
-			req.Path = append(req.Path, n.ID)
-			n.Output = append(n.Output, req)
-		} else {
-			req.LatencyMS += 50
-			req.Path = append(req.Path, n.ID)
-
-			for _, replica := range n.Replicas {
-				replicationReq := &Request{
-					ID:        req.ID + "-repl",
-					Timestamp: req.Timestamp,
-					LatencyMS: req.LatencyMS + 5,
-					Origin:    req.Origin,
-					Type:      "REPLICATION",
-					Path:      append(append([]string{}, req.Path...), "->"+replica.ID),
-				}
-				n.ReplicationQueue = append(n.ReplicationQueue, replicationReq)
-			}
-
-			n.Output = append(n.Output, req)
-		}
-	}
-
-	if len(n.Queue) > 10 {
-		for _, req := range n.Queue {
-			req.LatencyMS += 10
-		}
-	}
-}
-
-func (n *DatabaseNode) Receive(req *Request) {
-	if req == nil {
-		return
-	}
-	req.LatencyMS += 2 // DB connection overhead
-	n.Queue = append(n.Queue, req)
-}
-
-func (n *DatabaseNode) Emit() []*Request {
-	out := append(n.Output, n.ReplicationQueue...)
-	n.Output = n.Output[:0]
-	n.ReplicationQueue = n.ReplicationQueue[:0]
-	return out
-}
-
-func (n *DatabaseNode) GetTargets() []string {
-	return n.Targets
-}

internal/simulation/datapipelinenode.go

@@ -1,114 +0,0 @@
-package simulation
-
-import (
-	"math/rand"
-)
-
-type DataPipelineNode struct {
-	ID              string
-	Label           string
-	BatchSize       int
-	Transformation  string
-	CurrentLoad     int
-	Queue           []*Request
-	Alive           bool
-	Targets         []string
-	Output          []*Request
-	LastFlushTimeMS int
-}
-
-func (n *DataPipelineNode) GetID() string { return n.ID }
-
-func (n *DataPipelineNode) Tick(tick int, currentTimeMs int) {
-	if len(n.Queue) == 0 {
-		return
-	}
-
-	if len(n.Queue) < n.BatchSize {
-		if n.LastFlushTimeMS == 0 {
-			n.LastFlushTimeMS = currentTimeMs
-		}
-		if currentTimeMs-n.LastFlushTimeMS >= 5000 {
-			n.processBatch(len(n.Queue), currentTimeMs)
-			n.LastFlushTimeMS = currentTimeMs
-		}
-		return
-	}
-
-	batchesToProcess := len(n.Queue) / n.BatchSize
-	maxBatchesPerTick := 2
-	actualBatches := min(batchesToProcess, maxBatchesPerTick)
-
-	for i := 0; i < actualBatches; i++ {
-		n.processBatch(n.BatchSize, currentTimeMs)
-		n.LastFlushTimeMS = currentTimeMs
-	}
-}
-
-func (n *DataPipelineNode) processBatch(size int, currentTimeMs int) {
-	if size == 0 {
-		return
-	}
-
-	batch := n.Queue[:size]
-	n.Queue = n.Queue[size:]
-	batchLatency := 100 + (size * 5)
-
-	for _, req := range batch {
-		req.LatencyMS += batchLatency
-		req.Path = append(req.Path, n.ID+"(batch)")
-
-		switch n.Transformation {
-		case "aggregate":
-			req.Type = "AGGREGATED"
-			n.Output = append(n.Output, req)
-		case "filter":
-			if rand.Float64() < 0.9 {
-				n.Output = append(n.Output, req)
-			}
-			continue
-		case "enrich":
-			req.Type = "ENRICHED"
-			n.Output = append(n.Output, req)
-		case "normalize":
-			req.Type = "NORMALIZED"
-			n.Output = append(n.Output, req)
-		case "dedupe":
-			if rand.Float64() < 0.95 {
-				req.Type = "DEDUPED"
-				n.Output = append(n.Output, req)
-			}
-			continue
-		default:
-			n.Output = append(n.Output, req)
-		}
-	}
-}
-
-func (n *DataPipelineNode) Receive(req *Request) {
-	if req == nil {
-		return
-	}
-	req.LatencyMS += 1
-	n.Queue = append(n.Queue, req)
-
-	if n.LastFlushTimeMS == 0 {
-		n.LastFlushTimeMS = req.Timestamp
-	}
-}
-
-func (n *DataPipelineNode) Emit() []*Request {
-	out := append([]*Request(nil), n.Output...)
-	n.Output = n.Output[:0]
-	return out
-}
-
-func (n *DataPipelineNode) Type() string  { return "datapipeline" }
-func (n *DataPipelineNode) IsAlive() bool { return n.Alive }
-func (n *DataPipelineNode) GetTargets() []string {
-	return n.Targets
-}
-
-func (n *DataPipelineNode) GetQueue() []*Request {
-	return n.Queue
-}

internal/simulation/engine.go

@@ -2,10 +2,22 @@ package simulation
 
 import (
 	"fmt"
-	"math/rand"
 	"systemdesigngame/internal/design"
 )
 
+type NodeLogic interface {
+	Tick(props map[string]any, queue []*Request, tick int) ([]*Request, bool)
+}
+
+type NodeInstance struct {
+	ID    string
+	Type  string
+	Props map[string]any
+	Queue []*Request
+	Alive bool
+	Logic NodeLogic
+}
+
 // a unit that flows through the system
 type Request struct {
 	ID string
@@ -21,305 +33,149 @@ type Request struct {
 	Path []string
 }
 
-// Every node implements this interface and is used by the engine to operate all nodes in a uniform way.
-type SimulationNode interface {
-	GetID() string
-	Type() string
-	Tick(tick int, currentTimeMs int) // Advance the node's state
-	Receive(req *Request)             // Accept new requests
-	Emit() []*Request
-	IsAlive() bool
-	GetTargets() []string // Connection to other nodes
-	GetQueue() []*Request // Requests currently pending
-}
-
-type Engine struct {
-	// Map of Node ID -> actual node, Represents all nodes in the graph
-	Nodes map[string]SimulationNode
-	// all tick snapshots
-	Timeline []*TickSnapshot
-	// how many ticks to run
-	Duration int
-	// no used here but we could use it if we want it configurable
-	TickMs int
-}
-
 // what hte system looks like given a tick
 type TickSnapshot struct {
 	TickMs int
 	// Queue size at each node
 	QueueSizes map[string]int
-	NodeHealth map[string]NodeState
+	NodeHealth map[string]bool
 	// what each node output that tick before routing
 	Emitted map[string][]*Request
 }
 
-// used for tracking health/debugging each node at tick
+type SimulationEngine struct {
-type NodeState struct {
+	Nodes     map[string]*NodeInstance
-	QueueSize int
+	Edges     map[string][]string
-	Alive     bool
+	TickMS    int
+	EntryNode string // this should be the node ID where traffic should enter
+	RPS       int    // how many requests per second should be injected while running
 }
 
-// Takes a level design and produces a runnable engine from it.
+func NewEngineFromDesign(d design.Design, tickMS int) *SimulationEngine {
-func NewEngineFromDesign(design design.Design, duration int, tickMs int) *Engine {
+	nodes := make(map[string]*NodeInstance)
+	edges := make(map[string][]string)
 
-	// Iterate over each nodes and then construct the simulation nodes
+	for _, n := range d.Nodes {
-	// Each constructed simulation node is then stored in the nodeMap
+		logic := GetLogicForType(n.Type)
-	nodeMap := make(map[string]SimulationNode)
 
-	for _, n := range design.Nodes {
+		if logic == nil {
-		var simNode SimulationNode
-
-		switch n.Type {
-		case "webserver":
-			simNode = &WebServerNode{
-				ID:    n.ID,
-				Alive: true,
-				Queue: []*Request{},
-			}
-		case "loadBalancer":
-			props := n.Props
-			simNode = &LoadBalancerNode{
-				ID:        n.ID,
-				Label:     asString(props["label"]),
-				Algorithm: asString(props["algorithm"]),
-				Queue:     []*Request{},
-				Alive:     true,
-				Targets:   []string{},
-			}
-		case "cache":
-			simNode = &CacheNode{
-				ID:             n.ID,
-				Label:          asString(n.Props["label"]),
-				CacheTTL:       int(asFloat64(n.Props["cacheTTL"])),
-				MaxEntries:     int(asFloat64(n.Props["maxEntries"])),
-				EvictionPolicy: asString(n.Props["evictionPolicy"]),
-				CurrentLoad:    0,
-				Queue:          []*Request{},
-				Cache:          make(map[string]CacheEntry),
-				Alive:          true,
-			}
-		case "database":
-			simNode = &DatabaseNode{
-				ID:          n.ID,
-				Label:       asString(n.Props["label"]),
-				Replication: int(asFloat64(n.Props["replication"])),
-				Queue:       []*Request{},
-				Alive:       true,
-			}
-		case "cdn":
-			simNode = &CDNNode{
-				ID:              n.ID,
-				Label:           asString(n.Props["label"]),
-				TTL:             int(asFloat64(n.Props["ttl"])),
-				GeoReplication:  asString(n.Props["geoReplication"]),
-				CachingStrategy: asString(n.Props["cachingStrategy"]),
-				Compression:     asString(n.Props["compression"]),
-				HTTP2:           asString(n.Props["http2"]),
-				Queue:           []*Request{},
-				Alive:           true,
-				output:          []*Request{},
-				missQueue:       []*Request{},
-			}
-		case "messageQueue":
-			simNode = &MessageQueueNode{
-				ID:         n.ID,
-				Label:      asString(n.Props["label"]),
-				QueueSize:  int(asFloat64(n.Props["maxSize"])),
-				MessageTTL: int(asFloat64(n.Props["retentionSeconds"])),
-				DeadLetter: false,
-				Queue:      []*Request{},
-				Alive:      true,
-			}
-		case "microservice":
-			simNode = &MicroserviceNode{
-				ID:             n.ID,
-				Label:          asString(n.Props["label"]),
-				APIEndpoint:    asString(n.Props["apiVersion"]),
-				RateLimit:      int(asFloat64(n.Props["rpsCapacity"])),
-				CircuitBreaker: true,
-				Queue:          []*Request{},
-				CircuitState:   "closed",
-				Alive:          true,
-			}
-		case "third party service":
-			simNode = &ThirdPartyServiceNode{
-				ID:          n.ID,
-				Label:       asString(n.Props["label"]),
-				APIEndpoint: asString(n.Props["provider"]),
-				RateLimit:   int(asFloat64(n.Props["latency"])),
-				RetryPolicy: "exponential",
-				Queue:       []*Request{},
-				Alive:       true,
-			}
-		case "data pipeline":
-			simNode = &DataPipelineNode{
-				ID:             n.ID,
-				Label:          asString(n.Props["label"]),
-				BatchSize:      int(asFloat64(n.Props["batchSize"])),
-				Transformation: asString(n.Props["transformation"]),
-				Queue:          []*Request{},
-				Alive:          true,
-			}
-		case "monitoring/alerting":
-			simNode = &MonitoringNode{
-				ID:             n.ID,
-				Label:          asString(n.Props["label"]),
-				Tool:           asString(n.Props["tool"]),
-				AlertMetric:    asString(n.Props["metric"]),
-				ThresholdValue: int(asFloat64(n.Props["threshold"])),
-				ThresholdUnit:  asString(n.Props["unit"]),
-				Queue:          []*Request{},
-				Alive:          true,
-			}
-		default:
 			continue
 		}
 
-		if simNode != nil {
+		// create a NodeInstance using data from the json
-			nodeMap[simNode.GetID()] = simNode
+		nodes[n.ID] = &NodeInstance{
+			ID:    n.ID,
+			Type:  n.Type,
+			Props: n.Props,
+			Queue: []*Request{},
+			Alive: true,
+			Logic: logic,
 		}
 	}
 
-	// Wire up connections
+	// build a map of the connections (edges)
-	for _, conn := range design.Connections {
+	for _, c := range d.Connections {
-		if sourceNode, ok := nodeMap[conn.Source]; ok {
+		edges[c.Source] = append(edges[c.Source], c.Target)
-			if targetSetter, ok := sourceNode.(interface{ AddTarget(string) }); ok {
-				targetSetter.AddTarget(conn.Target)
-			}
-		}
 	}
 
-	return &Engine{
+	return &SimulationEngine{
-		Nodes:    nodeMap,
+		Nodes:     nodes,
-		Duration: duration,
+		Edges:     edges,
-		TickMs:   tickMs,
+		TickMS:    tickMS,
+		RPS:       0,  // ideally, this will come from the design (serialized json)
+		EntryNode: "", // default to empty, we check this later in the run method
 	}
 }
 
-func (e *Engine) Run() {
+func (e *SimulationEngine) Run(duration int, tickMs int) []*TickSnapshot {
-	// clear and set defaults
+	snapshots := []*TickSnapshot{}
-	const tickMS = 100
+	currentTime := 0
-	currentTimeMs := 0
-	e.Timeline = e.Timeline[:0]
-
-	// start ticking. This is really where the simulation begins
-	for tick := 0; tick < e.Duration; tick++ {
 
-		// find the entry points (where traffic enters) or else print a warning
+	for tick := 0; tick < duration; tick++ {
-		entries := e.findEntryPoints()
+		if e.RPS > 0 && e.EntryNode != "" {
-		if len(entries) == 0 {
+			count := int(float64(e.RPS) * float64(e.TickMS) / 1000.0)
-			fmt.Println("[ERROR] No entry points found! Simulation will not inject requests.")
+			reqs := make([]*Request, count)
-		}
 
-		// inject new requests of each entry node every tick
+			for i := 0; i < count; i++ {
-		for _, node := range entries {
+				reqs[i] = &Request{
-			if shouldInject(tick) {
+					ID:        fmt.Sprintf("req-%d-%d", tick, i),
-				req := &Request{
+					Origin:    e.EntryNode,
-					ID:        generateRequestID(tick),
-					Timestamp: currentTimeMs,
-					LatencyMS: 0,
-					Origin:    node.GetID(),
 					Type:      "GET",
-					Path:      []string{node.GetID()},
+					Timestamp: tick * e.TickMS,
+					Path:      []string{e.EntryNode},
 				}
-				node.Receive(req)
 			}
+
+			node := e.Nodes[e.EntryNode]
+			node.Queue = append(node.Queue, reqs...)
 		}
 
-		// snapshot to record what happened this tick
+		emitted := map[string][]*Request{}
 		snapshot := &TickSnapshot{
 			TickMs:     tick,
-			NodeHealth: make(map[string]NodeState),
+			QueueSizes: map[string]int{},
+			NodeHealth: map[string]bool{},
+			Emitted:    map[string][]*Request{},
 		}
 
 		for id, node := range e.Nodes {
-			// capture health data before processing
+			// if the node is not alive, don't even bother.
-			snapshot.NodeHealth[id] = NodeState{
+			if !node.Alive {
-				QueueSize: len(node.GetQueue()),
+				continue
-				Alive:     node.IsAlive(),
 			}
 
-			// tick all nodes
+			// this will preopulate some props so that we can use different load balancing algorithms
-			node.Tick(tick, currentTimeMs)
+			if node.Type == "loadbalancer" && node.Props["algorithm"] == "least-connection" {
-
+				queueSizes := make(map[string]interface{})
-			// get all processed requets and fan it out to all connected targets
+				for _, targetID := range e.Edges[id] {
-			for _, req := range node.Emit() {
+					queueSizes[targetID] = len(e.Nodes[targetID].Queue)
-				snapshot.Emitted[node.GetID()] = append(snapshot.Emitted[node.GetID()], req)
-
-				for _, targetID := range node.GetTargets() {
-					if target, ok := e.Nodes[targetID]; ok && target.IsAlive() && !hasVisited(req, targetID) {
-						// Deep copy request and update path
-						newReq := *req
-						newReq.Path = append([]string{}, req.Path...)
-						newReq.Path = append(newReq.Path, targetID)
-						target.Receive(&newReq)
-					}
 				}
+				node.Props["_queueSizes"] = queueSizes
 			}
 
-		}
+			// simulate the node. outputs is the emitted requests (request post-processing) and alive tells you if the node is healthy
+			outputs, alive := node.Logic.Tick(node.Props, node.Queue, tick)
 
-		// store the snapshot and advance time
+			// at this point, all nodes have ticked. record the emitted requests
-		e.Timeline = append(e.Timeline, snapshot)
+			emitted[id] = outputs
-		currentTimeMs += tickMS
-	}
-}
 
-func (e *Engine) findEntryPoints() []SimulationNode {
+			// clear the queue after processing. Queues should only contain requests for the next tick that need processing.
-	var entries []SimulationNode
+			node.Queue = nil
-	for _, node := range e.Nodes {
-		if node.Type() == "loadBalancer" {
-			entries = append(entries, node)
-		}
-	}
-	return entries
-}
 
-func (e *Engine) injectRequests(entries []SimulationNode, requests []*Request) {
+			// update if the node is still alive
-	for i, req := range requests {
+			node.Alive = alive
-		node := entries[i%len(entries)]
-		node.Receive(req)
-	}
-}
 
-func shouldInject(tick int) bool {
+			// populate snapshot
-	return tick%100 == 0
+			snapshot.QueueSizes[id] = len(node.Queue)
-}
+			snapshot.NodeHealth[id] = node.Alive
-
+			snapshot.Emitted[id] = outputs
-func generateRequestID(tick int) string {
+		}
-	return fmt.Sprintf("req-%d-%d", tick, rand.Intn(1000))
-}
 
-func asFloat64(v interface{}) float64 {
+		// iterate over each emitted request
-	switch val := v.(type) {
+		for from, reqs := range emitted {
-	case float64:
+			// figure out where those requests should go to
-		return val
+			for _, to := range e.Edges[from] {
-	case int:
+				// add those requests to the target node's input (queue)
-		return float64(val)
+				e.Nodes[to].Queue = append(e.Nodes[to].Queue, reqs...)
-	case int64:
+			}
-		return float64(val)
+		}
-	case float32:
-		return float64(val)
-	default:
-		return 0
-	}
-}
 
-func asString(v interface{}) string {
+		snapshots = append(snapshots, snapshot)
-	s, ok := v.(string)
+		currentTime += e.TickMS
-	if !ok {
-		return ""
 	}
 
-	return s
+	return snapshots
 }
 
-func hasVisited(req *Request, nodeID string) bool {
+func GetLogicForType(t string) NodeLogic {
-	for _, id := range req.Path {
+	switch t {
-		if id == nodeID {
+	case "webserver":
-			return true
+		return WebServerLogic{}
-		}
+	case "loadbalancer":
+		return LoadBalancerLogic{}
+	case "cdn":
+		return CDNLogic{}
+	default:
+		return nil
 	}
-	return false
 }

internal/simulation/engine_test.go

@@ -1,130 +1,107 @@
 package simulation
 
 import (
-	"fmt"
-	"os"
-	"path/filepath"
 	"testing"
 
-	"encoding/json"
 	"systemdesigngame/internal/design"
 )
 
-func TestNewEngineFromDesign(t *testing.T) {
+func TestSimpleChainSimulation(t *testing.T) {
-	designInput := &design.Design{
+	d := design.Design{
 		Nodes: []design.Node{
-			{
+			{ID: "a", Type: "webserver", Props: map[string]any{"capacityRPS": 1, "baseLatencyMs": 10}},
-				ID:   "web1",
+			{ID: "b", Type: "webserver", Props: map[string]any{"capacityRPS": 1, "baseLatencyMs": 10}},
-				Type: "webserver",
-				Props: map[string]interface{}{
-					"cpu":            float64(2),
-					"ramGb":          float64(4),
-					"rpsCapacity":    float64(100),
-					"monthlyCostUsd": float64(20),
-				},
-			},
-			{
-				ID:   "cache1",
-				Type: "cache",
-				Props: map[string]interface{}{
-					"label":          "L1 Cache",
-					"cacheTTL":       float64(60),
-					"maxEntries":     float64(1000),
-					"evictionPolicy": "LRU",
-				},
-			},
 		},
 		Connections: []design.Connection{
-			{
+			{Source: "a", Target: "b"},
-				Source: "web1",
-				Target: "cache1",
-			},
 		},
 	}
 
-	engine := NewEngineFromDesign(*designInput, 10, 100)
+	engine := NewEngineFromDesign(d, 100)
 
-	if len(engine.Nodes) != 2 {
+	engine.Nodes["a"].Queue = append(engine.Nodes["a"].Queue, &Request{
-		t.Fatalf("expected 2 nodes, got %d", len(engine.Nodes))
+		ID:        "req-1",
+		Origin:    "a",
+		Type:      "GET",
+		Timestamp: 0,
+		Path:      []string{"a"},
+	})
+
+	snaps := engine.Run(2, 100)
+
+	if len(snaps) != 2 {
+		t.Fatalf("expected 2 snapshots, got %d", len(snaps))
 	}
 
-	if len(engine.Nodes["web1"].GetTargets()) != 1 {
+	if len(snaps[0].Emitted["a"]) != 1 {
-		t.Fatalf("expected web1 to have 1 target, got %d", len(engine.Nodes["web1"].GetTargets()))
+		t.Errorf("expected a to emit 1 request at tick 0")
+	}
+	if snaps[0].QueueSizes["b"] != 0 {
+		t.Errorf("expected b's queue to be 0 after tick 0 (not yet processed)")
 	}
 
-	if engine.Nodes["web1"].GetTargets()[0] != "cache1" {
+	if len(snaps[1].Emitted["b"]) != 1 {
-		t.Fatalf("expected web1 target to be cache1")
+		t.Errorf("expected b to emit 1 request at tick 1")
 	}
 }
 
-func TestComplexSimulationRun(t *testing.T) {
+func TestSingleTickRouting(t *testing.T) {
-	filePath := filepath.Join("testdata", "complex_design.json")
+	d := design.Design{
-	data, err := os.ReadFile(filePath)
+		Nodes: []design.Node{
-	if err != nil {
+			{ID: "a", Type: "webserver", Props: map[string]any{"capacityRPS": 1.0, "baseLatencyMs": 10.0}},
-		t.Fatalf("Failed to read JSON file: %v", err)
+			{ID: "b", Type: "webserver", Props: map[string]any{"capacityRPS": 1.0, "baseLatencyMs": 10.0}},
+		},
+		Connections: []design.Connection{
+			{Source: "a", Target: "b"},
+		},
 	}
 
-	var d design.Design
+	engine := NewEngineFromDesign(d, 100)
-	if err := json.Unmarshal([]byte(data), &d); err != nil {
-		t.Fatalf("Failed to unmarshal JSON: %v", err)
-	}
 
-	engine := NewEngineFromDesign(d, 10, 100)
+	engine.Nodes["a"].Queue = append(engine.Nodes["a"].Queue, &Request{
-	if engine == nil {
+		ID:        "req-1",
-		t.Fatal("Engine should not be nil")
+		Origin:    "a",
-	}
+		Type:      "GET",
+		Timestamp: 0,
+		Path:      []string{"a"},
+	})
 
-	engine.Run()
+	snaps := engine.Run(1, 100)
 
-	if len(engine.Timeline) == 0 {
+	if len(snaps) != 1 {
-		t.Fatal("Expected timeline snapshots after Run, got none")
+		t.Fatalf("expected 1 snapshot, got %d", len(snaps))
 	}
 
-	// Optional: check that some nodes received or emitted requests
+	if len(snaps[0].Emitted["a"]) != 1 {
-	for id, node := range engine.Nodes {
+		t.Errorf("expected a to emit 1 request, got %d", len(snaps[0].Emitted["a"]))
-		if len(node.Emit()) > 0 {
-			t.Logf("Node %s has activity", id)
-		}
 	}
-}
 
-func TestSimulationRunEndToEnd(t *testing.T) {
+	if len(engine.Nodes["b"].Queue) != 1 {
-	data, err := os.ReadFile("testdata/simple_design.json")
+		t.Errorf("expected b to have 1 request queued for next tick, got %d", len(engine.Nodes["b"].Queue))
-	fmt.Print(data)
-	if err != nil {
-		t.Fatalf("Failed to read test data: %v", err)
 	}
+}
 
-	var design design.Design
+func TestHighRPSSimulation(t *testing.T) {
-	if err := json.Unmarshal(data, &design); err != nil {
+	d := design.Design{
-		t.Fatalf("Failed to unmarshal JSON: %v", err)
+		Nodes: []design.Node{
+			{ID: "entry", Type: "webserver", Props: map[string]any{"capacityRPS": 5000, "baseLatencyMs": 1}},
+		},
+		Connections: []design.Connection{},
 	}
 
-	engine := NewEngineFromDesign(design, 20, 100) // 20 ticks, 100ms per tick
+	engine := NewEngineFromDesign(d, 100)
-	engine.Run()
+	engine.EntryNode = "entry"
-
+	engine.RPS = 100000
-	if len(engine.Timeline) != 20 {
-		t.Errorf("Expected 20 timeline entries, got %d", len(engine.Timeline))
-	}
 
-	anyTraffic := false
+	snaps := engine.Run(10, 100)
-	for _, snapshot := range engine.Timeline {
-		for _, nodeState := range snapshot.NodeHealth {
-			if nodeState.QueueSize > 0 {
-				anyTraffic = true
-				break
-			}
-		}
-	}
 
-	if !anyTraffic {
+	totalEmitted := 0
-		t.Errorf("Expected at least one node to have non-zero queue size over time")
+	for _, snap := range snaps {
+		totalEmitted += len(snap.Emitted["entry"])
 	}
 
-	// Optional: check a few expected node IDs
+	expected := 10 * 5000 // capacity-limited output
-	for _, id := range []string{"node-1", "node-2"} {
+	if totalEmitted != expected {
-		if _, ok := engine.Nodes[id]; !ok {
+		t.Errorf("expected %d total emitted requests, got %d", expected, totalEmitted)
-			t.Errorf("Expected node %s to be present in simulation", id)
-		}
 	}
 }

internal/simulation/loadbalancer.go

@@ -1,91 +1,64 @@
 package simulation
 
 import (
-	"math/rand"
+	"fmt"
 )
 
-type LoadBalancerNode struct {
+type LoadBalancerLogic struct{}
-	// unique identifier for the node
-	ID string
-	// human readable name
-	Label string
-	// load balancing strategy
-	Algorithm string
-	// list of incoming requests to be processed
-	Queue []*Request
-	// IDs of downstream nodes (e.g. webservers)
-	Targets []string
-	// use to track round-robin state (i.e. which target is next)
-	Counter int
-	// bool for health check
-	Alive bool
-	// requests that this node has handled (ready to be emitted)
-	Processed []*Request
-}
 
-func (lb *LoadBalancerNode) GetID() string {
+func (l LoadBalancerLogic) Tick(props map[string]any, queue []*Request, tick int) ([]*Request, bool) {
-	return lb.ID
+	// Extract the load balancing algorithm from the props.
-}
+	algorithm := AsString(props["algorithm"])
+	// Number of downstream targets
+	targets := int(AsFloat64(props["_numTargets"]))
 
-func (lb *LoadBalancerNode) Type() string {
+	if len(queue) == 0 {
-	return "loadBalancer"
+		return nil, true
-}
+	}
-
-func (lb *LoadBalancerNode) IsAlive() bool {
-	return lb.Alive
-}
-
-// Acceps an incoming request by adding it to the Queue which will be processed on the next tick
-func (lb *LoadBalancerNode) Receive(req *Request) {
-	lb.Queue = append(lb.Queue, req)
-}
 
-func (lb *LoadBalancerNode) Tick(tick int, currentTimeMs int) {
+	// Hold the processed requests to be emitted
-	// clear out the process so it starts fresh
+	output := []*Request{}
-	lb.Processed = nil
 
-	// for each pending request...
+	switch algorithm {
-	for _, req := range lb.Queue {
+	case "least-connection":
-		// if there are no targets to forward to, skip processing
+		// extrat current queue sizes from downstream targets
-		if len(lb.Targets) == 0 {
+		queueSizesRaw, ok := props["_queueSizes"].(map[string]interface{})
-			continue
+		if !ok {
+			return nil, true
 		}
 
-		// placeholder for algorithm-specific logic. TODO.
+		// find target with smallest queue
-		switch lb.Algorithm {
+		for _, req := range queue {
-		case "random":
+			minTarget := "target-0"
-			fallthrough
+			minSize := int(AsFloat64(queueSizesRaw[minTarget]))
-		case "round-robin":
+			for i := 1; i < targets; i++ {
-			fallthrough
+				targetKey := fmt.Sprintf("target-%d", i)
-		default:
+				size := int(AsFloat64(queueSizesRaw[targetKey]))
-			lb.Counter++
+				if size < minSize {
+					minTarget = targetKey
+					minSize = size
+				}
+			}
+
+			// Clone the request and append the selected target to its path
+			reqCopy := *req
+			reqCopy.Path = append(reqCopy.Path, minTarget)
+			output = append(output, &reqCopy)
+		}
+	default:
+		// Retrieve the last used index
+		next := int(AsFloat64(props["_rrIndex"]))
+		for _, req := range queue {
+			// Clone ther equest and append the selected target to its path
+			reqCopy := *req
+			reqCopy.Path = append(reqCopy.Path, fmt.Sprintf("target-%d", next))
+			output = append(output, &reqCopy)
+			// Advance to next target
+			next = (next + 1) % targets
 		}
 
-		// Append the load balancer's ID to the request's path to record it's journey through the system
+		props["_rrIndex"] = float64(next)
-		req.Path = append(req.Path, lb.ID)
-
-		// Simulate networking delay
-		req.LatencyMS += 10
-
-		// Mark the request as processed so it can be emitted to targets
-		lb.Processed = append(lb.Processed, req)
 	}
 
-	// clear the queue after processing. Ready for next tick.
+	return output, true
-	lb.Queue = lb.Queue[:0]
-}
-
-// return the list of process requests and then clear the processed requests
-func (lb *LoadBalancerNode) Emit() []*Request {
-	out := lb.Processed
-	lb.Processed = nil
-	return out
-}
-
-func (lb *LoadBalancerNode) GetTargets() []string {
-	return lb.Targets
-}
-
-func (lb *LoadBalancerNode) GetQueue() []*Request {
-	return lb.Queue
 }

internal/simulation/loadbalancer_test.go

@@ -0,0 +1,70 @@
+package simulation
+
+import (
+	"systemdesigngame/internal/design"
+	"testing"
+)
+
+func TestLoadBalancerAlgorithms(t *testing.T) {
+	t.Run("round-rouble", func(t *testing.T) {
+		d := design.Design{
+			Nodes: []design.Node{
+				{ID: "lb", Type: "loadbalancer", Props: map[string]any{"algorithm": "round-robin"}},
+				{ID: "a", Type: "webserver", Props: map[string]any{"capacityRPS": 10}},
+				{ID: "b", Type: "webserver", Props: map[string]any{"capacityRPS": 10}},
+			},
+			Connections: []design.Connection{
+				{Source: "lb", Target: "a"},
+				{Source: "lb", Target: "b"},
+			},
+		}
+
+		e := NewEngineFromDesign(d, 100)
+		e.EntryNode = "lb"
+		e.RPS = 4
+
+		snaps := e.Run(1, 100)
+		if len(snaps[0].Emitted["lb"]) != 4 {
+			t.Errorf("expected lb to emit 4 requests")
+		}
+
+		path0 := snaps[0].Emitted["lb"][0].Path[1]
+		path1 := snaps[0].Emitted["lb"][1].Path[1]
+
+		if path0 == path1 {
+			t.Errorf("expecting alternating targets, got %s and %s", path0, path1)
+		}
+	})
+
+	t.Run("least-connection", func(t *testing.T) {
+		d := design.Design{
+			Nodes: []design.Node{
+				{ID: "lb", Type: "loadbalancer", Props: map[string]any{"algorithm": "least-connection"}},
+				{ID: "a", Type: "webserver", Props: map[string]any{"capacityRPS": 1}},
+				{ID: "b", Type: "webserver", Props: map[string]any{"capacityRPS": 1}},
+			},
+			Connections: []design.Connection{
+				{Source: "lb", Target: "a"},
+				{Source: "lb", Target: "b"},
+			},
+		}
+
+		e := NewEngineFromDesign(d, 100)
+		e.EntryNode = "lb"
+		e.RPS = 2
+
+		snaps := e.Run(1, 100)
+		if len(snaps[0].Emitted["lb"]) != 2 {
+			t.Errorf("expected lb to emit 2 requests")
+		}
+
+		paths := []string{
+			snaps[0].Emitted["lb"][0].Path[1],
+			snaps[0].Emitted["lb"][1].Path[1],
+		}
+
+		if paths[0] == paths[1] {
+			t.Errorf("expected requests to be balanced, go %v", paths)
+		}
+	})
+}

internal/simulation/messagequeuenode.go

@@ -1,101 +0,0 @@
-package simulation
-
-type MessageQueueNode struct {
-	ID               string
-	Label            string
-	QueueSize        int
-	MessageTTL       int // TTL in milliseconds
-	DeadLetter       bool
-	CurrentLoad      int
-	Queue            []*Request
-	Alive            bool
-	Targets          []string
-	output           []*Request
-	deadLetterOutput []*Request
-}
-
-func (n *MessageQueueNode) GetID() string { return n.ID }
-func (n *MessageQueueNode) Type() string  { return "messagequeue" }
-func (n *MessageQueueNode) IsAlive() bool { return n.Alive }
-
-func (n *MessageQueueNode) Tick(tick int, currentTimeMs int) {
-	if len(n.Queue) == 0 {
-		return
-	}
-
-	// Message queues have very high throughput
-	maxProcessPerTick := 20 // Higher than database (3) or CDN (10)
-	processCount := min(len(n.Queue), maxProcessPerTick)
-
-	// Check for queue overflow (simulate back pressure)
-	if len(n.Queue) > n.QueueSize {
-		// Move oldest messages to dead letter queue if enabled
-		if n.DeadLetter {
-			overflow := len(n.Queue) - n.QueueSize
-			for i := 0; i < overflow; i++ {
-				deadReq := n.Queue[i]
-				deadReq.Type = "DEAD_LETTER"
-				deadReq.Path = append(deadReq.Path, n.ID+"(dead)")
-				n.deadLetterOutput = append(n.deadLetterOutput, deadReq)
-			}
-			n.Queue = n.Queue[overflow:]
-		} else {
-			// Drop messages if no dead letter queue
-			n.Queue = n.Queue[:n.QueueSize]
-		}
-	}
-
-	// Process messages with TTL check
-	for i := 0; i < processCount; i++ {
-		req := n.Queue[0]
-		n.Queue = n.Queue[1:]
-
-		// Check TTL (time to live) - use current time in milliseconds
-		messageAgeMs := currentTimeMs - req.Timestamp
-		if messageAgeMs > n.MessageTTL {
-			// Message expired
-			if n.DeadLetter {
-				req.Type = "EXPIRED"
-				req.Path = append(req.Path, n.ID+"(expired)")
-				n.deadLetterOutput = append(n.deadLetterOutput, req)
-			}
-			// Otherwise drop expired message
-			continue
-		}
-
-		// Message queue adds minimal latency (very fast)
-		req.LatencyMS += 2
-		req.Path = append(req.Path, n.ID)
-		n.output = append(n.output, req)
-	}
-}
-
-func (n *MessageQueueNode) Receive(req *Request) {
-	if req == nil {
-		return
-	}
-
-	// Message queues have very low receive overhead
-	req.LatencyMS += 1
-
-	n.Queue = append(n.Queue, req)
-}
-
-func (n *MessageQueueNode) Emit() []*Request {
-	// Return both normal messages and dead letter messages
-	allRequests := append(n.output, n.deadLetterOutput...)
-
-	// Clear queues
-	n.output = n.output[:0]
-	n.deadLetterOutput = n.deadLetterOutput[:0]
-
-	return allRequests
-}
-
-func (n *MessageQueueNode) GetTargets() []string {
-	return n.Targets
-}
-
-func (n *MessageQueueNode) GetQueue() []*Request {
-	return n.Queue
-}

internal/simulation/microservicenode.go

@@ -1,75 +0,0 @@
-package simulation
-
-import "math/rand"
-
-type MicroserviceNode struct {
-	ID             string
-	Label          string
-	APIEndpoint    string
-	RateLimit      int // max requests per tick
-	CircuitBreaker bool
-	CircuitState   string // "closed", "open", "half-open"
-	ErrorCount     int
-	CurrentLoad    int
-	Queue          []*Request
-	Output         []*Request
-	Alive          bool
-	Targets        []string
-}
-
-func (n *MicroserviceNode) GetID() string { return n.ID }
-
-func (n *MicroserviceNode) Type() string { return "microservice" }
-
-func (n *MicroserviceNode) IsAlive() bool { return n.Alive }
-
-func (n *MicroserviceNode) Receive(req *Request) {
-	n.Queue = append(n.Queue, req)
-}
-
-func (n *MicroserviceNode) Emit() []*Request {
-	out := append([]*Request(nil), n.Output...)
-	n.Output = n.Output[:0]
-	return out
-}
-
-func (n *MicroserviceNode) GetTargets() []string {
-	return n.Targets
-}
-
-func (n *MicroserviceNode) Tick(tick int, currentTimeMs int) {
-	n.CurrentLoad = 0
-	n.ErrorCount = 0
-	n.Output = nil
-
-	toProcess := min(len(n.Queue), n.RateLimit)
-	for i := 0; i < toProcess; i++ {
-		req := n.Queue[i]
-
-		if rand.Float64() < 0.02 {
-			n.ErrorCount++
-			if n.CircuitBreaker {
-				n.CircuitState = "open"
-				n.Alive = false
-			}
-
-			continue
-		}
-
-		req.LatencyMS += 10
-		req.Path = append(req.Path, n.ID)
-		n.Output = append(n.Output, req)
-		n.CurrentLoad++
-	}
-
-	if n.CircuitState == "open" && tick%10 == 0 {
-		n.CircuitState = "closed"
-		n.Alive = true
-	}
-
-	n.Queue = n.Queue[toProcess:]
-}
-
-func (n *MicroserviceNode) GetQueue() []*Request {
-	return n.Queue
-}

internal/simulation/monitoringnode.go

@@ -1,73 +0,0 @@
-package simulation
-
-type MonitoringNode struct {
-	ID             string
-	Label          string
-	Tool           string
-	AlertMetric    string
-	ThresholdValue int
-	ThresholdUnit  string
-	Queue          []*Request
-	Alive          bool
-	Targets        []string
-	Metrics        map[string]int
-	Alerts         []*Request
-}
-
-func (n *MonitoringNode) GetID() string { return n.ID }
-
-func (n *MonitoringNode) Type() string { return "monitoring" }
-
-func (n *MonitoringNode) IsAlive() bool { return n.Alive }
-
-func (n *MonitoringNode) Receive(req *Request) {
-	n.Queue = append(n.Queue, req)
-}
-
-func (n *MonitoringNode) Emit() []*Request {
-	out := append([]*Request(nil), n.Alerts...)
-	n.Alerts = n.Alerts[:0]
-	return out
-}
-
-func (n *MonitoringNode) Tick(tick int, currentTimeMs int) {
-	if !n.Alive {
-		return
-	}
-
-	if n.Metrics == nil {
-		n.Metrics = make(map[string]int)
-	}
-
-	// Simulate processing requests as metrics
-	for _, req := range n.Queue {
-		// For now, pretend all requests are relevant to the AlertMetric
-		n.Metrics[n.AlertMetric] += 1
-		req.LatencyMS += 1
-		req.Path = append(req.Path, n.ID)
-
-		if n.Metrics[n.AlertMetric] > n.ThresholdValue {
-			alert := &Request{
-				ID:        "alert-" + req.ID,
-				Timestamp: currentTimeMs,
-				Origin:    n.ID,
-				Type:      "alert",
-				LatencyMS: 0,
-				Path:      []string{n.ID, "alert"},
-			}
-			n.Alerts = append(n.Alerts, alert)
-
-			// Reset after alert (or you could continue accumulating)
-			n.Metrics[n.AlertMetric] = 0
-		}
-	}
-
-	n.Queue = nil
-}
-func (n *MonitoringNode) GetTargets() []string {
-	return n.Targets
-}
-
-func (n *MonitoringNode) GetQueue() []*Request {
-	return n.Queue
-}

internal/simulation/testdata/simple_design.json

@@ -1,7 +1,7 @@
 {
   "nodes": [
     {
-      "id": "node-1",
+      "id": "loadbalancer",
       "type": "loadBalancer",
       "position": { "x": 0, "y": 0 },
       "props": {
@@ -10,19 +10,22 @@
       }
     },
     {
-      "id": "node-2",
+      "id": "webserver",
       "type": "webserver",
       "position": { "x": 100, "y": 0 },
       "props": {
         "label": "Web Server",
-        "instanceSize": "medium"
+        "instanceSize": "medium",
+        "capacityRPS": 5,
+        "baseLatencyMs": 50,
+        "penaltyPerRPS": 10
       }
     }
   ],
   "connections": [
     {
-      "source": "node-1",
+      "source": "loadbalancer",
-      "target": "node-2",
+      "target": "webserver",
       "label": "Traffic",
       "direction": "forward",
       "protocol": "HTTP",

internal/simulation/thirdpartyservicenode.go

@@ -1,87 +0,0 @@
-package simulation
-
-import "math/rand"
-
-type ThirdPartyServiceNode struct {
-	ID          string
-	Label       string
-	APIEndpoint string
-	RateLimit   int    // Max requests per tick
-	RetryPolicy string // "exponential", "fixed", etc.
-	CurrentLoad int
-	Queue       []*Request
-	ErrorCount  int
-	RetryCount  int
-	Alive       bool
-	Targets     []string
-	Output      []*Request
-}
-
-// --- Interface Methods ---
-
-func (n *ThirdPartyServiceNode) GetID() string { return n.ID }
-func (n *ThirdPartyServiceNode) Type() string  { return "thirdpartyservice" }
-func (n *ThirdPartyServiceNode) IsAlive() bool { return n.Alive }
-func (n *ThirdPartyServiceNode) Receive(req *Request) {
-	n.Queue = append(n.Queue, req)
-}
-func (n *ThirdPartyServiceNode) Emit() []*Request {
-	out := append([]*Request(nil), n.Output...)
-	n.Output = n.Output[:0]
-	return out
-}
-
-// Add missing Queue method for interface compliance
-func (n *ThirdPartyServiceNode) GetQueue() []*Request {
-	return n.Queue
-}
-
-// --- Simulation Logic ---
-
-func (n *ThirdPartyServiceNode) Tick(tick int, currentTimeMs int) {
-	if !n.Alive {
-		return
-	}
-
-	// Simulate third-party call behavior with success/failure
-	maxProcess := min(n.RateLimit, len(n.Queue))
-	newQueue := n.Queue[maxProcess:]
-	n.Queue = nil
-
-	for i := 0; i < maxProcess; i++ {
-		req := newQueue[i]
-		success := simulateThirdPartySuccess(req)
-
-		if success {
-			req.LatencyMS += 100 + randInt(0, 50) // simulate response time
-			req.Path = append(req.Path, n.ID)
-			n.Output = append(n.Output, req)
-		} else {
-			n.ErrorCount++
-			n.RetryCount++
-			if n.RetryPolicy == "exponential" && n.RetryCount < 3 {
-				n.Queue = append(n.Queue, req) // retry again next tick
-			}
-		}
-	}
-
-	// Simulate degradation if too many errors
-	if n.ErrorCount > 10 {
-		n.Alive = false
-	}
-}
-
-func (n *ThirdPartyServiceNode) GetTargets() []string {
-	return n.Targets
-}
-
-// --- Helpers ---
-
-func simulateThirdPartySuccess(req *Request) bool {
-	// 90% success rate
-	return randInt(0, 100) < 90
-}
-
-func randInt(min, max int) int {
-	return min + int(rand.Float64()*float64(max-min))
-}

internal/simulation/util.go

@@ -0,0 +1,24 @@
+package simulation
+
+func AsFloat64(v interface{}) float64 {
+	switch val := v.(type) {
+	case float64:
+		return val
+	case int:
+		return float64(val)
+	case int64:
+		return float64(val)
+	case float32:
+		return float64(val)
+	default:
+		return 0
+	}
+}
+
+func AsString(v interface{}) string {
+	s, ok := v.(string)
+	if !ok {
+		return ""
+	}
+	return s
+}

internal/simulation/webserver.go

@@ -0,0 +1,27 @@
+package simulation
+
+// keep this stateless. Trying to store state here is a big mistake.
+// This is meant to be a pure logic handler.
+type WebServerLogic struct {
+}
+
+func (l WebServerLogic) Tick(props map[string]any, queue []*Request, tick int) ([]*Request, bool) {
+	maxRPS := int(AsFloat64(props["capacityRPS"]))
+
+	toProcess := queue
+	if len(queue) > maxRPS {
+		toProcess = queue[:maxRPS]
+	}
+
+	var output []*Request
+	for _, req := range toProcess {
+		output = append(output, &Request{
+			ID:        req.ID,
+			Timestamp: req.Timestamp,
+			Origin:    req.Origin,
+			Type:      req.Type,
+		})
+	}
+
+	return output, true
+}

internal/simulation/webservernode.go

@@ -1,73 +0,0 @@
-package simulation
-
-type WebServerNode struct {
-	ID            string
-	Queue         []*Request
-	CapacityRPS   int
-	BaseLatencyMs int
-	PenaltyPerRPS float64
-	Processed     []*Request
-	Alive         bool
-	Targets       []string
-}
-
-func (ws *WebServerNode) GetID() string {
-	return ws.ID
-}
-
-func (ws *WebServerNode) Type() string {
-	return "webserver"
-}
-
-func (ws *WebServerNode) IsAlive() bool {
-	return ws.Alive
-}
-
-func (ws *WebServerNode) Tick(tick int, currentTimeMs int) {
-	toProcess := min(ws.CapacityRPS, len(ws.Queue))
-	for i := 0; i < toProcess; i++ {
-		req := ws.Queue[i]
-		req.LatencyMS += ws.BaseLatencyMs
-		ws.Processed = append(ws.Processed, req)
-	}
-
-	// Remove processed requests from the queue
-	ws.Queue = ws.Queue[toProcess:]
-
-	// Apply penalty for overload
-	if len(ws.Queue) > 0 {
-		overload := len(ws.Queue)
-		for _, req := range ws.Queue {
-			req.LatencyMS += int(ws.PenaltyPerRPS * float64(overload))
-		}
-	}
-}
-
-func (ws *WebServerNode) Receive(req *Request) {
-	ws.Queue = append(ws.Queue, req)
-}
-
-func (ws *WebServerNode) Emit() []*Request {
-	out := ws.Processed
-	ws.Processed = nil
-	return out
-}
-
-func (ws *WebServerNode) AddTarget(targetID string) {
-	ws.Targets = append(ws.Targets, targetID)
-}
-
-func min(a, b int) int {
-	if a < b {
-		return a
-	}
-	return b
-}
-
-func (ws *WebServerNode) GetQueue() []*Request {
-	return ws.Queue
-}
-
-func (ws *WebServerNode) GetTargets() []string {
-	return ws.Targets
-}

router/handlers/simulation.go

@@ -4,7 +4,6 @@ import (
 	"encoding/json"
 	"net/http"
 	"systemdesigngame/internal/design"
-	"systemdesigngame/internal/simulation"
 )
 
 type SimulationHandler struct{}
@@ -28,9 +27,6 @@ func (h *SimulationHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	engine := simulation.NewEngineFromDesign(design, 10000, 100)
-	engine.Run()
-
 	// For now, return a mock successful response but eventually, we want to go to the results page(s)
 	response := SimulationResponse{
 		Success: true,

static/game-mode.css

@@ -0,0 +1,440 @@
+/* Reusing the existing CSS from the game */
+@import url('https://fonts.googleapis.com/css2?family=JetBrains+Mono:wght@300;400;500;700&display=swap');
+
+/* === CSS VARIABLES === */
+:root {
+    /* Colors */
+    --color-bg-body: #161b22;
+    --color-bg-dark: #121212;
+    --color-bg-sidebar: #111;
+    --color-bg-component: #1e1e1e;
+    --color-bg-hover: #2a2a2a;
+    --color-bg-accent: #005f87;
+    --color-bg-tab-active: #1a3d2a;
+    --color-border: #444;
+    --color-border-accent: #00ff88;
+    --color-border-panel: #30363d;
+    --color-text-primary: #ccc;
+    --color-text-muted: #888;
+    --color-text-accent: #00ff88;
+    --color-text-white: #fff;
+    --color-text-dark: #333;
+    --color-button: #238636;
+    --color-button-disabled: #555;
+    --color-connection: #333;
+    --color-connection-selected: #007bff;
+    --color-tooltip-bg: #333;
+    --color-tooltip-text: #fff;
+
+    /* Sizes */
+    --radius-small: 4px;
+    --radius-medium: 6px;
+    --radius-large: 8px;
+    --font-family-mono: 'JetBrains Mono', monospace;
+    --font-family-code: 'Fira Code', monospace;
+    --component-padding: 8px;
+    --component-gap: 12px;
+}
+
+/* === RESET & BASE STYLES === */
+* {
+    box-sizing: border-box;
+}
+
+body {
+    margin: 0;
+    font-family: var(--font-family-mono);
+    background-color: var(--color-bg-body);
+    color: var(--color-text-primary);
+    min-height: 100vh;
+}
+
+/* === LAYOUT === */
+#page-container {
+    display: flex;
+    flex-direction: column;
+    width: 100%;
+    min-height: 100vh;
+}
+
+#sd-header {
+    width: 100%;
+    background: none;
+    padding: 12px 24px;
+    font-weight: bold;
+    color: var(--color-text-accent);
+    border-bottom: 1px solid var(--color-text-dark);
+    display: flex;
+    align-items: center;
+    justify-content: space-between;
+}
+
+.header-text {
+    font-size: 24px;
+    margin: 0;
+}
+
+#main-content {
+    display: flex;
+    flex-direction: column;
+    flex: 1;
+    padding: 60px 24px;
+    align-items: center;
+    background: radial-gradient(circle at 30% 50%, rgba(0, 255, 136, 0.1), transparent 50%),
+        radial-gradient(circle at 70% 80%, rgba(255, 107, 53, 0.1), transparent 50%);
+}
+
+/* === REUSED EXISTING CLASSES === */
+.requirements-section {
+    background: #161b22;
+    border: 1px solid #30363d;
+    border-radius: 8px;
+    padding: 20px;
+    margin-bottom: 20px;
+}
+
+.requirements-list {
+    margin: 0;
+    padding: 0;
+    list-style: none;
+}
+
+.requirement-item {
+    position: relative;
+    padding: 8px 0 8px 25px;
+    margin: 0;
+    border-bottom: 1px solid #30363d;
+}
+
+.requirement-item:before {
+    content: "✓";
+    color: #00ff88;
+    position: absolute;
+    left: 0;
+}
+
+.panel-title {
+    font-weight: bold;
+    color: var(--color-text-white);
+    font-size: 15px;
+    margin-bottom: 0.5rem;
+}
+
+.panel-metric {
+    margin-bottom: 0.4rem;
+}
+
+.panel-metric .label {
+    display: inline-block;
+    width: 140px;
+    color: var(--color-text-muted);
+}
+
+#github-login-btn {
+    display: inline-flex;
+    align-items: center;
+    gap: 8px;
+    padding: 8px 14px;
+    background-color: #fff;
+    color: #000000;
+    text-decoration: none;
+    border-radius: var(--radius-medium);
+    font-weight: 500;
+    font-family: var(--font-family-mono);
+    font-size: 12px;
+    border: 1px solid #2ea043;
+    transition: background-color 0.2s ease;
+}
+
+#github-login-btn:hover {
+    background-color: #ccc;
+}
+
+/* === CUSTOM STYLES FOR GAME MODE SELECTION === */
+.hero-section {
+    text-align: center;
+    margin-bottom: 60px;
+}
+
+.hero-section h1 {
+    font-size: 48px;
+    font-weight: 700;
+    margin: 0 0 20px 0;
+    background: linear-gradient(135deg, var(--color-text-accent), var(--color-connection-selected));
+    -webkit-background-clip: text;
+    -webkit-text-fill-color: transparent;
+    background-clip: text;
+}
+
+.hero-section p {
+    font-size: 18px;
+    color: var(--color-text-muted);
+    max-width: 600px;
+    margin: 0 auto;
+    line-height: 1.6;
+}
+
+.game-mode-grid {
+    display: grid;
+    grid-template-columns: repeat(2, 1fr);
+    gap: 30px;
+    max-width: 1000px;
+    width: 100%;
+    margin-bottom: 60px;
+}
+
+.game-mode-card {
+    background: var(--color-bg-component);
+    border: 2px solid var(--color-border);
+    border-radius: var(--radius-large);
+    padding: 30px;
+    transition: all 0.3s ease;
+    position: relative;
+    overflow: hidden;
+}
+
+.game-mode-card::before {
+    content: '';
+    position: absolute;
+    top: 0;
+    left: 0;
+    right: 0;
+    bottom: 0;
+    background: linear-gradient(135deg, transparent, rgba(255, 255, 255, 0.05));
+    opacity: 0;
+    transition: opacity 0.3s ease;
+}
+
+.game-mode-card:hover {
+    transform: translateY(-5px);
+    box-shadow: 0 10px 30px rgba(0, 0, 0, 0.3);
+    background-color: var(--color-bg-hover);
+}
+
+.game-mode-card:hover::before {
+    opacity: 1;
+}
+
+.game-mode-card.campaign {
+    border-color: var(--color-border-accent);
+}
+
+.game-mode-card.campaign:hover {
+    border-color: var(--color-border-accent);
+    box-shadow: 0 10px 30px rgba(0, 255, 136, 0.3);
+}
+
+.game-mode-card.practice {
+    border-color: #ff6b35;
+}
+
+.game-mode-card.practice:hover {
+    border-color: #ff6b35;
+    box-shadow: 0 10px 30px rgba(255, 107, 53, 0.3);
+}
+
+.mode-header {
+    display: flex;
+    align-items: center;
+    gap: 15px;
+    margin-bottom: 25px;
+}
+
+.mode-icon {
+    width: 60px;
+    height: 60px;
+    border-radius: var(--radius-medium);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    font-size: 28px;
+    font-weight: bold;
+    background: var(--color-bg-dark);
+    border: 1px solid var(--color-border-panel);
+}
+
+.game-mode-card.campaign .mode-icon {
+    color: var(--color-text-accent);
+    border-color: var(--color-border-accent);
+}
+
+.game-mode-card.practice .mode-icon {
+    color: #ff6b35;
+    border-color: #ff6b35;
+}
+
+.mode-title {
+    font-size: 28px;
+    font-weight: 700;
+    margin: 0 0 5px 0;
+    color: var(--color-text-white);
+}
+
+.mode-subtitle {
+    font-size: 16px;
+    color: var(--color-text-muted);
+    margin: 0;
+}
+
+.mode-features {
+    margin-bottom: 25px;
+}
+
+.mode-features .requirements-list .requirement-item {
+    border-bottom: 1px solid var(--color-border-panel);
+}
+
+.game-mode-card.campaign .mode-features .requirement-item:before {
+    color: var(--color-text-accent);
+}
+
+.game-mode-card.practice .mode-features .requirement-item:before {
+    color: #ff6b35;
+}
+
+.mode-progress {
+    background: var(--color-bg-dark);
+    border: 1px solid var(--color-border-panel);
+    border-radius: var(--radius-medium);
+    padding: 16px;
+    margin-bottom: 25px;
+}
+
+.progress-bar {
+    background: var(--color-border);
+    height: 8px;
+    border-radius: var(--radius-small);
+    overflow: hidden;
+    margin-top: 8px;
+}
+
+.progress-fill {
+    height: 100%;
+    background: linear-gradient(90deg, var(--color-text-accent), var(--color-connection-selected));
+    width: 33%;
+    border-radius: var(--radius-small);
+}
+
+.recent-activity {
+    font-size: 14px;
+}
+
+.recent-activity .activity-item {
+    margin-bottom: 4px;
+}
+
+.recent-activity .activity-item.active {
+    color: #ff6b35;
+}
+
+.recent-activity .activity-item.inactive {
+    color: var(--color-text-muted);
+}
+
+.mode-button {
+    width: 100%;
+    padding: 15px;
+    border: none;
+    border-radius: var(--radius-medium);
+    font-size: 16px;
+    font-weight: 600;
+    font-family: var(--font-family-mono);
+    cursor: pointer;
+    transition: all 0.3s ease;
+    text-transform: uppercase;
+    letter-spacing: 0.5px;
+    text-decoration: none;
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    gap: 8px;
+}
+
+.game-mode-card.campaign .mode-button {
+    background: linear-gradient(90deg, var(--color-text-accent), var(--color-connection-selected));
+    color: var(--color-bg-body);
+}
+
+.game-mode-card.practice .mode-button {
+    background: linear-gradient(90deg, #ff6b35, #f7931e);
+    color: var(--color-text-white);
+}
+
+.mode-button:hover {
+    opacity: 0.9;
+    transform: translateY(-2px);
+}
+
+/* === STATS GRID === */
+.stats-grid {
+    display: grid;
+    grid-template-columns: repeat(4, 1fr);
+    gap: 20px;
+    max-width: 800px;
+    width: 100%;
+}
+
+.stat-card {
+    background: var(--color-bg-component);
+    border: 1px solid var(--color-border);
+    border-radius: var(--radius-large);
+    padding: 20px;
+    text-align: center;
+}
+
+.stat-value {
+    font-size: 28px;
+    font-weight: 700;
+    margin-bottom: 8px;
+}
+
+.stat-card:nth-child(1) .stat-value {
+    color: var(--color-text-accent);
+}
+
+.stat-card:nth-child(2) .stat-value {
+    color: var(--color-connection-selected);
+}
+
+.stat-card:nth-child(3) .stat-value {
+    color: #ff6b35;
+}
+
+.stat-card:nth-child(4) .stat-value {
+    color: #f7931e;
+}
+
+.stat-label {
+    font-size: 14px;
+    color: var(--color-text-muted);
+}
+
+/* === RESPONSIVE === */
+@media (max-width: 1024px) {
+    .game-mode-grid {
+        grid-template-columns: 1fr;
+        gap: 20px;
+    }
+
+    .stats-grid {
+        grid-template-columns: repeat(2, 1fr);
+    }
+}
+
+@media (max-width: 768px) {
+    .hero-section h1 {
+        font-size: 36px;
+    }
+
+    .game-mode-card {
+        padding: 20px;
+    }
+
+    #main-content {
+        padding: 40px 16px;
+    }
+
+    .stats-grid {
+        grid-template-columns: 1fr;
+    }
+}