add one minute chart + refactor rpc

- add one minute charts
- update disk io to use bytes
- update hub and agent connection interfaces / handlers to be more
flexible
- change agent cache to use cache time instead of session id
- refactor collection of metrics which require deltas to track
separately per cache time

agent/agent_cache.go

@@ -1,37 +1,55 @@
 package agent
 
 import (
+	"sync"
 	"time"
 
 	"github.com/henrygd/beszel/internal/entities/system"
 )
 
-// Not thread safe since we only access from gatherStats which is already locked
-type SessionCache struct {
-	data           *system.CombinedData
-	lastUpdate     time.Time
-	primarySession string
-	leaseTime      time.Duration
+type systemDataCache struct {
+	sync.RWMutex
+	cache map[uint16]*cacheNode
 }
 
-func NewSessionCache(leaseTime time.Duration) *SessionCache {
-	return &SessionCache{
-		leaseTime: leaseTime,
-		data:      &system.CombinedData{},
+type cacheNode struct {
+	data       *system.CombinedData
+	lastUpdate time.Time
+}
+
+// NewSystemDataCache creates a cache keyed by the polling interval in milliseconds.
+func NewSystemDataCache() *systemDataCache {
+	return &systemDataCache{
+		cache: make(map[uint16]*cacheNode),
 	}
 }
 
-func (c *SessionCache) Get(sessionID string) (stats *system.CombinedData, isCached bool) {
-	if sessionID != c.primarySession && time.Since(c.lastUpdate) < c.leaseTime {
-		return c.data, true
+// Get returns cached combined data when the entry is still considered fresh.
+func (c *systemDataCache) Get(cacheTimeMs uint16) (stats *system.CombinedData, isCached bool) {
+	c.RLock()
+	defer c.RUnlock()
+
+	node, ok := c.cache[cacheTimeMs]
+	if !ok {
+		return &system.CombinedData{}, false
 	}
-	return c.data, false
+	// allowedSkew := time.Second
+	// isFresh := time.Since(node.lastUpdate) < time.Duration(cacheTimeMs)*time.Millisecond-allowedSkew
+	// allow a 50% skew of the cache time
+	isFresh := time.Since(node.lastUpdate) < time.Duration(cacheTimeMs/2)*time.Millisecond
+	return node.data, isFresh
 }
 
-func (c *SessionCache) Set(sessionID string, data *system.CombinedData) {
-	if data != nil {
-		*c.data = *data
+// Set stores the latest combined data snapshot for the given interval.
+func (c *systemDataCache) Set(data *system.CombinedData, cacheTimeMs uint16) {
+	c.Lock()
+	defer c.Unlock()
+
+	node, ok := c.cache[cacheTimeMs]
+	if !ok {
+		node = &cacheNode{}
+		c.cache[cacheTimeMs] = node
 	}
-	c.primarySession = sessionID
-	c.lastUpdate = time.Now()
+	node.data = data
+	node.lastUpdate = time.Now()
 }