Periodic tasks refactor (#62)

* Once every second the session run a list of periodic tasks on every
instance concurrently. We use these tasks to do things like:
- Collect mem and cpu stats
- Check if instance is part of a swarm cluster
- Broadcast information to connected clients

services/collect_stats_task.go

@@ -0,0 +1,64 @@
+package services
+
+import (
+	"encoding/json"
+	"fmt"
+	"log"
+
+	"github.com/docker/docker/api/types"
+	units "github.com/docker/go-units"
+)
+
+type collectStatsTask struct {
+	mem        float64
+	memLimit   float64
+	memPercent float64
+	v          *types.StatsJSON
+
+	cpuPercent     float64
+	previousCPU    uint64
+	previousSystem uint64
+}
+
+func (c *collectStatsTask) Run(i *Instance) {
+	reader, err := GetContainerStats(i.Name)
+	if err != nil {
+		log.Println("Error while trying to collect instance stats", err)
+		return
+	}
+	dec := json.NewDecoder(reader)
+	e := dec.Decode(&c.v)
+	if e != nil {
+		log.Println("Error while trying to collect instance stats", e)
+		return
+	}
+	// Memory
+	if c.v.MemoryStats.Limit != 0 {
+		c.memPercent = float64(c.v.MemoryStats.Usage) / float64(c.v.MemoryStats.Limit) * 100.0
+	}
+	c.mem = float64(c.v.MemoryStats.Usage)
+	c.memLimit = float64(c.v.MemoryStats.Limit)
+
+	i.Mem = fmt.Sprintf("%.2f%% (%s / %s)", c.memPercent, units.BytesSize(c.mem), units.BytesSize(c.memLimit))
+
+	// cpu
+	c.previousCPU = c.v.PreCPUStats.CPUUsage.TotalUsage
+	c.previousSystem = c.v.PreCPUStats.SystemUsage
+	c.cpuPercent = calculateCPUPercentUnix(c.previousCPU, c.previousSystem, c.v)
+	i.Cpu = fmt.Sprintf("%.2f%%", c.cpuPercent)
+}
+
+func calculateCPUPercentUnix(previousCPU, previousSystem uint64, v *types.StatsJSON) float64 {
+	var (
+		cpuPercent = 0.0
+		// calculate the change for the cpu usage of the container in between readings
+		cpuDelta = float64(v.CPUStats.CPUUsage.TotalUsage) - float64(previousCPU)
+		// calculate the change for the entire system between readings
+		systemDelta = float64(v.CPUStats.SystemUsage) - float64(previousSystem)
+	)
+
+	if systemDelta > 0.0 && cpuDelta > 0.0 {
+		cpuPercent = (cpuDelta / systemDelta) * float64(len(v.CPUStats.CPUUsage.PercpuUsage)) * 100.0
+	}
+	return cpuPercent
+}