SwarmKit Package Reference

pkg/swarmkit/ — Executor, Controller, VMM Manager, and Task Translator.

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Overview

The pkg/swarmkit package implements the SwarmKit executor interface, transforming SwarmKit container tasks into Firecracker microVMs. It is the core integration point between SwarmKit’s orchestration and Firecracker’s VM execution.

Package Structure:

pkg/swarmkit/
├── executor.go         # Executor implementation (SwarmKit interface)
├── vmm.go              # VMM Manager (Firecracker process management)
├── translator.go       # Task → Firecracker config translation
├── interfaces.go       # Interface definitions
├── mocks.go            # Test mocks
└── configs/            # SwarmKit configuration helpers

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Executor

File: executor.go

The Executor struct implements SwarmKit’s swarmkit_exec.Executor interface, providing the bridge between SwarmKit task scheduling and Firecracker VM execution.

Type Definition

type Executor struct {
    config        *Config
    imagePrep     types.ImagePreparer
    networkMgr    types.NetworkManager
    volumeMgr     *storage.VolumeManager
    secretMgr     *storage.SecretManager
    vmmMgr        VMMManagerInterface
    controllers   map[string]*Controller
    executorMu    sync.RWMutex
    cleanupCancel context.CancelFunc
    cleanupDone   chan struct{}
    networkKeys   []*api.EncryptionKey
    cleanupMu     sync.Mutex
}

Configuration

type Config struct {
    FirecrackerPath  string   `yaml:"firecracker_path"`
    KernelPath       string   `yaml:"kernel_path"`
    RootfsDir        string   `yaml:"rootfs_dir"`
    SocketDir        string   `yaml:"socket_dir"`
    DefaultVCPUs     int      `yaml:"default_vcpus"`
    DefaultMemoryMB  int      `yaml:"default_memory_mb"`
    BridgeName       string   `yaml:"bridge_name"`
    Subnet           string   `yaml:"subnet"`
    BridgeIP         string   `yaml:"bridge_ip"`
    IPMode           string   `yaml:"ip_mode"`
    NATEnabled       bool     `yaml:"nat_enabled"`
    VXLANEnabled     bool     `yaml:"vxlan_enabled"`
    VXLANPeers       []string `yaml:"vxlan_peers"`
    Debug            bool     `yaml:"debug"`
    ReservedCPUs     int      `yaml:"reserved_cpus"`
    ReservedMemoryMB int      `yaml:"reserved_memory_mb"`
    MaxImageAgeDays  int      `yaml:"max_image_age_days"`
    StateDir         string   `yaml:"state_dir"`

    // Jailer configuration
    EnableJailer    bool   `yaml:"enable_jailer"`
    JailerPath      string `yaml:"jailer_path"`
    JailerUID       int    `yaml:"jailer_uid"`
    JailerGID       int    `yaml:"jailer_gid"`
    JailerChrootDir string `yaml:"jailer_chroot_dir"`
    ParentCgroup    string `yaml:"parent_cgroup"`
    CgroupVersion   string `yaml:"cgroup_version"`
    EnableCgroups   bool   `yaml:"enable_cgroups"`

    // Network identity
    Hostname      string `yaml:"hostname"`
    JoinAddr      string `yaml:"join_addr"`
    AdvertiseAddr string `yaml:"advertise_addr"`

    // Consul service discovery
    ConsulEnabled bool   `yaml:"consul_enabled"`
    ConsulAddress string `yaml:"consul_address"`
}

Constructor

func NewExecutor(config *Config) (*Executor, error)

Parameters:

• config — Executor configuration (required)

Defaults Applied:

Field	Default Value
FirecrackerPath	"firecracker"
KernelPath	"/usr/share/firecracker/vmlinux"
RootfsDir	"/var/lib/firecracker/rootfs"
SocketDir	"/var/run/firecracker"
DefaultVCPUs	1
DefaultMemoryMB	512
BridgeName	"swarm-br0"
Subnet	"192.168.127.0/24"
BridgeIP	"192.168.127.1/24"
IPMode	"static"

Example:

config := &swarmkit.Config{
    KernelPath:       "/usr/share/firecracker/vmlinux",
    RootfsDir:        "/var/lib/firecracker/rootfs",
    DefaultVCPUs:     2,
    DefaultMemoryMB:  1024,
    BridgeName:       "swarm-br0",
    ConsulEnabled:    true,
    ConsulAddress:    "127.0.0.1:8500",
}

exec, err := swarmkit.NewExecutor(config)
if err != nil {
    log.Fatal(err)
}

SwarmKit Interface Methods

The Executor implements these SwarmKit executor interface methods:

Configure

func (e *Executor) Configure(ctx context.Context, driver swarmkit_exec.Driver) error

Purpose: Initialize executor with SwarmKit driver for task management.

Parameters:

• ctx — Context for cancellation
• driver — SwarmKit driver for task queue operations

Side Effects:

• Creates network infrastructure (bridge, VXLAN)
• Starts Consul peer discovery watcher (if enabled)
• Initializes cleanup goroutine

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Create

func (e *Executor) Create(ctx context.Context, task *api.Task) (swarmkit_exec.Controller, error)

Purpose: Create a controller for a new task.

Parameters:

• ctx — Context for cancellation
• task — SwarmKit task specification

Returns:

• Controller — Task controller for lifecycle management
• error — Creation error (e.g., unsupported runtime)

Implementation:

func (e *Executor) Create(ctx context.Context, task *api.Task) (swarmkit_exec.Controller, error) {
    // Check task runtime type
    container := task.Spec.GetContainer()
    if container == nil {
        return nil, fmt.Errorf("unsupported runtime type")
    }

    // Create controller
    ctrl := &Controller{
        task:       task,
        executor:   e,
        vmm:        e.vmmMgr,
        preparer:   e.imagePrep,
        networkMgr: e.networkMgr,
    }

    // Store controller
    e.executorMu.Lock()
    e.controllers[task.ID] = ctrl
    e.executorMu.Unlock()

    return ctrl, nil
}

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Close

func (e *Executor) Close() error

Purpose: Shutdown executor and cleanup all resources.

Side Effects:

• Stops cleanup goroutine
• Removes all controllers
• Cleanup network infrastructure

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Controller

File: executor.go

The Controller manages the lifecycle of a single task/VM.

Type Definition

type Controller struct {
    task       *api.Task
    executor   *Executor
    vmm        VMMManagerInterface
    preparer   types.ImagePreparer
    networkMgr types.NetworkManager

    mu         sync.RWMutex
    closed     bool
}

Methods

Prepare

func (c *Controller) Prepare(ctx context.Context) error

Purpose: Prepare task for execution (image prep, network setup).

Steps:

1. Validate task runtime (must be container)
2. Prepare rootfs image via ImagePreparer.Prepare()
3. Setup TAP device and network via NetworkManager.CreateTapDevice()
4. Allocate IP address
5. Prepare volumes via VolumeManager
6. Inject secrets/configs

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Start

func (c *Controller) Start(ctx context.Context) error

Purpose: Start the Firecracker VM.

Steps:

1. Translate task to Firecracker config
2. Start Firecracker process via VMMManager.Start()
3. Configure VM (kernel, rootfs, network, machine config)
4. Send InstanceStart action
5. Wait for VM to reach running state

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Wait

func (c *Controller) Wait(ctx context.Context) error

Purpose: Wait for task completion.

Returns:

• nil — Task completed successfully
• error — Task failed or context canceled

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Stop

func (c *Controller) Stop(ctx context.Context) error

Purpose: Gracefully stop the VM.

Steps:

1. Send graceful shutdown signal
2. Wait for init grace period
3. Force kill if still running

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Remove

func (c *Controller) Remove(ctx context.Context) error

Purpose: Remove task and cleanup resources.

Cleanup Steps:

1. Stop VM (if running)
2. Remove TAP device
3. Release IP allocation
4. Cleanup jailer directory (if enabled)
5. Remove controller from executor

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Close

func (c *Controller) Close() error

Purpose: Close controller without cleanup (for failed tasks).

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VMMManager

File: vmm.go

The VMMManager manages Firecracker VM processes and API communication.

Type Definition

type VMMManager struct {
    config    *VMMConfig
    vms       map[string]*VMInstance
    mu        sync.RWMutex
    socketDir string
}

type VMInstance struct {
    ID             string
    PID            int
    Config         interface{}
    state          VMState
    CreatedAt      time.Time
    SocketPath     string
    InitSystem     string
    GracePeriodSec int
    mu             sync.RWMutex
}

VM States

type VMState string

const (
    VMStateNew      VMState = "new"
    VMStateStarting VMState = "starting"
    VMStateRunning  VMState = "running"
    VMStateStopping VMState = "stopping"
    VMStateStopped  VMState = "stopped"
    VMStateCrashed  VMState = "crashed"
)

Constructor

func NewVMMManager(config interface{}) VMMManager

Parameters:

• config — VMMConfig or any config interface

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Methods

Start

func (vm *VMMManager) Start(ctx context.Context, task *types.Task, config interface{}) error

Purpose: Start a Firecracker VM for the task.

Steps:

1. Find Firecracker binary
2. Start process with API socket
3. Wait for API server ready (10s timeout)
4. Configure VM via HTTP API
5. Send InstanceStart action
6. Track VM instance

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Stop

func (vm *VMMManager) Stop(ctx context.Context, taskID string) error

Purpose: Stop a running VM.

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Pause

func (vm *VMMManager) Pause(ctx context.Context, taskID string) error

Purpose: Pause VM (for snapshot).

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Resume

func (vm *VMMManager) Resume(ctx context.Context, taskID string) error

Purpose: Resume paused VM.

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GetInfo

func (vm *VMMManager) GetInfo(taskID string) (*VMInstance, error)

Purpose: Get VM instance info.

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List

func (vm *VMMManager) List() []string

Purpose: List all managed VM IDs.

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Cleanup

func (vm *VMMManager) Cleanup(ctx context.Context, taskID string) error

Purpose: Cleanup VM resources (socket, state).

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Firecracker API Types

type BootSource struct {
    KernelImagePath string  `json:"kernel_image_path"`
    BootArgs        string  `json:"boot_args,omitempty"`
}

type Drive struct {
    DriveID      string `json:"drive_id"`
    IsRootDevice bool   `json:"is_root_device"`
    IsReadOnly   bool   `json:"is_read_only"`
    PathOnHost   string `json:"path_on_host"`
}

type MachineConfig struct {
    VCPUs      int  `json:"vcpu_count"`
    MemSizeMib int  `json:"mem_size_mib"`
    HtEnabled  bool `json:"ht_enabled"`
}

type ActionsType struct {
    ActionType string `json:"action_type"`
}

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Translator

File: translator.go

The Translator converts SwarmKit task specifications into Firecracker VM configurations.

Type Definition

type TaskTranslator struct {
    config *Config
}

type Config struct {
    KernelPath    string
    InitrdPath    string
    DefaultVCPUs  int
    DefaultMemMB  int
    InitSystem    string
    NetworkConfig types.NetworkConfig
}

Constructor

func NewTaskTranslator(config *Config) *TaskTranslator

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Methods

Translate

func (t *TaskTranslator) Translate(task *api.Task) (*types.VMConfig, error)

Purpose: Convert SwarmKit task to Firecracker VM config.

Translation Steps:

1. Extract container spec from task
2. Determine vCPUs and memory from task resources or defaults
3. Build kernel boot args
4. Configure drives (rootfs path)
5. Setup network config
6. Apply init system settings

Example Output:

vmConfig := &types.VMConfig{
    KernelPath: "/usr/share/firecracker/vmlinux",
    BootArgs:   "console=ttyS0 reboot=k panic=1 pci=off ip=dhcp",
    RootfsPath: "/var/lib/firecracker/rootfs/nginx-alpine.ext4",
    VCPUs:      2,
    MemoryMB:   1024,
    Network: &types.NetworkConfig{
        TapDevice:  "tap-abc123",
        IPAddress:  "192.168.127.42",
        Gateway:    "192.168.127.1",
    },
    InitSystem:     "tini",
    GracePeriodSec: 10,
}

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Interfaces

File: interfaces.go

VMMManagerInterface

type VMMManagerInterface interface {
    Start(ctx context.Context, task *types.Task, config interface{}) error
    Stop(ctx context.Context, taskID string) error
    Pause(ctx context.Context, taskID string) error
    Resume(ctx context.Context, taskID string) error
    GetInfo(taskID string) (*VMInstance, error)
    List() []string
    Cleanup(ctx context.Context, taskID string) error
}

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Consul Integration

When ConsulEnabled is true, the executor:

1. Registers service in Consul catalog
2. Starts peer watcher via WatchPeers()
3. Updates VXLAN FDB on peer changes

// Consul client creation
consulClient, err := discovery.NewConsulClient(discovery.ConsulConfig{
    Address:       config.ConsulAddress,
    ServiceID:     config.Hostname,
    LocalIP:       localIP,
    LocalHostname: config.Hostname,
    VXLANPort:     4789,
})

// Register and watch
consulClient.RegisterService(vxlanID, config.BridgeIP)
networkMgr.SetNodeDiscovery(consulClient)

go consulClient.WatchPeers(ctx, func(peers []string) {
    networkMgr.UpdateVXLANPeers(peers)
})

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Error Handling

Common Errors

Error	Cause	Resolution
"config cannot be nil"	Nil config passed	Provide valid Config struct
"unsupported runtime type"	Task not container	Use container runtime tasks
"VM already exists for task"	Duplicate task ID	Wait for cleanup or force remove
"firecracker binary not found"	Firecracker not installed	Install Firecracker v1.15+
"firecracker API server not ready"	Socket timeout	Check Firecracker process

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Testing

The package includes comprehensive test coverage with mocks:

// Create mock executor for testing
mockVMM := &MockVMMManager{}
mockNetwork := &MockNetworkManager{}
mockPreparer := &MockImagePreparer{}

exec := &Executor{
    vmmMgr:      mockVMM,
    networkMgr:  mockNetwork,
    imagePrep:   mockPreparer,
    controllers: make(map[string]*Controller),
}

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Related Documentation

Topic	Document
Network internals	Network Reference
VM lifecycle	Lifecycle Reference
Image preparation	Image Reference
Architecture overview	Architecture Overview

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See Also: SwarmKit Integration Guide | SwarmKit User Guide