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DockerはどうやってコンテナのIDを決定しているか

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Dockerがコンテナを作成したときのIDをどうやって決定しているかを説明します。

結論から言うと、ランダムなIDを生成しています。

newContainer関数の中でgenerateIDAndNameが呼ばれ、同ファイルのgenerateID関数が呼ばれてここでランダムなIDが生成されています。


newContainer

daemon/container.go#L128

func (daemon *Daemon) newContainer(name string, operatingSystem string, config *containertypes.Config, hostConfig *containertypes.HostConfig, imgID image.ID, managed bool) (*container.Container, error) {

var (
id string
err error
noExplicitName = name == ""
)
id, name, err = daemon.generateIDAndName(name)
if err != nil {
return nil, err
}

if hostConfig.NetworkMode.IsHost() {
if config.Hostname == "" {
config.Hostname, err = os.Hostname()
if err != nil {
return nil, errdefs.System(err)
}
}
} else {
daemon.generateHostname(id, config)
}
entrypoint, args := daemon.getEntrypointAndArgs(config.Entrypoint, config.Cmd)

base := daemon.newBaseContainer(id)
base.Created = time.Now().UTC()
base.Managed = managed
base.Path = entrypoint
base.Args = args //FIXME: de-duplicate from config
base.Config = config
base.HostConfig = &containertypes.HostConfig{}
base.ImageID = imgID
base.NetworkSettings = &network.Settings{IsAnonymousEndpoint: noExplicitName}
base.Name = name
base.Driver = daemon.imageService.GraphDriverForOS(operatingSystem)
base.OS = operatingSystem
return base, err
}


GenerateNonCryptoID

github.com/docker/docker/pkg/stringid/stringid.go#L69

// GenerateNonCryptoID generates unique id without using cryptographically

// secure sources of random.
// It helps you to save entropy.
func GenerateNonCryptoID() string {
return generateID(readerFunc(rand.Read))
}


generateID

github.com/docker/docker/pkg/stringid/stringid.go#L44

func generateID(r io.Reader) string {

b := make([]byte, 32)
for {
if _, err := io.ReadFull(r, b); err != nil {
panic(err) // This shouldn't happen
}
id := hex.EncodeToString(b)
// if we try to parse the truncated for as an int and we don't have
// an error then the value is all numeric and causes issues when
// used as a hostname. ref #3869
if _, err := strconv.ParseInt(TruncateID(id), 10, 64); err == nil {
continue
}
return id
}
}