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Do.Once docker client initialization #556

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Dec 11, 2015
Merged

Do.Once docker client initialization #556

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54 changes: 34 additions & 20 deletions client/driver/docker.go
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@ import (
"path/filepath"
"strconv"
"strings"
"sync"

docker "github.com/fsouza/go-dockerclient"

Expand All @@ -20,6 +21,10 @@ import (
"github.com/mitchellh/mapstructure"
)

// We store the client globally to cache the connection to the docker daemon.
var createClient sync.Once
var client *docker.Client

type DockerDriver struct {
DriverContext
fingerprint.StaticFingerprinter
Expand Down Expand Up @@ -83,28 +88,37 @@ func NewDockerDriver(ctx *DriverContext) Driver {
// to connect to the docker daemon. In production mode we will read
// docker.endpoint from the config file.
func (d *DockerDriver) dockerClient() (*docker.Client, error) {
// Default to using whatever is configured in docker.endpoint. If this is
// not specified we'll fall back on NewClientFromEnv which reads config from
// the DOCKER_* environment variables DOCKER_HOST, DOCKER_TLS_VERIFY, and
// DOCKER_CERT_PATH. This allows us to lock down the config in production
// but also accept the standard ENV configs for dev and test.
dockerEndpoint := d.config.Read("docker.endpoint")
if dockerEndpoint != "" {
cert := d.config.Read("docker.tls.cert")
key := d.config.Read("docker.tls.key")
ca := d.config.Read("docker.tls.ca")

if cert+key+ca != "" {
d.logger.Printf("[DEBUG] driver.docker: using TLS client connection to %s", dockerEndpoint)
return docker.NewTLSClient(dockerEndpoint, cert, key, ca)
} else {
d.logger.Printf("[DEBUG] driver.docker: using standard client connection to %s", dockerEndpoint)
return docker.NewClient(dockerEndpoint)
if client != nil {
return client, nil
}

var err error
createClient.Do(func() {
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I'm curious if Once.Do has implications with periodic fingerprinting. For example if we run the fingerprinter once and the client fails because docker is not running yet, does that mean we can't connect later?

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Well if it failed, then we would return false on the fingerprinter anyways so the node wouldn't say it had docker capabilities.

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I thought we ran fingerprinters periodically, though. If so it could fail if nomad starts before Docker, and would then succeed on a subsequent fingerprint pass.

So on the second call I think client would still be nil but the Once.Do would prevent the initialization from firing again.

We would fall through and return nil, nil.

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I thought we ran fingerprinters periodically, though. If so it could fail if nomad starts before Docker, and would then succeed on a subsequent fingerprint pass.

I'm currently using Systemd to make sure Nomad is started after Docker. The same could be done in Upstart. I think that's a fair assumption.

// Default to using whatever is configured in docker.endpoint. If this is
// not specified we'll fall back on NewClientFromEnv which reads config from
// the DOCKER_* environment variables DOCKER_HOST, DOCKER_TLS_VERIFY, and
// DOCKER_CERT_PATH. This allows us to lock down the config in production
// but also accept the standard ENV configs for dev and test.
dockerEndpoint := d.config.Read("docker.endpoint")
if dockerEndpoint != "" {
cert := d.config.Read("docker.tls.cert")
key := d.config.Read("docker.tls.key")
ca := d.config.Read("docker.tls.ca")

if cert+key+ca != "" {
d.logger.Printf("[DEBUG] driver.docker: using TLS client connection to %s", dockerEndpoint)
client, err = docker.NewTLSClient(dockerEndpoint, cert, key, ca)
} else {
d.logger.Printf("[DEBUG] driver.docker: using standard client connection to %s", dockerEndpoint)
client, err = docker.NewClient(dockerEndpoint)
}
return
}
}

d.logger.Println("[DEBUG] driver.docker: using client connection initialized from environment")
return docker.NewClientFromEnv()
d.logger.Println("[DEBUG] driver.docker: using client connection initialized from environment")
client, err = docker.NewClientFromEnv()
})
return client, err
}

func (d *DockerDriver) Fingerprint(cfg *config.Config, node *structs.Node) (bool, error) {
Expand Down