Added functionality for blue-green releases on deploy

command/deploy.go

@@ -62,6 +62,10 @@ General Options:
     Use the taskgroup count from the Nomad jobfile instead of the count that
     is currently set in a running job.
 
+  -blue-green
+	Set the max-parallel and the canary count of each task group to the current
+	count for a blue-green type release
+
   -ignore-no-changes
     By default if no changes are detected when running a deployment Levant will
     exit with a status 1 to indicate a deployment didn't happen. This behaviour
@@ -120,6 +124,7 @@ func (c *DeployCommand) Run(args []string) int {
 	flags.BoolVar(&config.Deploy.Force, "force", false, "")
 	flags.BoolVar(&config.Deploy.ForceBatch, "force-batch", false, "")
 	flags.BoolVar(&config.Deploy.ForceCount, "force-count", false, "")
+	flags.BoolVar(&config.Deploy.BlueGreen, "blue-green", false, "")
 	flags.BoolVar(&config.Plan.IgnoreNoChanges, "ignore-no-changes", false, "")
 	flags.StringVar(&level, "log-level", "INFO", "")
 	flags.StringVar(&format, "log-format", "HUMAN", "")
@@ -179,6 +184,18 @@ func (c *DeployCommand) Run(args []string) int {
 		}
 	}
 
+	if config.Deploy.BlueGreen {
+		if err = c.checkBlueGreen(config.Template.Job); err != nil {
+			c.UI.Error(fmt.Sprintf("[ERROR] levant/command: %v", err))
+			return 1
+		}
+		for _, taskGroup := range config.Template.Job.TaskGroups {
+			blueGreenCount := taskGroup.Count
+			taskGroup.Update.MaxParallel = blueGreenCount
+			taskGroup.Update.Canary = blueGreenCount
+		}
+	}
+
 	if !config.Deploy.Force {
 		p := levant.PlanConfig{
 			Client:   config.Client,
@@ -230,3 +247,15 @@ func (c *DeployCommand) checkForceBatch(job *nomad.Job, forceBatch bool) error {
 
 	return fmt.Errorf("force-batch passed but job is not periodic")
 }
+
+func (c *DeployCommand) checkBlueGreen(job *nomad.Job) error {
+
+	if job.IsPeriodic() {
+		return fmt.Errorf("blue-green passed but job is periodic")
+	}
+	if *job.Type == "system" {
+		return fmt.Errorf(`blue-green passed but job type is "system"`)
+	}
+
+	return nil
+}

docs/commands.md

@@ -20,6 +20,8 @@ Levant supports a number of command line arguments which provide control over th
 
 * **-force-count** (bool: false) Use the taskgroup count from the Nomad job file instead of the count that is obtained from the running job count.
 
+* **-blue-green** (bool: false) Set the max-parallel and the canary count of each task group to the current count for a blue-green type release
+
 * **-ignore-no-changes** (bool: false) By default if no changes are detected when running a deployment Levant will exit with a status 1 to indicate a deployment didn't happen. This behaviour can be changed using this flag so that Levant will exit cleanly ensuring CD pipelines don't fail when no changes are detected
 
 * **-log-level** (string: "INFO") The level at which Levant will log to. Valid values are DEBUG, INFO, WARN, ERROR and FATAL.

levant/structs/config.go

@@ -39,6 +39,10 @@ type DeployConfig struct {
 	// and force the count based on the rendered job file.
 	ForceCount bool
 
+	// BlueGreen is a boolean flag that can be used to run a blue-green release,
+	// overwriting the max-parallel and the canary count variables
+	BlueGreen bool
+
 	// EnvVault is a boolean flag that can be used to enable reading the VAULT_TOKEN
 	// from the enviromment.
 	EnvVault bool