From 28d9ee7d457c938fc94f88462a61c390273b139f Mon Sep 17 00:00:00 2001
From: Theodor-Alexandru Irimia <Theodor-Alexandru.Irimia@trivago.com>
Date: Mon, 1 Jul 2019 13:08:45 +0200
Subject: [PATCH] Added functionality for blue-green releases

---
 command/deploy.go        | 29 +++++++++++++++++++++++++++++
 docs/commands.md         |  2 ++
 levant/structs/config.go |  4 ++++
 3 files changed, 35 insertions(+)

diff --git a/command/deploy.go b/command/deploy.go
index 06db75058..6f6d08926 100644
--- a/command/deploy.go
+++ b/command/deploy.go
@@ -59,6 +59,10 @@ 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
@@ -116,6 +120,7 @@ func (c *DeployCommand) Run(args []string) int {
 	flags.StringVar(&config.Client.ConsulAddr, "consul-address", "", "")
 	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", "")
@@ -175,6 +180,18 @@ 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
+		}
+	}
+
 	p := levant.PlanConfig{
 		Client:   config.Client,
 		Plan:     config.Plan,
@@ -224,3 +241,15 @@ 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
+}
diff --git a/docs/commands.md b/docs/commands.md
index 184a8d0f6..01c3e7314 100644
--- a/docs/commands.md
+++ b/docs/commands.md
@@ -18,6 +18,8 @@ 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.
diff --git a/levant/structs/config.go b/levant/structs/config.go
index 4839c25c9..36064a44b 100644
--- a/levant/structs/config.go
+++ b/levant/structs/config.go
@@ -35,6 +35,10 @@ 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
+
 	// VaultToken is a string with the vault token.
 	VaultToken string