Option to retain versions

Author: mcjfunk

README.md

@@ -44,7 +44,7 @@ alias, this option should not be used.)
 From now on you can reference the aliased versions on Lambda invokes with the
 stage qualifier. The aliased version is read only in the AWS console, so it is
 guaranteed that the environment and function parameters (memory, etc.) cannot
-be changed for a deployed version by accident, as it can be done with the 
+be changed for a deployed version by accident, as it can be done with the
 `$LATEST` qualifier.This adds an additional level of stability to your deployment
 process.
 
@@ -68,6 +68,12 @@ Example:
 
 See the `alias remove` command below.
 
+## Maintain versions
+
+By default, when you deploy, the version of the function gets assigned the retention policy of 'Delete'. This means any subsequent deploys will delete any version without an alias. This was done because you can no longer tell which alias it came from.
+
+There are usecases where retaining versions is less risky and as such, you can opt into retaining these versions by deploying with the `--retain` flag.
+
 ## Remove a service
 
 To remove a complete service, all deployed user aliases have to be removed first,

lib/stackops/functions.js

@@ -110,8 +110,9 @@ module.exports = function(currentTemplate, aliasStackTemplates, currentAliasStac
 
 			// Reference correct function name in version
 			version.Properties.FunctionName = { 'Fn::ImportValue': `${stackName}-${functionName}-LambdaFunctionArn` };
-			// With alias support we do not want to retain the versions
-			version.DeletionPolicy = 'Delete';
+
+			// With alias support we do not want to retain the versions unless explicitly asked to do so
+			version.DeletionPolicy = this._retain ? 'Retain' : 'Delete';
 
 			// Add alias to alias stack. Reference the version export in the stage stack
 			// to prevent version deletion.

lib/validate.js

@@ -20,6 +20,7 @@ module.exports = {
 		this._masterAlias = this._options.masterAlias || this._stage;
 		this._alias = this._options.alias || this._masterAlias;
 		this._stackName = this._provider.naming.getStackName();
+		this._retain = this._options.retain || false;
 
 		// Make alias available as ${self:provider.alias}
 		this._serverless.service.provider.alias = this._alias;