Kubernetes で Deploymentを使うときにマニフェストを修正して kubectl applyコマンドを繰り返し実行すると，ReplicaSetのリビジョンが増えていく．サンプルとして Deploymentの imageを nginx:1.8-alpine➔ nginx:1.9-alpine➔ nginx:1.10-alpineと修正しながらマニフェストを適用していくと以下のようになる．kubectl rollout historyコマンドを使うとリビジョンも確認できる．また kubectl applyに --recordオプションを付けると操作履歴を記録できる．
# nginx:1.8-alpine $ kubectl apply -f deployment.yaml --record deployment.apps/sandbox-deployment-revision-history-limit created # nginx:1.9-alpine $ kubectl apply -f deployment.yaml --record deployment.apps/sandbox-deployment-revision-history-limit configured # nginx:1.10-alpine $ kubectl apply -f deployment.yaml --record deployment.apps/sandbox-deployment-revision-history-limit configured $ kubectl rollout history deployment sandbox-deployment-revision-history-limit deployment.apps/sandbox-deployment-revision-history-limit REVISION CHANGE-CAUSE 1 kubectl apply --filename=deployment.yaml--record=true2 kubectl apply --filename=deployment.yaml--record=true3 kubectl apply --filename=deployment.yaml--record=true $ kubectl get deployments,replicasets NAME READY UP-TO-DATE AVAILABLE AGE deployment.apps/sandbox-deployment-revision-history-limit 3/333 91s NAME DESIRED CURRENT READY AGE replicaset.apps/sandbox-deployment-revision-history-limit-769f75d55b 000 90s replicaset.apps/sandbox-deployment-revision-history-limit-7b94db8c49 000 51s replicaset.apps/sandbox-deployment-revision-history-limit-7f87f5f547 333 28s
また Deploymentは kubectl rollout undoコマンドを使うと過去のリビジョンにロールバックできる．さらに --to-revisionオプションと組み合わせれば，指定したリビジョンにロールバックすることもできる．ロールバックをしてもリビジョンは増えていく．
$ kubectl rollout undo deployment sandbox-deployment-revision-history-limit deployment.apps/sandbox-deployment-revision-history-limit rolled back $ kubectl rollout undo deployment sandbox-deployment-revision-history-limit --to-revision1 deployment.apps/sandbox-deployment-revision-history-limit rolled back $ kubectl rollout history deployment sandbox-deployment-revision-history-limit deployment.apps/sandbox-deployment-revision-history-limit REVISION CHANGE-CAUSE 3 kubectl apply --filename=deployment.yaml--record=true4 kubectl apply --filename=deployment.yaml--record=true5 kubectl apply --filename=deployment.yaml--record=true
リビジョン数のデフォルト値は 10
残る ReplicaSetのリビジョン数は Deploymentの .spec.revisionHistoryLimitフィールドで設定できる．デフォルトは 10となる．ドキュメントの表現だと「古いリビジョン (old revision)」となる．よって DESIRED = 0になって残っている ReplicaSetのことを指す．
まず「デフォルト値」を確認するために，引き続き nginx:1.11-alpineから nginx:1.18-alpineまで修正しながらマニフェストを繰り返し適用していく．すると，ReplicaSetの古いリビジョン数は「10個」になった．
# nginx:1.11-alpine $ kubectl apply -f deployment.yaml --record deployment.apps/sandbox-deployment-revision-history-limit configured # nginx:1.12-alpine $ kubectl apply -f deployment.yaml --record deployment.apps/sandbox-deployment-revision-history-limit configured # nginx:1.13-alpine $ kubectl apply -f deployment.yaml --record deployment.apps/sandbox-deployment-revision-history-limit configured # nginx:1.14-alpine $ kubectl apply -f deployment.yaml --record deployment.apps/sandbox-deployment-revision-history-limit configured # nginx:1.15-alpine $ kubectl apply -f deployment.yaml --record deployment.apps/sandbox-deployment-revision-history-limit configured # nginx:1.16-alpine $ kubectl apply -f deployment.yaml --record deployment.apps/sandbox-deployment-revision-history-limit configured # nginx:1.17-alpine $ kubectl apply -f deployment.yaml --record deployment.apps/sandbox-deployment-revision-history-limit configured # nginx:1.18-alpine $ kubectl apply -f deployment.yaml --record deployment.apps/sandbox-deployment-revision-history-limit configured $ kubectl get deployments,replicasets NAME READY UP-TO-DATE AVAILABLE AGE deployment.apps/sandbox-deployment-revision-history-limit 3/333 5m NAME DESIRED CURRENT READY AGE replicaset.apps/sandbox-deployment-revision-history-limit-54f45b4c75 000 98s replicaset.apps/sandbox-deployment-revision-history-limit-59b7cf78b9 000 2m16s replicaset.apps/sandbox-deployment-revision-history-limit-6cb4d85b97 000 72s replicaset.apps/sandbox-deployment-revision-history-limit-6d9d558bb6 000 84s replicaset.apps/sandbox-deployment-revision-history-limit-6dfdf56756 000 60s replicaset.apps/sandbox-deployment-revision-history-limit-769f75d55b 000 4m59s replicaset.apps/sandbox-deployment-revision-history-limit-7b94db8c49 000 4m20s replicaset.apps/sandbox-deployment-revision-history-limit-7dccdcc475 333 37s replicaset.apps/sandbox-deployment-revision-history-limit-7f87f5f547 000 3m57s replicaset.apps/sandbox-deployment-revision-history-limit-8565fb55f8 000 2m35s replicaset.apps/sandbox-deployment-revision-history-limit-c544754bd 000 117s
次に nginx:1.19-alpineに修正してマニフェストを適用すると，以下のように最初に作られた replicaset.apps/sandbox-deployment-revision-history-limit-769f75d55bが削除された．なお，ドキュメントには The rest will be garbage-collected in the background.と書かれているため，kubectl applyのタイミングではなく，裏側で削除されていると言える．
# nginx:1.19-alpine $ kubectl apply -f deployment.yaml --record deployment.apps/sandbox-deployment-revision-history-limit configured $ kubectl get deployments,replicasets NAME READY UP-TO-DATE AVAILABLE AGE deployment.apps/sandbox-deployment-revision-history-limit 3/333 6m NAME DESIRED CURRENT READY AGE replicaset.apps/sandbox-deployment-revision-history-limit-54f45b4c75 000 2m38s replicaset.apps/sandbox-deployment-revision-history-limit-59b7cf78b9 000 3m16s replicaset.apps/sandbox-deployment-revision-history-limit-6cb4d85b97 000 2m12s replicaset.apps/sandbox-deployment-revision-history-limit-6d9d558bb6 000 2m24s replicaset.apps/sandbox-deployment-revision-history-limit-6dfdf56756 000 2m replicaset.apps/sandbox-deployment-revision-history-limit-7b94db8c49 000 5m20s replicaset.apps/sandbox-deployment-revision-history-limit-7dccdcc475 000 97s replicaset.apps/sandbox-deployment-revision-history-limit-7f87f5f547 000 4m57s replicaset.apps/sandbox-deployment-revision-history-limit-8565fb55f8 000 3m35s replicaset.apps/sandbox-deployment-revision-history-limit-c544754bd 000 2m57s replicaset.apps/sandbox-deployment-revision-history-limit-f9f7485cd 333 25s
.spec.revisionHistoryLimitを設定する
最後に Deploymentのマニフェストに .spec.revisionHistoryLimit: 2を追加する．
apiVersion: apps/v1 kind: Deployment metadata:name: sandbox-deployment-revision-history-limit labels:app: nginx spec:replicas:3revisionHistoryLimit:2selector:matchLabels:app: nginx template:metadata:labels:app: nginx spec:containers:- name: nginx image: nginx:1.19-alpine ports:- containerPort:80
マニフェストを適用して，リビジョン数を確認すると，自動的に削除されていた．
$ kubectl apply -f deployment.yaml --record deployment.apps/sandbox-deployment-revision-history-limit configured $ kubectl get deployments,replicasets NAME READY UP-TO-DATE AVAILABLE AGE deployment.apps/sandbox-deployment-revision-history-limit 3/333 8m NAME DESIRED CURRENT READY AGE replicaset.apps/sandbox-deployment-revision-history-limit-6dfdf56756 000 4m replicaset.apps/sandbox-deployment-revision-history-limit-7dccdcc475 000 3m37s replicaset.apps/sandbox-deployment-revision-history-limit-f9f7485cd 333 2m25s
まとめ
Kubernetes で Deploymentに .spec.revisionHistoryLimitフィールドを設定するとリビジョン数を制御できる．ドキュメントにはリビジョンを多く残すと，etcdのリソースを消費したり，kubectl get replicasetsコマンドの結果が見にくくなると書いてある．最適値は状況によって異なるとは思うけど，デフォルト値 10は多すぎるようにも感じる．.spec.revisionHistoryLimitフィールドを覚えておこう！