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=true 2 kubectl apply --filename=deployment.yaml --record=true 3 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 3/3 3 3 91s NAME DESIRED CURRENT READY AGE replicaset.apps/sandbox-deployment-revision-history-limit-769f75d55b 0 0 0 90s replicaset.apps/sandbox-deployment-revision-history-limit-7b94db8c49 0 0 0 51s replicaset.apps/sandbox-deployment-revision-history-limit-7f87f5f547 3 3 3 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-revision 1 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=true 4 kubectl apply --filename=deployment.yaml --record=true 5 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 3/3 3 3 5m NAME DESIRED CURRENT READY AGE replicaset.apps/sandbox-deployment-revision-history-limit-54f45b4c75 0 0 0 98s replicaset.apps/sandbox-deployment-revision-history-limit-59b7cf78b9 0 0 0 2m16s replicaset.apps/sandbox-deployment-revision-history-limit-6cb4d85b97 0 0 0 72s replicaset.apps/sandbox-deployment-revision-history-limit-6d9d558bb6 0 0 0 84s replicaset.apps/sandbox-deployment-revision-history-limit-6dfdf56756 0 0 0 60s replicaset.apps/sandbox-deployment-revision-history-limit-769f75d55b 0 0 0 4m59s replicaset.apps/sandbox-deployment-revision-history-limit-7b94db8c49 0 0 0 4m20s replicaset.apps/sandbox-deployment-revision-history-limit-7dccdcc475 3 3 3 37s replicaset.apps/sandbox-deployment-revision-history-limit-7f87f5f547 0 0 0 3m57s replicaset.apps/sandbox-deployment-revision-history-limit-8565fb55f8 0 0 0 2m35s replicaset.apps/sandbox-deployment-revision-history-limit-c544754bd 0 0 0 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 3/3 3 3 6m NAME DESIRED CURRENT READY AGE replicaset.apps/sandbox-deployment-revision-history-limit-54f45b4c75 0 0 0 2m38s replicaset.apps/sandbox-deployment-revision-history-limit-59b7cf78b9 0 0 0 3m16s replicaset.apps/sandbox-deployment-revision-history-limit-6cb4d85b97 0 0 0 2m12s replicaset.apps/sandbox-deployment-revision-history-limit-6d9d558bb6 0 0 0 2m24s replicaset.apps/sandbox-deployment-revision-history-limit-6dfdf56756 0 0 0 2m replicaset.apps/sandbox-deployment-revision-history-limit-7b94db8c49 0 0 0 5m20s replicaset.apps/sandbox-deployment-revision-history-limit-7dccdcc475 0 0 0 97s replicaset.apps/sandbox-deployment-revision-history-limit-7f87f5f547 0 0 0 4m57s replicaset.apps/sandbox-deployment-revision-history-limit-8565fb55f8 0 0 0 3m35s replicaset.apps/sandbox-deployment-revision-history-limit-c544754bd 0 0 0 2m57s replicaset.apps/sandbox-deployment-revision-history-limit-f9f7485cd 3 3 3 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: 3 revisionHistoryLimit: 2 selector: 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 3/3 3 3 8m NAME DESIRED CURRENT READY AGE replicaset.apps/sandbox-deployment-revision-history-limit-6dfdf56756 0 0 0 4m replicaset.apps/sandbox-deployment-revision-history-limit-7dccdcc475 0 0 0 3m37s replicaset.apps/sandbox-deployment-revision-history-limit-f9f7485cd 3 3 3 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！
