The REST API requests , GET , POST , PUT etc to Kubernetes API server are request , responses and simple to understand , such as kubectl create <something>. I wonder how the API server serves the pod logs when I do kubectl logs -f <pod-name> ( and similar operations like kubectl attach <pod> ), Is it just an http response to GET in a loop?
My advice is to always check what kubectl does under the cover, and for that use -v=9 with your command. It will provide you with full request and responses that are going between the client and the server.
If you extract any Kubernetes object using kubectl on the highest debugging level -v 9 with a streaming option -f, as for example kubectl logs -f <pod-name> -v 9, you can realize that kubectl passing follow=true flag to API request by acquiring logs from target Pod accordingly, and stream to the output as well:
curl -k -v -XGET -H "Accept: application/json, /" -H "User-Agent: kubectl/v1.12.1 (linux/amd64) kubernetes/4ed3216" 'https://API_server_IP/api/v1/namespaces/default/pods/Pod-name/log?follow=true'
You can consider launching own API requests by following the next steps:
Obtain token for authorization purpose:
MY_TOKEN="$(kubectl get secret <default-secret> -o jsonpath='{$.data.token}' | base64 -d)"Then you can retrieve manually the required data from API server directly:
curl -k -v -H "Authorization : Bearer $MY_TOKEN" https://API_server_IP/api/v1/namespaces/default/podsYep, it looks like it's currently just a HTTP GET that kubectl is using, when looking at the source of logs.go although there seems to be a desire to unify and upgrade a couple of commands (exec, port-forward, logs, etc.) to WebSockets.
Showing Maciej's excellent suggestion in action:
$ kubectl run test --image centos:7 \
-- sh -c "while true ; do echo Work ; sleep 2 ; done"
$ kubectl get po
NAME READY STATUS RESTARTS AGE
test-769f6f8c9f-2nx7m 1/1 Running 0 2m
$ kubectl logs -v9 -f test-769f6f8c9f-2nx7m
I1019 13:49:34.282007 71247 loader.go:359] Config loaded from file /Users/mhausenblas/.kube/config
I1019 13:49:34.284698 71247 loader.go:359] Config loaded from file /Users/mhausenblas/.kube/config
I1019 13:49:34.292620 71247 loader.go:359] Config loaded from file /Users/mhausenblas/.kube/config
I1019 13:49:34.293136 71247 round_trippers.go:386] curl -k -v -XGET -H "Accept: application/json, */*" -H "User-Agent: kubectl/v1.12.0 (darwin/amd64) kubernetes/0ed3388" 'https://192.168.64.13:8443/api/v1/namespaces/default/pods/test-769f6f8c9f-2nx7m'
I1019 13:49:34.305016 71247 round_trippers.go:405] GET https://192.168.64.13:8443/api/v1/namespaces/default/pods/test-769f6f8c9f-2nx7m 200 OK in 11 milliseconds
I1019 13:49:34.305039 71247 round_trippers.go:411] Response Headers:
I1019 13:49:34.305047 71247 round_trippers.go:414] Date: Fri, 19 Oct 2018 12:49:34 GMT
I1019 13:49:34.305054 71247 round_trippers.go:414] Content-Type: application/json
I1019 13:49:34.305062 71247 round_trippers.go:414] Content-Length: 2390
I1019 13:49:34.305125 71247 request.go:942] Response Body: {"kind":"Pod","apiVersion":"v1","metadata":{"name":"test-769f6f8c9f-2nx7m","generateName":"test-769f6f8c9f-","namespace":"default","selfLink":"/api/v1/namespaces/default/pods/test-769f6f8c9f-2nx7m","uid":"0581b0fa-d39d-11e8-9827-42a64713caf8","resourceVersion":"892912","creationTimestamp":"2018-10-19T12:46:39Z","labels":{"pod-template-hash":"3259294759","run":"test"},"ownerReferences":[{"apiVersion":"apps/v1","kind":"ReplicaSet","name":"test-769f6f8c9f","uid":"057f3ad4-d39d-11e8-9827-42a64713caf8","controller":true,"blockOwnerDeletion":true}]},"spec":{"volumes":[{"name":"default-token-fbx4m","secret":{"secretName":"default-token-fbx4m","defaultMode":420}}],"containers":[{"name":"test","image":"centos:7","args":["sh","-c","while true ; do echo Work ; sleep 2 ; done"],"resources":{},"volumeMounts":[{"name":"default-token-fbx4m","readOnly":true,"mountPath":"/var/run/secrets/kubernetes.io/serviceaccount"}],"terminationMessagePath":"/dev/termination-log","terminationMessagePolicy":"File","imagePullPolicy":"IfNotPresent"}],"restartPolicy":"Always","terminationGracePeriodSeconds":30,"dnsPolicy":"ClusterFirst","serviceAccountName":"default","serviceAccount":"default","nodeName":"minikube","securityContext":{},"schedulerName":"default-scheduler","tolerations":[{"key":"node.kubernetes.io/not-ready","operator":"Exists","effect":"NoExecute","tolerationSeconds":300},{"key":"node.kubernetes.io/unreachable","operator":"Exists","effect":"NoExecute","tolerationSeconds":300}]},"status":{"phase":"Running","conditions":[{"type":"Initialized","status":"True","lastProbeTime":null,"lastTransitionTime":"2018-10-19T12:46:39Z"},{"type":"Ready","status":"True","lastProbeTime":null,"lastTransitionTime":"2018-10-19T12:46:40Z"},{"type":"ContainersReady","status":"True","lastProbeTime":null,"lastTransitionTime":null},{"type":"PodScheduled","status":"True","lastProbeTime":null,"lastTransitionTime":"2018-10-19T12:46:39Z"}],"hostIP":"192.168.64.13","podIP":"172.17.0.11","startTime":"2018-10-19T12:46:39Z","containerStatuses":[{"name":"test","state":{"running":{"startedAt":"2018-10-19T12:46:39Z"}},"lastState":{},"ready":true,"restartCount":0,"image":"centos:7","imageID":"docker-pullable://centos@sha256:67dad89757a55bfdfabec8abd0e22f8c7c12a1856514726470228063ed86593b","containerID":"docker://5c25f5fce576d68d743afc9b46a9ea66f3cd245f5075aa95def623b6c2d93256"}],"qosClass":"BestEffort"}}
I1019 13:49:34.316531 71247 loader.go:359] Config loaded from file /Users/mhausenblas/.kube/config
I1019 13:49:34.317000 71247 round_trippers.go:386] curl -k -v -XGET -H "Accept: application/json, */*" -H "User-Agent: kubectl/v1.12.0 (darwin/amd64) kubernetes/0ed3388" 'https://192.168.64.13:8443/api/v1/namespaces/default/pods/test-769f6f8c9f-2nx7m/log?follow=true'
I1019 13:49:34.339341 71247 round_trippers.go:405] GET https://192.168.64.13:8443/api/v1/namespaces/default/pods/test-769f6f8c9f-2nx7m/log?follow=true 200 OK in 22 milliseconds
I1019 13:49:34.339380 71247 round_trippers.go:411] Response Headers:
I1019 13:49:34.339390 71247 round_trippers.go:414] Content-Type: text/plain
I1019 13:49:34.339407 71247 round_trippers.go:414] Date: Fri, 19 Oct 2018 12:49:34 GMT
Work
Work
Work
^C