CRI: Log management for container stdout/stderr streams

Goals and non-goals

Container Runtime Interface (CRI) is an ongoing project to allow container runtimes to integrate with kubernetes via a newly-defined API. The goal of this proposal is to define how container’s stdout/stderr log streams should be handled in CRI.

The explicit non-goal is to define how (non-stdout/stderr) application logs should be handled. Collecting and managing arbitrary application logs is a long-standing issue [1] in kubernetes and is worth a proposal of its own. Even though this proposal does not touch upon these logs, the direction of this proposal is aligned with one of the most-discussed solutions, logging volumes [1], for general logging management.

In this proposal, “logs” refer to the stdout/stderr streams of the containers, unless specified otherwise.

Previous CRI logging issues: - Tracking issue: https://github.com/kubernetes/kubernetes/issues/30709 - Proposal (by @tmrtfs): https://github.com/kubernetes/kubernetes/pull/33111

The scope of this proposal is narrower than the #33111 proposal, and hopefully this will encourage a more focused discussion.


Below is a brief overview of logging in kubernetes with docker, which is the only container runtime with fully functional integration today.

Log lifecycle and management

Docker supports various logging drivers (e.g., syslog, journal, and json-file), and allows users to configure the driver by passing flags to the docker daemon at startup. Kubernetes defaults to the “json-file” logging driver, in which docker writes the stdout/stderr streams to a file in the json format as shown below.

{“log”: “The actual log line”, “stream”: “stderr”, “time”: “2016-10-05T00:00:30.082640485Z”}

Docker deletes the log files when the container is removed, and a cron-job (or systemd timer-based job) on the node is responsible to rotate the logs (using logrotate). To preserve the logs for introspection and debuggability, kubelet keeps the terminated container until the pod object has been deleted from the apiserver.

Container log retrieval

The kubernetes CLI tool, kubectl, allows users to access the container logs using kubectl logs command. kubectl logs supports flags such as --since that requires understanding of the format and the metadata (i.e., timestamps) of the logs. In the current implementation, kubelet calls docker logs with parameters to return the log content. As of now, docker only supports log operations for the “journal” and “json-file” drivers [2]. In other words, the support of kubectl logs is not universal in all kubernetes deployments.

Cluster logging support

In a production cluster, logs are usually collected, aggregated, and shipped to a remote store where advanced analysis/search/archiving functions are supported. In kubernetes, the default cluster-addons includes a per-node log collection daemon, fluentd. To facilitate the log collection, kubelet creates symbolic links to all the docker containers logs under /var/log/containers with pod and container metadata embedded in the filename.


The fluentd daemon watches the /var/log/containers/ directory and extract the metadata associated with the log from the path. Note that this integration requires kubelet to know where the container runtime stores the logs, and will not be directly applicable to CRI.


  1. Provide ways for CRI-compliant runtimes to support all existing logging features, i.e., kubectl logs.

  2. Allow kubelet to manage the lifecycle of the logs to pave the way for better disk management in the future. This implies that the lifecycle of containers and their logs need to be decoupled.

  3. Allow log collectors to easily integrate with Kubernetes across different container runtimes while preserving efficient storage and retrieval.

Requirement (1) provides opportunities for runtimes to continue support kubectl logs --since and related features. Note that even though such features are only supported today for a limited set of log drivers, this is an important usability tool for a fresh, basic kubernetes cluster, and should not be overlooked. Requirement (2) stems from the fact that disk is managed by kubelet as a node-level resource (not per-pod) today, hence it is difficult to delegate to the runtime by enforcing per-pod disk quota policy. In addition, container disk quota is not well supported yet, and such limitation may not even be well-perceived by users. Requirement (1) is crucial to the kubernetes’ extensibility and usability across all deployments.

Proposed solution

This proposal intends to satisfy the requirements by

  1. Enforce where the container logs should be stored on the host filesystem. Both kubelet and the log collector can interact with the log files directly.

  2. Ask the runtime to decorate the logs in a format that kubelet understands.

Log directories and structures

Kubelet will be configured with a root directory (e.g., /var/log/pods or `/var/lib/kubelet/logs/) to store all container logs. Below is an example of a path to the log of a container in a pod.


In CRI, this is implemented by setting the pod-level log directory when creating the pod sandbox, and passing the relative container log path when creating a container.

PodSandboxConfig.LogDirectory: /var/log/pods/<podUID>/
ContainerConfig.LogPath: <containerName>_<instance#>.log

Because kubelet determines where the logs are stores and can access them directly, this meets requirement (1). As for requirement (2), the log collector can easily extract basic pod metadata (e.g., pod UID, container name) from the paths, and watch the directly for any changes. In the future, we can extend this by maintaining a metadata file in the pod directory.

Log format

The runtime should decorate each log entry with a RFC 3339Nano timestamp prefix, the stream type (i.e., “stdout” or “stderr”), the tags of the log entry, the log content that ends with a newline.

The tags fields can support multiple tags, delimited by :. Currently, only one tag is defined in CRI to support multi-line log entries: partial or full. Partial (P) is used when a log entry is split into multiple lines by the runtime, and the entry has not ended yet. Full (F) indicates that the log entry is completed – it is either a single-line entry, or this is the last line of the muiltple-line entry.

For example,

2016-10-06T00:17:09.669794202Z stdout F The content of the log entry 1
2016-10-06T00:17:09.669794202Z stdout P First line of log entry 2
2016-10-06T00:17:09.669794202Z stdout P Second line of the log entry 2
2016-10-06T00:17:10.113242941Z stderr F Last line of the log entry 2

With the knowledge, kubelet can parses the logs and serve them for kubectl logs requests. This meets requirement (3). Note that the format is defined deliberately simple to provide only information necessary to serve the requests. We do not intend for kubelet to host various logging plugins. It is also worth mentioning again that the scope of this proposal is restricted to stdout/stderr streams of the container, and we impose no restriction to the logging format of arbitrary container logs.

Who should rotate the logs?

We assume that a separate task (e.g., cron job) will be configured on the node to rotate the logs periodically, similar to today’s implementation.

We do not rule out the possibility of letting kubelet or a per-node daemon (DaemonSet) to take up the responsibility, or even declare rotation policy in the kubernetes API as part of the PodSpec, but it is beyond the scope of the this proposal.

What about non-supported log formats?

If a runtime chooses to store logs in non-supported formats, it essentially opts out of kubectl logs features, which is backed by kubelet today. It is assumed that the user can rely on the advanced, cluster logging infrastructure to examine the logs.

It is also possible that in the future, kubectl logs can contact the cluster logging infrastructure directly to serve logs [1a]. Note that this does not eliminate the need to store the logs on the node locally for reliability.

How can existing runtimes (docker/rkt) comply to the logging requirements?

In the short term, the ongoing docker-CRI integration [3] will support the proposed solution only partially by (1) creating symbolic links for kubelet to access, but not manage the logs, and (2) add support for json format in kubelet. A more sophisticated solution that either involves using a custom plugin or launching a separate process to copy and decorate the log will be considered as a mid-term solution.

For rkt, implementation will rely on providing external file-descriptors for stdout/stderr to applications via systemd [4]. Those streams are currently managed by a journald sidecar, which collects stream outputs and store them in the journal file of the pod. This will replaced by a custom sidecar which can produce logs in the format expected by this specification and can handle clients attaching as well.


There are ad-hoc solutions/discussions that addresses one or two of the requirements, but no comprehensive solution for CRI specifically has been proposed so far (with the exception of @tmrtfs’s proposal #33111, which has a much wider scope). It has come up in discussions that kubelet can delegate all the logging management to the runtime to allow maximum flexibility. However, it is difficult for this approach to meet either requirement (1) or (2), without defining complex logging API.

There are also possibilities to implement the current proposal by imposing the log file paths, while leveraging the runtime to access and/or manage logs. This requires the runtime to expose knobs in CRI to retrieve, remove, and examine the disk usage of logs. The upside of this approach is that kubelet needs not mandate the logging format, assuming runtime already includes plugins for various logging formats. Unfortunately, this is not true for existing runtimes such as docker, which supports log retrieval only for a very limited number of log drivers [2]. On the other hand, the downside is that we would be enforcing more requirements on the runtime through log storage location on the host, and a potentially premature logging API that may change as the disk management evolves.


[1] Log management issues: - a. https://github.com/kubernetes/kubernetes/issues/17183 - b. https://github.com/kubernetes/kubernetes/issues/24677 - c. https://github.com/kubernetes/kubernetes/pull/13010

[2] Docker logging drivers: - https://docs.docker.com/engine/admin/logging/overview/

[3] Docker CRI integration: - https://github.com/kubernetes/kubernetes/issues/31459

[4] rkt support: https://github.com/systemd/systemd/pull/4179