Recall that one of our design goals was to avoid making changes to each individual container definition to use the new logging system. Log downloading programs connect to a query socket (labelled “memlogdq.sock”), read logs from the internal buffer and write them somewhere else. Logging clients send log entries via a file descriptor (labelled “linuxkit-external-logging.sock”). The following diagram shows the initial design: However if the buffer does fill, then the oldest log lines will be dropped. The buffer is bounded to prevent a spammy logging client consuming too much memory in the VM. We decided to use the Go library container/ring to create a bounded circular buffer. The use of memlogd is entirely optional in LinuxKit if it is not included in the image then logs are written to the console and directly to open files as before. A log downloader starts later, after the network and persistent storage is available, connects to memlogd and streams the logs somewhere permanent.Īs long as the logs are streamed before the in-memory buffer is full, no lines will be lost. Since it is only buffering in memory, memlogd does not require any network or persistent storage. We decided to implement first-class support for logging by adding a “memory log daemon” called memlogd which starts before the first on-boot action and buffers in memory the last few thousand lines of console output from each container. would not force existing LinuxKit users to rewrite their YAML definitions or modify their existing code.could be developed within the upstream LinuxKit project and.could rotate the log files so they don’t grow forever.could write the logs to separate files to make them easier to read in a diagnostics report.was able to capture all the logs - both the on-boot actions and the service logs.In summary we wanted to add a logging mechanism to Docker Desktop that: Ideally we would start the logging daemon at the beginning so that no logs are lost, but then we would not have access to persistent disks or the network to store the logs anywhere useful. Running a syslog daemon as an “on-boot” action would avoid the race with services, but we would have to choose where to put it in the “on-boot” actions list. Furthermore, since services start concurrently, there would be a race between the syslog daemon starting and syslog clients starting: either logs would be lost or each client startup would have to block waiting for the syslog service to start. Unfortunately a logging daemon running in a service would start later - and therefore miss all the logs from - the “on-boot” actions. Other services would then connect to syslog to write logs. Initially we considered adding logging to the VM by running a syslog compatible logging daemon as a regular service that exposes /dev/log or a port (or both). The following diagram shows a simplified high-level view of the boot process:īy default the “on-boot” actions’ stdout and stderr are written both to the VM console and files in /var/log/onboot.* while the “services” stdout and stderr are connected directly to open files in /var/log which are left to grow forever. The second phase contains “services” which are started concurrently and run forever as containerd tasks.
These actions typically format disks, enable swap, configure sysctl settings and network interfaces.
#Postfix docker syslog series#
The LinuxKit VM in Docker Desktop boots in two phases: in the first phase, the init process executes a series of one-shot “on-boot” actions sequentially using runc to isolate them in containers. For a minimal example of a LinuxKit VM definition, see - more examples and documentation are available in the LinuxKit repository.
#Postfix docker syslog iso#
The Docker Desktop VM boots from an ISO which is built using LinuxKit from a list of Docker images together with a list of capabilities and bind mounts. This extra layer of isolation introduces an interesting new problem: how do we capture all the logs so we can include them in Docker Desktop diagnostic reports? If we do nothing then the logs will be written separately into each individual container which obviously isn’t very useful! Each component within the VM (including the Docker engine itself) runs as a separate isolated container. Docker Desktop runs a Virtual Machine to host Docker containers.
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