Skip to content

Nodes#

Node object is one of the containerlab' pillars. Essentially, it is nodes and links what constitute the lab topology. To let users build flexible and customizable labs the nodes are meant to be configurable.

The node configuration is part of the topology definition file and may consist of the following fields that we explain in details below.

# part of topology definition file
topology:
  nodes:
    node1:  # node name
      kind: nokia_srlinux
      type: ixrd2l
      image: ghcr.io/nokia/srlinux
      startup-config: /root/mylab/node1.cfg
      binds:
        - /usr/local/bin/gobgp:/root/gobgp
        - /root/files:/root/files:ro
      ports:
      - 80:8080
      - 55555:43555/udp
      - 55554:43554/tcp
      user: test
      env:
        ENV1: VAL1
      cmd: /bin/bash script.sh

kind#

The kind property selects which kind this node is of. Kinds are essentially a way of telling containerlab how to treat the nodes properties considering the specific flavor of the node. We dedicated a separate section to discuss kinds in details.

Note

Kind must be defined either by setting the kind for a node specifically (as in the example above), or by setting the default kind: 

topology:
  defaults:
    kind: nokia_srlinux
  nodes:
    node1:
    # kind value of `srl` is inherited from defaults section

type#

With type the user sets a type of the node. Types work in combination with the kinds, such as the type value of ixrd2l sets the chassis type for SR Linux node, thus this value only makes sense to nodes of kind nokia_srlinux.

Other nodes might treat type field differently, that will depend on the kind of the node. The type values and effects defined in the documentation for a specific kind.

group#

group is a freeform string that denotes which group a node belongs to. The grouping is currently only used to sort topology elements on a graph.

image#

The common image attribute sets the container image name that will be used to start the node. The image name should be provided in a well-known format of repository(:tag).

We use <repository> image name throughout the docs articles. This means that the image with <repository>:latest name will be looked up. A user will need to add the latest tag if they want to use the same loose-tag naming:

# tagging srlinux:20.6.1-286 as srlinux:latest
# after this change it's possible to use `srlinux:latest` or `srlinux` image name
docker tag srlinux:20.6.1-286 srlinux:latest

image-pull-policy#

With image-pull-policy a user defines the container image pull policy.

Valid values are:

  • IfNotPresent - Pull container image if it is not already present. (If e.g. the :latest tag has been updated on a remote registry, containerlab will not re-pull it, if an image with the same tag is already present)
  • Never - Do not at all try to pull the image from a registry. An error will be thrown and the execution is stopped if the image is not available locally.
  • Always - Always try to pull the new image from a registry. An error will be thrown if pull fails. This will ensure fetching latest image version even if it exists locally.

The default value is IfNotPresent.

topology:
  nodes:
    srl:
      image: ghcr.io/nokia/srlinux
      image-pull-policy: Always

restart-policy#

With restart-policy a user defines the restart policy of a container as per docker docs.

Valid values are:

  • no - Don't automatically restart the container.
  • on-failure - Restart the container if it exits due to an error, which manifests as a non-zero exit code. The on-failure policy only prompts a restart if the container exits with a failure. It doesn't restart the container if the daemon restarts.
  • always - Always restart the container if it stops. If it's manually stopped, it's restarted only when Docker daemon restarts or the container itself is manually restarted.
  • unless-stopped Similar to always, except that when the container is stopped (manually or otherwise), it isn't restarted even after Docker daemon restarts.

no is the default restart policy value for all kinds, but linux. Linux kind defaults to always.

topology:
  nodes:
    srl:
      image: ghcr.io/nokia/srlinux
      kind: nokia_srlinux
      restart-policy: always
    alpine:
      kind: linux
      image: alpine
      restart-policy: "no"

license#

Some containerized NOSes require a license to operate or can leverage a license to lift-off limitations of an unlicensed version. With license property a user sets a path to a license file that a node will use. The license file will then be mounted to the container by the path that is defined by the kind/type of the node.

startup-config#

For all Network OS kinds, it's possible to provide startup configuration that the node applies on boot. The startup config can be provided in two ways:

  1. As a path to a file that is available on the host machine and contains the config blob that the node understands.
  2. As an embedded config blob that is provided as a multiline string.

path to a startup-config file#

When a path to a startup-config file is provided, containerlab either mounts the file to the container by a path that NOS expects to have its startup-config file, or it will apply the config via using the NOS-dependent method.

topology:
  nodes:
    srl:
      startup-config: ./some/path/to/startup-config.cfg

Check the particular kind documentation to see if the startup-config is supported and how it is applied.

embedded startup-config#

It is possible to embed the startup configuraion in the topology file itself. This is done by providing the startup-config as a multiline string.

topology:
  nodes:
    srl:
      startup-config: |
        system information location "I am embedded config"

Note

If a config file exists in the lab directory for a given node, then it will take preference over the startup config passed with this setting. If it is desired to discard the previously saved config and use the startup config instead, use the enforce-startup-config setting or deploy a lab with the reconfigure flag.

remote startup-config#

It is possible to specify a remote http(s) location for a startup-config file. Simply provide a URL that can be accessed from the containerlab host.

topology:
  kinds:
    nokia_srlinux:
      type: ixrd3
      image: ghcr.io/nokia/srlinux
      startup-config: https://raw.githubusercontent.com/srl-labs/containerlab/main/tests/02-basic-srl/srl2-startup.cli

The remote file will be downloaded to the containerlab's temp directory at $TMP/.clab/<filename> path and provided to the node as a locally available startup-config file. The filename will have a generated name that follows the pattern <lab-name>-<node-name>-<filename-from-url>, where <filename-from-url> is the last element of the URL path.

Note

  • Upon deletion of a lab, the downloaded startup-config files will not be removed. A manual cleanup should be performed if required.
  • If a lab is redeployed with the lab name and startup-config paths unchanged, the local file will be overwritten.
  • For https locations the certificates won't be verified to allow fetching artifacts from servers with self-signed certificates.

enforce-startup-config#

By default, containerlab will use the config file that is available in the lab directory for a given node even if the startup config parameter points to another file. To make a node to boot with the config set with startup-config parameter no matter what, set the enforce-startup-config to true.

suppress-startup-config#

By default, containerlab will create a startup-config when initially creating a lab. To prevent a startup-config file from being created (in a Zero-Touch Provisioning lab, for example), set the suppress-startup-config to true.

auto-remove#

By default, containerlab will not remove the failed or stopped nodes so that you can read their logs and understand the reason of a failure. If it is required to remove the failed/stopped nodes, use auto-remove: true property.

The property can be set on all topology levels.

startup-delay#

To make certain node(s) to boot/start later than others use the startup-delay config element that accepts the delay amount in seconds.

This setting can be applied on node/kind/default levels.

binds#

Users can leverage the bind mount capability to expose host files to the containerized nodes.

Binds instructions are provided under the binds container of a default/kind/node configuration section. The format of those binding instructions follows the same of the docker's --volume parameter.

topology:
  nodes:
    testNode:
      kind: linux
      # some other node parameters
      binds:
        - /usr/local/bin/gobgp:/root/gobgp # (1)!
        - /root/files:/root/files:ro # (2)!
        - somefile:/somefile # (3)!
        - ~/.ssh/id_rsa:/root/.ssh/id_rsa # (4)!
        - /var/run/somedir # (5)!
  1. mount a host file found by the path /usr/local/bin/gobgp to a container under /root/gobgp (implicit RW mode)
  2. mount a /root/files directory from a host to a container in RO mode
  3. when a host path is given in a relative format, the path is considered relative to the topology file and not a current working directory.
  4. The ~ char will be expanded to a user's home directory.
  5. mount an anonymous volume to a container under /var/run/somedir (implicit RW mode)
Bind variables

By default, binds are either provided as an absolute or a relative (to the current working dir) path. Although the majority of cases can be very well covered with this, there are situations in which it is desirable to use a path that is relative to the node-specific example.

Consider a two-node lab mylab.clab.yml with node-specific files, such as state information or additional configuration artifacts. A user could create a directory for such files similar to that:

.
├── cfgs
│   ├── n1
│   │   └── conf
│   └── n2
│       └── conf
└── mylab.clab.yml

3 directories, 3 files

Then to mount those files to the nodes, the nodes would have been configured with binds like that:

name: mylab
topology:
  nodes:
    n1:
      binds:
        - cfgs/n1/conf:/conf
    n2:
      binds:
        - cfgs/n2/conf:/conf

while this configuration is correct, it might be considered verbose as the number of nodes grows. To remove this verbosity, the users can use a special variable __clabNodeDir__ in their bind paths. This variable will expand to the node-specific directory that containerlab creates for each node.

This means that you can create a directory structure that containerlab will create anyhow and put the needed files over there. With the lab named mylab and the nodes named n1 and n2 the structure containerlab uses is as follows:

.
├── clab-mylab
│   ├── n1
│   │   └── conf
│   └── n2
│       └── conf
└── mylab.clab.yml

3 directories, 3 files

With this structure in place, the clab file can leverage the __clabNodeDir__ variable:

name: mylab
topology:
  nodes:
    n1:
      binds:
        - __clabNodeDir__/conf:/conf
    n2:
      binds:
        - __clabNodeDir__/conf:/conf

Notice how __clabNodeDir__ hides the directory structure and node names and removes the verbosity of the previous approach.

Another special variable the containerlab topology file can use is __clabDir__. In the example above, it would expand into clab-mylab folder. With __clabDir__ variable it becomes convenient to bind files like ansible-inventory.yml or topology-data.json that containerlab automatically creates:

name: mylab
topology:
  nodes:
    ansible:
      binds:
        - __clabDir__/ansible-inventory.yml:/ansible-inventory.yml:ro
    graphite:
      binds:
        - __clabDir__/topology-data.json:/htdocs/clab/topology-data.json:ro

Binds defined on multiple levels (defaults -> kind -> node) will be merged with the duplicated values removed (the lowest level takes precedence).

When a bind with the same destination is defined on multiple levels, the lowest level takes precedence. This allows to override the binds defined on the higher levels.

ports#

To bind the ports between the lab host and the containers the users can populate the ports object inside the node:

ports:
  - 80:8080 # tcp port 80 of the host is mapped to port 8080 of the container
  - 55555:43555/udp
  - 55554:43554/tcp

The list of port bindings consists of strings in the same format that is acceptable by docker run command's -p/--export flag.

This option is only configurable under the node level.

env#

To add environment variables to a node use the env container that can be added at defaults, kind and node levels.

The variables values are merged when the same vars are defined on multiple levels with nodes level being the most specific.

topology:
  defaults:
    env:
      ENV1: 3 # ENV1=3 will be set if it's not set on kind or node level
      ENV2: glob # ENV2=glob will be set for all nodes
  kinds:
    nokia_srlinux:
      env:
        ENV1: 2 # ENV1=2 will be set to if it's not set on node level
        ENV3: kind # ENV3=kind will be set for all nodes of srl kind
  nodes:
    node1:
      env:
        ENV1: 1 # ENV1=1 will be set for node1
        # env vars expansion is available, for example
        # ENV2 variable will be set to the value of the environment variable SOME_ENV
        # that is defined for the shell you run containerlab with
        ENV2: ${SOME_ENV}

You can also specify a magic ENV VAR - __IMPORT_ENVS: true - which will import all environment variables defined in your shell to the relevant topology level.

env-files#

To add environment variables defined in a file use the env-files property that can be defined at defaults, kind and node levels.

The variable defined in the files are merged across all of them wtit more specific definitions overwriting less specific. Node level is the most specific one.

Files can either be specified with their absolute path or a relative path. The base path for the relative path resolution is the directory that holds the topology definition file.

topology:
  defaults:
    env-files:
      - envfiles/defaults
      - /home/user/clab/default-env
  kinds:
    nokia_srlinux:
      env-files:
        - envfiles/common
        - ~/spines
  nodes:
    node1:
      env-files:
        - /home/user/somefile

user#

To set a user which will be used to run a containerized process use the user configuration option. Can be defined at node, kind and global levels.

topology:
  defaults:
    user: alice # alice user will be used for all nodes unless set on kind or node levels
  kinds:
    nokia_srlinux:
      user: bob # bob user will be used for nodes of kind srl unless it is set on node level
  nodes:
    node1:
      user: clab # clab user will be used for node1

entrypoint#

Changing the entrypoint of the container is done with entrypoint config option. It accepts the "shell" form and can be set on all levels.

topology:
  defaults:
    entrypoint: entrypoint.sh
  kinds:
    nokia_srlinux:
      entrypoint: entrypoint.sh
  nodes:
    node1:
      entrypoint: entrypoint.sh

cmd#

It is possible to set/override the command of the container image with cmd configuration option. It accepts the "shell" form and can be set on all levels.

topology:
  defaults:
    cmd: bash cmd.sh
  kinds:
    nokia_srlinux:
      cmd: bash cmd2.sh
  nodes:
    node1:
      cmd: bash cmd3.sh

labels#

To add container labels to a node use the labels container that can be added at defaults, kind and node levels.

The label values are merged when the same vars are defined on multiple levels with nodes level being the most specific.

Consider the following example, where labels are defined on different levels to show value propagation.

topology:
  defaults:
    labels:
      label1: value1
      label2: value2
  kinds:
    nokia_srlinux:
      labels:
        label1: kind_value1
        label3: value3
  nodes:
    node1:
      labels:
        label1: node_value1

As a result of such label distribution, node1 will have the following labels:

label1: node_value1 # most specific label wins
label2: value2 # inherited from defaults section
label3: value3 # inherited from kinds section

Note

Both user-defined and containerlab-assigned labels also promoted to environment variables prefixed with CLAB_LABEL_ prefix.

mgmt-ipv4#

To make a node to boot with a user-specified management IPv4 address, the mgmt-ipv4 setting can be used. Note, that the static management IP address should be part of the subnet that is used within the lab.

Read more about user-defined management addresses here.

nodes:
    r1:
      kind: nokia_srlinux
      mgmt-ipv4: 172.20.20.100

mgmt_ipv6#

To make a node to boot with a user-specified management IPv4 address, the mgmt_ipv6 setting can be used. Note, that the static management IP address should be part of the subnet that is used within the lab.

Read more about user-defined management addresses here.

nodes:
    r1:
      kind: nokia_srlinux
      mgmt_ipv6: 3fff:172:20:20::100

DNS#

To influence the DNS configuration a particular node uses, the dns configuration knob should be used. Within this blob, DNS server addresses, options and search domains can be provisioned.

topology:
  nodes:
    r1:
      kind: nokia_srlinux
      image: ghcr.io/nokia/srlinux
      dns:
        servers:
          - 1.1.1.1
          - 8.8.4.4
        search:
          - foo.com
        options:
          - some-opt

publish#

Container lab integrates with border0.com service to allow for private, Internet-reachable tunnels created for ports of containerlab nodes. This enables effortless access sharing with customers/partners/colleagues.

This integration is extensively covered on Publish ports page.

name: demo
topology:
  nodes:
    r1:
      kind: nokia_srlinux
      publish:
        - tcp/22     # tcp port 22 will be published
        - tcp/57400  # tcp port 57400 will be published
        - http/8080  # http port 8080 will be published

network-mode#

By default containerlab nodes use bridge-mode driver - nodes are created with their first interface connected to a docker network (management network).

It is possible to change this behavior using network-mode property of a node.

host mode#

The network-mode configuration option set to host will launch the node in the host networking mode.

# example node definition with host networking mode
my-node:
  image: alpine:3
  network-mode: host

container mode#

Additionally, a node can join network namespace of another container - by referencing the node in the format of container:parent_node_name2:

# example node definition with shared network namespace
my-node:
  kind: linux
sidecar-node:
  kind: linux
  network-mode: container:my-node # (1)
  startup-delay: 10 # (2)
  1. my-node portion of a network-mode property instructs sidecar-node to join the network namespace of a my-node.
  2. startup-delay is required in this case in order to properly initialize the namespace of a parent container.

Container name used after container: portion can refer to a node defined in containerlab topology or can refer to a name of a container that was launched outside of containerlab. This is useful when containerlab node needs to connect to a network namespace of a container deployed by 3rd party management tool (e.g. k8s kind).

none mode#

If you want to completely disable the networking stack on a container, you can use the none network mode. In this mode containerlab will deploy nodes without eth0 interface and docker networking. See docker docs for more details.

runtime#

By default containerlab nodes will be started by docker container runtime. Besides that, containerlab has experimental support for podman, and ignite runtimes.

It is possible to specify a global runtime with a global --runtime flag, or set the runtime on a per-node basis:

Options for the runtime parameter are:

  • docker
  • podman
  • ignite

The default runtime can also be influenced via the CLAB_RUNTIME environment variable, which takes the same values as mentioned above.

# example node definition with per-node runtime definition
my-node:
  image: alpine:3
  runtime: podman

exec#

Containers typically have some process that is launched inside the sandboxed environment. The said process and its arguments are provided via container instructions such as entrypoint and cmd in Docker's case.

Quite often, it is needed to run additional commands inside the containers when they finished booting. Instead of modifying the entrypoint and cmd it is possible to use the exec parameter and specify a list of commands to execute:

# two commands will be executed for node `my-node` once it finishes booting
my-node:
  image: alpine:3
  kind: linux
  binds:
    - myscript.sh:/myscript.sh
  exec:
    - echo test123
    - bash /myscript.sh

The exec is particularly helpful to provide some startup configuration for linux nodes such as IP addressing and routing instructions.

exec and access to env vars

When you want the exec command to have access to the env variables defined in the topology file or in the container' environment you have to escape the $ sign:

  nodes:
    test:
      kind: linux
      image: alpine:3
      env:
        FOO: BAR
      exec:
        - ash -c 'echo $$FOO'

memory#

By default, container runtimes do not impose any memory resource constraints1. A container can use too much of the host's memory, making the host OS unstable.

The memory parameter can be used to limit the amount of memory a node/container can use.

# my-node will have access to at most 1Gb of memory.
my-node:
  image: alpine:3
  kind: linux
  memory: 1Gb

Supported memory suffixes (case insensitive): b, kib, kb, mib, mb, gib, gb.

cpu#

By default, container runtimes do not impose any CPU resource constraints1. A container can use as much as the host's scheduler allows.

The cpu parameter can be used to limit the number of CPUs a node/container can use.

# my-node will have access to at most 1.5 of the CPUs
# available in the host machine.
my-node:
  image: alpine:3
  kind: linux
  cpu: 1.5

cpu-set#

The cpu-set parameter can be used to limit the node CPU usage to specific cores of the host system.

Valid syntaxes:

  • 0-3: Cores 0, 1, 2 and 3
  • 0,3: Cores 0 and 3
  • 0-1,4-5: Cores 0, 1, 4 and 5
# my-node will have access to CPU cores 0, 1, 4 and 5.
my-node:
  image: alpine:3
  kind: linux
  cpu-set: 0-1,4-5

sysctls#

The sysctl container' setting can be set via the sysctls knob under the defaults, kind and node levels.

The sysctl values will be merged. Certain kinds already set up sysctl values in the background, which take precedence over the user-defined values.

The following is an example on how to setup the sysctls.

topology:
  defaults:
    sysctls:
      net.ipv4.ip_forward: 1
      net.ipv6.icmp.ratelimi: 100
  kinds:
    nokia_srlinux:
      sysctls:
        net.ipv4.ip_forward: 0

  nodes:
    node1:
      sysctls:
        net.ipv6.icmp.ratelimit: 1000

stages#

Stages are a way to define stages a node goes through during its lifecycle and the interdependencies between the different stages of different nodes in the lab.

The stages are currently mainly used to host the wait-for knob, which is used to define the startup dependencies between nodes.

The following stages have been defined for a node:

  • create - a node enters this stage when containerlab is about to create the node's container. The node finishes this stage when the container is created and is in the created state.
  • create-links - a node enters this stage when containerlab is about to attach the links to the node. The node finishes this stage when all the links have been attached to the node.
  • configure - a node enters this stage when containerlab is about to run post-deploy commands associated with the node. The node finishes this stage when post deployment commands have been completed.
  • healthy - this stage has no distinctive enter/exit points. It is used to define a stage where a node is considered healthy. The healthiness of a node is defined by the healthcheck configuration of the node and the appropriate container status.
  • exit - a node reaches the exit state when the container associated with the node has exited status.

Stages can be defined on the defaults, kind and node levels.

wait-for#

For the explicit definition of interstage dependencies between the nodes, the wait-for knob under the stages level can be used.

In the example below node four nodes are defined with different stages and wait-for dependencies between the stages.

  1. node1 will enter the create stage only after node2 has finished its create stage.
  2. node2 will enter its create stage only after node3 has finished its create-links stage. This means that all the links associated with node3 has been attached to the node3 node.
  3. node3 will enter its create stage only after node4 has been found healthy. This means that node4 container must be healthy for node3 to enter the creation stage.
  4. node4 doesn't "wait" for any of the nodes, but it defines its own healthcheck configuration.
  nodes:
    node1:
      stages:
        create:
          wait-for:
            - node: node2
              stage: create

    node2:
      stages:
        create:
          wait-for:
            - node: node3
              stage: create-links

    node3:
      stages:
        create:
          wait-for:
            - node: node4
              stage: healthy

    node4:
      healthcheck:
        start-period: 5
        interval: 1
        test:
          - CMD-SHELL
          - cat /etc/os-release

Containerlab's built-in Dependency Manger takes care of all the dependencies, both explicitly-defined and implicit ones. It will inspect the dependency graph and make sure it is acyclic. The output of the Dependency Manager graph is visible in the debug mode.

Note, that wait-for is a list, a node's stage may depend on several other nodes' stages.

Usage scenarios

One of the use cases where wait-for might be crucial is when a number of VM-based nodes are deployed. Typically, simultaneous deployment of VMs might lead to shortage of CPU resources and VMs might fail to boot. In such cases, wait-for can be used to define the order of VM deployment, thus ensuring that certain VMs enter their create stage after certain nodes have reached healthy status.

Per-stage command execution#

The existing exec node configuration parameter is used to run commands when then node has finished all its deployment stages. Whilst this is the most common use case, it has its limitations, namely you can't run commands when the node is about to deploy its links, or when it is about to enter the healthy stage.

These more advanced command execution scenarios are enabled in the per-stage command execution feature.

With per-stage command execution the user can define exec block under each stage; moreover, it is possible to specify when the commands should be run on-enter or on-exit of the stage. And if that is not enough, you can also specify where the command should be executed, on the host or in the container.

nodes:
  node1:
    stages:
      create-links:
        exec:
          - command: ls /sys/class/net/
            target: container #(1)!
            phase: on-enter #(2)!
  1. target defaults to "container" and can be omitted. Possible values container or host
  2. phase defaults to "on-enter" and can be omitted. Possible values on-enter or on-exit

In the example above, the ls /sys/class/net/ command will be executed when node1 is about to enter the create-links stage. As expected, the command will list only interfaces provisioned by docker (eth0 and lo), but none of the containerlab-provisioned interfaces, since the create-links stage has not been finished yet.

Per-stage command execution gives you additional flexibility in terms of when the commands are executed, and what commands are executed at each stage.

Host exec#

The stage's exec property runs the commands in the container namespace and therefore targets the container node itself. This is super useful in itself, but sometimes you need to run a command on the host as a reaction to a stage enter/exit event.

This is what target property of the stage's exec is designed for. It runs the command in the host namespace and therefore targets the host itself.

nodes:
  node1:
    stages:
      create-links:
        exec:
          - command: touch /tmp/hello
            target: host
            phase: on-enter

In the example above, containerlab will run touch /tmp/hello command when the node1 is about to enter the create-links stage.

certificate#

To automatically generate a TLS certificate for a node and sign it with the Certificate Authority created by containerlab, use certificate.issue: true parameter.
The signed certificate will be stored in the Lab directory under the .tls/<NODE_NAME>/ folder.

Note, that nodes which by default rely on TLS-enabled interfaces will generate a certificate regardless of this parameter.

name: cert-gen

topology:
  nodes:
    a1:
      kind: linux
      image: alpine:latest
      certificate:
        issue: true

To configure key size and certificate validity duration use the following options:

  certificate:
    issue: true
    key-size: 4096
    validity-duration: 1h

subject alternative names (SAN)#

With SANs field of the certificate block the user sets the Subject Alternative Names that will be added to the node's certificate.

For a topology node named "srl" in a lab named "srl01", the following SANs are set by default:

  • srl
  • clab-srl01-srl
  • srl.srl01.io
  • IPv4/6 addresses of the node
topology:

  nodes:
    srl:
      kind: nokia_srlinux
      certificate:
        sans:
          - "test.com"
          - 192.168.96.155

healthcheck#

Containerlab supports the docker healthcheck configuration for the nodes. The healthcheck instruction can be set on the defaults, kind or node level, with the node level likely being the most used one.

Healtcheck allows containerlab users to define the healthcheck configuration that will be used by the container runtime to check the health of the container.

topology:
  nodes:
    l1:
      kind: linux
      image: alpine:3
      healthcheck:
        test:
          - CMD-SHELL
          - cat /etc/os-release
        start-period: 3
        retries: 1
        interval: 5
        timeout: 2

The healthcheck instruction is a dictionary that can contain the following keys:

  • test - the command to run to check the health of the container. The command is provided as a list of strings. The first element of the list is the type of the command - either CMD or CMD-SHELL, the rest are the arguments.
    When CMD type is used, the command and its arguments should be provided as a separate list elements. The CMD-SHELL allows you to specify the command that will be evaluated by the container's shell.
  • start-period - the time in seconds to wait for the container to bootstrap before running the first health check. The default value is 0 seconds.
  • interval - the time interval between the health checks. The default value is 30 seconds.
  • timeout - the time to wait for a single health check operation to complete. The default value is 30 seconds.
  • retries - the number of consecutive healthcheck failures needed to report the container as unhealthy. The default value is 3.

When the node is configured with a healthcheck the health status is visible in the docker inspect and docker ps outputs.

  1. docker runtime resources constraints. ↩↩

  2. this deployment model makes two containers to use a shared network namespace, similar to a Kubernetes pod construct. ↩