Java Driver

Name: java

The java driver is used to execute Java applications packaged into a Java Jar file. The driver requires the Jar file to be accessible from the Nomad client via the artifact downloader.

Task Configuration

task "webservice" {
  driver = "java"

  config {
    jar_path    = "local/example.jar"
    jvm_options = ["-Xmx2048m", "-Xms256m"]
  }
}

The java driver supports the following configuration in the job spec:

class - (Optional) The name of the class to run. If jar_path is specified and the manifest specifies a main class, this is optional. If shipping classes rather than a Jar, please specify the class to run and the class_path.
class_path - (Optional) The class_path specifies the class path used by Java to lookup classes and Jars.
jar_path - (Optional) The path to the downloaded Jar. In most cases this will just be the name of the Jar. However, if the supplied artifact is an archive that contains the Jar in a subfolder, the path will need to be the relative path (subdir/from_archive/my.jar).
args - (Optional) A list of arguments to the Jar's main method. References to environment variables or any interpretable Nomad variables will be interpreted before launching the task.
jvm_options - (Optional) A list of JVM options to be passed while invoking java. These options are passed without being validated in any way by Nomad.
pid_mode - (Optional) Set to "private" to enable PID namespace isolation for this task, or "host" to disable isolation. If left unset, the behavior is determined from the default_pid_mode in plugin configuration.

Warning: If set to "host", other processes running as the same user will be able to access sensitive process information like environment variables.

ipc_mode - (Optional) Set to "private" to enable IPC namespace isolation for this task, or "host" to disable isolation. If left unset, the behavior is determined from the default_ipc_mode in plugin configuration.

Warning: If set to "host", other processes running as the same user will be able to make use of IPC features, like sending unexpected POSIX signals.

cap_add - (Optional) A list of Linux capabilities to enable for the task. Effective capabilities (computed from cap_add and cap_drop) must be a subset of the allowed capabilities configured with allow_caps. Note that "all" is not permitted here if the allow_caps field in the driver configuration doesn't also allow all capabilities.

config {
  cap_add = ["net_raw", "sys_time"]
}

cap_drop - (Optional) A list of Linux capabilities to disable for the task. Effective capabilities (computed from cap_add and cap_drop) must be a subset of the allowed capabilities configured with allow_caps.

config {
  cap_drop = ["all"]
  cap_add  = ["chown", "sys_chroot", "mknod"]
}

work_dir - (Optional) Sets a custom working directory for the task. This path must be absolute and within the task's chroot or in a host volume mounted with a volume_mount block. This will also change the working directory when using nomad alloc exec.

Examples

A simple config block to run a Java Jar:

task "web" {
  driver = "java"

  config {
    jar_path    = "local/hello.jar"
    jvm_options = ["-Xmx2048m", "-Xms256m"]
  }

  # Specifying an artifact is required with the "java" driver. This is the
  # mechanism to ship the Jar to be run.
  artifact {
    source = "https://internal.file.server/hello.jar"

    options {
      checksum = "md5:123445555555555"
    }
  }
}

A simple config block to run a Java class:

task "web" {
  driver = "java"

  config {
    class       = "Hello"
    class_path  = "${NOMAD_TASK_DIR}"
    jvm_options = ["-Xmx2048m", "-Xms256m"]
  }

  # Specifying an artifact is required with the "java" driver. This is the
  # mechanism to ship the Jar to be run.
  artifact {
    source = "https://internal.file.server/Hello.class"

    options {
      checksum = "md5:123445555555555"
    }
  }
}

Capabilities

The java driver implements the following capabilities.

Feature	Implementation
`nomad alloc signal`	false
`nomad alloc exec`	false
filesystem isolation	none, chroot (only for linux)
network isolation	host, group
volume mounting	none, all (only for linux)

Plugin Options

default_pid_mode (string: optional) - Defaults to "private". Set to "private" to enable PID namespace isolation for tasks by default, or "host" to disable isolation.

Warning: If set to "host", other processes running as the same user will be able to access sensitive process information like environment variables.

default_ipc_mode (string: optional) - Defaults to "private". Set to "private" to enable IPC namespace isolation for tasks by default, or "host" to disable isolation.

Warning: If set to "host", other processes running as the same user will be able to make use of IPC features, like sending unexpected POSIX signals.

allow_caps - A list of allowed Linux capabilities. Defaults to

["audit_write", "chown", "dac_override", "fowner", "fsetid", "kill", "mknod",
 "net_bind_service", "setfcap", "setgid", "setpcap", "setuid", "sys_chroot"]

which is modeled after the capabilities allowed by docker by default (without NET_RAW). Allows the operator to control which capabilities can be obtained by tasks using cap_add and cap_drop options. Supports the value "all" as a shortcut for allow-listing all capabilities supported by the operating system.

Warning: Allowing more capabilities beyond the default may lead to undesirable consequences, including untrusted tasks being able to compromise the host system.

Client Requirements

The java driver requires Java to be installed and in your system's $PATH. On Linux, Nomad must run as root since it will use chroot and cgroups which require root privileges. The task must also specify at least one artifact to download, as this is the only way to retrieve the Jar being run.

Client Attributes

The java driver will set the following client attributes:

driver.java - Set to 1 if Java is found on the host node. Nomad determines this by executing java -version on the host and parsing the output
driver.java.version - Version of Java, ex: 1.6.0_65
driver.java.runtime - Runtime version, ex: Java(TM) SE Runtime Environment (build 1.6.0_65-b14-466.1-11M4716)
driver.java.vm - Virtual Machine information, ex: Java HotSpot(TM) 64-Bit Server VM (build 20.65-b04-466.1, mixed mode)

Here is an example of using these properties in a job file:

job "docs" {
  # Only run this job where the JVM is higher than version 1.6.0.
  constraint {
    attribute = "${attr.driver.java.version}"
    operator  = ">"
    value     = "1.6.0"
  }
}

Resource Isolation

The resource isolation provided varies by the operating system of the client and the configuration.

On Linux, Nomad will attempt to use cgroups, namespaces, and chroot to isolate the resources of a process. If the Nomad agent is not running as root, many of these mechanisms cannot be used.

As a baseline, the Java jars will be run inside a Java Virtual Machine, providing a minimum amount of isolation.

Nomad can only use cgroups to control resources if all the required controllers are available. If one or more required cgroups are unavailable, Nomad will disable resource controls that require cgroups entirely. See the documentation on cgroup controller requirements for more details.

Chroot

The chroot created on Linux is populated with data in the following directories from the host machine:

[
  "/bin",
  "/etc",
  "/lib",
  "/lib32",
  "/lib64",
  "/run/resolvconf",
  "/sbin",
  "/usr",
]

The task's chroot is populated by linking or copying the data from the host into the chroot. Note that this can take considerable disk space. Since Nomad v0.5.3, the client manages garbage collection locally which mitigates any issue this may create.

This list is configurable through the agent client configuration file.