Go has been increasingly used to write new configuration management tools. Narcissus provides a library to abstract configuration files as Go structures. https://github.com/raphink/narcissus

1. Narcissus: mapping configs in Go
Raphaël Pinson

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Who am I?
■ Raphaël Pinson (@raphink)
○ Infrastructure Developer & Training Leader
○ Augeas & Augeasproviders developer
○ Various contributions to Puppet & Ecosystem

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Camptocamp
■ Switzerland / France / Germany
■ Open-source development/integration expert
■ ~ 90 employees
■ Puppet user and contributor since 2008
■ Main contributor to the Puppet Forge

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Augeas: generic C library for config edition
■ Config mgmt "scalpel"
■ Bindings for many langages
■ Used in libvirt, nut, etc.
■ Integrated in Puppet, Ansible,
Salt, etc.
■ > 300 parsers (lenses)
augeas.net

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Augeasproviders: Puppet library for config providers
■ Provides resource abstraction
■ Declarative API to link
parameters to tree
nodes/labels
■ ~20 existing types/providers
augeasproviders.com

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Augeasproviders: Declarative API
resource_path do |resource|
service = resource[:service]
type = resource[:type]
mod = resource[:module]
control_cond = (resource[:control_is_param] == :true) ? "and control='#{resource[:control]}'" : ''
if target == '/etc/pam.conf'
"$target/*[service='#{service}' and type='#{type}' and module='#{mod}' #{control_cond}]"
else
"$target/*[type='#{type}' and module='#{mod}' #{control_cond}]"
end
end

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What about go?
■ Many new cfgmgmt tools
written in go
■ Augeas bindings for go
■ No abstraction for generic
config files yet

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Augeas bindings for go
■ github.com/dominikh/go-
augeas
■ Simple mapping to
Augeas API
■ No config file abstraction
package main
import (
"honnef.co/go/augeas"
"fmt"
)
func main() {
ag, err := augeas.New("/", "", augeas.None)
if err != nil {
panic(err)
}
version, err := ag.Get("/augeas/version")
fmt.Println(version, err)
}

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Go uses tags on structs
■ YAML/JSON/XML parsing
■ Flags/interfaces
(.e.g. go-flags)
// OpenstackConfigAuth stores authentication
informations
type OpenstackConfigAuth struct {
AuthURL string `yaml:"auth_url,omitempty"`
Password string `yaml:"password,omitempty"`
ProjectID string `yaml:"project_id,omitempty"`
ProjectName string `yaml:"project_name,omitempty"`
Username string `yaml:"username,omitempty"`
UserDomainName string
`yaml:"user_domain_name,omitempty"`
}

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Using tags with Augeas
■ Use a narcissus tag
■ Map tags to Augeas tree
■ Expose configuration files
as go structs

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Augeas parsing /etc/group
augtool> print /files/etc/group/adm
/files/etc/group/adm
/files/etc/group/adm/password = "x"
/files/etc/group/adm/gid = "4"
/files/etc/group/adm/user[1] = "syslog"
/files/etc/group/adm/user[2] = "raphink"
■ Group names are node
labels
■ Sub-nodes for password and
gid
■ Sub-nodes for users

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Narcissus simple struct declaration
type group struct {
augeasPath string
Name string `narcissus:".,value-from-label"`
Password string `narcissus:"password"`
GID int `narcissus:"gid"`
Users []string `narcissus:"user"`
}

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Parsing /etc/group with Narcissus
aug, err := augeas.New("/", "", augeas.None)
if err != nil {
log.Fatal("Failed to create Augeas handler")
}
n := narcissus.New(&aug)
group := &group{
augeasPath: "/files/etc/group/docker",
}
if err = n.Parse(group) ; err != nil {
log.Fatalf("Failed to retrieve group: %v", err)
}

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Modifying files
user := n.NewPasswdUser("raphink")
// Modify UID
user.UID = 42
err = n.Write(user)
if err != nil {
log.Fatalf("Failed to save user: %v", err)
}

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Mapping /etc/services
// Services maps a /etc/services file
type Services struct {
augeasPath string `default:"/files/etc/services"`
Comments []struct {
Comment string `narcissus:"."`
} `narcissus:"#comment"`
Services []Service `narcissus:"service-name"`
}
// Service maps a Services entry
type Service struct {
augeasPath string
Name string `narcissus:"."`
Port int `narcissus:"port"`
Protocol string `narcissus:"protocol"`
Comment string `narcissus:"#comment"`
}
augtool> print /files/etc/services/
/files/etc/services
/files/etc/services/#comment[1] = "Network services,
Internet style"
/files/etc/services/service-name[1] = "tcpmux"
/files/etc/services/service-name[1]/port = "1"
/files/etc/services/service-name[1]/protocol = "tcp"
/files/etc/services/service-name[1]/#comment = "TCP
port service multiplexer"

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Mapping /etc/hosts (seq)
// Hosts maps a /etc/hosts file
type Hosts struct {
augeasPath string `default:"/files/etc/hosts"`
Comments []struct {
Comment string `narcissus:"."`
} `narcissus:"#comment"`
Hosts []Host `narcissus:"seq"`
}
// Host maps an Hosts entry
type Host struct {
augeasPath string
IPAddress string `narcissus:"ipaddr"`
Canonical string `narcissus:"canonical"`
Aliases []string `narcissus:"alias"`
Comment string `narcissus:"#comment"`
}
augtool> print /files/etc/hosts
/files/etc/hosts
/files/etc/hosts/1
/files/etc/hosts/1/ipaddr = "127.0.0.1"
/files/etc/hosts/1/canonical = "localhost"
/files/etc/hosts/2
/files/etc/hosts/2/ipaddr = "127.0.1.1"
/files/etc/hosts/2/canonical =
"wrk8.wrk.cby.camptocamp.com"
/files/etc/hosts/2/alias = "wrk8"

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Write your own
■ Use existing Augeas lenses (or write your own)
■ Map tree to go struct
■ Profit!