consul-template

README

Consul Template

This project provides a convenient way to populate values from Consul into the file system using the consul-template daemon.

The daemon consul-template queries a Consul instance and updates any number of specified templates on the file system. As an added bonus, consul-template can optionally run arbitrary commands when the update process completes. See the Examples fikder for some scenarios where this functionality might prove useful.

The documentation in this README corresponds to the master branch of Consul Template. It may contain unreleased features or different APIs than the most recently released version. Please see the Git tag that corresponds to your version of Consul Template for the proper documentation.

Installation

You can download a released consul-template artifact from the Consul Template release page. If you wish to compile from source, please see the instructions in the Contributing section.

Usage

Options

For the full list of options that correspond with your release, run:

consul-template -h

Command Line

The CLI interface supports all of the options detailed above.

Query the demo.consul.io Consul instance, rendering the template on disk at /tmp/template.ctmpl to /tmp/result, running Consul Template as a service until stopped:

$ consul-template \
  -consul demo.consul.io \
  -template "/tmp/template.ctmpl:/tmp/result"

Query a local Consul instance, rendering the template and restarting Nginx if the template has changed, once, polling 30s if Consul is unavailable:

$ consul-template \
  -consul 127.0.0.1:8500 \
  -template "/tmp/template.ctmpl:/var/www/nginx.conf:service nginx restart" \
  -retry 30s \
  -once

Query a Consul instance, rendering multiple templates and commands as a service until stopped:

$ consul-template \
  -consul my.consul.internal:6124 \
  -template "/tmp/nginx.ctmpl:/var/nginx/nginx.conf:service nginx restart" \
  -template "/tmp/redis.ctmpl:/var/redis/redis.conf:service redis restart" \
  -template "/tmp/haproxy.ctmpl:/var/haproxy/haproxy.conf"

Query a Consul instance that requires authentication, dumping the templates to stdout instead of writing to disk. In this example, the second and third parameters to the -template option are required but ignored. The file will not be written to disk and the optional command will not be executed:

$ consul-template \
  -consul my.consul.internal:6124 \
  -template "/tmp/template.ctmpl:/tmp/result:service nginx restart"
  -dry

Query a Consul that uses custom SSL certificates:

$ consul-template \
  -consul 127.0.0.1:8543 \
  -ssl \
  -ssl-cert /path/to/client/cert.pem \
  -ssl-ca-cert /path/to/ca/cert.pem \
  -template "/tmp/template.ctmpl:/tmp/result" \
  -dry \
  -once

Query Consul and act as a supervisor for the given child process:

$ consul-template \
  -template "/tmp/in.ctmpl:/tmp/result" \
  -exec "/sbin/my-server"

For more information, please see the Exec Mode documentation.

Configuration File(s)

The Consul Template configuration files are written in HashiCorp Configuration Language (HCL). By proxy, this means the Consul Template configuration file is JSON-compatible. For more information, please see the HCL specification.

The Configuration file syntax interface supports all of the options detailed above, unless otherwise noted in the table.

// This is the address of the Consul agent. By default, this is 127.0.0.1:8500,
// which is the default bind and port for a local Consul agent. It is not
// recommended that you communicate directly with a Consul server, and instead
// communicate with the local Consul agent. There are many reasons for this,
// most importantly the Consul agent is able to multiplex connections to the
// Consul server and reduce the number of open HTTP connections. Additionally,
// it provides a "well-known" IP address for which clients can connect.
consul = "127.0.0.1:8500"

// This is the ACL token to use when connecting to Consul. If you did not
// enable ACLs on your Consul cluster, you do not need to set this option.
//
// This option is also available via the environment variable CONSUL_TOKEN.
token = "abcd1234"

// This is the signal to listen for to trigger a reload event. The default
// value is shown below. Setting this value to the empty string will cause CT
// to not listen for any reload signals.
reload_signal = "SIGHUP"

// This is the signal to listen for to trigger a core dump event. The default
// value is shown below. Setting this value to the empty string will cause CT
// to not listen for any core dump signals.
dump_signal = "SIGQUIT"

// This is the signal to listen for to trigger a graceful stop. The default
// value is shown below. Setting this value to the empty string will cause CT
// to not listen for any graceful stop signals.
kill_signal = "SIGINT"

// This is the amount of time to wait before retrying a connection to Consul.
// Consul Template is highly fault tolerant, meaning it does not exit in the
// face of failure. Instead, it uses exponential back-off and retry functions to
// wait for the cluster to become available, as is customary in distributed
// systems.
retry = "10s"

// This is the maximum interval to allow "stale" data. By default, only the
// Consul leader will respond to queries; any requests to a follower will
// forward to the leader. In large clusters with many requests, this is not as
// scalable, so this option allows any follower to respond to a query, so long
// as the last-replicated data is within these bounds. Higher values result in
// less cluster load, but are more likely to have outdated data.
max_stale = "10m"

// This is the log level. If you find a bug in Consul Template, please enable
// debug logs so we can help identify the issue. This is also available as a
// command line flag.
log_level = "warn"

// This is the path to store a PID file which will contain the process ID of the
// Consul Template process. This is useful if you plan to send custom signals
// to the process.
pid_file = "/path/to/pid"

// This is the quiescence timers; it defines the minimum and maximum amount of
// time to wait for the cluster to reach a consistent state before rendering a
// template. This is useful to enable in systems that have a lot of flapping,
// because it will reduce the the number of times a template is rendered.
wait = "5s:10s"

// This denotes the start of the configuration section for Vault. All values
// contained in this section pertain to Vault.
vault {
  // This is the address of the Vault leader. The protocol (http(s)) portion
  // of the address is required.
  address = "https://vault.service.consul:8200"

  // This is the token to use when communicating with the Vault server.
  // Like other tools that integrate with Vault, Consul Template makes the
  // assumption that you provide it with a Vault token; it does not have the
  // incorporated logic to generate tokens via Vault's auth methods.
  //
  // This value can also be specified via the environment variable VAULT_TOKEN.
  token = "abcd1234"

  // This tells Consul Template that the provided token is actually a wrapped
  // token that should be unwrapped using Vault's cubbyhole response wrapping
  // before being used. Please see Vault's cubbyhole response wrapping
  // documentation for more information.
  unwrap_token = true

  // This option tells Consul Template to automatically renew the Vault token
  // given. If you are unfamiliar with Vault's architecture, Vault requires
  // tokens be renewed at some regular interval or they will be revoked. Consul
  // Template will automatically renew the token at half the lease duration of
  // the token. The default value is true, but this option can be disabled if
  // you want to renew the Vault token using an out-of-band process.
  //
  // Note that secrets specified in a template (using {{secret}} for example)
  // are always renewed, even if this option is set to false. This option only
  // applies to the top-level Vault token itself.
  renew_token = true

  // This section details the SSL options for connecting to the Vault server.
  // Please see the SSL options below for more information (they are the same).
  ssl {
    // ...
  }
}

// This block specifies the basic authentication information to pass with the
// request. For more information on authentication, please see the Consul
// documentation.
auth {
  enabled  = true
  username = "test"
  password = "test"
}

// This block configures the SSL options for connecting to the Consul server.
ssl {
  // This enables SSL. Specifying any option for SSL will also enable it.
  enabled = true

  // This enables SSL peer verification. The default value is "true", which
  // will check the global CA chain to make sure the given certificates are
  // valid. If you are using a self-signed certificate that you have not added
  // to the CA chain, you may want to disable SSL verification. However, please
  // understand this is a potential security vulnerability.
  verify = false

  // This is the path to the certificate to use to authenticate. If just a
  // certificate is provided, it is assumed to contain both the certificate and
  // the key to convert to an X509 certificate. If both the certificate and
  // key are specified, Consul Template will automatically combine them into an
  // X509 certificate for you.
  cert = "/path/to/client/cert"
  key = "/path/to/client/key"

  // This is the path to the certificate authority to use as a CA. This is
  // useful for self-signed certificates or for organizations using their own
  // internal certificate authority.
  ca_cert = "/path/to/ca"
}

// This block defines the configuration for connecting to a syslog server for
// logging.
syslog {
  // This enables syslog logging. Specifying any other option also enables
  // syslog logging.
  enabled = true

  // This is the name of the syslog facility to log to.
  facility = "LOCAL5"
}

// This block defines the configuration for de-duplication mode. Please see the
// de-duplication mode documentation later in the README for more information
// on how de-duplication mode operates.
deduplicate {
  // This enables de-duplication mode. Specifying any other options also enables
  // de-duplication mode.
  enabled = true

  // This is the prefix to the path in Consul's KV store where de-duplication
  // templates will be pre-rendered and stored.
  prefix = "consul-template/dedup/"
}

// This block defines the configuration for exec mode. Please see the exec mode
// documentation at the bottom of this README for more information on how exec
// mode operates and the caveats of this mode.
exec {
  // This is the command to exec as a child process. There can be only one
  // command per Consul Template process.
  command = "/usr/bin/app"

  // This is a random splay to wait before killing the command. The default
  // value is 0 (no wait), but large clusters should consider setting a splay
  // value to prevent all child processes from reloading at the same time when
  // data changes occur. When this value is set to non-zero, Consul Template
  // will wait a random period of time up to the splay value before reloading
  // or killing the child process. This can be used to prevent the thundering
  // herd problem on applications that do not gracefully reload.
  splay = "5s"

  // This defines the signal that will be sent to the child process when a
  // change occurs in a watched template. The signal will only be sent after
  // the process is started, and the process will only be started after all
  // dependent templates have been rendered at least once. The default value
  // is "" (empty or nil), which tells Consul Template to restart the child
  // process instead of sending it a signal. This is useful for legacy
  // applications or applications that cannot properly reload their
  // configuration without a full reload.
  reload_signal = "SIGUSR1"

  // This defines the signal sent to the child process when Consul Template is
  // gracefully shutting down. The application should begin a graceful cleanup.
  // If the application does not terminate before the `kill_timeout`, it will
  // be terminated (effectively "kill -9"). The default value is "SIGTERM".
  kill_signal = "SIGINT"

  // This defines the amount of time to wait for the child process to gracefully
  // terminate when Consul Template exits. After this specified time, the child
  // process will be force-killed (effectively "kill -9"). The default value is
  // "30s".
  kill_timeout = "2s"
}

// This block defines the configuration for a template. Unlike other blocks,
// this block may be specified multiple times to configure multiple templates.
// It is also possible to configure templates via the CLI directly.
template {
  // This is the source file on disk to use as the input template. This is often
  // called the "Consul Template template". This option is required.
  source = "/path/on/disk/to/template.ctmpl"

  // This is the destination path on disk where the source template will render.
  // If the parent directories do not exist, Consul Template will attempt to
  // create them.
  destination = "/path/on/disk/where/template/will/render.txt"

  // This is the optional command to run when the template is rendered. The
  // command will only run if the resulting template changes. The command must
  // return within 30s (configurable), and it must have a successful exit code.
  // Consul Template is not a replacement for a process monitor or init system.
  command = "restart service foo"

  // This is the maximum amount of time to wait for the optional command to
  // return. Default is 30s.
  command_timeout = "60s"

  // This is the permission to render the file. If this option is left
  // unspecified, Consul Template will attempt to match the permissions of the
  // file that already exists at the destination path. If no file exists at that
  // path, the permissions are 0644.
  perms = 0600

  // This option backs up the previously rendered template at the destination
  // path before writing a new one. It keeps exactly one backup. This option is
  // useful for preventing accidental changes to the data without having a
  // rollback strategy.
  backup = true

  // These are the delimiters to use in the template. The default is "{{" and
  // "}}", but for some templates, it may be easier to use a different delimiter
  // that does not conflict with the output file itself.
  left_delimiter  = "{{"
  right_delimiter = "}}"

  // This is the `minimum(:maximum)` to wait before rendering a new template to
  // disk and triggering a command, separated by a colon (`:`). If the optional
  // maximum value is omitted, it is assumed to be 4x the required minimum value.
  // This is a numeric time with a unit suffix ("5s"). There is no default value.
  // The wait value for a template takes precedence over any globally-configured
  // wait.
  wait = "2s:6s"
}

Note: Not all fields are required. For example, if you are not using Vault secrets, you do not need to specify a vault configuration. Similarly, if you are not logging to syslog, you do not need to specify a syslog configuration.

For additional security, token may also be read from the environment using the CONSUL_TOKEN or VAULT_TOKEN environment variables respectively. It is highly recommended that you do not put your tokens in plain-text in a configuration file.

Query the nyc3 demo Consul instance, rendering the template on disk at /tmp/template.ctmpl to /tmp/result, running Consul Template as a service until stopped:

consul = "nyc3.demo.consul.io"

template {
  source      = "/tmp/template.ctmpl"
  destination = "/tmp/result"
}

If a directory is given instead of a file, all files in the directory (recursively) will be merged in lexical order. So if multiple files declare a "consul" key for instance, the last one will be used. Please note that symbolic links are not followed.

Commands specified on the command line take precedence over those defined in a config file!

Templating Language

Consul Template consumes template files in the Go Template format. If you are not familiar with the syntax, we recommend reading the documentation, but it is similar in appearance to Mustache, Handlebars, or Liquid.

In addition to the Go-provided template functions, Consul Template exposes the following functions:

API Functions

datacenters

Query Consul for all data centers in the catalog. Data centers are queried using the following syntax:

{{datacenters}}

file

Read and output the contents of a local file on disk. If the file cannot be read, an error will occur. Files are read using the following syntax:

{{file "/path/to/local/file"}}

This example will out the entire contents of the file at /path/to/local/file into the template. Note: this does not process nested templates.

key

Query Consul for the value at the given key. If the key cannot be converted to a string-like value, an error will occur. Keys are queried using the following syntax:

{{key "service/redis/maxconns@east-aws"}}

The example above is querying Consul for the service/redis/maxconns in the east-aws data center. If you omit the data center attribute, the local Consul data center will be queried:

{{key "service/redis/maxconns"}}

The beauty of Consul is that the key-value structure is entirely up to you!

key_or_default

Query Consul for the value at the given key. If no key exists at the given path, the default value will be used instead. The existing constraints and usage for keys apply:

{{key_or_default "service/redis/maxconns@east-aws" "5"}}

Please note that Consul Template uses a multi-phase evaluation. During the first phase of evaluation, Consul Template will have no data from Consul and thus will always fall back to the default value. Subsequent reads from Consul will pull in the real value from Consul (if the key exists) on the next template pass. This is important because it means that Consul Template will never "block" the rendering of a template due to a missing key from a key_or_default. Even if the key exists, if Consul has not yet returned data for the key, the default value will be used instead.

ls

Query Consul for all top-level key-value pairs at the given prefix. If any of the values cannot be converted to a string-like value, an error will occur:

{{range ls "service/redis@east-aws"}}
{{.Key}} {{.Value}}{{end}}

If the Consul instance had the correct structure at service/redis in the east-aws data center, the resulting template could look like:

minconns 2
maxconns 12

If you omit the data center attribute on ls, the local Consul data center will be queried.

node

Query Consul for a single node in the catalog.

{{node "node1"}}

When called without any arguments then the node for the current agent is returned.

{{node}}

You can specify an optional parameter to the nodes call to specify the data center:

{{node "node1" "@east-aws"}}

If the node specified is not found then nil is returned. If the node is found then you are provided with information about the node and a list of services that it provides.

{{with node}}{{.Node.Node}} ({{.Node.Address}}){{range .Services}}
  {{.Service}} {{.Port}} ({{.Tags | join ","}}){{end}}
{{end}}

nodes

Query Consul for all nodes in the catalog. Nodes are queried using the following syntax:

{{nodes}}

This example will query Consul's default data center. You can specify an optional parameter to the nodes call to specify the data center:

{{nodes "@east-aws"}}

This will query Consul for all nodes in the east-aws data center.

secret

Query Vault for the secret data at the given path. If the path does not exist or if the configured Vault token does not have permission to read the path, an error will be returned. If the path exists, but the key does not exist, <no value> will be returned.

{{with secret "secret/passwords"}}{{.Data.password}}{{end}}

The following fields are available:

• LeaseID - the unique lease identifier
• LeaseDuration - the number of seconds the lease is valid
• Renewable - if the secret is renewable
• Data - the raw data - this is a map[string]interface{}, so it can be queried using Go's templating "dot notation"

If the map key has dots "." in it, you need to access the value using the index function:

{{index .Data "my.key.with.dots"}}

If additional arguments are passed to the function, then the operation is assumed to be a write operation instead of a read operation. The write operation must return data in order to be valid. This is especially useful for the PKI secret backend, for example.

{{ with secret "pki/issue/my-domain-dot-com" "common_name=foo.example.com" }}
{{ .Data.certificate }}
{{ end }}

The parameters must be key=value pairs, and each pair must be its own argument to the function:

{{ secret "path/" "a=b" "c=d" "e=f" }}

Please always consider the security implications of having the contents of a secret in plain-text on disk. If an attacker is able to get access to the file, they will have access to plain-text secrets.

Please note that Vault does not support blocking queries. As a result, Consul Template will not immediately reload in the event a secret is changed as it does with Consul's key-value store. Consul Template will fetch a new secret at half the lease duration of the original secret. For example, most items in Vault's generic secret backend have a default 30 day lease. This means Consul Template will renew the secret every 15 days. As such, it is recommended that a smaller lease duration be used when generating the initial secret to force Consul Template to renew more often.

secrets

Query Vault to list the secrets at the given path. Please note this requires Vault 0.5+ and the endpoint you want to list secrets must support listing. Not all endpoints support listing. The result is the list of secret names as strings.

{{range secrets "secret/"}}{{.}}{{end}}

The trailing slash is optional in the template, but the generated secret dependency will always have a trailing slash in log output.

To iterate and list over every secret in the generic secret backend in Vault, for example, you would need to do something like this:

{{range secrets "secret/"}}
{{with secret (printf "secret/%s" .)}}
{{range $k, $v := .Data}}
{{$k}}: {{$v}}
{{end}}
{{end}}
{{end}}

You should probably never do this. Please also note that Vault does not support blocking queries. To understand the implications, please read the note at the end of the secret function.

service

Query Consul for the service group(s) matching the given pattern. Services are queried using the following syntax:

{{service "release.web@east-aws"}}

The example above is querying Consul for healthy "web" services, in the "east-aws" data center. The tag and data center attributes are optional. To query all nodes of the "web" service (regardless of tag) for the current data center:

{{service "web"}}

The function returns a []*HealthService struct which can be used for ranging in a template:

{{range service "web@data center"}}
server {{.Name}} {{.Address}}:{{.Port}}{{end}}

which would produce something like:

server nyc_web_01 123.456.789.10:8080
server nyc_web_02 456.789.101.213:8080

By default only healthy services are returned. If you want to get all services, you can pass the "any" option:

{{service "web" "any"}}

This will return all services registered to the agent, regardless of their status.

If you want to filter services by a specific health or health(s), you can specify a comma-separated list of health check statuses:

{{service "web" "passing, warning"}}

This will returns services which are deemed "passing" or "warning" according to their node and service-level checks defined in Consul. Please note that the comma implies an "or", not an "and".

Specifying more than one status filter while "any" is used will return an error, since "any" is the superset of all status filters.

There is an architectural difference between the following:

{{service "web"}}
{{service "web" "passing"}}

The former will return all services which Consul considers "healthy" and passing. The latter will return all services registered with the Consul agent and perform client-side filtering. As a general rule, you should not use the "passing" argument alone if you want only healthy services - simply omit the second argument instead. However, the extra argument is useful if you want "passing or warning" services like:

{{service "web" "passing, warning"}}

The service's status is also exposed if you need to do additional filtering:

{{range service "web" "any"}}
{{if eq .Status "critical"}}
// Critical state!{{end}}
{{if eq .Status "passing"}}
// Ok{{end}}

To put a service into maintenance mode in Consul around executing the command, simply wrap your command in a consul maint call:

#!/bin/sh
set -e
consul maint -enable -service web -reason "Consul Template updated"
service nginx reload
consul maint -disable -service web

Alternatively, if you do not have the Consul agent installed, you can make the API requests directly (advanced):

#!/bin/sh
set -e
curl -X PUT "http://$CONSUL_HTTP_ADDR/v1/agent/service/maintenance/web?enable=true&reason=Consul+Template+Updated"
service nginx reload
curl -X PUT "http://$CONSUL_HTTP_ADDR/v1/agent/service/maintenance/web?enable=false"

services

Query Consul for all services in the catalog. Services are queried using the following syntax:

{{services}}

This example will query Consul's default data center. You can specify an optional parameter to the services call to specify the data center:

{{services "@east-aws"}}

Please be advised: the services function is different than service, which accepts more parameters and queries the health endpoints for a list of services. This endpoint queries the Consul catalog which returns a map of services to its list of tags. For example:

{{range services}}
{{.Name}}
{{range .Tags}}
  {{.}}{{end}}
{{end}}

tree

Query Consul for all key-value pairs at the given prefix. If any of the values cannot be converted to a string-like value, an error will occur:

{{range tree "service/redis@east-aws"}}
{{.Key}} {{.Value}}{{end}}

If the Consul instance had the correct structure at service/redis in the east-aws data center, the resulting template could look like:

minconns 2
maxconns 12
nested/config/value "value"

Unlike ls, tree returns all keys under the prefix, just like the Unix tree command.

If you omit the data center attribute on tree, the local Consul data center will be queried.

---

Helper Functions

byKey

Takes the list of key pairs returned from a tree function and creates a map that groups pairs by their top-level directory. For example, if the Consul KV store contained the following structure:

groups/elasticsearch/es1
groups/elasticsearch/es2
groups/elasticsearch/es3
services/elasticsearch/check_elasticsearch
services/elasticsearch/check_indexes

With the following template:

{{range $key, $pairs := tree "groups" | byKey}}{{$key}}:
{{range $pair := $pairs}}  {{.Key}}={{.Value}}
{{end}}{{end}}

The result would be:

elasticsearch:
  es1=1
  es2=1
  es3=1

Note that the top-most key is stripped from the Key value. Keys that have no prefix after stripping are removed from the list.

The resulting pairs are keyed as a map, so it is possible to look up a single value by key:

{{$weights := tree "weights"}}
{{range service "release.web"}}
  {{$weight := or (index $weights .Node) 100}}
  server {{.Node}} {{.Address}}:{{.Port}} weight {{$weight}}{{end}}

byTag

Takes the list of services returned by the service or services function and creates a map that groups services by tag.

{{range $tag, $services := service "web" | byTag}}{{$tag}}
{{range $services}} server {{.Name}} {{.Address}}:{{.Port}}
{{end}}{{end}}

contains

Determines if a needle is within an iterable element.

{{ if .Tags | contains "production" }}
# ...
{{ end }}

env

Reads the given environment variable accessible to the current process.

{{env "CLUSTER_ID"}}

This function can be chained to manipulate the output:

{{env "CLUSTER_ID" | toLower}}

explode

Takes the result from a tree or ls call and converts it into a deeply-nested map for parsing/traversing.

{{ tree "config" | explode }}

Note: You will lose any metadata about the keypair after it has been exploded.

You can also access deeply nested values:

{{ with tree "config" | explode }}
{{.a.b.c}}{{ end }}

Note: You will need to have a reasonable format about your data in Consul. Please see Golang's text/template package for more information.

in

Determines if a needle is within an iterable element.

{{ if in .Tags "production" }}
# ...
{{ end }}

loop

Accepts varying parameters and differs its behavior based on those parameters.

If loop is given one integer, it will return a goroutine that begins at zero and loops up to but not including the given integer:

{{range loop 5}}
# Comment{{end}}

If given two integers, this function will return a goroutine that begins at the first integer and loops up to but not including the second integer:

{{range $i := loop 5 8}}
stanza-{{$i}}{{end}}

which would render:

stanza-5
stanza-6
stanza-7

Note: It is not possible to get the index and the element since the function returns a goroutine, not a slice. In other words, the following is not valid:

# Will NOT work!
{{range $i, $e := loop 5 8}}
# ...{{end}}

join

Takes the given list of strings as a pipe and joins them on the provided string:

{{$items | join ","}}

trimSpace

Takes the provided input and trims all whitespace, tabs and newlines:

{{ file "/etc/ec2_version"| trimSpace }}

parseBool

Takes the given string and parses it as a boolean:

{{"true" | parseBool}}

This can be combined with a key and a conditional check, for example:

{{if key "feature/enabled" | parseBool}}{{end}}

parseFloat

Takes the given string and parses it as a base-10 float64:

{{"1.2" | parseFloat}}

parseInt

Takes the given string and parses it as a base-10 int64:

{{"1" | parseInt}}

This can be combined with other helpers, for example:

{{range $i := loop key "config/pool_size" | parseInt}}
# ...{{end}}

parseJSON

Takes the given input (usually the value from a key) and parses the result as JSON:

{{with $d := key "user/info" | parseJSON}}{{$d.name}}{{end}}

Note: Consul Template evaluates the template multiple times, and on the first evaluation the value of the key will be empty (because no data has been loaded yet). This means that templates must guard against empty responses. For example:

{{with $d := key "user/info" | parseJSON}}
{{if $d}}
...
{{end}}
{{end}}

It just works for simple keys. But fails if you want to iterate over keys or use index function. Wrapping code that access object with {{ if $d }}...{{end}} is good enough.

Alternatively you can read data from a local JSON file:

{{with $d := file "/path/to/local/data.json" | parseJSON}}{{$d.some_key}}{{end}}

parseUint

Takes the given string and parses it as a base-10 int64:

{{"1" | parseUint}}

See parseInt for examples.

plugin

Takes the name of a plugin and optional payload and executes a Consul Template plugin.

{{plugin "my-plugin"}}

This is most commonly combined with a JSON filter for customization:

{{tree "foo" | explode | toJSON | plugin "my-plugin"}}

Please see the plugins section for more information about plugins.

regexMatch

Takes the argument as a regular expression and will return true if it matches on the given string, or false otherwise.

{{"foo.bar" | regexMatch "foo([.a-z]+)"}}

regexReplaceAll

Takes the argument as a regular expression and replaces all occurrences of the regex with the given string. As in go, you can use variables like $1 to refer to subexpressions in the replacement string.

{{"foo.bar" | regexReplaceAll "foo([.a-z]+)" "$1"}}

replaceAll

Takes the argument as a string and replaces all occurrences of the given string with the given string.

{{"foo.bar" | replaceAll "." "_"}}

This function can be chained with other functions as well:

{{service "web"}}{{.Name | replaceAll ":" "_"}}{{end}}

split

Splits the given string on the provided separator:

{{"foo\nbar\n" | split "\n"}}

This can be combined with chained and piped with other functions:

{{key "foo" | toUpper | split "\n" | join ","}}

timestamp

Returns the current timestamp as a string (UTC). If no arguments are given, the result is the current RFC3339 timestamp:

{{timestamp}} // e.g. 1970-01-01T00:00:00Z

If the optional parameter is given, it is used to format the timestamp. The magic reference date Mon Jan 2 15:04:05 -0700 MST 2006 can be used to format the date as required:

{{timestamp "2006-01-02"}} // e.g. 1970-01-01

See Go's time.Format() for more information.

As a special case, if the optional parameter is "unix", the unix timestamp in seconds is returned as a string.

{{timestamp "unix"}} // e.g. 0

toJSON

Takes the result from a tree or ls call and converts it into a JSON object.

{{ tree "config" | explode | toJSON }} // e.g. {"admin":{"port":1234},"maxconns":5,"minconns":2}

Note: This functionality should be considered final. If you need to manipulate keys, combine values, or perform mutations, that should be done outside of Consul. In order to keep the API scope limited, we likely will not accept Pull Requests that focus on customizing the toJSON functionality.

toJSONPretty

Takes the result from a tree or ls call and converts it into a pretty-printed JSON object, indented by two spaces.

{{ tree "config" | explode | toJSONPretty }}
/*
{
  "admin": {
    "port": 1234
  },
  "maxconns": 5,
  "minconns": 2,
}
*/

Note: This functionality should be considered final. If you need to manipulate keys, combine values, or perform mutations, that should be done outside of Consul. In order to keep the API scope limited, we likely will not accept Pull Requests that focus on customizing the toJSONPretty functionality.

toLower

Takes the argument as a string and converts it to lowercase.

{{key "user/name" | toLower}}

See Go's strings.ToLower() for more information.

toTitle

Takes the argument as a string and converts it to titlecase.

{{key "user/name" | toTitle}}

See Go's strings.Title() for more information.

toUpper

Takes the argument as a string and converts it to uppercase.

{{key "user/name" | toUpper}}

See Go's strings.ToUpper() for more information.

toYAML

Takes the result from a tree or ls call and converts it into a pretty-printed YAML object, indented by two spaces.

{{ tree "config" | explode | toYAML }}
/*
admin:
  port: 1234
maxconns: 5
minconns: 2
*/

Note: This functionality should be considered final. If you need to manipulate keys, combine values, or perform mutations, that should be done outside of Consul. In order to keep the API scope limited, we likely will not accept Pull Requests that focus on customizing the toYAML functionality.

---

Math Functions

The following functions are available on floats and integer values.

add

Returns the sum of the two values.

{{ add 1 2 }} // 3

This can also be used with a pipe function.

{{ 1 | add 2 }} // 3

subtract

Returns the difference of the second value from the first.

{{ subtract 2 5 }} // 3

This can also be used with a pipe function.

{{ 5 | subtract 2 }}

Please take careful note of the order of arguments.

multiply

Returns the product of the two values.

{{ multiply 2 2 }} // 4

This can also be used with a pipe function.

{{ 2 | multiply 2 }} // 4

divide

Returns the division of the second value from the first.

{{ divide 2 10 }} // 5

This can also be used with a pipe function.

{{ 10 | divide 2 }} // 5

Please take careful note of the order or arguments.

Plugins

Authoring Plugins

For some use cases, it may be necessary to write a plugin that offloads work to another system. This is especially useful for things that may not fit in the "standard library" of Consul Template, but still need to be shared across multiple instances.

Consul Template plugins must have the following API:

$ NAME [INPUT...]

• NAME - the name of the plugin - this is also the name of the binary, either a full path or just the program name. It will be executed in a shell with the inherited PATH so e.g. the plugin cat will run the first executable cat that is found on the PATH.
• INPUT - input from the template - this will always be JSON if provided

Important Notes

• Plugins execute user-provided scripts and pass in potentially sensitive data from Consul or Vault. Nothing is validated or protected by Consul Template, so all necessary precautions and considerations should be made by template authors
• Plugin output must be returned as a string on stdout. Only stdout will be parsed for output. Be sure to log all errors, debugging messages onto stderr to avoid errors when Consul Template returns the value.
• Always exit 0 or Consul Template will assume the plugin failed to execute
• Ensure the empty input case is handled correctly (see Multi-phase execution)
• Data piped into the plugin is appended after any parameters given explicitly (eg {{ "sample-data" | plugin "my-plugin" "some-parameter"}} will call my-plugin some-parameter sample-data)

Here is a sample plugin in a few different languages that removes any JSON keys that start with an underscore and returns the JSON string:

#! /usr/bin/env ruby
require "json"

if ARGV.empty?
  puts JSON.fast_generate({})
  Kernel.exit(0)
end

hash = JSON.parse(ARGV.first)
hash.reject! { |k, _| k.start_with?("_")  }
puts JSON.fast_generate(hash)
Kernel.exit(0)

func main() {
  arg := []byte(os.Args[1])

  var parsed map[string]interface{}
  if err := json.Unmarshal(arg, &parsed); err != nil {
    fmt.Fprintln(os.Stderr, fmt.Sprintf("err: %s", err))
    os.Exit(1)
  }

  for k, _ := range parsed {
    if string(k[0]) == "_" {
      delete(parsed, k)
    }
  }

  result, err := json.Marshal(parsed)
  if err != nil {
    fmt.Fprintln(os.Stderr, fmt.Sprintf("err: %s", err))
    os.Exit(1)
  }

  fmt.Fprintln(os.Stdout, fmt.Sprintf("%s", result))
  os.Exit(0)
}

Caveats

Once Mode

In Once mode, Consul Template will wait for all dependencies to be rendered. If a template specifies a dependency (a request) that does not exist in Consul, once mode will wait until Consul returns data for that dependency. Please note that "returned data" and "empty data" are not mutually exclusive.

When you query for all healthy services named "foo" ({{ service "foo" }}), you are asking Consul - "give me all the healthy services named foo". If there are no services named foo, the response is the empty array. This is also the same response if there are no healthy services named foo.

Consul template processes input templates multiple times, since the first result could impact later dependencies:

{{ range services }}
{{ range service .Name }}
{{ end }}
{{ end }}

In this example, we have to process the output of services before we can lookup each service, since the inner loops cannot be evaluated until the outer loop returns a response. Consul Template waits until it gets a response from Consul for all dependencies before rendering a template. It does not wait until that response is non-empty though.

The most common point of confusion here is with the key API. In all modes (not just once), Consul Template will not block if a key does not exist:

{{ key "foo" }}

This is because Consul returns a result for the key named "foo", but that result is the empty string (no value).

Exec Mode

As of version 0.16.0, Consul Template has the ability to maintain an arbitrary child process (similar to envconsul). This mode is most beneficial when running Consul Template in a container or on a scheduler like Nomad or Kubernetes. When activated, Consul Template will spawn and manage the lifecycle of the child process.

This mode is best-explained through example. Consider a simple application that reads a configuration file from disk and spawns a server from that configuration.

$ consul-template \
    -template="/tmp/config.ctmpl:/tmp/server.conf" \
    -exec="/bin/my-server -config /tmp/server.conf"

When Consul Template starts, it will pull the required dependencies and populate the /tmp/server.conf, which the my-server binary consumes. After that template is rendered completely the first time, Consul Template spawns and manages a child process. When any of the list templates change, Consul Template will send the configurable reload signal to that child process. If no reload signal is provided, Consul Template will kill and restart the process. Additionally, in this mode, Consul Template will proxy any signals it receives to the child process. This enables a scheduler to control the lifecycle of the process and also eases the friction of running inside a container.

A common point of confusion is that the command string behaves the same as the shell; it does not. In the shell, when you run foo | bar or foo > bar, that is actually running as a subprocess of your shell (bash, zsh, csh, etc.). When Consul Template spawns the exec process, it runs outside of your shell. This behavior is different from when Consul Template executes the template-specific reload command. If you want the ability to pipe or redirect in the exec command, you will need to spawn the process in subshell, for example:

exec {
  command = "$SHELL -c 'my-server > /var/log/my-server.log'"
}

Note that when spawning like this, most shells do not proxy signals to their child by default, so your child process will not receive the signals that Consul Template sends to the shell. You can avoid this by writing a tiny shell wrapper and executing that instead:

#!/usr/bin/env bash
trap "kill -TERM $child" SIGTERM

/bin/my-server -config /tmp/server.conf
child=$!
wait "$child"

Alternatively, you can use your shell's exec function directly, if it exists:

#!/usr/bin/env bash
exec /bin/my-server -config /tmp/server.conf > /var/log/my-server.log

There are some additional caveats with Exec Mode, which should be considered carefully before use:

• If the child process dies, the Consul Template process will also die. Consul Template does not supervise the process! This is generally the responsibility of the scheduler or init system.
• The child process must remain in the foreground. This is a requirement for Consul Template to manage the process and send signals.
• The exec command will only start after all templates have been rendered at least once. One may have multiple templates for a single Consul Template process, all of which must be rendered before the process starts. Consider something like an nginx or apache configuration where both the process configuration file and individual site configuration must be written in order for the service to successfully start.
• After the child process is started, any change to any dependent template will cause the reload signal to be sent to the child process. This reload signal defaults to nil, in which Consul Template will not kill and respawn the child. The reload signal can be specified and customized via the CLI or configuration file.
• When Consul Template is stopped gracefully, it will send the configurable kill signal to the child process. The default value is SIGTERM, but it can be customized via the CLI or configuration file.
• Consul Template will forward all signals it receives to the child process except its defined reload_signal, dump_signal, and kill_signal. If you disable these signals, Consul Template will forward them to the child process.
• It is not possible to have more than one exec command (although each template can still have its own reload command).
• Individual template reload commands still fire independently of the exec command.

De-Duplication Mode

Consul Template works by parsing templates to determine what data is needed and then watching Consul for any changes to that data. This allows Consul Template to efficiently re-render templates when a change occurs. However, if there are many instances of Consul Template rendering a common template there is a linear duplication of work as each instance is querying the same data.

To make this pattern more efficient Consul Template supports work de-duplication across instances. This can be enabled with the -dedup flag or via the deduplicate configuration block. Once enabled, Consul Template uses leader election on a per-template basis to have only a single node perform the queries. Results are shared among other instances rendering the same template by passing compressed data through the Consul K/V store.

Please note that no Vault data will be stored in the compressed template. Because ACLs around Vault are typically more closely controlled than those ACLs around Consul's KV, Consul Template will still request the secret from Vault on each iteration.

Termination on Error

By default Consul Template is highly fault-tolerant. If Consul is unreachable or a template changes, Consul Template will happily continue running. The only exception to this rule is if the optional command exits non-zero. In this case, Consul Template will also exit non-zero. The reason for this decision is so the user can easily configure something like Upstart or God to manage Consul Template as a service.

If you want Consul Template to continue watching for changes, even if the optional command argument fails, you can append || true to your command. For example:

$ consul-template \
  -template "in.ctmpl:out.file:service nginx restart || true"

In this example, even if the Nginx restart command returns non-zero, the overall function will still return an OK exit code; Consul Template will continue to run as a service. Additionally, if you have complex logic for restarting your service, you can intelligently choose when you want Consul Template to exit and when you want it to continue to watch for changes. For these types of complex scripts, we recommend using a custom sh or bash script instead of putting the logic directly in the consul-template command or configuration file.

Command Environment

The current processes environment is used when executing commands with the following additional environment variables:

• CONSUL_HTTP_ADDR
• CONSUL_HTTP_TOKEN
• CONSUL_HTTP_AUTH
• CONSUL_HTTP_SSL
• CONSUL_HTTP_SSL_VERIFY

These environment variables are exported with their current values when the command executes. Other Consul tooling reads these environment variables, providing smooth integration with other Consul tools (like consul maint or consul lock). Additionally, exposing these environment variables gives power users the ability to further customize their command script.

Multi-phase Execution

Consul Template does an n-pass evaluation of templates, accumulating dependencies on each pass. This is required due to nested dependencies, such as:

{{range services}}
{{range service .Name}}
  {{.Address}}
{{end}}{{end}}

During the first pass, Consul Template does not know any of the services in Consul, so it has to perform a query. When those results are returned, the inner-loop is then evaluated with that result, potentially creating more queries and watches.

Because of this implementation, template functions need a default value that is an acceptable parameter to a range function (or similar), but does not actually execute the inner loop (which would cause a panic). This is important to mention because complex templates must account for the "empty" case. For example, the following will not work:

{{with index (service "foo") 0}}
# ...
{{end}}

This will raise an error like:

<index $services 0>: error calling index: index out of range: 0

That is because, during the first evaluation of the template, the service key is returning an empty slice. You can account for this in your template like so:

{{if service "foo"}}
{{with index (service "foo") 0}}
{{.Node}}
{{ end }}
{{ end }}

This will still add the dependency to the list of watches, but Go will not evaluate the inner-if, avoiding the out-of-index error.

Running and Process Lifecycle

While there are multiple ways to run Consul Template, the most common pattern is to run Consul Template as a system service. When Consul Template first starts, it reads any configuration files and templates from disk and loads them into memory. From that point forward, changes to the files on disk do not propagate to running process without a reload.

The reason for this behavior is simple and aligns with other tools like haproxy. A user may want to perform pre-flight validation checks on the configuration or templates before loading them into the process. Additionally, a user may want to update configuration and templates simultaneously. Having Consul Template automatically watch and reload those files on changes is both operationally dangerous and against some of the paradigms of modern infrastructure. Instead, Consul Template listens for the SIGHUP syscall to trigger a configuration reload. If you update configuration or templates, simply send HUP to the running Consul Template process and Consul Template will reload all the configurations and templates from disk.

Debugging

Consul Template can print verbose debugging output. To set the log level for Consul Template, use the -log-level flag:

$ consul-template -log-level info ...

<timestamp> [INFO] (cli) received redis from Watcher
<timestamp> [INFO] (cli) invoking Runner
# ...

You can also specify the level as debug:

$ consul-template -log-level debug ...

<timestamp> [DEBUG] (cli) creating Runner
<timestamp> [DEBUG] (cli) creating Consul API client
<timestamp> [DEBUG] (cli) creating Watcher
<timestamp> [DEBUG] (cli) looping for data
<timestamp> [DEBUG] (watcher) starting watch
<timestamp> [DEBUG] (watcher) all pollers have started, waiting for finish
<timestamp> [DEBUG] (redis) starting poll
<timestamp> [DEBUG] (service redis) querying Consul with &{...}
<timestamp> [DEBUG] (service redis) Consul returned 2 services
<timestamp> [DEBUG] (redis) writing data to channel
<timestamp> [DEBUG] (redis) starting poll
<timestamp> [INFO] (cli) received redis from Watcher
<timestamp> [INFO] (cli) invoking Runner
<timestamp> [DEBUG] (service redis) querying Consul with &{...}
# ...

FAQ

Q: How is this different than confd?
A: The answer is simple: Service Discovery as a first class citizen. You are also encouraged to read this Pull Request on the project for more background information. We think confd is a great project, but Consul Template fills a missing gap. Additionally, Consul Template has first class integration with Vault, making it easy to incorporate secret material like database credentials or API tokens into configuration files.

Q: How is this different than Puppet/Chef/Ansible/Salt?
A: Configuration management tools are designed to be used in unison with Consul Template. Instead of rendering a stale configuration file, use your configuration management software to render a dynamic template that will be populated by Consul.

Contributing

To build and install Consul Template locally, you will need a modern Go (Go 1.6+) environment.

First, clone the repo:

$ git clone https://github.com/hashicorp/consul-template.git

To compile the consul-template binary and run the test suite:

$ make dev

This will compile the consul-template binary into bin/consul-template as well as your $GOPATH and run the test suite.

If you just want to run the tests:

$ make test

Or to run a specific test in the suite:

go test ./... -run SomeTestFunction_name

Submit Pull Requests and Issues to the Consul Template project on GitHub.