View Source mix release (Mix v1.18.0-dev)
Assembles a self-contained release for the current project:
$ MIX_ENV=prod mix release
$ MIX_ENV=prod mix release NAME
Once a release is assembled, it can be packaged and deployed to a
target, as long as the target runs on the same operating system (OS)
distribution and version as the machine running the mix release
command.
A release can be configured in your mix.exs
file under the :releases
key inside def project
:
def project do
[
releases: [
demo: [
include_executables_for: [:unix],
applications: [runtime_tools: :permanent]
],
...
]
]
end
You can specify multiple releases where the key is the release name and the value is a keyword list with the release configuration. Releasing a certain name is done with:
$ MIX_ENV=prod mix release demo
If the given name does not exist, an error is raised.
If mix release
is invoked, without specifying a release name, and
there are multiple releases configured, an error will be raised
unless you set default_release: NAME
at the root of your project
configuration.
If mix release
is invoked and there are no releases configured, a
release is assembled using the application name and default values.
Why releases?
Releases allow developers to precompile and package all of their code and the runtime into a single unit. The benefits of releases are:
Code preloading. The VM has two mechanisms for loading code: interactive and embedded. By default, it runs in the interactive mode which dynamically loads modules when they are used for the first time. The first time your application calls
Enum.map/2
, the VM will find theEnum
module and load it. There's a downside: when you start a new server in production, it may need to load many other modules, causing the first requests to have an unusual spike in response time. With releases, the system preloads all modules and guarantees your system is ready to handle requests after booting.Configuration and customization. Releases give developers fine grained control over system configuration and the VM flags used to start the system.
Self-contained. A release does not require the source code to be included in your production artifacts. All of the code is precompiled and packaged. Releases do not even require Erlang or Elixir in your servers, as it includes the Erlang VM and its runtime by default. Furthermore, both Erlang and Elixir standard libraries are stripped to bring only the parts you are actually using.
Multiple releases. You can assemble different releases with different configuration per application or even with different applications altogether.
Management scripts. Releases come with scripts to start, restart, connect to the running system remotely, execute RPC calls, run as daemon, run as a Windows service, and more.
Running the release
Once a release is assembled, you can start it by calling
bin/RELEASE_NAME start
inside the release. In production, you would do:
$ MIX_ENV=prod mix release
$ _build/prod/rel/my_app/bin/my_app start
bin/my_app start
will start the system connected to the current standard
input/output, where logs are also written to by default. This is the
preferred way to run the system. Many tools, such as systemd
, platforms
as a service, such as Heroku, and many containers platforms, such as Docker,
are capable of processing the standard input/output and redirecting
the log contents elsewhere. Those tools and platforms also take care
of restarting the system in case it crashes.
You can also execute one-off commands, run the release as a daemon on
Unix-like system, or install it as a service on Windows. We will take a
look at those next. You can also list all available commands by invoking
bin/RELEASE_NAME
.
One-off commands (eval and rpc)
If you want to invoke specific modules and functions in your release,
you can do so in two ways: using eval
or rpc
.
$ bin/RELEASE_NAME eval "IO.puts(:hello)"
$ bin/RELEASE_NAME rpc "IO.puts(:hello)"
The eval
command starts its own instance of the VM but without
starting any of the applications in the release and without starting
distribution. For example, if you need to do some prep work before
running the actual system, like migrating your database, eval
can
be a good fit. Just keep in mind any application you may use during
eval has to be explicitly started.
You can start an application by calling Application.ensure_all_started/1
.
From Elixir v1.16, it is guaranteed the applications have been
at least loaded. In earlier versions, if you needed to load applications
but not start them, you also needed to call Application.load/1
.
Another way to run commands is with rpc
, which will connect to the
system currently running and instruct it to execute the given
expression. This means you need to guarantee the system was already
started and be careful with the instructions you are executing.
You can also use remote
to connect a remote IEx session to the
system.
Helper module
As you operate your system, you may find yourself running some piece of code as a one-off command quite often. You may consider creating a module to group these tasks:
# lib/my_app/release_tasks.ex
defmodule MyApp.ReleaseTasks do
def eval_purge_stale_data() do
# Eval commands needs to start the app before
# Or Application.load(:my_app) if you can't start it
Application.ensure_all_started(:my_app)
# Code that purges stale data
...
end
def rpc_print_connected_users() do
# Code that print users connected to the current running system
...
end
end
In the example above, we prefixed the function names with the command name used to execute them, but that is entirely optional.
And to run them:
$ bin/RELEASE_NAME eval "MyApp.ReleaseTasks.eval_purge_stale_data()"
$ bin/RELEASE_NAME rpc "MyApp.ReleaseTasks.rpc_print_connected_users()"
Daemon mode (Unix-like)
You can run the release in daemon mode with the command:
$ bin/RELEASE_NAME daemon
In daemon mode, the system is started on the background via
run_erl
. You may also
want to enable heart
in daemon mode so it automatically restarts the system in case
of crashes. See the generated releases/RELEASE_VSN/env.sh
file.
The daemon will write all of its standard output to the "tmp/log/"
directory in the release root. You can watch the log file by doing
tail -f tmp/log/erlang.log.1
or similar. Once files get too large,
the index suffix will be incremented. A developer can also attach
to the standard input of the daemon by invoking "to_erl tmp/pipe/"
from the release root. However, note that attaching to the system
should be done with extreme care, since the usual commands for
exiting an Elixir system, such as hitting Ctrl+C twice or Ctrl+\,
will actually shut down the daemon. Therefore, using
bin/RELEASE_NAME remote
should be preferred, even in daemon mode.
You can customize the tmp directory used both for logging and for
piping in daemon mode by setting the RELEASE_TMP
environment
variable. See the "Customization" section.
Services mode (Windows)
While daemons are not available on Windows, it is possible to install a
released system as a service on Windows with the help of
erlsrv
.
This can be done by running:
$ bin/RELEASE_NAME install
Once installed, the service must be explicitly managed via the erlsrv
executable, which is included in the erts-VSN/bin
directory.
The service is not started automatically after installing.
For example, if you have a release named demo
, you can install
the service and then start it from the release root as follows:
$ bin/demo install
$ erts-VSN/bin/erlsrv.exe start demo_demo
The name of the service is demo_demo
because the name is built
by concatenating the node name with the release name. Since Elixir
automatically uses the same name for both, the service will be
referenced as demo_demo
.
The install
command must be executed as an administrator.
bin/RELEASE_NAME
commands
The following commands are supported by bin/RELEASE_NAME
:
start Starts the system
start_iex Starts the system with IEx attached
daemon Starts the system as a daemon (Unix-like only)
daemon_iex Starts the system as a daemon with IEx attached (Unix-like only)
install Installs this system as a Windows service (Windows only)
eval "EXPR" Executes the given expression on a new, non-booted system
rpc "EXPR" Executes the given expression remotely on the running system
remote Connects to the running system via a remote shell
restart Restarts the running system via a remote command
stop Stops the running system via a remote command
pid Prints the operating system PID of the running system via a remote command
version Prints the release name and version to be booted
Deployments
Requirements
A release is built on a host, a machine which contains Erlang, Elixir, and any other dependencies needed to compile your application. A release is then deployed to a target, potentially the same machine as the host, but usually separate, and often there are many targets (either multiple instances, or the release is deployed to heterogeneous environments).
To deploy straight from a host to a separate target, the following must be the same between the host and the target:
- Target architecture (for example, x86_64 or ARM)
- Target vendor + operating system (for example, Windows, Linux, or Darwin/macOS)
- Target ABI (for example, musl or gnu)
This is often represented in the form of target triples, for example,
x86_64-unknown-linux-gnu
, x86_64-unknown-linux-musl
, x86_64-apple-darwin
.
If you are building on a MacBook (x86_64-apple-darwin
) and trying to deploy
to a typical Ubuntu machine (x86_64-unknown-linux-gnu
), the release will not
work. Instead you should build the release on a x86_64-unknown-linux-gnu
host.
Typically, different versions of Erlang VM and Elixir are available for different targets via package managers, precompiled artifacts, and similar. However, to deploy from a host to a separate target, you must also guarantee that any dependency with NIFs (Natively-Implemented Functions) are compiled for the same triplet. As we will see, this can be done in different ways, such as releasing on the target itself, or by using virtual machines or containers, usually as part of your release pipeline.
In addition to matching the target triple, it is also important that the
target has all of the system packages that your application will need at
runtime. A common one is the need for OpenSSL when building an application
that uses :crypto
or :ssl
, which is dynamically linked to the Erlang VM.
Project dependencies containing NIFs (natively-implemented functions) may
also dynamically link to system libraries, so check those accordingly.
Of course, some operating systems and package managers can differ between versions, so if your goal is to have full compatibility between host and target, it is best to ensure the operating system and system package manager have the same versions on host and target. This may even be a requirement in some systems, especially so with package managers that try to create fully reproducible environments (Nix, Guix).
Using matching host and target
There are a couple of ways to guarantee that a release is built on a host with the same properties as the target. A simple option is to fetch the source, compile the code and assemble the release on the target itself. It would be something like this:
$ git clone remote://path/to/my_app.git my_app_source
$ cd my_app_source
$ mix deps.get --only prod
$ MIX_ENV=prod mix release
$ _build/prod/rel/my_app/bin/my_app start
If you prefer, you can also compile the release to a separate directory, so you can erase all source after the release is assembled:
$ git clone remote://path/to/my_app.git my_app_source
$ cd my_app_source
$ mix deps.get --only prod
$ MIX_ENV=prod mix release --path ../my_app_release
$ cd ../my_app_release
$ rm -rf ../my_app_source
$ bin/my_app start
However, this option can be expensive if you have multiple production nodes or if the release assembling process is a long one, as each node needs to individually assemble the release.
You can automate this process in a couple different ways. One option
is to make it part of your Continuous Integration (CI) / Continuous
Deployment (CD) pipeline. When you have a CI/CD pipeline, it is common
that the machines in your CI/CD pipeline run on the exact same target
triple as your production servers (if they don't, they should).
In this case, you can assemble the release at the end of your CI/CD
pipeline by calling MIX_ENV=prod mix release
and push the artifact
to S3 or any other network storage. To perform the deployment, your
production machines can fetch the deployment from the network storage
and run bin/my_app start
.
Using images
Another mechanism to automate deployments is to use images, such as
Amazon Machine Images, or container platforms, such as Docker.
For instance, you can use Docker to run locally a system with the
exact same target triple as your production servers. Inside the
container, you can invoke MIX_ENV=prod mix release
and build
a complete image and/or container with the operating system, all
dependencies as well as the releases.
However, when building such images on your machine, those technologies use emulation which may not interplay well with Erlang VM's JIT (just-in time) compiler. To address this, you can set this environment variable on your build stage:
ENV ERL_AFLAGS "+JMsingle true"
Shutting down
Once a system is deployed, shutting down the system can be done by
sending SIGINT/SIGTERM to the system, which is what most containers,
platforms and tools do, or by explicitly invoking bin/RELEASE_NAME stop
.
Once the system receives the shutdown request, each application and
their respective supervision trees will stop, one by one, in the
opposite order that they were started.
Customization
There are a couple ways in which developers can customize the generated artifacts inside a release.
Options
The following options can be set inside your mix.exs
on each release definition:
:applications
- a keyword list with application names as keys and their mode as value. By default:applications
includes the current application and all applications the current application depends on, recursively. You can include new applications or change the mode of existing ones by listing them here.The order of the applications given will be preserved as much as possible, with only
:kernel
,:stdlib
,:sasl
, and:elixir
listed before the given application list. The supported values are::permanent
(default) - the application is started and the node shuts down if the application terminates, regardless of reason:transient
- the application is started and the node shuts down if the application terminates abnormally:temporary
- the application is started and the node does not shut down if the application terminates:load
- the application is only loaded:none
- the application is part of the release but it is neither loaded nor started
If you change the mode of an application, the mode will apply to all its child applications. However, if an application has two parents, the mode of the parent with highest priority wins (where
:permanent
has the highest priority, according to the list above).:strip_beams
- controls if BEAM files should have their debug information, documentation chunks, and other non-essential metadata removed. Defaults totrue
. May be set tofalse
to disable stripping. Also accepts[keep: ["Docs", "Dbgi"]]
to keep certain chunks that are usually stripped. You can also set the:compress
option to true to enable individual compression of BEAM files, although it is typically preferred to compress the whole release instead.:cookie
- a string representing the Erlang Distribution cookie. If this option is not set, a random cookie is written to thereleases/COOKIE
file when the first release is assembled. At runtime, we will first attempt to fetch the cookie from theRELEASE_COOKIE
environment variable and then we'll read thereleases/COOKIE
file.If you are setting this option manually, we recommend the cookie option to be a long and randomly generated string, such as:
Base.encode32(:crypto.strong_rand_bytes(40))
. We also recommend restricting the characters in the cookie to only alphanumeric characters and underscore.:validate_compile_env
- by default a release will match all runtime configuration against any configuration that was marked at compile time in your application of its dependencies via theApplication.compile_env/3
function. If there is a mismatch between those, it means your system is misconfigured and unable to boot. You can disable this check by setting this option to false.:path
- the path the release should be installed to. Defaults to"_build/MIX_ENV/rel/RELEASE_NAME"
.:version
- the release version as a string or{:from_app, app_name}
. Defaults to the current application version. The{:from_app, app_name}
format can be used to easily reference the application version from another application. This is particularly useful in umbrella applications.:quiet
- a boolean that controls if releases should write steps to the standard output. Defaults tofalse
.:include_erts
- a boolean, string, or anonymous function of arity zero. If a boolean, it indicates whether the Erlang Runtime System (ERTS), which includes the Erlang VM, should be included in the release. The default istrue
, which is also the recommended value. If a string, it represents the path to an existing ERTS installation. If an anonymous function of arity zero, it's a function that returns any of the above (boolean or string).You may also set this option to
false
if you desire to use the ERTS version installed on the target. Note, however, that the ERTS version on the target must have the exact version as the ERTS version used when the release is assembled. Setting it tofalse
also disables hot code upgrades. Therefore,:include_erts
should be set tofalse
with caution and only if you are assembling the release on the same server that runs it.:include_executables_for
- a list of atoms detailing for which Operating Systems executable files should be generated for. By default, it is set to[:unix, :windows]
. You can customize those as follows:releases: [ demo: [ include_executables_for: [:unix] # Or [:windows] or [] ] ]
:rel_templates_path
- the path to find template files that are copied to the release, such as "vm.args.eex", "remote.vm.args.eex", "env.sh.eex" (or "env.bat.eex"), and "overlays". Defaults to "rel" in the project root.:overlays
- a list of directories with extra files to be copied as is to the release. The "overlays" directory at:rel_templates_path
is always included in this list by default (typically at "rel/overlays"). See the "Overlays" section for more information.:steps
- a list of steps to execute when assembling the release. See the "Steps" section for more information.:skip_mode_validation_for
- a list of application names (atoms) specifying applications to skip strict validation of "unsafe" modes. An "unsafe" case is when a parent application mode is:permanent
but one of the applications it depends on is set to:load
. Use this with care, as a release with invalid modes may no longer boot without additional tweaks. Defaults to[]
.
Note each release definition can be given as an anonymous function. This is useful if some release attributes are expensive to compute:
releases: [
demo: fn ->
[version: @version <> "+" <> git_ref()]
end
]
Besides the options above, it is possible to customize the generated release with custom files, by tweaking the release steps or by running custom options and commands on boot. We will detail both approaches next.
Overlays
Often it is necessary to copy extra files to the release root after
the release is assembled. This can be easily done by placing such
files in the rel/overlays
directory. Any file in there is copied
as is to the release root. For example, if you have placed a
"rel/overlays/Dockerfile" file, the "Dockerfile" will be copied as
is to the release root.
If you want to specify extra overlay directories, you can do so
with the :overlays
option. If you need to copy files dynamically,
see the "Steps" section.
Steps
It is possible to add one or more steps before and after the release is
assembled. This can be done with the :steps
option:
releases: [
demo: [
steps: [&set_configs/1, :assemble, ©_extra_files/1]
]
]
The :steps
option must be a list and it must always include the
atom :assemble
, which does most of the release assembling. You
can pass anonymous functions before and after the :assemble
to
customize your release assembling pipeline. Those anonymous functions
will receive a Mix.Release
struct and must return the same or
an updated Mix.Release
struct. It is also possible to build a tarball
of the release by passing the :tar
step anywhere after :assemble
.
If the release :path
is not configured, the tarball is created in
_build/MIX_ENV/RELEASE_NAME-RELEASE_VSN.tar.gz
Otherwise it is
created inside the configured :path
.
See Mix.Release
for more documentation on the struct and which
fields can be modified. Note that the :steps
field itself can be
modified and it is updated every time a step is called. Therefore,
if you need to execute a command before and after assembling the
release, you only need to declare the first steps in your pipeline
and then inject the last step into the release struct. The steps
field can also be used to verify if the step was set before or
after assembling the release.
vm.args and env.sh (env.bat)
Developers may want to customize the VM flags and environment variables
given when the release starts. The simplest way to customize those files
is by running mix release.init
. The Mix task will copy custom
rel/vm.args.eex
, rel/remote.vm.args.eex
, rel/env.sh.eex
, and
rel/env.bat.eex
files to your project root. You can modify those files
and they will be evaluated every time you perform a new release. Those
files are regular EEx templates and they have a single assign, called
@release
, with the Mix.Release
struct.
The vm.args
and remote.vm.args
files may contain any of the VM flags
accepted by the erl
command.
The env.sh
and env.bat
is used to set environment variables.
In there, you can set vars such as RELEASE_NODE
, RELEASE_COOKIE
,
and RELEASE_TMP
to customize your node name, cookie and tmp
directory respectively. Whenever env.sh
or env.bat
is invoked,
the variables RELEASE_ROOT
, RELEASE_NAME
, RELEASE_VSN
, and
RELEASE_COMMAND
have already been set, so you can rely on them.
See the section on environment variables for more information.
Furthermore, while the vm.args
files are static, you can use
env.sh
and env.bat
to dynamically set VM options. For example,
if you want to make sure the Erlang Distribution listens only on
a given port known at runtime, you can set the following:
case $RELEASE_COMMAND in
start*|daemon*)
ELIXIR_ERL_OPTIONS="-kernel inet_dist_listen_min $BEAM_PORT inet_dist_listen_max $BEAM_PORT"
export ELIXIR_ERL_OPTIONS
;;
*)
;;
esac
Note we only set the port on start/daemon commands. If you also limit
the port on other commands, such as rpc
, then you will be unable
to establish a remote connection as the port will already be in use
by the node.
On Windows, your env.bat
would look like this:
IF NOT %RELEASE_COMMAND:start=%==%RELEASE_COMMAND% (
set ELIXIR_ERL_OPTIONS="-kernel inet_dist_listen_min %BEAM_PORT% inet_dist_listen_max %BEAM_PORT%"
)
Inside env.sh
and env.bat
files you can access command-line arguments given to release commands.
For example, given this env.sh.eex
:
echo $@
or this env.bat.eex
:
echo %*
starting the release with bin/myapp start --foo bar baz
will print start --foo bar baz
.
epmd
-less deployment
When a distributed Erlang/Elixir node starts, it runs a separate daemon called EPMD (Erlang Port Mapper Daemon) and registers the node name within EPMD. It is possible to skip this additional Operating System process by setting the following flags in your vm.args files:
# In vm.args.eex
-start_epmd false -erl_epmd_port 6789
# In remote.vm.args.eex
-start_epmd false -erl_epmd_port 6789 -dist_listen false
You can pick any port of your choice.
Application configuration
Mix provides two mechanisms for configuring the application environment
of your application and your dependencies: build-time and runtime. On this
section, we will learn how those mechanisms apply to releases. An introduction
to this topic can be found in the "Configuration" section of the Mix
module.
Build-time configuration
Whenever you invoke a mix
command, Mix loads the configuration in
config/config.exs
, if said file exists. We say that this configuration
is a build-time configuration as it is evaluated whenever you compile your
code or whenever you assemble the release.
In other words, if your configuration does something like:
import Config
config :my_app, :secret_key, System.fetch_env!("MY_APP_SECRET_KEY")
The :secret_key
key under :my_app
will be computed on the
host machine, whenever the release is built. Therefore if the machine
assembling the release not have access to all environment variables used
to run your code, loading the configuration will fail as the environment
variable is missing. Luckily, Mix also provides runtime configuration,
which should be preferred and we will see next.
Runtime configuration
To enable runtime configuration in your release, all you need to do is
to create a file named config/runtime.exs
:
import Config
config :my_app, :secret_key, System.fetch_env!("MY_APP_SECRET_KEY")
This file will be executed whenever your Mix project or your release starts.
Your config/runtime.exs
file needs to follow three important rules:
- It MUST
import Config
at the top instead of the deprecateduse Mix.Config
- It MUST NOT import any other configuration file via
import_config
- It MUST NOT access
Mix
in any way, asMix
is a build tool and it is not available inside releases
If a config/runtime.exs
exists, it will be copied to your release
and executed early in the boot process, when only Elixir and Erlang's
main applications have been started.
You can change the path to the runtime configuration file by setting
:runtime_config_path
inside each release configuration. This path is
resolved at build time as the given configuration file is always copied
to inside the release:
releases: [
demo: [
runtime_config_path: ...
]
]
By setting :runtime_config_path
to false
it can be used to prevent
a runtime configuration file to be included in the release.
Config providers
Releases also supports custom mechanisms, called config providers, to load
any sort of runtime configuration to the system while it boots. For instance,
if you need to access a vault or load configuration from a JSON file, it can
be achieved with config providers. The runtime configuration outlined in the
previous section is handled by the Config.Reader
provider. See the
Config.Provider
module for more information and more examples.
The following options can be set inside your releases key in your mix.exs
to control how config providers work:
:reboot_system_after_config
- reboot the system after configuration so you can configure system applications, such as:kernel
and:stdlib
, in yourconfig/runtime.exs
. Generally speaking, it is best to configure:kernel
and:stdlib
using thevm.args
file but this option is available for those who need more complex configuration. When set totrue
, the release will first boot in interactive mode to compute a config file and write it to the "tmp" directory. Then it reboots in the configuredRELEASE_MODE
. You can configure the "tmp" directory by setting theRELEASE_TMP
environment variable, either explicitly or inside yourreleases/RELEASE_VSN/env.sh
(orenv.bat
on Windows). Defaults totrue
if using the deprecatedconfig/releases.exs
,false
otherwise.:prune_runtime_sys_config_after_boot
- if:reboot_system_after_config
is set, every time your system boots, the release will write a config file to your tmp directory. These configuration files are generally small. But if you are concerned with disk space or if you have other restrictions, you can ask the system to remove said config files after boot. The downside is that you will no longer be able to restart the system internally (neither viaSystem.restart/0
norbin/RELEASE_NAME restart
). If you need a restart, you will have to terminate the Operating System process and start a new one. Defaults tofalse
.:start_distribution_during_config
- if:reboot_system_after_config
is set, releases only start the Erlang VM distribution features after the config files are evaluated. You can set it totrue
if you need distribution during configuration. Defaults tofalse
.:config_providers
- a list of tuples with custom config providers. SeeConfig.Provider
for more information. Defaults to[]
.
Customization and configuration summary
Generally speaking, the following files are available for customizing and configuring the running system:
config/config.exs
(andconfig/prod.exs
) - provides build-time application configuration, which are executed when the release is assembledconfig/runtime.exs
- provides runtime application configuration. It is executed every time your Mix project or your release boots and is further extensible via config providers. If you want to detect you are inside a release, you can check for release specific environment variables, such asRELEASE_NODE
orRELEASE_MODE
rel/vm.args.eex
andrel/remote.vm.args.eex
- template files that are copied into every release and provides static configuration of the Erlang Virtual Machine and other runtime flags.vm.args
runs onstart
,daemon
, andeval
commands.remote.vm.args
configures the VM forremote
andrpc
commandsrel/env.sh.eex
andrel/env.bat.eex
- template files that are copied into every release and are executed on every command to set up environment variables, including specific ones to the VM, and the general environment
Directory structure
A release is organized as follows:
bin/
RELEASE_NAME
erts-ERTS_VSN/
lib/
APP_NAME-APP_VSN/
ebin/
include/
priv/
releases/
RELEASE_VSN/
consolidated/
elixir
elixir.bat
env.bat
env.sh
iex
iex.bat
remote.vm.args
runtime.exs
start.boot
start.script
start_clean.boot
start_clean.script
sys.config
vm.args
COOKIE
start_erl.data
tmp/
We document this structure for completeness. In practice, developers should not modify any of those files after the release is assembled. Instead use env scripts, custom config provider, overlays, and all other mechanisms described here to configure how your release works.
Environment variables
The system sets different environment variables. The following variables
are set early on and can only be read by env.sh
and env.bat
:
RELEASE_ROOT
- points to the root of the release. If the system includes ERTS, then it is the same as:code.root_dir/0
. This variable is always computed and it cannot be set to a custom valueRELEASE_COMMAND
- the command given to the release, such as"start"
,"remote"
,"eval"
, and so on. This is typically accessed insideenv.sh
andenv.bat
to set different environment variables under different conditions. Note, however, thatRELEASE_COMMAND
has not been validated by the timeenv.sh
andenv.bat
are called, so it may be empty or contain invalid values. This variable is always computed and it cannot be set to a custom valueRELEASE_NAME
- the name of the release. It can be set to a custom value when invoking the releaseRELEASE_VSN
- the version of the release, otherwise the latest version is used. It can be set to a custom value when invoking the release. The custom value must be an existing release version in thereleases/
directoryRELEASE_PROG
- the command line executable used to start the release
The following variables can be set before you invoke the release or
inside env.sh
and env.bat
:
RELEASE_COOKIE
- the release cookie. By default uses the value inreleases/COOKIE
. It can be set to a custom valueRELEASE_NODE
- the release node name, in the formatname
or optionallyname@host
if running in distributed mode. It can be set to a custom value. The name part must be made only of letters, digits, underscores, and hyphensRELEASE_SYS_CONFIG
- the location of the sys.config file. It can be set to a custom path and it must not include the.config
extensionRELEASE_VM_ARGS
- the location of the vm.args file. It can be set to a custom pathRELEASE_REMOTE_VM_ARGS
- the location of the remote.vm.args file. It can be set to a custom pathRELEASE_TMP
- the directory in the release to write temporary files to. It can be set to a custom directory. It defaults to$RELEASE_ROOT/tmp
RELEASE_MODE
- if the release should load code on demand (interactive) or preload it (embedded). Defaults to "embedded", which increases boot time but it means the runtime will respond faster as it doesn't have to load code. Choose interactive if you need to decrease boot time and reduce memory usage on boot. It applies only to start/daemon/install commandsRELEASE_DISTRIBUTION
- how do we want to run the distribution. May bename
(long names),sname
(short names) ornone
(distribution is not started automatically). Defaults tosname
. When connecting nodes across hosts, you typically want to set this toname
(required to use IPs as host names)RELEASE_BOOT_SCRIPT
- the name of the boot script to use when starting the release. This script is used when running commands such asstart
anddaemon
. The boot script is expected to be located at the pathreleases/RELEASE_VSN/RELEASE_BOOT_SCRIPT.boot
. Defaults tostart
RELEASE_BOOT_SCRIPT_CLEAN
- the name of the boot script used when starting the release clean, without your application or its dependencies. This script is used by commands such aseval
,rpc
, andremote
. The boot script is expected to be located at the pathreleases/RELEASE_VSN/RELEASE_BOOT_SCRIPT_CLEAN.boot
. Defaults tostart_clean
Umbrellas
Releases are well integrated with umbrella projects, allowing you to release one or more subsets of your umbrella children. The only difference between performing a release in the umbrella project compared to a regular application is that umbrellas require you to explicitly list your release and the starting point for each release. For example, imagine this umbrella applications:
my_app_umbrella/
apps/
my_app_core/
my_app_event_processing/
my_app_web/
where both my_app_event_processing
and my_app_web
depend on
my_app_core
but they do not depend on each other.
Inside your umbrella, you can define multiple releases:
releases: [
web_and_event_processing: [
applications: [
my_app_event_processing: :permanent,
my_app_web: :permanent
]
],
web_only: [
applications: [my_app_web: :permanent]
],
event_processing_only: [
applications: [my_app_event_processing: :permanent]
]
]
Note you don't need to define all applications in :applications
,
only the entry points. Also remember that the recommended mode
for all applications in the system is :permanent
.
Finally, keep in mind it is not required for you to assemble the release from the umbrella root. You can also assemble the release from each child application individually. Doing it from the root, however, allows you to include two applications that do not depend on each other as part of the same release.
Hot Code Upgrades
Erlang and Elixir are sometimes known for the capability of upgrading a node that is running in production without shutting down that node. However, this feature is not supported out of the box by Elixir releases.
The reason we don't provide hot code upgrades is because they are very complicated to perform in practice, as they require careful coding of your processes and applications as well as extensive testing. Given most teams can use other techniques that are language agnostic to upgrade their systems, such as Blue/Green deployments, Canary deployments, Rolling deployments, and others, hot upgrades are rarely a viable option. Let's understand why.
In a hot code upgrade, you want to update a node from version A to
version B. To do so, the first step is to write recipes for every application
that changed between those two releases, telling exactly how the application
changed between versions, those recipes are called .appup
files.
While some of the steps in building .appup
files can be automated,
not all of them can. Furthermore, each process in the application needs
to be explicitly coded with hot code upgrades in mind. Let's see an example.
Imagine your application has a counter process as a GenServer:
defmodule Counter do
use GenServer
def start_link(_) do
GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
end
def bump do
GenServer.call(__MODULE__, :bump)
end
## Callbacks
def init(:ok) do
{:ok, 0}
end
def handle_call(:bump, counter) do
{:reply, :ok, counter + 1}
end
end
You add this process as part of your supervision tree and ship version
0.1.0 of your system. Now let's imagine that on version 0.2.0 you added
two changes: instead of bump/0
, that always increments the counter by
one, you introduce bump/1
that passes the exact value to bump the
counter. You also change the state, because you want to store the maximum
bump value:
defmodule Counter do
use GenServer
def start_link(_) do
GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
end
def bump(by) do
GenServer.call(__MODULE__, {:bump, by})
end
## Callbacks
def init(:ok) do
{:ok, {0, 0}}
end
def handle_call({:bump, by}, {counter, max}) do
{:reply, :ok, {counter + by, max(max, by)}}
end
end
If you were to perform a hot code upgrade in such an application, it would
crash, because in the initial version the state was just a counter
but in the new version the state is a tuple. Furthermore, you changed
the format of the call
message from :bump
to {:bump, by}
and
the process may have both old and new messages temporarily mixed, so
we need to handle both. The final version would be:
defmodule Counter do
use GenServer
def start_link(_) do
GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
end
def bump(by) do
GenServer.call(__MODULE__, {:bump, by})
end
## Callbacks
def init(:ok) do
{:ok, {0, 0}}
end
def handle_call(:bump, {counter, max}) do
{:reply, :ok, {counter + 1, max(max, 1)}}
end
def handle_call({:bump, by}, {counter, max}) do
{:reply, :ok, {counter + by, max(max, by)}}
end
def code_change(_, counter, _) do
{:ok, {counter, 0}}
end
end
Now you can proceed to list this process in the .appup
file and
hot code upgrade it. This is one of the many steps necessary
to perform hot code upgrades and it must be taken into account by
every process and application being upgraded in the system.
The .appup
cookbook
provides a good reference and more examples.
Once .appup
s are created, the next step is to create a .relup
file with all instructions necessary to update the release itself.
Erlang documentation does provide a chapter on
Creating and upgrading a target system.
Learn You Some Erlang has a chapter on hot code upgrades.
Overall, there are many steps, complexities and assumptions made
during hot code upgrades, which is ultimately why they are not
provided by Elixir out of the box. However, hot code upgrades can
still be achieved by teams who desire to implement those steps
on top of mix release
in their projects or as separate libraries.
Command line options
--force
- forces recompilation--no-archives-check
- does not check archive--no-deps-check
- does not check dependencies--no-elixir-version-check
- does not check Elixir version--no-compile
- does not compile before assembling the release--overwrite
- overwrite existing files instead of prompting the user for action--path
- the path of the release--quiet
- does not write progress to the standard output--version
- the version of the release