Kernel(Application)

📦
version 29.1.1 · 5 versions
, in arch rolling, nixos 26.05, nixpkgs unstable

🎀 Kernel Application 🎀

okay so bestie, this is the Kernel(Application) manual page — brought to you by Ericsson AB, no cap. It's giving “foundational Erlang/OTP energy,” and we're gonna get through every single detail together. periodt. 💅

🌸 NAME 🌸

Kernel Application — that's literally the name, and yes it's iconic. 🫶

✨ DESCRIPTION ✨

💖 Description 💖

The Kernel application has all the code necessary to run the Erlang runtime system: file servers, code servers, and so on.

The Kernel application is the first application started. It is mandatory in the sense that the minimal system based on Erlang/OTP consists of Kernel and STDLIB. Kernel contains the following functional areas:

  • Start, stop, supervision, configuration, and distribution of applications
  • Code loading
  • Logging
  • Global name service
  • Supervision of Erlang/OTP
  • Communication with sockets
  • Operating system interface

Like… she does a lot. main character energy fr fr. 👑

🧚 Logger Handlers 🧚

Two standard logger handlers are defined in the Kernel application. These are described in the Kernel User's Guide, and in the logger_std_h and logger_disk_log_h manual pages.

🚦 OS Signal Event Handler 🚦

Asynchronous OS signals may be subscribed to via the Kernel application's event manager (see OTP Design Principles and gen_event) registered as erl_signal_server. A default signal handler is installed which handles the following signals:

  • sigusr1 — The default handler will halt Erlang and produce a crashdump with slogan "Received SIGUSR1". This is equivalent to calling erlang:halt("Received SIGUSR1").
  • sigquit — The default handler will halt Erlang immediately. This is equivalent to calling erlang:halt().
  • sigterm — The default handler will terminate Erlang normally. This is equivalent to calling init:stop().

🦋 Events 🦋

Any event handler added to erl_signal_server must handle the following events.

  • sighup — Hangup detected on controlling terminal or death of controlling process
  • sigquit — Quit from keyboard
  • sigabrt — Abort signal from abort
  • sigalrm — Timer signal from alarm
  • sigterm — Termination signal
  • sigusr1 — User-defined signal 1
  • sigusr2 — User-defined signal 2
  • sigchld — Child process stopped or terminated
  • sigstop — Stop process
  • sigtstp — Stop typed at terminal
  • sigcont — Continue after stop
  • sigwinch — Window size change
  • siginfo — Status request from keyboard. Note several operating systems (Linux in particular) do not support this signal. os:set_signal/2 will throw a badarg exception if support is missing.

The way to set OS signals is described in os:set_signal/2. 🛰️

⚙️ Configuration ⚙️

The following configuration parameters are defined for the Kernel application. For more information about configuration parameters, see the app file.

  • connect_all = true | false — If enabled (true), which also is the default, global will actively connect to all nodes that becomes known to it. Note that you also want to enable prevent_overlapping_partitions in order for global to ensure that a fully connected network is maintained. prevent_overlapping_partitions will also prevent inconsistencies in global's name registration and locking.

The now deprecated command line argument -connect_all <boolean> has the same effect as the connect_all configuration parameter. If this configuration parameter is defined, it will override the command line argument.

  • distributed = [Distrib] — Specifies which applications that are distributed and on which nodes they are allowed to execute. In this parameter:
    • Distrib = {App,Nodes} | {App,Time,Nodes}
    • App = atom()
    • Time = integer()>0
    • Nodes = [node() | {node(),...,node()}]

The parameter is described in application:load/2.

  • dist_auto_connect = Value — Specifies when nodes are automatically connected. If this parameter is not specified, a node is always automatically connected, for example, when a message is to be sent to that node. Value is one of:
    • never — Connections are never automatically established, they must be explicitly connected. See net_kernel.
    • once — Connections are established automatically, but only once per node. If a node goes down, it must thereafter be explicitly connected. See net_kernel.
  • epmd_module = module() — Configures the module responsible for communication with epmd. If this parameter is undefined, it defaults to erl_epmd.

The now deprecated command line argument -epmd_module <module> has the same effect as the epmd_module configuration parameter. If this configuration parameter is defined, it will override the command line argument.

  • erl_epmd_node_listen_port = integer() — Configures the port used by erl_epmd to listen for connection and connect to other nodes. If this flag is set, the Erlang VM will boot in distributed mode even if EPMD is not available. If not set, a port is chosen automatically (equivalent to port 0). See erl_epmd for more details.

The now deprecated command line argument erl_epmd_port <module> has the same effect as the erl_epmd_node_listen_port configuration parameter. If this configuration parameter is defined, it will override the command line argument.

  • permissions = [Perm] — Specifies the default permission for applications when they are started. In this parameter:
    • Perm = {ApplName,Bool}
    • ApplName = atom()
    • Bool = boolean()

Permissions are described in application:permit/2.

  • logger = [Config] — Specifies the configuration for Logger, except the primary log level, which is specified with logger_level, and the compatibility with SASL Error Logging, which is specified with logger_sasl_compatible.

The logger parameter is described in section Logging in the Kernel User's Guide.

  • logger_level = Level — Specifies the primary log level for Logger. Log events with the same, or a more severe level, pass through the primary log level check. See section Logging in the Kernel User's Guide for more information about Logger and log levels.

Level = emergency | alert | critical | error | warning | notice | info | debug | all | none

To change the primary log level at runtime, use logger:set_primary_config(level, Level).

Defaults to notice.

  • logger_metadata = Metadata — Specifies primary metadata for log events.

Metadata = map()

Defaults to #{}.

  • logger_sasl_compatible = true | false — Specifies if Logger behaves backwards compatible with the SASL error logging functionality from releases prior to Erlang/OTP 21.0.

If this parameter is set to true, the default Logger handler does not log any progress-, crash-, or supervisor reports. If the SASL application is then started, it adds a Logger handler named sasl, which logs these events according to values of the SASL configuration parameter sasl_error_logger and sasl_errlog_type.

See section Deprecated Error Logger Event Handlers and Configuration in the sasl(6) manual page for information about the SASL configuration parameters.

See section SASL Error Logging in the SASL User's Guide, and section Backwards Compatibility with error_logger in the Kernel User's Guide for information about the SASL error logging functionality, and how Logger can be backwards compatible with this.

Defaults to false.

NOTE: If this parameter is set to true, sasl_errlog_type indicates that progress reports shall be logged, and the configured primary log level is notice or more severe, then SASL automatically sets the primary log level to info. That is, this setting can potentially overwrite the value of the Kernel configuration parameter logger_level. This is to allow progress reports, which have log level info, to be forwarded to the handlers. 😭 important lil caveat bestie.
  • global_groups = [GroupTuple] — Defines global groups, see global_group. In this parameter:
    • GroupTuple = {GroupName, [Node]} | {GroupName, PublishType, [Node]}
    • GroupName = atom()
    • PublishType = normal | hidden
    • Node = node()
  • inet_default_connect_options = [{Opt, Val}] — Specifies default options for connect sockets, see inet.
  • inet_default_listen_options = [{Opt, Val}] — Specifies default options for listen (and accept) sockets, see inet.
  • inet_dist_use_interface = ip_address() — If the host of an Erlang node has many network interfaces, this parameter specifies which one to listen on. For the type definition of ip_address(), see inet.
  • inet_dist_listen_min = First, inet_dist_listen_max = Last — Defines the First..Last port range for the listener socket of a distributed Erlang node.
  • inet_dist_listen_options = Opts — Defines a list of extra socket options to be used when opening the listening socket for a distributed Erlang node. See gen_tcp:listen/2.
  • inet_dist_connect_options = Opts — Defines a list of extra socket options to be used when connecting to other distributed Erlang nodes. See gen_tcp:connect/4.
  • inet_parse_error_log = silent — If set, no log events are issued when erroneous lines are found and skipped in the various Inet configuration files.
  • inetrc = Filename — The name (string) of an Inet user configuration file. For details, see section Inet Configuration in the ERTS User's Guide.
  • net_setuptime = SetupTime — SetupTime must be a positive integer or floating point number, and is interpreted as the maximum allowed time for each network operation during connection setup to another Erlang node. The maximum allowed value is 120. If higher values are specified, 120 is used. Default is 7 seconds if the variable is not specified, or if the value is incorrect (for example, not a number).

Notice that this value does not limit the total connection setup time, but rather each individual network operation during the connection setup and handshake. ⏱️

  • log_missed_net_ticks = true | false — Controls whether a warning is logged for each missed sub-tick on a distribution connection. A sub-tick is missed when no data has been received from a connected node during one tick interval. A warning is emitted on every subsequent missed sub-tick until the node is declared down after net_tickintensity consecutive missed sub-ticks, at which point a final timeout warning is always logged regardless of this setting. Defaults to false.

okay so we are still deep in the config-parameter rabbit hole, and this next batch is giving “important little knobs that keep the distributed node from acting brand new” 💅. Stay with me, bestie — every flag and default is right here.

  • net_ticker_spawn_options = Opts — Defines a list of extra spawn options for net ticker processes. There is one such process for each connection to another node. A net ticker process is responsible for supervising the connection it is associated with. These processes also execute the distribution handshake protocol when setting up connections. When there is a large number of distribution connections, setting up garbage collection options can be helpful to reduce memory usage. Default is [link, {priority, max}], and these two options cannot be changed. The monitor and {monitor, MonitorOpts} options are not allowed and will be dropped if present. See the documentation of the erlang:spawn_opt/4 BIF for information about valid options. If the Opts list is not a proper list, or containing invalid options the setup of connections will fail.

Note that the behavior described above is only true if the distribution carrier protocol used is implemented as described in ERTS User's Guide ➜ How to implement an Alternative Carrier for the Erlang Distribution ➜ Distribution Module without further alterations. The implementer of the distribution carrier protocol used may have chosen to ignore the net_ticker_spawn_options parameter or altered its behavior. Currently all distribution modules shipped with OTP do, however, behave as described above.

  • net_tickintensity = NetTickIntensity — Net tick intensity specifies how many ticks to send during a net tick time period when no other data is sent over a connection to another node. This also determines how often to check for data from the other node. The higher net tick intensity, the closer to the chosen net tick time period the node will detect an unresponsive node. The net tick intensity defaults to 4. The value of NetTickIntensity should be an integer in the range 4..1000. If NetTickIntensity is not an integer or an integer less than 4, 4 will silently be used. If NetTickIntensity is an integer larger than 1000, 1000 will silently be used.
NOTE: Note that all communicating nodes are expected to use the same net tick intensity as well as the same net tick time.
WARNING: Be careful not to set a too high net tick intensity, since you can overwhelm the node with work if it is set too high.
  • net_ticktime = NetTickTime — Specifies the net tick time in seconds. This is the approximate time a connected node may be unresponsive until it is considered down and thereby disconnected.

Net tick time together with net tick intensity determines an interval TickInterval = NetTickTime/NetTickIntensity. Once every TickInterval seconds, each connected node is ticked if nothing has been sent to it during that last TickInterval seconds. A tick is a small package sent on the connection. A connected node is considered to be down if no ticks or payload packages have been received during the last NetTickIntensity number of TickInterval seconds intervals. This ensures that nodes that are not responding, for reasons such as hardware errors, are considered to be down.

As the availability is only checked every TickInterval seconds, the actual time T a node has been unresponsive when detected may vary between MinT and MaxT, where:

MinT = NetTickTime - NetTickTime / NetTickIntensity
MaxT = NetTickTime + NetTickTime / NetTickIntensity

NetTickTime defaults to 60 seconds and NetTickIntensity defaults to 4. Thus, 45 < T < 75 seconds.

NOTE: Notice that all communicating nodes are to have the same NetTickTime and NetTickIntensity values specified, as it determines both the frequency of outgoing ticks and the expected frequency of incoming ticks.

NetTickTime needs to be a multiple of NetTickIntensity. If the configured values are not, NetTickTime will internally be rounded up to the nearest millisecond. net_kernel:get_net_ticktime() will, however, report net tick time truncated to the nearest second.

Normally, a terminating node is detected immediately by the transport protocol (like TCP/IP).

  • prevent_overlapping_partitions = true | false — If enabled (true), global will actively prevent overlapping partitions from forming when connections are lost between nodes. This fix is enabled by default. If you are about to disable this fix, make sure to read the global documentation about this fix for more important information about this.
  • shutdown_timeout = integer() | infinity — Specifies the time application_controller waits for an application to terminate during node shutdown. If the timer expires, application_controller brutally kills application_master of the hanging application. If this parameter is undefined, it defaults to infinity.
  • sync_nodes_mandatory = [NodeName] — Specifies which other nodes must be alive for this node to start properly. If some node in the list does not start within the specified time, this node does not start either. If this parameter is undefined, it defaults to [].
  • sync_nodes_optional = [NodeName] — Specifies which other nodes can be alive for this node to start properly. If some node in this list does not start within the specified time, this node starts anyway. If this parameter is undefined, it defaults to the empty list.
  • sync_nodes_timeout = integer() | infinity — Specifies the time (in milliseconds) that this node waits for the mandatory and optional nodes to start. If this parameter is undefined, no node synchronization is performed. This option ensures that global is synchronized.
  • start_distribution = true | false — Starts all distribution services, such as rpc, global, and net_kernel if the parameter is true. This parameter is to be set to false for systems who want to disable all distribution functionality.

Defaults to true.

  • start_dist_ac = true | false — Starts the dist_ac server if the parameter is true. This parameter is to be set to true for systems using distributed applications.

Defaults to false. If this parameter is undefined, the server is started if parameter distributed is set.

  • start_boot_server = true | false — Starts the boot_server if the parameter is true (see erl_boot_server). This parameter is to be set to true in an embedded system using this service.

Defaults to false.

  • boot_server_slaves = [SlaveIP] — If configuration parameter start_boot_server is true, this parameter can be used to initialize boot_server with a list of slave IP addresses:
SlaveIP = string() | atom | {integer(),integer(),integer(),integer()},

where 0 <= integer() <=255.

Examples of SlaveIP in atom, string, and tuple form:

'150.236.16.70', "150,236,16,70", {150,236,16,70}.

Defaults to [].

  • start_disk_log = true | false — Starts the disk_log_server if the parameter is true (see disk_log). This parameter is to be set to true in an embedded system using this service.

Defaults to false.

  • start_pg = true | false — Starts the default pg scope server (see pg) if the parameter is true. This parameter is to be set to true in an embedded system that uses this service.

Defaults to false.

  • start_timer = true | false — Starts the timer_server if the parameter is true (see timer). This parameter is to be set to true in an embedded system using this service.

Defaults to false.

  • shell_docs_ansi = boolean() — Specifies whether the documentation rendered in the shell should use ANSI escape codes.

See also shell_docs:config/0.

  • shell_history = enabled | disabled | module() — Specifies whether shell history should be logged to disk between usages of erl (enabled), not logged at all (disabled), or a user-specified module will be used to log shell history. This module should export load() -> [string()] returning a list of strings to load in the shell when it starts, and add(iodata()) -> ok. called every time a new line is entered in the shell. By default logging is disabled.
  • shell_history_drop = [string()] — Specific log lines that should not be persisted. For example ["q().", "init:stop()."] will allow ignoring commands that shut the node down. Defaults to [].
  • shell_history_file_bytes = integer() — How many bytes the shell should remember. By default, the value is set to 512kb, and the minimal value is 50kb.
  • shell_history_path = string() — Specifies where the shell history files will be stored. Defaults to the user's cache directory as returned by filename:basedir(user_cache, "erlang-history").
  • shutdown_func = {Mod :: atom(), Func :: atom()} — Sets a function that application_controller calls when it starts to terminate. The function is called as Mod:Func(Reason), where Reason is the terminate reason for application_controller, and it must return as soon as possible for application_controller to terminate properly.
  • source_search_rules = [DirRule] | [SuffixRule] — Where:
    • DirRule = {ObjDirSuffix,SrcDirSuffix}
    • SuffixRule = {ObjSuffix,SrcSuffix,[DirRule]}
    • ObjDirSuffix = string()
    • SrcDirSuffix = string()
    • ObjSuffix = string()
    • SrcSuffix = string()

Specifies a list of rules for use by filelib:find_file/2 filelib:find_source/2 If this is set to some other value than the empty list, it replaces the default rules. Rules can be simple pairs of directory suffixes, such as {"ebin", "src"}, which are used by filelib:find_file/2, or triples specifying separate directory suffix rules depending on file name extensions, for example [{".beam", ".erl", [{"ebin", "src"}]}, which are used by filelib:find_source/2. Both kinds of rules can be mixed in the list.

The interpretation of ObjDirSuffix and SrcDirSuffix is as follows: if the end of the directory name where an object is located matches ObjDirSuffix, then the name created by replacing ObjDirSuffix with SrcDirSuffix is expanded by calling filelib:wildcard/1, and the first regular file found among the matches is the source file.

  • standard_io_encoding = Encoding — Set whether bytes sent or received via standard_io should be interpreted as unicode or latin1. By default input and output is interpreted as Unicode if it is supported on the host. With this flag you may configure the encoding on startup.

This works similarly to io:setopts(standard_io, {encoding, Encoding}) but is applied before any bytes on standard_io may have been read.

Encoding is one of:

  • unicode — Configure standard_io to use unicode mode.
  • latin1 — Configure standard_io to use latin1 mode.

Anything other than unicode or latin1 will be ignored, and the system will configure the encoding by itself — typically unicode on modern systems. No cap, it just handles it 💅

See Escripts and non-interactive I/O in Unicode Usage in Erlang for more details. ✨

  • os_cmd_shell = string() — tells the system which shell to use when invoking system commands via os:cmd/2. By default, the shell is detected automatically. 🐚

🪦 Deprecated Configuration Parameters 🪦

Okay bestie, history lesson: in Erlang/OTP 21.0, a new API for logging was added. The old error_logger event manager, and event handlers running on this manager, still work — but they are no longer used by default. Lowkey vintage, highkey still functional 😭

The following application configuration parameters can still be set, but they are only used if the corresponding configuration parameters for Logger are not set. So like… Logger gets first dibs, periodt.

  • error_logger — replaced by setting the type, and possibly file and modes parameters of the default logger_std_h handler. Example:
erl -kernel logger '[{handler,default,logger_std_h,#{config=>#{file=>"/tmp/erlang.log"}}}]'
  • error_logger_format_depth — replaced by setting the depth parameter of the default handlers formatter. Example:
erl -kernel logger '[{handler,default,logger_std_h,#{formatter=>{logger_formatter,#{legacy_header=>true,template=>[{logger_formatter,header},"\n",msg,"\n"],depth=>10}}}]'

See Backwards compatibility with error_logger for more information. 🫶

k bye bestie! this footer energy is literally source/version kernel 11.0.4, date 6, page Kernel(Application) — iconic, no notes 💅✨

📦 where this page comes from