Configuring Actor Applications

CAF configures applications at startup using an actor_system_config or a user-defined subclass of that type. The config objects allow users to add custom types, to load modules (optional components), and to fine-tune the behavior of loaded modules with command line options or configuration files (see Command Line Options and INI Configuration Files).

The following code example is a minimal CAF application with a Middleman but without any custom configuration options.

void caf_main(actor_system& system) {
  // ...

The compiler expands this example code to the following.

void caf_main(actor_system& system) {
  // ...
int main(int argc, char** argv) {
  return exec_main<io::middleman>(caf_main, argc, argv);

The function exec_main performs several steps:

  1. Initialize all meta objects for the type ID blocks listed in CAF_MAIN.
  2. Create a config object. If caf_main has two arguments, then CAF assumes that the second argument is the configuration and the type gets derived from that argument. Otherwise, CAF uses actor_system_config.
  3. Parse command line arguments and configuration file (if present).
  4. Load all modules requested in CAF_MAIN.
  5. Create an actor system.
  6. Call caf_main with the actor system and optionally with config.

When implementing the steps performed by CAF_MAIN by hand, the main function would resemble the following (pseudo) code:

int main(int argc, char** argv) {
  // Create the config.
  actor_system_config cfg;
  // Add runtime-type information for user-defined types.
  // Read CLI options.
  if (auto err = cfg.parse(argc, argv)) {
    std::cerr << "error while parsing CLI and file options: "
              << to_string(err) << std::endl;
    return EXIT_FAILURE;
  // Return immediately if a help text was printed.
  if (cfg.cli_helptext_printed)
    return EXIT_SUCCESS;
  // Load modules.
  // Create the actor system.
  actor_system sys{cfg};
  // Run user-defined code.
  caf_main(sys, cfg);
  return EXIT_SUCCESS;

Using CAF_MAIN simply automates that boilerplate code. A minimal example with a custom type ID block as well as a custom configuration class with the I/O module loaded looks as follows:

CAF_BEGIN_TYPE_ID_BLOCK(my, first_custom_type_id)

  // ...


class my_config : public actor_system_config {
  my_config() {
    // ...

void caf_main(actor_system& system, const my_config& cfg) {
  // ...

CAF_MAIN(id_block::my, io::middleman)


Using type ID blocks is optional. Users can also call add_message_type for each user-defined type in the constructor of my_config.

Loading Modules

The simplest way to load modules is to use the macro CAF_MAIN and to pass a list of all requested modules, as shown below.

void caf_main(actor_system& system) {
  // ...
CAF_MAIN(mod1, mod2, ...)

Alternatively, users can load modules in user-defined config classes.

class my_config : public actor_system_config {
  my_config() {
    // ...

The third option is to simply call x.load<mod1>() on a config object before initializing an actor system with it.

Command Line Options and INI Configuration Files

CAF organizes program options in categories and parses CLI arguments as well as INI files. CLI arguments override values in the INI file which override hard-coded defaults. Users can add any number of custom program options by implementing a subtype of actor_system_config. The example below adds three options to the global category.

class config : public actor_system_config {
  uint16_t port = 0;
  std::string host = "localhost";
  bool server_mode = false;

  config() {
    opt_group{custom_options_, "global"}
      .add(port, "port,p", "set port")
      .add(host, "host,H", "set host (ignored in server mode)")
      .add(server_mode, "server-mode,s", "enable server mode");

We create a new global category in custom_options_. Each following call to add then appends individual options to the category. The first argument to add is the associated variable. The second argument is the name for the parameter, optionally suffixed with a comma-separated single-character short name. The short name is only considered for CLI parsing and allows users to abbreviate commonly used option names. The third and final argument to add is a help text.

The custom config class allows end users to set the port for the application to 42 with either -p 42 (short name) or --port=42 (long name). The long option name is prefixed by the category when using a different category than global. For example, adding the port option to the category foo means end users have to type --foo.port=42 when using the long name. Short names are unaffected by the category, but have to be unique.

Boolean options do not require arguments. The member variable server_mode is set to true if the command line contains either --server-mode or -s.

The example uses member variables for capturing user-provided settings for simplicity. However, this is not required. For example, add<bool>(...) allows omitting the first argument entirely. All values of the configuration are accessible with get_or. Note that all global options can omit the "global." prefix.

CAF adds the program options help (with short names -h and -?) as well as long-help to the global category.

The default name for the INI file is caf-application.ini. Users can change the file name and path by passing --config-file=<path> on the command line.

INI files are organized in categories. No value is allowed outside of a category (no implicit global category). The parses uses the following syntax:

Syntax Type
key=true Boolean
key=1 Integer
key=1.0 Floating point number
key=1ms Timespan
key='foo' Atom
key="foo" String
key=[0, 1, ...] List
key={a=1, b=2, ...} Dictionary (map)

The following example INI file lists all standard options in CAF and their default value. Note that some options such as scheduler.max-threads are usually detected at runtime and thus have no hard-coded default.

; This file shows all possible parameters with defaults.
; Values enclosed in <> are detected at runtime unless defined by the user.

; when using the default scheduler
; accepted alternative: 'sharing'
; configures whether the scheduler generates profiling output
; forces a fixed number of threads if set
max-threads=<number of cores>
; maximum number of messages actors can consume in one run
; measurement resolution in milliseconds (only if profiling is enabled)
; output file for profiler data (only if profiling is enabled)

; when using 'stealing' as scheduler policy
; number of zero-sleep-interval polling attempts
; frequency of steal attempts during aggressive polling
; number of moderately aggressive polling attempts
; frequency of steal attempts during moderate polling
; sleep interval between poll attempts
; frequency of steal attempts during relaxed polling
; sleep interval between poll attempts

; when loading io::middleman
; configures whether MMs try to span a full mesh
; application identifier of this node, prevents connection to other CAF
; instances with different identifier
; maximum number of consecutive I/O reads per broker
; heartbeat message interval in ms (0 disables heartbeating)
; configures whether the MM attaches its internal utility actors to the
; scheduler instead of dedicating individual threads (needed only for
; deterministic testing)
; configures whether the MM starts a background thread for I/O activity,
; setting this to true allows fully deterministic execution in unit test and
; requires the user to trigger I/O manually
; disables communication via TCP
; enable communication via UDP
; configures how many background workers are spawned for deserialization,
; by default CAF uses 1-4 workers depending on the number of cores
workers=<min(3, number of cores / 4) + 1>

; when compiling with logging enabled
; file name template for output log file files (empty string disables logging)
; format for rendering individual log file entries
file-format="%r %c %p %a %t %C %M %F:%L %m%n"
; configures the minimum severity of messages that are written to the log file
; (quiet|error|warning|info|debug|trace)
; mode for console log output generation (none|colored|uncolored)
; format for printing individual log entries to the console
; configures the minimum severity of messages that are written to the console
; (quiet|error|warning|info|debug|trace)
; excludes listed components from logging (list of atoms)

Adding Custom Message Types

CAF requires serialization support for all of its message types (see Type Inspection). In addition, CAF also needs a mapping of unique type names to user-defined types at runtime. This is required to deserialize arbitrary messages from the network.

The function actor_system_config::add_message_type adds runtime-type information for a single type. It takes a template parameter (the message type) and one function argument (the type name). For example, cfg.add_message_type<foo>("foo") would add runtime-type information for the type foo. However, calling add_message_type for each type individually is both verbose and prone to error.

For new code, we strongly recommend using the new type ID blocks. When setting the CMake option CAF_ENABLE_TYPE_ID_CHECKS to ON (or calling the configure script with --enable-type-id-checks), CAF raises a static assertion that prohibits any message type that does not appear in such a type ID block. When using this API, users can instead call add_message_types once per message block. Combined with the type ID checks, this makes sure that the runtime-type information never runs out of sync when adding new message types.

Passing the type ID blocks to CAF_MAIN also automates the setup steps for adding new message types.

A type ID block in CAF starts by calling CAF_BEGIN_TYPE_ID_BLOCK. Inside the block appear any number of CAF_ADD_TYPE_ID and CAF_ADD_ATOM statements. The type ID block only requires forward declarations. The block ends at CAF_END_TYPE_ID_BLOCK, as shown in the example below.

struct foo;
struct foo2;

CAF_BEGIN_TYPE_ID_BLOCK(custom_types_1, first_custom_type_id)

  CAF_ADD_TYPE_ID(custom_types_1, (foo))
  CAF_ADD_TYPE_ID(custom_types_1, (foo2))
  CAF_ADD_TYPE_ID(custom_types_1, (std::pair<int32_t, int32_t>) )



The second argument to CAF_ADD_TYPE_ID (the type) must appear in extra parentheses. This unusual syntax is an artifact of argument handling in macros. Without the extra set of parentheses, we would not be able to add types with commas such as std::pair.

The first argument to all macros is the name of the type ID block. The macro expands a name X to caf::id_block::X. In the example above, we can refer to the custom type ID block with caf::id_block::custom_types_1. To add the required runtime-type information to CAF, we can either call cfg.add_message_types<caf::id_block::custom_types_1>() on a config object pass the ID block to CAF_MAIN:



At the point of calling CAF_MAIN or add_message_types, the compiler must have the type declaration plus all inspect overloads available for each type in the type ID block.

Adding Custom Actor Types experimental

Adding actor types to the configuration allows users to spawn actors by their name. In particular, this enables spawning of actors on a different node (see Remote Spawning of Actors experimental). For our example configuration, we consider the following simple calculator actor.

using calculator = caf::typed_actor<
  caf::replies_to<caf::add_atom, int32_t, int32_t>::with<int32_t>,
  caf::replies_to<caf::sub_atom, int32_t, int32_t>::with<int32_t>>;

Adding the calculator actor type to our config is achieved by calling add_actor_type. After calling this in our config, we can spawn the calculator anywhere in the distributed actor system (assuming all nodes use the same config). Note that the handle type still requires a type ID (see Adding Custom Message Types).

After adding the actor type to the config, we can spawn our calculator by name. Unlike the regular spawn overloads, this version requires wrapping the constructor arguments into a message and the function might fail and thus returns an expected:

if (auto x = system.spawn<calculator>("calculator", make_message())) {
  //  ... do something with *x ...
} else {
  std::cerr << "*** unable to spawn a calculator: " << to_string(x.error())
            << std::endl;
  // ...

Adding dynamically typed actors to the config is achieved in the same way. When spawning a dynamically typed actor in this way, the template parameter is simply actor. For example, spawning an actor “foo” which requires one string is created with:

auto worker = system.spawn<actor>("foo", make_message("bar"));

Because constructor (or function) arguments for spawning the actor are stored in a message, only actors with appropriate input types are allowed. Pointer types, for example, are illegal in messages.

Log Output

CAF comes with a logger integrated into the actor system. By default, CAF itself won’t emit any log messages. Developers can set the verbosity of CAF itself at build time by setting the CMake option CAF_LOG_LEVEL manually or by passing --with-log-level=... to the configure script. The available verbosity levels are (from least to most output):

  • error
  • warning
  • info
  • debug
  • trace

The logging infrastructure is always available to users, regardless of the verbosity level of CAF itself.

File Name

The output file is generated from the template configured by logger-file-name. This template supports the following variables.

Variable Output
[PID] The OS-specific process ID.
[TIMESTAMP] The UNIX timestamp on startup.
[NODE] The node ID of the CAF system.


Console output is disabled per default. Setting logger-console to either uncolored or colored prints log events to std::clog. Using the colored option will print the log events in different colors depending on the severity level.

Format Strings

CAF uses log4j-like format strings for configuring printing of individual events via logger-file-format and logger-console-format. Note that format modifiers are not supported at the moment. The recognized field identifiers are:

Pattern Output
%c The category/component.
%C The full qualifier of the current function. For example, the function void ns::foo::bar() would print
%d The date in ISO 8601 format, i.e., "YYYY-MM-DDThh:mm:ss".
%F The file name.
%L The line number.
%m The user-defined log message.
%M The name of the current function. For example, the name of void ns::foo::bar() is printed as bar.
%n A newline.
%p The priority (severity level).
%r Elapsed time since starting the application in milliseconds.
%t ID of the current thread.
%a ID of the current actor (or actor0 when not logging inside an actor).
%% A single percent sign.


The two configuration options logger.component-blacklist and logger.(file|console)-verbosity reduce the amount of generated log events. The former is a list of excluded component names and the latter can increase the reported severity level (but not decrease it beyond the level defined at compile time).