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Lists, Durations, and Dictionaries in Configs

Introduction

The actor_system_config with its CLI and INI parsers exist because CAF needs powerful and flexible ways to tweak CAF applications without recompilations. Configuring the applications themselves is also easy to achieve since the configuration is easily extensible. However, the current implementation lacks a few basic types and additional features would greatly improve its usefulness.

Motivation

This proposal discusses improvements to CAF's configuration mechanism in various categories.

Missing Duration Type

Configurations commonly include time intervals. CAF exposes the polling intervals of the scheduler via its config, for example. However, durations are not treated as first-class entity. As a result, users have to pick a resolution (milliseconds, for example) and read integers from the config.

Data Structures

Recursive config types are unsupported in CAF. Any key/value pair in the INI file creates a pair of string (key) and config_value (value). The latter is defined as variant<string, double, int64_t, bool, atom_value>. This building block lacks any support for data structures like lists and maps. An undocumented feature of CAF's parsers already enables lists, albeit very inelegantly. This example illustrates a custom configuration with a list of values:

struct config : actor_system_config {
  std::vector<string> hosts;
  config() {
    opt_group{custom_options_, "global"}
    .add(hosts, "hosts,h", "set hosts");
  }
};

Now, each time the value is set using an CLI parameter CAF actually appends a value to the list (an undocumented feature). For example, ./app -h host1 -h host2 -h host3 will fill the list with the strings "host1", "host2", and "host3". This behavior is counterintuitive. Even more so when doing the same per config file:

[global]
hosts="host1"
hosts="host2"
hosts="host3"

CAF happily adds each entry to the list, but that's a "feature" (hack, really) of the config reader. The CLI and INI parsers are completely unaware of lists, let alone maps. Maps could be easily supported by adding pairs and lists.

Finally, reading a dictionary (i.e., std::map::<std::string, config_value>) from the config is not supported at all. While the configuration itself is organized in this way, there is no abstraction in place for allowing users to re-use this structure.

Error Handling and Dynamic Configuration Parameters

The actor system config drops all unknown parameters, printing error messages for unrecognized options. This has several downsides:

  1. There is no generic way to access the whole configuration. For example, simply iterating the configuration as-is with proper type information is not possible. One can iterate the set of all options, but that isn't straightforward.
  2. Actors can only access the configuration as actor_system_config via self->system().config(). This means users have no easy access to any option specified in a subtype (unless using dynamic_cast and friends).
  3. Fully dynamic parsing of the received config is simply impossible because all "unknown" options are dropped. The only supported way to read a config is by subtyping actor_system_config and filling custom_options_.

Having a map<string, map<string, config_value>> (i.e. mapping groups to the key/value pairs for options) in the actor_system_config for storing all parameters given by the user (or present by default) would pave the way to give users access to a "raw" representation of the whole system configuration.

The one case where the configuration should drop configuration values with an error message is when trying to override a value with a different type. The following example should raise an error:

[scheduler]
max-threads="foo" ; type mismatch: trying to override an integer with a string

Proposed Solution

The parsers should receive first-class support for lists, durations, and dictionaries.

Data Structures and Missing Duration Type

This is the current definition of the config_value building block:

using config_value = variant<string, double, int64_t, bool, atom_value>;

The variant should include caf::timespan, lists, and dictionaries:

using config_value = variant<string, double, int64_t, bool, atom_value,
                             timespan, vector<config_value>,
                             map<string, config_value>>;

(Note: the above is pseudo-code and not legal C++.)

The parses should accept the following syntax:

  • key=true is a boolean
  • key=1 is an integer
  • key=1.0 is an floating point number
  • key=1ms is an timespan
  • key='foo' is an atom
  • key="foo" is a string
  • key=[0, 1, ...] is as a list
  • key={a=1, b=2, ...} is a dictionary

Additional State in actor_system_config

The system config should behave like a map<string, map<string, config_value>> and store all user-defined values. Further, the config class should provide functions for conveniently looking up values. In particular, a get_or function can greatly improve user experience. For example, auto has_udp = get_or(cfg, "middleman.enable-udp", false) either returns a user-defined value for the key middleman.enable-udp or false if 1) the user did not provide a value or 2) the config contains a non-boolean value for the key.

Impact on Existing Code

  1. The config_value type becomes more complex.
  2. INI and CLI parsers need to be updated and improved.
  3. The class actor_system_config gets more state and convenience functions.

Alternatives

Lists already have quirky support. Building on this would avoid complexity in the parsers. However, the long term inconvenience for users outweighs any short-term benefits of not updating the parsers.

Alternative to the proposed syntax changes could be considered. For example, PHP's INI parser accepts the following notation for lists and maps:

[somegroup]
somelist[] = "a"
somelist[] = "b"
somelist[] = "c"
somelist[] = "d"
urls[svn] = "http://svn.php.net"
urls[git] = "http://git.php.net"

This style of list support is easy to add on top of the current undocumented behavior. The corresponding syntax using our proposed solution reads:

[somegroup]
somelist = ["a", "b", "c", "d"]
urls = {
  svn = "http://svn.php.net",
  git = "http://git.php.net",
}

The proposed solution is less redundant and more declarative.

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