# Modules API reference

## Supported languages

Currently modules written in C and Lua(JIT) are supported.

## The anatomy of an extension

A module is a shared object or script defining specific functions/fields; here’s an overview.

C | Lua | Params | Comment
--- | --- | --- | ---
`X_api()` [1] |  |  | API version
`X_init()` | `X.init()` | `module` | Constructor
`X_deinit()` | `X.deinit()` | `module` | Destructor
`X_config()` | `X.config()` | `module, str` | Configuration
`X_layer` | `X.layer` |  | Module layer
`X_props` |  |  | List of properties

[1]

Mandatory symbol; defined by using `KR_MODULE_EXPORT()`.

The `X` corresponds to the module name; if the module name is `hints`, the prefix for constructor would be `hints_init()`. More details are in docs for the `kr_module` and `kr_layer_api` structures.

Note

The modules get ordered – by default in the same as the order in which they were loaded. The loading command can specify where in the order the module should be positioned.

## Writing a module in Lua

The probably most convenient way of writing modules is Lua since you can use already installed modules from system and have first-class access to the scripting engine. You can also tap to all the events, that the C API has access to, but keep in mind that transitioning from the C to Lua function is slower than the other way round, especially when JIT-compilation is taken into account.

Note

The Lua functions retrieve an additional first parameter compared to the C counterparts - a “state”. Most useful C functions and structures have lua FFI wrappers, sometimes with extra sugar.

The modules follow the Lua way, where the module interface is returned in a named table.

    --- @module Count incoming queries
    local counter = {}

    function counter.init(module)
            counter.total = 0
            counter.last = 0
            counter.failed = 0
    end

    function counter.deinit(module)
            print('counted', counter.total, 'queries')
    end

    -- @function Run the q/s counter with given interval.
    function counter.config(conf)
            -- We can use the scripting facilities here
            if counter.ev then event.cancel(counter.ev)
            event.recurrent(conf.interval, function ()
                    print(counter.total - counter.last, 'q/s')
                    counter.last = counter.total
            end)
    end

    return counter

The created module can be then loaded just like any other module, except it isn’t very useful since it doesn’t provide any layer to capture events. The Lua module can however provide a processing layer, just like its C counterpart.

    -- Notice it isn't a function, but a table of functions
    counter.layer = {
            begin = function (state, data)
                            counter.total = counter.total + 1
                            return state
                    end,
            finish = function (state, req, answer)
                            if state == kres.FAIL then
                                    counter.failed = counter.failed + 1
                            end
                            return state
                    end
    }

There is currently an additional “feature” in comparison to C layer functions: some functions do not get called at all if `state == kres.FAIL`; see docs for details: `kr_layer_api`.

Since the modules are like any other Lua modules, you can interact with them through the CLI and and any interface.

Tip

Module discovery: `kres_modules.` is prepended to the module name and lua search path is used on that.

## Writing a module in C

As almost all the functions are optional, the minimal module looks like this:

    #include "lib/module.h"
    /* Convenience macro to declare module ABI. */
    KR_MODULE_EXPORT(mymodule)

Let’s define an observer thread for the module as well. It’s going to be stub for the sake of brevity, but you can for example create a condition, and notify the thread from query processing by declaring module layer (see the Writing layers).

    static void* observe(void *arg)
    {
            /* ... do some observing ... */
    }

    int mymodule_init(struct kr_module *module)
    {
            /* Create a thread and start it in the background. */
            pthread_t thr_id;
            int ret = pthread_create(&thr_id, NULL, &observe, NULL);
            if (ret != 0) {
                    return kr_error(errno);
            }

            /* Keep it in the thread */
            module->data = thr_id;
            return kr_ok();
    }

    int mymodule_deinit(struct kr_module *module)
    {
            /* ... signalize cancellation ... */
            void *res = NULL;
            pthread_t thr_id = (pthread_t) module->data;
            int ret = pthread_join(thr_id, res);
            if (ret != 0) {
                    return kr_error(errno);
            }

            return kr_ok();
    }

This example shows how a module can run in the background, this enables you to, for example, observe and publish data about query resolution.

## Configuring modules

There is a callback `X_config()` that you can implement, see hints module.

## Exposing C module properties

A module can offer NULL-terminated list of properties, each property is essentially a callable with free-form JSON input/output. JSON was chosen as an interchangeable format that doesn’t require any schema beforehand, so you can do two things - query the module properties from external applications or between modules (e.g. statistics module can query cache module for memory usage). JSON was chosen not because it’s the most efficient protocol, but because it’s easy to read and write and interface to outside world.

Note

The `void *env` is a generic module interface. Since we’re implementing daemon modules, the pointer can be cast to `struct engine*`. This is guaranteed by the implemented API version (see Writing a module in C).

Here’s an example how a module can expose its property:

    char* get_size(void *env, struct kr_module *m,
                   const char *args)
    {
            /* Get cache from engine. */
            struct engine *engine = env;
            struct kr_cache *cache = &engine->resolver.cache;
            /* Read item count */
            int count = (cache->api)->count(cache->db);
            char *result = NULL;
            asprintf(&result, "{ \"result\": %d }", count);

            return result;
    }

    struct kr_prop *cache_props(void)
    {
            static struct kr_prop prop_list[] = {
                    /* Callback,   Name,   Description */
                    {&get_size, "get_size", "Return number of records."},
                    {NULL, NULL, NULL}
            };
            return prop_list;
    }

    KR_MODULE_EXPORT(cache)

Once you load the module, you can call the module property from the interactive console. Note: the JSON output will be transparently converted to Lua tables.

    $ kresd
    ...
    [system] started in interactive mode, type 'help()'
    > modules.load('cached')
    > cached.get_size()
    [size] => 53

### Special properties

If the module declares properties `get` or `set`, they can be used in the Lua interpreter as regular tables.