Modules API reference

Supported languages

Currently modules written in C and LuaJIT are supported. There is also a support for writing modules in Go 1.5+ — the library has no native Go bindings, library is accessible using CGO.

The anatomy of an extension

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

Note — the Modules header documents the module loading and API.

C/Go Lua Params Comment
X_api() [1]     API version
X_init() X.init() module Constructor
X_deinit() X.deinit() module, key Destructor
X_config() X.config() module Configuration
X_layer() X.layer module Module layer
X_props()     List of properties
[1]Mandatory symbol.

The X corresponds to the module name, if the module name is hints, then the prefix for constructor would be hints_init(). This doesn’t apply for Go, as it for now always implements main and requires capitalized first letter in order to export its symbol.


The resolution context struct kr_context holds loaded modules for current context. A module can be registered with kr_context_register(), which triggers module constructor immediately after the load. Module destructor is automatically called when the resolution context closes.

If the module exports a layer implementation, it is automatically discovered by kr_resolver() on resolution init and plugged in. The order in which the modules are registered corresponds to the call order of layers.

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.


The Lua functions retrieve an additional first parameter compared to the C counterparts - a “state”. There is no Lua wrapper for C structures used in the resolution context, until they’re implemented you can inspect the structures using the ffi library.

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) = 0
        counter.last = 0
        counter.failed = 0

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

-- @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.last, 'q/s')
                counter.last =

return counter


The API functions may return an integer value just like in other languages, but they may also return a coroutine that will be continued asynchronously. A good use case for this approach is is a deferred initialization, e.g. loading a chunks of data or waiting for I/O.

function counter.init(module) = 0
        counter.last = 0
        counter.failed = 0
        return coroutine.create(function ()
                for line in io.lines('/etc/hosts') do
                        load(module, line)

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)
               = + 1
                        return state
        finish = function (state, req, answer)
                        if state == kres.FAIL then
                                counter.failed = counter.failed + 1
                        return state

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


The module can be placed anywhere in the Lua search path, in the working directory or in the MODULESDIR.

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 API. */

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.

Writing a module in Go

The Go modules use CGO to interface C resolver library, there are no native bindings yet. Second issue is that layers are declared as a structure of function pointers, which are not present in Go, the workaround is to declare them in CGO header. Each module must be the main package, here’s a minimal example:

package main

#include "lib/module.h"
import "C"
import "unsafe"

/* Mandatory functions */

//export mymodule_api
func mymodule_api() C.uint32_t {
        return C.KR_MODULE_API
func main() {}


Do not forget to prefix function declarations with //export symbol_name, as only these will be exported in module.

In order to integrate with query processing, you have to declare a helper function with function pointers to the the layer implementation. Since the code prefacing import "C" is expanded in headers, you need the static inline trick to avoid multiple declarations. Here’s how the preface looks like:

#include "lib/layer.h"
#include "lib/module.h"
// Need a forward declaration of the function signature
int finish(kr_layer_t *);
// Workaround for layers composition
static inline const kr_layer_api_t *_layer(void)
        static const kr_layer_api_t api = {
                .finish = &finish
        return &api;
import "C"
import "unsafe"

Now we can add the implementations for the finish layer and finalize the module:

//export finish
func finish(ctx *C.kr_layer_t) {
        // Since the context is unsafe.Pointer, we need to cast it
        var param *C.struct_kr_request = (*C.struct_kr_request)(
        // Now we can use the C API as well
        fmt.Printf("[go] resolved %d queries\n", C.list_size(&param.rplan.resolved))
        return 0

//export mymodule_layer
func mymodule_layer(module *C.struct_kr_module) *C.kr_layer_api_t {
        // Wrapping the inline trampoline function
        return C._layer()

See the CGO for more information about type conversions and interoperability between the C/Go.


  • main() function is mandatory in each module, otherwise it won’t compile.
  • Module layer function implementation must be done in C during import "C", as Go doesn’t support pointers to functions.
  • The library doesn’t have a Go-ified bindings yet, so interacting with it requires CGO shims, namely structure traversal and type conversions (strings, numbers).
  • Other modules can be called through C call C.kr_module_call(kr_context, module_name, module_propery, input)

Configuring modules

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

Exposing C/Go 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 (i.e. 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.


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;


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

Note — this relies on function pointers, so the same static inline trick as for the Layer() is required for C/Go.

Special properties

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