diff --git a/src/filters/filter.h b/src/filters/filter.h
index 5f921ea..1d7ab1d 100644
--- a/src/filters/filter.h
+++ b/src/filters/filter.h
@@ -15,18 +15,138 @@
 
 #include "../strlcpy.h"
 
+/**
+ * @file
+ * This header describes module API of type 'filter'
+ *
+ * Sample workflow of module usage:
+ *
+ * @msc
+ *   f2b, filter;
+ *   f2b =>  filter [label="create(id)"];
+ *   f2b <<  filter [label="module handler, cfg_t *cfg"];
+ *       |||;
+ *   f2b =>  filter [label="config(cfg, param, value)"];
+ *   f2b <<  filter [label="true"];
+ *   f2b =>  filter [label="config(cfg, param, value)"];
+ *   f2b <<  filter [label="true"];
+ *   f2b =>  filter [label="config(cfg, param, value)"];
+ *   f2b <<  filter [label="false"];
+ *   f2b =>  filter [label="error(cfg)"];
+ *   f2b <<  filter [label="const char *error"];
+ *       ---        [label="filter is ready for append()"];
+ *   f2b =>  filter [label="append(cfg, pattern)"];
+ *   f2b <<  filter [label="true"];
+ *   f2b =>  filter [label="append(cfg, pattern)"];
+ *   f2b <<  filter [label="false"];
+ *   f2b =>  filter [label="error(cfg)"];
+ *   f2b <<  filter [label="const char *error"];
+ *       |||        [label="more calls of append()"];
+ *   f2b =>  filter [label="ready(cfg)"];
+ *   f2b <<  filter [label="true"];
+ *       ---        [label="filter is ready to use"];
+ *   f2b =>  filter [label="match(cfg, line, buf, sizeof(buf))"];
+ *   f2b <<  filter [label="false"];
+ *       ...        [label="no match"];
+ *   f2b =>  filter [label="match(cfg, line, buf, sizeof(buf))"];
+ *   f2b <<  filter [label="true"];
+ *       ...        [label="match found, buf filled with ipaddr"];
+ *   f2b >>  filter [label="stats(cfg, &matches, &pattern, true)"];
+ *   f2b <<  filter [label="true"];
+ *   f2b >>  filter [label="stats(cfg, &matches, &pattern, false)"];
+ *   f2b <<  filter [label="true"];
+ *   f2b >>  filter [label="stats(cfg, &matches, &pattern, false)"];
+ *   f2b <<  filter [label="false"];
+ *       ...        [label="no more stats"];
+ *   f2b =>  filter [label="flush(cfg)"];
+ *   f2b <<  filter [label="true"];
+ *       ---        [label="now you may config(), append() or destroy() filter"];
+ *   f2b =>  filter [label="destroy(cfg)"];
+ * @endmsc
+ */
+
+/**
+ * @def ID_MAX
+ * Maximum length of name for usage in @a start()
+ */
 #define ID_MAX 32
+/**
+ * @def PATTERN_MAX
+ * Maximum length of regex
+ */
 #define PATTERN_MAX 256
+/**
+ * @def HOST_TOKEN
+ * Use this string in place where to search for ip address
+ */
 #define HOST_TOKEN "<HOST>"
 
+/**
+ * Opaque module handler, contains module internal structs
+ */
 typedef struct _config cfg_t;
 
+/**
+ * @brief Create instance of module
+ * @param id Module name string
+ * @returns Opaque module handler or NULL on failure
+ */
 extern cfg_t *create(const char *id);
-extern const char *error(cfg_t *c);
-extern bool   config(cfg_t *c, const char *key, const char *value);
-extern bool   append(cfg_t *c, const char *pattern);
-extern bool    ready(cfg_t *c);
-extern bool    stats(cfg_t *c, int *matches, char **pattern, bool reset);
-extern bool    match(cfg_t *c, const char *line, char *buf, size_t bufsize);
-extern void    flush(cfg_t *c);
-extern void  destroy(cfg_t *c);
+/**
+ * @brief Returns last error description
+ * @param cfg Module handler
+ * @returns Pointer to string with description of last error
+ * @note Returned pointer not marked with const, because libdl complains,
+ *       but contents on pointer should not be modified or written in any way
+ */
+extern const char *error(cfg_t *cfg);
+/**
+ * @brief Contigure module instance
+ * @param cfg Module handler
+ * @param key Parameter name
+ * @param value Parameter value
+ * @returns true on success, false on error with setting intenal error buffer
+ */
+extern bool   config(cfg_t *cfg, const char *key, const char *value);
+/**
+ * @brief Add match pattern
+ * @param cfg Module handler
+ * @param pattern Regex expression
+ * @returns true on success, false on error with setting intenal error buffer
+ */
+extern bool   append(cfg_t *cfg, const char *pattern);
+/**
+ * @brief Checks is module ready for usage
+ * @param cfg Module handler
+ * @returns true if ready, false if not
+ */
+extern bool    ready(cfg_t *cfg);
+/**
+ * @brief Fetch match stats by-pattern
+ * @param cfg Module handler
+ * @param matches Pointer to storage for matches count
+ * @param pattern Associated pattern
+ * @param reset Reset to start of statistics (for later calls)
+ * @returns false on no more stats or true otherwise with filling @a matches and @a pattern
+ */
+extern bool    stats(cfg_t *cfg, int *matches, char **pattern, bool reset);
+/**
+ * @brief Match given line against configured regexps
+ * @param cfg Module handler
+ * @param line Source line of data
+ * @param buf Buffer for storing match result
+ * @param bufsize Size of buffer above
+ * @returns false if no match or true otherwise with filling @a buf
+ */
+extern bool    match(cfg_t *cfg, const char *line, char *buf, size_t bufsize);
+/**
+ * @brief Destroy all added patterns and free it's resources
+ * @param cfg Module handler
+ */
+extern void    flush(cfg_t *cfg);
+/**
+ * @brief Free module handle
+ * @param cfg Module handler
+ * @note Module handler becomes invalid after calling this function on it
+ */
+extern void  destroy(cfg_t *cfg);