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Krabby

A simple HTTP/WebSocket Lua scriptable webserver written in modern C++.

Dependencies

Most of the dependencies are in git submodules under the lib directory. To fetch Krabby with all its dependencies do this:

$ git clone https://github.com/godexsoft/krabby.git --recurse-submodules

You will also need SQLite3 development package and Lua5.1 or newer with its development package installed on your system. Krabby will use whatever Lua you have available (through sol3).

For example, on Ubuntu:

$ sudo apt-get install lua5.1-dev
$ sudo apt-get install libsqlite3-dev

CMake is used for generation of your preferred build files. By default it will generate a Makefile. You will need CMake 3.15 or newer in order to succesfully configure Krabby.

Compile

$ mkdir build
$ cd build
$ cmake -DENABLE_LOG=ON -DCRAB_TLS=ON ..
$ make

NOTE:: CRAB_TLS option is only needed if you wish to include SSL support for the client request. For it to work you need to have OpenSSL installed.

NOTE:: You can specify the root of your OpenSSL installation (for MacOSX with brew for example): -DOPENSSL_ROOT_DIR=/usr/local/opt/openssl

Usage:

See examples/scripts directory for Lua code.

Krabby accepts a data root directory as first (optional) argument. It will load all the .lua scripts it can find recursively.

$ krabby -h
$ krabby path/to/data/root

NOTE: Krabby will use current directory as data root if no path is specified.

Docker

Docker image is available at https://hub.docker.com/r/godexsoft/krabby

$ docker run -d --rm -v /path/to/data/root:/data:rw -p 8080:8080 --name krabby godexsoft/krabby:0.0.4

API

JSON

JSON manipulation is supported via the json type:

local data = json.new()      
local other = json.parse("{'some':'thing'}")

data:str("strParam", "some string")
data:int("intParam", 420)
data:dbl("doubleParam", 42.0)
data:bool("boolParam", true)
data:obj("objOrArray", other)

-- note: same goes for str, int, dbl and bool
local otherCopy = data:obj("objOrArray")

Templates

Output produced by Krabby can be generated using Inja templates by passing a filepath and a JSON object with data:

local output = template:render_file("templates/index.j2", data)    
print(output)

Timers

Timers can be set to fire like so:

local t = timer.new(
    function()
        print("timer fired")
    end)

t:once(1.23)

Key-Value Storage

Krabby creates and maintains an sqlite3 database which can be used as a high-efficient key-value storage. It operates on strings, arrays of strings (string_vector) or JSON objects.

Example using JSON directly:

-- loads a JSON array with fallback to empty array
local records = storage:load("your_key", json.array())

local data = json.new()
data:str("name", "Krabby")
data:int("age", 420)

-- add a new record and save it back to storage
records:push_back(data)
storage:save("your_key", records)

Example using string_vector:

local data = storage:load("your key", string_vector.new())
local new_user = json.new()
new_user:str("name", name)

local new_key = generate_key(16)        
storage:save(new_key, new_user) -- save JSON object under key
        
data:push_back(new_key)
storage:save("list key", data)

Removing data:

-- `user_key` is a string key for an existing user
storage:remove(user_key) -- remove the JSON object itself
storage:remove("user list key", user_key) -- remove it from a string_vector list of users

Live Reload/Recompile

Krabby is maintaining two contexts, one master and one staging context. You can reload the staging context at any time by using Reload() anywhere in your Lua code. This will rediscover and recompile all lua scripts under data root path and if everything compiles fine it will swap the current master context with the newly created staging:

local output = Reload()
if #output > 0 then
    print("compilation errors", output)
end

Basic HTTP

Writing Responses

Once you will learn about routes below you will know how to apply these functions, but for now here is what is available for writing data back to clients:

respond(who, 200, "text/html", "<html><body>plain html data</body></html>")
respond_html(who, 404, "<p>wrapped html 404 page</p>")
respond_text(who, 401, "not authorized text here")
respond_msg(who, "arbitrary text data for websocket")
Mountpoints

Mountpoints are used to expose static content at a given location on the filesystem. The paths are relative to data root specified at startup.

To expose data/root/path/public at /public/* on your server you can do this:

Mount( "/public", "public" )
Routes

Routes can be used to setup dynamic endpoints. For example to make a RESTful API. Supported routes are

  • Get
  • Post
  • Delete
  • Put
  • Patch

The following example sets up a route at /krabby/[3-16 characters long string without spaces] that will be expecting par1 and par2 to be passed in the query string (i.e. localhost:8080/krabby/hello?par1=first&par2=second). If the parameters are not passed Krabby will automatically return an error page. All the parameters passed to this route will end up in params regardless of them being required or not. The matches table will contain all the matches for the route path regular expression.

Get( "/krabby/(\\w{3,16})", {"par1", "par2"},
    function(who, req, matches, params)
        -- if this was called via 'localhost:8080/krabby/hello?par1=first&par2=second'
        -- matches[1] == "/krabby/hello"
        -- matches[2] == "hello"
        -- params["par1"] == "first"
        -- params["par2"] == "second"
    end )

Note: the route path does not have to be a regular expression

Note: if you don't require any parameters just pass {} for the list.

WebSockets

WebSocket communication is possible in Krabby. The upgrade function upgrades a HTTP socket to WebSocket. It accepts two functions as its arguments: a onMessage callback and a onDisconnect callback. Here is an example of the server script for a WebSocket API:

Get( "/api", {},
    function(who, req, matches, params)
    who:upgrade(
        function(msg)
            respond_msg(who, "hello") -- send a message                
            return true -- you need to return true to keep the connection alive                
        end,
        function()
            print("web socket closed")
        end )
    end )

Note: Proper documentation will be written eventually.

Handling Disconnect

When a connection is established via a route it's expected that your code does one of the following:

  • write a response with one of the functions listed in the corresponding section
  • call upgrade to initiate WebSocket communication
  • call postpone_response if you need more time to generate a response (e.g. via timer or a client request)

NOTE: If none of the above was done the client will automatically receive a Not found response.

The function that was setup with postpone_response on its respective who will be called on disconnect:

who:postpone_response(
    function()
        -- code to handle disconnect of who
    end )

Client Requests

You can also fetch data from a remote server using ClientGet. This works for both http and https:

handle = ClientGet("https://yourhost.com/api.json", 
    function(resp) -- called on response                
        local data = json.parse(resp.body)        
        local output = template:render_file("templates/mytemplate.j2", data)
        respond(who, 200, "text/html", output)
    end, 
    function(err) -- called on error
        respond_html(who, 500, "Could not query api: "..err)        
    end )

-- to cancel the request:
handle:cancel()

Note: The request will automatically get destroyed with no callbacks if it is not saved in a variable/table.

Note: See examples/scripts/http_request.lua for a more detailed example.

Utils

There are a few utils included with Krabby

  • generate_key(size) - generates a size long random alphanumeric key
  • hash_sha1(str) - computes sha1 for a string (raw bytes)
  • hmac_sha1(str, secret) - computes hmac-sha1 for a string with a shared secret
  • string_to_hex(bytes) - computes a hex string for input bytes
  • escape_html(html) - escapes html string

Note: See examples/scripts/utils.lua for usage examples.

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