How Lua runs in HAProxy
=======================

HAProxy Lua running contexts
----------------------------

The Lua code executed in HAProxy can be processed in 2 main modes. The first one
is the **initialisation mode**, and the second is the **runtime mode**.

* In the **initialisation mode**, we can perform DNS solves, but we cannot
  perform socket I/O. In this initialisation mode, HAProxy still blocked during
  the execution of the Lua program.

* In the **runtime mode**, we cannot perform DNS solves, but we can use sockets.
  The execution of the Lua code is multiplexed with the requests processing, so
  the Lua code seems to be run in blocking, but it is not the case.

The Lua code is loaded in one or more files. These files contains main code and
functions. Lua have 6 execution context.

1. The Lua file **body context**. It is executed during the load of the Lua file
   in the HAProxy `[global]` section with the directive `lua-load`. It is
   executed in initialisation mode. This section is use for configuring Lua
   bindings in HAProxy.

2. The Lua **init context**. It is an Lua function executed just after the
   HAProxy configuration parsing. The execution is in initialisation mode. In
   this context the HAProxy environment are already initialized. It is useful to
   check configuration, or initializing socket connections or tasks. These
   functions are declared in the body context with the Lua function
   `core.register_init()`. The prototype of the function is a simple function
   without return value and without parameters, like this: `function fcn()`.

3. The Lua **task context**. It is an Lua function executed after the start
   of the HAProxy scheduler, and just after the declaration of the task with the
   Lua function `core.register_task()`. This context can be concurrent with the
   traffic processing. It is executed in runtime mode. The prototype of the
   function is a simple function without return value and without parameters,
   like this: `function fcn()`.

4. The **action context**. It is an Lua function conditionally executed. These
   actions are declared by the HAProxy directives "`tcp-request content lua
   <function>`", "`tcp-response content lua <function>`", "`http-request lua
   <function>`" and "`http-response lua <function>`". The prototype of the
   Lua called function is a function with doesn't returns anything and that take
   an object of class TXN as entry. `function fcn(txn)`

5. The **sample-fetch context**. This function takes a TXN object as entry
   argument and returns a string. These types of function cannot execute any
   blocking function. They are useful to aggregate some of original HAProxy
   sample-fetches and return the result. The prototype of the function is
   `function string fcn(txn)`. These functions can be registered with the Lua
   function `core.register_fetches()`. Each declared sample-fetch is prefixed by
   the string "lua.".

   ***NOTE**: It is possible that this function cannot found the required data
   in the original HAProxy sample-fetches, in this case, it cannot return the
   result. This case is not yet supported*

6. The **converter context**. It is an Lua function that takes a string as input
   and returns another string as output. These types of function are stateless,
   it cannot access to any context. They don't execute any blocking function.
   The call prototype is `function string fcn(string)`. This function can be
   registered with the Lua function `core.register_converters()`. Each declared
   converter is prefixed by the string "lua.".

HAProxy Lua Hello world
-----------------------

HAProxy configuration file (`hello_world.conf`):

    global
       lua-load hello_world.lua

    listen proxy
       bind 127.0.0.1:10001
       tcp-request content lua hello_world

HAProxy Lua file (`hello_world.lua`):

    function hello_world(txn)
       local res = txn:res_channel()
       res:send("hello world\n")
    end

How to start HAProxy for testing this configuration:

    ./haproxy -f hello_world.conf

On other terminal, you can test with telnet:

    #:~ telnet 127.0.0.1 10001
    coucou

core class
==========

*The "core" class contains all the HAProxy core functions. These function are
useful for the controlling the execution flow, registering hooks, manipulating
global maps or ACL, ...*

### `core.add_acl(filename, key)`

  **context**: *runtime*, *init* except *main code*

  Add the ACL *key* in the ACLs list referenced by the file *filename*.

### `core.del_acl(filename, key)`

  **context**: *runtime*, *init* except *main code*

  Delete the ACL entry referenced by the key *key* in the list of ACLs
  referenced by *filename*.

### `core.del_map(filename, key)`

  **context**: *runtime*, *init* except *main code*

  Delete the map entry indexed with the key *key* in the list of maps referenced
  by *filename*.

### `core.msleep(int milliseconds)`

  **context**: *runtime* except *sample-fetch* and *converters*

  The core.sleep stop the Lua execution between *milliseconds* argument.

### `core.register_converters(string name, function func)`

  **context**: *init*

  Register an Lua function executed as converter. All the registered converters
  can be used in HAProxy with the prefix "lua.". An converter get a string as
  input and return a string as output. The registered function can take up to 9
  values as parameter. All the value are strings.

  The prototype of the Lua function used as argument is:

    function(object txn, string param1, string param2, ...)

### `core.register_fetches(string name, function func)`

  **context**: *init*

  Register an Lua function executed as sample fetch. All the registered sample
  fetchs can be used in HAProxy with the prefix "lua.". A Lua sample fetch
  return a string as output. The registered function can take up to 9 values as
  parameter. All the value are strings.

  The prototype of the Lua function used as argument is:

    string function(object txn, string param1, string param2, ...)

  lua example code:

    core.register_fetches("hello", function()
        return "hello"
    end)

  HAProxy example configuration:

    frontend example
       http-request redirect location "/%[lua.hello]"

### `core.register_init(function func)`

  **context**: *init*

  Register a function executed after the configuration parsing. This is useful
  to check any parameters.

  The prototype of the Lua function used as argument is:

    function()

### `core.register_task(function func)`

  **context**: *init*, *runtime*

  Register and start independent task. The task is started when the HAProxy
  main scheduler starts. For example this type of tasks can be executed to
  perform complex health checks. 

  The prototype of the Lua function used as argument is:

    function()

### `core.set_nice(int nice)`

  **context**: *runtime*, *init* except *main code*

  Change the nice of the current task or current session. The *nice* must be
  between -1024 and 1024.

### `core.set_map(filename, key, value)`

  **context**: *runtime*, *init* except *main code*

  set the value *value* associated to the key *key* in the map referenced by
  *filename*.

### `core.sleep(int seconds)`

  **context**: *init*, *runtime* except *sample-fetch* and *converters*

  The `core.sleep()` functions stop the Lua execution between *seconds* argument.

### `socket core.tcp()`

  **context**: *runtime*, *init* except *main code*

  This function returns a new object of a *socket* class.

### `socket core.yield()`

  **context**: *runtime*

  Give back the hand at the HAProxy scheduler. It is used when the LUA
  processing consumes a lot of processing time.

txn class
=========

*The txn class contain all the functions relative to the http or tcp
transaction (Note than a tcp stream is the same than a tcp transaction, but an
HTTP transaction is not the same than a tcp stream).*

*The functions found permits to retrieve data from the requests, alter it and
forward it.*

*All the functions provided by this class are available in the converters,
sample-fetches, http-request, http-response and tcp-request.*

### `data txn.get_priv(txn)`

  Return Lua data stored in the current transaction. If no data are stored, it
  returns a nil value.

### `txn.set_priv(txn, data)`

  Store any data in the current HAProxy transaction. This action replace the
  old stored data.

### `txn.<fetch>(txn, ...)`

  The transaction class (txn) contains a function for each HAProxy native
  sample-fetch. The list of usable sample-fetches and their usage are available
  in the HAProxy main documention.

### `txn.get_headers(txn)`

  This function returns an array of headers.

### `channel txn.get_req_buf(txn)`

  This function returns an object able to manipulate the request buffer.

### `channel txn.get_res_buf(txn)`

  This function returns an object able to manipulate the response buffer.

### `txn.finish(txn)`

  This function finish the current transaction. The data waiting in the buffers
  is send and the transaction is terminated.

### `txn.close(txn)`

  This function close the transaction and the associated session. It can be
  used when a critical error is detected.

### `txn.http.redirect(txn, location)`
### `txn.http.req.add_header(txn, name, value)`
### `txn.http.req.set_method(txn, string)`
### `txn.http.req.set_path(txn, string)`
### `txn.http.req.set_query(txn, string)`
### `txn.http.req.set_uri(txn, string)`
### `txn.http.req.set_header(txn, name, value)`
### `txn.http.req.del_header(txn, name)`
### `txn.http.req.replace_header(txn, name, regex, string)`
### `txn.http.req.replace_value(txn, name, regex, string)`

### `txn.http.res.set_header(txn, name, value)`
### `txn.http.res.del_header(txn, name)`
### `txn.http.res.replace_header(txn, name, regex, string)`
### `txn.http.res.replace_value(txn, name, regex, string)`

channel class
=============

*HAProxy uses two buffers for the processing of the requests. The first one is
used with the request data (from the client to the server) and the second is
used for the response data (from the server to the client).*

*Each buffer contains two types of data. The first type is the incoming data
waiting for a processing. The second part is the outgoing data already
processed. Usually, the incoming data is processed, after it is tagged as
outgoing data, and finally it is sent. The following functions provides tools
for manipulating these data in a buffer.*

*The following diagram shows where the channel class function are applied.*

![Channel manipulation points](channel.png)


### `string channel.dup(channel)`

   This function returns a string that contain the entire buffer. The data is
   not remove from the buffer and can be reprocessed later.

   If the buffer cant receive more data, a 'nil' value is returned.

### `string channel.get(channel)`

   This function returns a string that contain the entire buffer. The data is
   consumed from the buffer.

   If the buffer cant receive more data, a 'nil' value is returned.

### `string channel.get_line(channel)`

   This function returns a string that contain the first line of the buffer. The
   data is consumed. If the data returned doesn't contains a final '\n' its
   assumed than its the last available data in the buffer.

   If the buffer cant receive more data, a 'nil' value is returned.

### `int channel.set(channel, string)`

   This function replace the content of the buffer by the string. The function
   returns the copied length, otherwise, it returns -1.

   The data set with this function are not send. They wait for the end of
   HAProxy processing, so the buffer can be full.

### `int channel.append(channel, string)`

   This function append the string argument to the content of the buffer. The
   function returns the copied length, otherwise, it returns -1.

   The data set with this function are not send. They wait for the end of
   HAProxy processing, so the buffer can be full.

### `int channel.send(channel, string)`

   This function required immediate send of the data. Unless if the connection
   is close, the buffer is regularly flushed and all the string can be sent.

### `int channel.get_in_length(channel)`

   This function returns the length of the input part of the buffer.

### `int channel.get_out_length(channel)`

   This function returns the length of the output part of the buffer.

### `channel.forward(channel, int)`

   This function transfer bytes from the input part of the buffer to the output
   part.

socket class
============

*This class must be compatible with the Lua Socket class. Only the 'client'
functions are available. See the Lua Socket documentation:*

[http://w3.impa.br/~diego/software/luasocket/tcp.html](http://w3.impa.br/~diego/software/luasocket/tcp.html)

### `socket.close(socket)`

  Closes a TCP object. The internal socket used by the object is closed and the
  local address to which the object was bound is made available to other
  applications. No further operations (except for further calls to the close
  method) are allowed on a closed socket.

  Note: It is important to close all used sockets once they are not needed,
  since, in many systems, each socket uses a file descriptor, which are limited
  system resources. Garbage-collected objects are automatically closed before
  destruction, though.

### `socket.connect(socket, address, port)`

  Attempts to connect a socket object to a remote host.

  Address can be an IP address or a host name. Port must be an integer number
  in the range [1..64K).

  In case of error, the method returns nil followed by a string describing the
  error. In case of success, the method returns 1.

  Note: The function socket.connect is available and is a shortcut for the
  creation of client sockets.

  Note: Starting with LuaSocket 2.0, the settimeout method affects the behavior
  of connect, causing it to return with an error in case of a timeout. If that
  happens, you can still call socket.select with the socket in the sendt table.
  The socket will be writable when the connection is established.

### `socket.connect_ssl(socket, address, port)`

  Same behavior than the function socket:connect, but uses SSL.

### `socket.getpeername(socket)`

  Returns information about the remote side of a connected client object.

  Returns a string with the IP address of the peer, followed by the port number
  that peer is using for the connection. In case of error, the method returns
  nil.

### `socket.getsockname(socket)`

  Returns the local address information associated to the object.

  The method returns a string with local IP address and a number with the port.
  In case of error, the method returns nil.

### `socket.receive(socket, [pattern [, prefix]])`

  Reads data from a client object, according to the specified read pattern.
  Patterns follow the Lua file I/O format, and the difference in performance
  between all patterns is negligible.

  Pattern can be any of the following:

  * **`*a`**: reads from the socket until the connection is closed. No
              end-of-line translation is performed;

  * **`*l`**: reads a line of text from the socket. The line is terminated by a
              LF character (ASCII 10), optionally preceded by a CR character
              (ASCII 13). The CR and LF characters are not included in the
              returned line.  In fact, all CR characters are ignored by the
              pattern. This is the default pattern.

  * **number**: causes the method to read a specified number of bytes from the
                socket. Prefix is an optional string to be concatenated to the
                beginning of any received data before return.

If successful, the method returns the received pattern. In case of error, the
  method returns nil followed by an error message which can be the string
  'closed' in case the connection was closed before the transmission was
  completed or the string 'timeout' in case there was a timeout during the
  operation. Also, after the error message, the function returns the partial
  result of the transmission.

  Important note: This function was changed severely. It used to support
  multiple patterns (but I have never seen this feature used) and now it
  doesn't anymore.  Partial results used to be returned in the same way as
  successful results. This last feature violated the idea that all functions
  should return nil on error.  Thus it was changed too.

### `socket.send(socket, data [, i [, j]])`

  Sends data through client object.

  Data is the string to be sent. The optional arguments i and j work exactly
  like the standard string.sub Lua function to allow the selection of a
  substring to be sent.

  If successful, the method returns the index of the last byte within [i, j]
  that has been sent. Notice that, if i is 1 or absent, this is effectively the
  total number of bytes sent. In case of error, the method returns nil,
  followed by an error message, followed by the index of the last byte within
  [i, j] that has been sent. You might want to try again from the byte
  following that. The error message can be 'closed' in case the connection was
  closed before the transmission was completed or the string 'timeout' in case
  there was a timeout during the operation.

  Note: Output is not buffered. For small strings, it is always better to
  concatenate them in Lua (with the '..' operator) and send the result in one
  call instead of calling the method several times.

### `socket.setoption(socket, option [, value])`

  Just implemented for compatibility, this cal does nothing.

### `socket.settimeout(socket, value [, mode])`

  Changes the timeout values for the object. All I/O operations are blocking.
  That is, any call to the methods send, receive, and accept will block
  indefinitely, until the operation completes. The settimeout method defines a
  limit on the amount of time the I/O methods can block. When a timeout time
  has elapsed, the affected methods give up and fail with an error code.

  The amount of time to wait is specified as the value parameter, in seconds.

  The timeout modes are bot implemented, the only settable timeout is the
  inactivity time waiting for complete the internal buffer send or waiting for
  receive data.

External Lua libraries
======================

A lot of useful lua libraries can be found here:

* [https://lua-toolbox.com/](https://lua-toolbox.com/)

Redis acces:

* [https://github.com/nrk/redis-lua](https://github.com/nrk/redis-lua)

Operations on bits:

* [http://bitop.luajit.org/](http://bitop.luajit.org/)

OpenSSL:

* [http://mkottman.github.io/luacrypto/index.html](http://mkottman.github.io/luacrypto/index.html)

* [https://github.com/brunoos/luasec/wiki](https://github.com/brunoos/luasec/wiki)