Apache HTTP Server Version 2.5

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# Creating hooks and scripts with mod_lua

Available Languages: en

This document expands on the `mod_lua` documentation and explores additional ways of using mod_lua for writing hooks and scripts.

### See also

- mod_lua
- Developing modules for Apache 2.4
- Request Processing in Apache 2.4
- Apache 2.x Hook Functions
- Comments

## Introduction ¶

### What is mod_lua

Stuff about what `mod_lua` is goes here.

### What we will be discussing in this document

This document will discuss several cases where `mod_lua` can be used to either ease up a phase of the request processing or create more transparency in the logic behind a decision made in a phase.

### Prerequisites

First and foremost, you are expected to have a basic knowledge of how the Lua programming language works. In most cases, we will try to be as pedagogical as possible and link to documents describing the functions used in the examples, but there are also many cases where it is necessary to either just assume that "it works" or do some digging yourself into what the hows and whys of various function calls.

## Optimizing mod_lua for production servers ¶

### Setting a scope for Lua states

Setting the right `LuaScope` setting for your Lua scripts can be essential to your server's performance. By default, the scope is set to `once`, which means that every call to a Lua script will spawn a new Lua state that handles that script and is destroyed immediately after. This option keeps the memory footprint of mod_lua low, but also affects the processing speed of a request. If you have the memory to spare, you can set the scope to `thread`, which will make mod_lua spawn a Lua state that lasts the entirety of a thread's lifetime, speeding up request processing by 2-3 times. Since mod_lua will create a state for each script, this may be an expensive move, memory-wise, so to compromise between speed and memory usage, you can choose the `server` option to create a pool of Lua states to be used. Each request for a Lua script or a hook function will then acquire a state from the pool and release it back when it's done using it, allowing you to still gain a significant performance increase, while keeping your memory footprint low. Some examples of possible settings are:

    LuaScope once
    LuaScope thread
    LuaScope server 5 40

As a general rule of thumb: If your server has none to low usage, use `once` or `request`, if your server has low to medium usage, use the `server` pool, and if it has high usage, use the `thread` setting. As your server's load increases, so will the number of states being actively used, and having your scope set to `once/request/conn` will stop being beneficial to your memory footprint.

Note: The `min` and `max` settings for the `server` scope denotes the minimum and maximum states to keep in a pool per server process, so keep this below your `ThreadsPerChild` limit.

### Using code caching

By default, `mod_lua` stats each Lua script to determine whether a reload (and thus, a re-interpretation and re-compilation) of a script is required. This is managed through the `LuaCodeCache` directive. If you are running your scripts on a production server, and you do not need to update them regularly, it may be advantageous to set this directive to the `forever` value, which will cause mod_lua to skip the stat process and always reuse the compiled byte-code from the first access to the script, thus speeding up the processing. For Lua hooks, this can prove to increase peformance, while for scripts handled by the `lua-script` handler, the increase in performance may be negligible, as files httpd will stat the files regardless.

### Keeping the scope clean

For maximum performance, it is generally recommended that any initialization of libraries, constants and master tables be kept outside the handle's scope:

    --[[ This is good practice ]]--
    require "string"
    require "someLibrary"
    local masterTable = {}
    local constant = "Foo bar baz"

    function handle(r)
        do_stuff()
    end

    --[[ This is bad practice ]]--
    require "string"

    function handle(r)
        require "someLibrary"
        local masterTable = {}
        local constant = "Foo bar baz"
        do_stuff()
    end

## Example 1: A basic remapping module ¶

These first examples show how mod_lua can be used to rewrite URIs in the same way that one could do using `Alias` or `RewriteRule`, but with more clarity on how the decision-making takes place, as well as allowing for more complex decisions than would otherwise be allowed with said directives.

    LuaHookTranslateName /path/too/foo.lua remap

    --[[
        Simple remap example.
        This example will rewrite /foo/test.bar to the physical file
        /internal/test, somewhat like how mod_alias works.
    ]]--

    function remap(r)
        -- Test if the URI matches our criteria
        local barFile =  r.uri:match("/foo/([a-zA-Z0-9]+)%.bar")
        if barFile then
            r.filename = "/internal/" .. barFile
        end
        return apache2.OK
    end

    --[[
        Advanced remap example.
        This example will evaluate some conditions, and based on that,
        remap a file to one of two destinations, using a rewrite map.
        This is similar to mixing AliasMatch and ProxyPass, but
        without them clashing in any way. Assuming we are on example.com, then:

        http://example.com/photos/test.png will be rewritten as /uploads/www/test.png
        http://example.com/ext/foo.html will be proxied to http://www.external.com/foo.html
        URIs that do not match, will be served by their respective default handlers
    ]]--

    local map = {
          photos = {
                       source = [[^/photos/(.+)\.png$]],
                       destination = [[/uploads/www/$1.png]],
                       proxy = false
                    },
          externals = {
                       source = [[^/ext/(.*)$]],
                       destination = [[http://www.external.com/$1]],
                       proxy = true
                    }
    }

    function interpolateString(s,v)
        return s:gsub("%$(%d+)", function(a) return v[tonumber(a)] end)
    end

    function remap(r)
        -- browse through the rewrite map
        for key, entry in pairs(map) do
            -- Match source regex against URI
            local match = r:regex(entry.source, r.uri) then
            if match and match[0] then
                r.filename = interpolateString(entry.destination, match)
                -- Is this a proxied remap?
                if entry.proxy then
                    r.handler = "proxy-server" -- tell mod_proxy to handle this
                    r.proxyreq = apache2.PROXYREQ_REVERSE -- We'll want to do a reverse proxy
                    r.filename = "proxy:" .. r.filename -- Add the proxy scheme to the destination
                end
                return apache2.OK
            end
        end
        return apache2.DECLINED
    end

bla bla

## Example 2: Mass virtual hosting ¶

As with simple and advanced rewriting, you can use mod_lua for dynamically assigning a hostname to a specific document root, much like `mod_vhost_alias` does, but with more control over what goes where. This could be as simple as a table holding the information about which host goes into which folder, or more advanced, using a database holding the document roots of each hostname.

    LuaHookTranslateName /path/too/foo.lua mass_vhost

    --[[
        Simple mass vhost script
        This example will check a map for a virtual host and rewrite filename and
        document root accordingly.
    ]]--

    local vhosts = {
        { domain = "example.com", home = "/www/example.com" },
        { domain = "example.org", home = "/nfs/ext1/example.org" }
    }

    function mass_vhost(r)
        -- Match against our hostname
        for key, entry in pairs(vhosts) do
            -- match against either host or *.host:
            if apache2.strcmp_match(r.hostname, entry.domain) or
               apache2.strcmp_match(r.hostname, "*." .. entry.domain) then
                -- If it matches, rewrite filename and set document root
                local filename = r.filename:sub(r.document_root:len()+1)
                r.filename = entry.home .. filename
                apahce2.set_document_root(entry.home)
                return apache2.OK
            end
        end
        return apache2.DECLINED
    end

    --[[
        Advanced mass virtual hosting
        This example will query a database for vhost entries and save them for
        60 seconds before checking for updates. For best performance, such scripts
        should generally be run with LuaScope set to 'thread' or 'server'
    ]]--

    local cached_vhosts = {}
    local timeout = 60

    -- Function for querying the database for saved vhost entries
    function query_vhosts(r)
        local host = r.hostname
        if not cached_vhosts[host] or (cached_vhosts[host] and cached_vhosts[host].updated < os.time() - timeout) then
            local db,err = ap.dbopen(r,"mod_dbd")
            local _host = db:escape(r,host)
            local res, err = db:query(r, ("SELECT `destination` FROM `vhosts` WHERE `hostname` = '%s' LIMIT 1"):format(_host) )
            if res and #res == 1 then
                cached_vhosts[host] = { updated = os.time(), destination = res[1][1] }
            else
                cached_vhosts[host] = { updated = os.time(), destination = nil } -- don't re-query whenever there's no result, wait a while.
            end
            db:close()
        end
        if cached_vhosts[host] then
            return cached_vhosts[host].destination
        else
            return nil
        end
    end

    function mass_vhost(r)
        -- Check whether the hostname is in our database
        local destination = query_vhosts(r)
        if destination then
            -- If found, rewrite and change document root
            local filename = r.filename:sub(r.document_root:len()+1)
            r.filename = destination .. filename
            ap.set_document_root(r,destination)
            return apache2.OK
        end
        return apache2.DECLINED
    end

## Example 3: A basic authorization hook ¶

With the authorization hooks, you can add custom auth phases to your request processing, allowing you to either add new requirements that were not previously supported by httpd, or tweaking existing ones to accommodate your needs.

    LuaHookAuthChecker /path/too/foo.lua check_auth

    --[[
         A simple authentication hook that checks a table containing usernames and
         passwords of two accounts.
    ]]--
    local accounts = {
        bob  = 'somePassword',
        jane = 'Iloveponies'
    }

    -- Function for parsing the Authorization header into a username and a password
    function parse_auth(str)
        local user,pass = nil, nil
        if str and str:len() > 0 then
            str = apache2.base64_decode(auth):sub(7));
            user, pass = auth:match("([^:]+)%:([^:]+)")
        end
        return user, pass
    end

    -- The authentication hook
    function check_auth(r)
        local user, pass = parse_auth(r.headers_in['Authorization'])
        local authenticated = false
        if user and pass then
            if accounts[user] and accounts[user] == pass then
                authenticated = true
                r.user = user
            end
        end
        r.headers_out["WWW-Authenticate"] = 'Basic realm="Super secret zone"'
        if not authenticated then
            return 401
        else
            return apache2.OK
        end
    end

    --[[
         An advanced authentication checker with a database backend,
         caching account entries for 1 minute
    ]]--

    local timeout = 60 -- Set account info to be refreshed every minute
    local accounts = {}

    -- Function for parsing the Authorization header into a username and a password
    function parse_auth(str)
        local user,pass = nil, nil
        if str and str:len() > 0 then
            str = apache2.base64_decode(auth):sub(7));
            user, pass = auth:match("([^:]+)%:([^:]+)")
        end
        return user, pass
    end

    -- Function for querying the database for the account's password (stored as a salted SHA-1 hash)
    function fetch_password(user)
        if not accounts[user] or (accounts[user] and accounts[user].updated < os.time() - timeout) then
            local db = apache2.dbopen(r, "mod_dbd")
            local usr = db:escape(user)
            local res, err = db:query( ("SELECT `password` FROM `accounts` WHERE `user` = '%s' LIMIT 1"):format(usr) )
            if res and #res == 1 then
                accounts[user] = { updated = os.time(), password = res[1][1] }
            else
                accounts[user] = nil
            end
            db:close()
        end
        if accounts[user] then
            return accounts[user].password
        else
            return nil
        end
    end

    -- The authentication hook
    function check_auth(r)
        local user, pass = parse_auth(r.headers_in['Authorization'])
        local authenticated = false
        if user and pass then
            pass = apache2.sha1("addSomeSalt" .. pass)
            local stored_pass = fetch_password(user)
            if stored_pass and pass == stored_pass then
                authenticated = true
                r.user = user
            end
        end
        r.headers_out["WWW-Authenticate"] = 'Basic realm="Super secret zone"'
        if not authenticated then
            return 401
        else
            return apache2.OK
        end
    end

## Example 4: Authorization using LuaAuthzProvider ¶

If you require even more advanced control over your authorization phases, you can add custom authz providers to help you manage your server. The example below shows you how you can split a single htpasswd file into groups with different permissions:

    LuaAuthzProvider rights /path/to/lua/script.lua rights_handler
    <Directory "/www/private">
        Require rights member
    </Directory>
    <Directory "/www/admin">
        Require rights admin
    </Directory>

    --[[
         This script has two user groups; members and admins, and whichever
         is referred to by the "Require rights" directive is checked to see
         if the authenticated user belongs to this group.
    ]]--

    local members = { "rbowen", "humbedooh", "igalic", "covener" }
    local admins = { "humbedooh" }

    function rights_handler(r, what)
        if r.user == nil then
            return apache2.AUTHZ_AUTHZ_DENIED_NO_USER
        end
        if what == "member" then
            for k, v in pairs(members) do
                if r.user == v then
                    return apache2.AUTHZ_GRANTED
                end
            end
        elseif what == "admin" then
            for k, v in pairs(admins) do
                if r.user == v then
                    return apache2.AUTHZ_GRANTED
                end
            end
        end
        return apache2.AUTHZ_DENIED
    end

## Example 5: A rudimentary load balancer ¶

This is an example of how you can create a load balancing mechanism. In this example, we will be setting/getting the number of requests served by each backend using IVM variables, and preferring the backend with least requests served in total:

    LuaHookTranslateName /path/to/script.lua proxy_handler

    --[[
         This script uses a basic IVM table to determine where to
         send the request.
    ]]--

    local backends = {
      "http://backend1.foo.com/",
      "http://backend2.foo.com/",
      "http://backend3.foo.com/"
    }

    function pick_backend(r)
        local chosen_backend = 1 -- default to backend1
        local lowest_count = nil
        for i = 1, #backends, 1 do -- Loop through all backends
            local count = r:ivm_get("proxy_request_count_" .. i)
            if not count then -- If this backend hasn't been used at all, prefer it
                chosen_backend = i
                lowest_count = 0
                break
            end
            if not lowest_count or lowest_count > count then -- If this backend has had less requests, pick it for now
                chosen_backend = i
                lowest_count = count
            end
        end
        lowest_count = lowest_count + 1
        r:ivm_set("proxy_request_count_" .. chosen_backend, lowest_count)
        return chosen_backend
    end

    function proxy_handler(r)
        local backend = pick_backend(r) -- Pick a backend based on no. of requests served
        r.handler  = "proxy-server"
        r.proxyreq = apache2.PROXYREQ_REVERSE
        r.filename = "proxy:" .. backends[backend] .. r.uri
        return apache2.DECLINED -- let the proxy handler do this instead
    end

## Example 6: Overlays using LuaMapHandler ¶

Coming soon!

    LuaMapHandler ^/portal/([a-z]+)/   /path/to/lua/script.lua handle_$1

## Example 6: Basic Lua scripts ¶

Also coming soon

Available Languages: en

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