REST Lua Service

Introduction

The RestLuaService is a service for initiating Lua scripts running within the LogicApp.

The RestLuaService receives messages from one or more instances of the RestServerApp which is configured to receive REST/HTTP(S) Requests from an external client.

The RestLuaService communicates with the RestServerApp using the REST-S-… messages.

Configuring RestLuaService

The RestLuaService is configured within a LogicApp.

    <?xml version="1.0" encoding="utf-8"?>
    <n2svcd>
      ...
      <applications>
        ...
        <application name="Logic" module="LogicApp">
          <include>
            <lib>../apps/logic/lib</lib>
          </include>
          <parameters>
            ...
          </parameters>
          <config>
            <services>
              <service module="RestServerApp::RestLuaService" libs="../apps/rest_s/lib" script_dir="/var/lib/n2svcd/logic/rest">
                <triggers>
                    ...
                </triggers>
              </service>
            </services>
            <agents>
              ...
            </agents>
          </config>
        </application>
        ...
      </application>
      ...
    </n2svcd>

In addition to the Common LogicApp Service Configuration, note the following specific attribute notes, and service-specific attributes.

Under normal installation, the following service attributes apply:

Script Selection (REST Request)

Script selection is optionally configurable using a <triggers> list in the service configuration. The service will choose how to handle any request received by:

1. First attempting to match the incoming request against a <trigger> definition. Triggers are attempted in the order defined in the <triggers> list.
2. Attempting to run a script whoes name is simply the part after the final “/” of the path.

For example, without a matching trigger, an inbound request with path /APP/rest/echo will map to a script name of echo, which will be satisfied by a file named echo.lua or echo.lc in the script_dir directory configured for the service.

However, if triggers are defined, for example:

<triggers>
    <trigger method="get"        path="/APP/rest/echo" script_key="simple_echo"/>
    <trigger              path_prefix="/APP/rest"      script_key="rest_api"/>
</trigger>

Then an inbound GET request with a path exactly matching /APP/rest/echo will be handled by the Lua script in a file called simple_echo.lua or simple_echo.lc. Any other requests where the HTTP path’s prefix is /APP/rest, regardless of HTTP method, will be handled by the Lua script rest_api.

Refer to the LogicApp configuration for more information on directories, library paths, and script caching parameters.

Script Trigger Configuration

The following configuration attributes are required for each <trigger> defined:

Note that attributes path_prefix, path and method are all optional. It is valid to define a trigger with only a script_key:

For example this triggers configuration:

<triggers>
  <trigger script_key="api">
</triggers>

causes all inbound HTTP requests to be handled by the Lua script api.[lc|lua]

Script Global Variables

Scripts run with this service have access to the Common LUA Service Global Variables.

There are no service-specific global variables.

Script Entry Parameters (REST Request)

The Lua script must be a Lua chunk such as the following example, which includes a single user-level HTTP header named Echo-Flag:

local n2svcd = require "n2.n2svcd"

local rest = ...

n2svcd.debug ("Echo Supplied REST Inputs:")
n2svcd.debug_var (rest)

return { 
    content_type = rest.content_type, 
    content = rest.content,
    http_headers = { { name = 'Echo-Flag', value = 'true' } }
}

The chunk will be executed with a single rest entry parameter which is an object with the following attributes:

Each object in the .http_headers Array for the rest entry parameter has the following structure.

Script Return Parameters (REST Response)

The Lua script is responsible for determing the REST Response which will be sent back in reply to the original REST Request.

The simplest way to do this is by the return value given back to the service at the end of script execution.

For full control over the REST response, the script return value may be a response object with the following attributes:

Each object in the .http_headers Array for the response object has the following structure.

Example (returning a Table REST Response):

    local n2svcd = require "n2.n2svcd"

    local rest = ...
        return ({ content_type = "application/json", content = '{ "numrows": 1 }'})

Alternatively, a script may return a simple string instead of a Table.

Example (returning a String REST Response):

    local n2svcd = require "n2.n2svcd"

    local rest = ...
        return "I AM OK YOU ARE OK"

This is a shorthand for code = 200, content_type = text/plain.

Alternatively, a script may return nil.

Example (returning a 200 OK with no Content):

    local n2svcd = require "n2.n2svcd"

    local rest = ...
        return nil

This is a shorthand for code = 200, no Content, no Content-Type.

The RestLuaService API

The REST Service API can be loaded as follows.

    local rest_service_api = require "n2.n2svcd.rest_service"

It is not necessary to load the REST Service API if you are only using the simple response mechanism described above. It is only required if you wish to use any of the extended features described below.

.response

When a Lua Script needs to perform extended processing, it may wish to send an early REST response before the script completes. This can be done with the response method on the REST API.

The response method takes a single response parameter. The structure of this response parameter is identical to the response parameter which may be returned as the script return parameter, as described above in Script Return Parameters (REST Response)

The response method returns true.

[Fragment] Example (403 Early Response with text/plain content and cookie):

    ...
    local headers = {}
    http.response_headers_set_cookie (headers, "SESSION$", "<none>", { httponly = 1 })

    rest_service_api.response ({ 
        code = 403, 
        content = "ACCESS DENIED!", 
        content_type = "text/plain", 
        http_headers = headers
    })
    ...
    [post-processing after REST transaction is concluded]
    ...

[Fragment] Example (200 Early Response with text/plain content):

    ...
    rest_service_api.response ("THANKS FOR CALLING")
    ...
    [post-processing after REST transaction is concluded]