Implementing Custom Logic

Note: This chapter assumes that you are familiar with Lua.

A Kong Gateway plugin allows you to inject custom logic (in Lua) at several entry-points in the life-cycle of a request/response or a tcp stream connection as it is proxied by Kong Gateway. To do so, the file kong.plugins.<plugin_name>.handler must return a table with one or more functions with predetermined names. Those functions will be invoked by Kong Gateway at different phases when it processes traffic.

The first parameter they take is always self. All functions except init_worker can receive a second parameter which is a table with the plugin configuration.

Module

  1. kong.plugins.<plugin_name>.handler

Available contexts

If you define any of the following functions in your handler.lua file you’ll implement custom logic at various entry-points of Kong Gateway’s execution life-cycle:

  • HTTP Module is used for plugins written for HTTP/HTTPS requests
Function namePhaseRequest ProtocolDescription
init_workerinit_workerExecuted upon every Nginx worker process’s startup.
certificatessl_certificatehttps, grpcs, wssExecuted during the SSL certificate serving phase of the SSL handshake.
rewriterewriteExecuted for every request upon its reception from a client as a rewrite phase handler.
In this phase, neither the Service nor the Consumer have been identified, hence this handler will only be executed if the plugin was configured as a global plugin.
accessaccesshttp(s), grpc(s), ws(s)Executed for every request from a client and before it is being proxied to the upstream service.
ws_handshakeaccessws(s)Executed for every request to a WebSocket service just before completing the WebSocket handshake.
responseaccesshttp(s), grpc(s)Replaces both header_filter() and body_filter(). Executed after the whole response has been received from the upstream service, but before sending any part of it to the client.
header_filterheader_filterhttp(s), grpc(s)Executed when all response headers bytes have been received from the upstream service.
ws_client_framecontentws(s)Executed for each WebSocket message received from the client.
ws_upstream_framecontentws(s)Executed for each WebSocket message received from the upstream service.
body_filterbody_filterhttp(s), grpc(s)Executed for each chunk of the response body received from the upstream service. Since the response is streamed back to the client, it can exceed the buffer size and be streamed chunk by chunk. This function can be called multiple times if the response is large. See the lua-nginx-module documentation for more details.
logloghttp(s), grpc(s)Executed when the last response byte has been sent to the client.
ws_closelogws(s)Executed after the WebSocket connection has been terminated.

Note: If a module implements the response function, Kong Gateway will automatically activate the “buffered proxy” mode, as if the kong.service.request.enable_buffering() function had been called. Because of a current Nginx limitation, this doesn’t work for HTTP/2 or gRPC upstreams.

To reduce unexpected behaviour changes, Kong Gateway does not start if a plugin implements both response and either header_filter or body_filter.

  • Stream Module is used for Plugins written for TCP and UDP stream connections
Function namePhaseDescription
init_workerinit_workerExecuted upon every Nginx worker process’s startup.
prereadprereadExecuted once for every connection.
loglogExecuted once for each connection after it has been closed.
certificatessl_certificateExecuted during the SSL certificate serving phase of the SSL handshake.

All of those functions, except init_worker, take one parameter which is given by Kong Gateway upon its invocation: the configuration of your plugin. This parameter is a Lua table, and contains values defined by your users, according to your plugin’s schema (described in the schema.lua module). More on plugins schemas in the next chapter.

Note that UDP streams don’t have real connections. Kong Gateway will consider all packets with the same origin and destination host and port as a single connection. After a configurable time without any packet, the connection is considered closed and the log function is executed.

handler.lua specifications

Kong Gateway processes requests in phases. A plugin is a piece of code that gets activated by Kong Gateway as each phase is executed while the request gets proxied.

Phases are limited in what they can do. For example, the init_worker phase does not have access to the config parameter because that information isn’t available when kong is initializing each worker.

A plugin’s handler.lua must return a table containing the functions it must execute on each phase.

Kong Gateway can process HTTP and stream traffic. Some phases are executed only when processing HTTP traffic, others when processing stream, and some (like init_worker and log) are invoked by both kinds of traffic.

In addition to functions, a plugin must define two fields:

  • VERSION is an informative field, not used by Kong Gateway directly. It usually matches the version defined in a plugin’s Rockspec version, when it exists.
  • PRIORITY is used to sort plugins before executing each of their phases. Plugins with a higher priority are executed first. See the plugin execution order below for more info about this field.

The following example handler.lua file defines custom functions for all the possible phases, in both http and stream traffic. It has no functionality besides writing a message to the log every time a phase is invoked. Note that a plugin doesn’t need to provide functions for all phases.

  1. local CustomHandler = {
  2. VERSION = "1.0.0",
  3. PRIORITY = 10,
  4. }
  5. function CustomHandler:init_worker()
  6. -- Implement logic for the init_worker phase here (http/stream)
  7. kong.log("init_worker")
  8. end
  9. function CustomHandler:preread(config)
  10. -- Implement logic for the preread phase here (stream)
  11. kong.log("preread")
  12. end
  13. function CustomHandler:certificate(config)
  14. -- Implement logic for the certificate phase here (http/stream)
  15. kong.log("certificate")
  16. end
  17. function CustomHandler:rewrite(config)
  18. -- Implement logic for the rewrite phase here (http)
  19. kong.log("rewrite")
  20. end
  21. function CustomHandler:access(config)
  22. -- Implement logic for the access phase here (http)
  23. kong.log("access")
  24. end
  25. function CustomHandler:ws_handshake(config)
  26. -- Implement logic for the WebSocket handshake here
  27. kong.log("ws_handshake")
  28. end
  29. function CustomHandler:header_filter(config)
  30. -- Implement logic for the header_filter phase here (http)
  31. kong.log("header_filter")
  32. end
  33. function CustomHandler:ws_client_frame(config)
  34. -- Implement logic for WebSocket client messages here
  35. kong.log("ws_client_frame")
  36. end
  37. function CustomHandler:ws_upstream_frame(config)
  38. -- Implement logic for WebSocket upstream messages here
  39. kong.log("ws_upstream_frame")
  40. end
  41. function CustomHandler:body_filter(config)
  42. -- Implement logic for the body_filter phase here (http)
  43. kong.log("body_filter")
  44. end
  45. function CustomHandler:log(config)
  46. -- Implement logic for the log phase here (http/stream)
  47. kong.log("log")
  48. end
  49. function CustomHandler:ws_close(config)
  50. -- Implement logic for WebSocket post-connection here
  51. kong.log("ws_close")
  52. end
  53. -- return the created table, so that Kong can execute it
  54. return CustomHandler

Note that in the example above we are using Lua’s : shorthand syntax for functions taking self as a first parameter. An equivalent non-shorthand version of the access function would be:

  1. function CustomHandler.access(self, config)
  2. -- Implement logic for the rewrite phase here (http)
  3. kong.log("access")
  4. end

The plugin’s logic doesn’t need to be all defined inside the handler.lua file. It can be split into several Lua files (also called modules). The handler.lua module can use require to include other modules in your plugin.

For example, the following plugin splits the functionality into three files. access.lua and body_filter.lua return functions. They are in the same folder as handler.lua, which requires and uses them to build the plugin:

  1. -- handler.lua
  2. local access = require "kong.plugins.my-custom-plugin.access"
  3. local body_filter = require "kong.plugins.my-custom-plugin.body_filter"
  4. local CustomHandler = {
  5. VERSION = "1.0.0",
  6. PRIORITY = 10
  7. }
  8. CustomHandler.access = access
  9. CustomHandler.body_filter = body_filter
  10. return CustomHandler
  1. -- access.lua
  2. return function(self, config)
  3. kong.log("access phase")
  4. end
  1. -- body_filter.lua
  2. return function(self, config)
  3. kong.log("body_filter phase")
  4. end

See the source code of the Key-Auth Plugin for an example of a real-life handler code.

Migrating from BasePlugin module

The BasePlugin module is deprecated and has been removed from Kong Gateway. If you have an old plugin that uses this module, replace the following section:

  1. -- DEPRECATED --
  2. local BasePlugin = require "kong.plugins.base_plugin"
  3. local CustomHandler = BasePlugin:extend()
  4. CustomHandler.VERSION = "1.0.0"
  5. CustomHandler.PRIORITY = 10

with the current equivalent:

  1. local CustomHandler = {
  2. VERSION = "1.0.0",
  3. PRIORITY = 10,
  4. }

You don’t need to add a :new() method or call any of the CustomHandler.super.XXX:(self) methods.

WebSocket Plugin Development

Warning: The WebSocket PDK is under active development and is considered unstable at this time. Backwards-incompatible changes may be made to these functions.

Handler Functions

Requests to services with the ws or wss protocol take a different path through the proxy than regular http requests. Therefore, there are some differences in behavior that must be accounted for when developing plugins for them.

The following handlers are not executed for WebSocket services:

  • access
  • response
  • header_filter
  • body_filter
  • log

The following handlers are unique to WebSocket services:

  • ws_handshake
  • ws_client_frame
  • ws_upstream_frame
  • ws_close

The following handlers are executed for both WebSocket and non-Websocket services:

  • init_worker
  • certificate (TLS/SSL requests only)
  • rewrite

Even with these differences, it is possible to develop plugins that support both WebSocket and non-WebSocket services. For example:

  1. -- handler.lua
  2. --
  3. -- I am a plugin that implements both WebSocket and non-WebSocket handlers.
  4. --
  5. -- I can be enabled for ws/wss services, http/https/grpc/grpcs services, or
  6. -- even as global plugin.
  7. local MultiProtoHandler = {
  8. VERSION = "0.1.0",
  9. PRIORITY = 1000,
  10. }
  11. function MultiProtoHandler:access()
  12. kong.ctx.plugin.request_type = "non-WebSocket"
  13. end
  14. function MultiProtoHandler:ws_handshake()
  15. kong.ctx.plugin.request_type = "WebSocket"
  16. end
  17. function MultiProtoHandler:log()
  18. kong.log("finishing ", kong.ctx.plugin.request_type, " request")
  19. end
  20. -- the `ws_close` handler for this plugin does not implement any WebSocket-specific
  21. -- business logic, so it can simply be aliased to the `log` handler
  22. MultiProtoHandler.ws_close = MultiProtoHandler.log
  23. return MultiProtoHandler

As seen above, the log and ws_close handlers are parallel to each other. In many cases, one can be aliased to the other without having to write any additional code. The access and ws_handshake handlers are also very similar in this regard. The notable difference lies in which PDK functions are/aren’t available in each context. For instance, the kong.request.get_body() PDK function cannot be used in an access handler because it is fundamentally incompatible with this kind of request.

WebSocket requests to non-WebSocket services

When WebSocket traffic is proxied via an http/https service, it is treated as a non-WebSocket request. Therefore, the http handlers (access, header_filter, etc) will be executed and not the WebSocket handlers (ws_handshake, ws_close, etc).

Plugin Development Kit

Logic implemented in those phases will most likely have to interact with the request/response objects or core components (e.g. access the cache, and database). Kong Gateway provides a Plugin Development Kit (or “PDK”) for such purposes: a set of Lua functions and variables that can be used by Plugins to execute various gateway operations in a way that is guaranteed to be forward-compatible with future releases of Kong Gateway.

When you are trying to implement some logic that needs to interact with Kong Gateway (e.g. retrieving request headers, producing a response from a plugin, logging some error or debug information), you should consult the Plugin Development Kit Reference.

Plugins execution order

Some plugins might depend on the execution of others to perform some operations. For example, plugins relying on the identity of the consumer have to run after authentication plugins. Considering this, Kong Gateway defines priorities between plugins execution to ensure that order is respected.

Your plugin’s priority can be configured via a property accepting a number in the returned handler table:

  1. CustomHandler.PRIORITY = 10

The higher the priority, the sooner your plugin’s phases will be executed in regard to other plugins’ phases (such as :access(), :log(), etc.).

Kong plugins

All of the plugins bundled with Kong Gateway have a static priority. This can be adjusted dynamically using the ordering option. See Dynamic Plugin Ordering for more information.

Open-source or Free mode

Enterprise

The following list includes all plugins bundled with open-source Kong Gateway or Kong Gateway running in Free mode.

Note: The correlation-id plugin’s execution order is different depending on whether you’re running Kong Gateway in Free mode or using the open-source package.

The current order of execution for the bundled plugins is:

PluginPriority
pre-function1000000
correlation-id100001
zipkin100000
bot-detection2500
cors2000
session1900
acme1705
jwt1450
oauth21400
key-auth1250
ldap-auth1200
basic-auth1100
hmac-auth1030
grpc-gateway998
ip-restriction990
request-size-limiting951
acl950
rate-limiting910
response-ratelimiting900
request-transformer801
response-transformer800
aws-lambda750
azure-functions749
upstream-timeout400
opentelemetry14
prometheus13
http-log12
statsd11
datadog10
file-log9
udp-log8
tcp-log7
loggly6
syslog4
grpc-web3
request-termination2
correlation-id1
post-function-1000

The following list includes all plugins bundled with a Kong Gateway Enterprise subscription.

The current order of execution for the bundled plugins is:

PluginPriority
pre-function1000000
correlation-id100001
zipkin100000
exit-transformer9999
bot-detection2500
cors2000
session1900
oauth2-introspection1700
acme1705
mtls-auth1600
degraphql1500
jwt1450
oauth21400
vault-auth1350
key-auth1250
key-auth-enc1250
ldap-auth1200
ldap-auth-advanced1200
basic-auth1100
openid-connect1050
hmac-auth1030
jwt-signer1020
request-validator999
websocket-size-limit999
websocket-validator999
grpc-gateway998
tls-handshake-modifier997
tls-metadata-headers996
application-registration995
ip-restriction990
request-size-limiting951
acl950
opa920
rate-limiting910
rate-limiting-advanced910
graphql-rate-limiting-advanced902
response-ratelimiting900
route-by-header850
jq811
request-transformer-advanced802
request-transformer801
response-transformer-advanced800
response-transformer800
route-transformer-advanced780
kafka-upstream751
aws-lambda750
azure-functions749
upstream-timeout400
proxy-cache-advanced100
proxy-cache100
graphql-proxy-cache-advanced99
forward-proxy50
canary20
opentelemetry14
prometheus13
http-log12
statsd11
statsd-advanced11
datadog10
file-log9
udp-log8
tcp-log7
loggly6
kafka-log5
syslog4
grpc-web3
request-termination2
mocking-1
post-function-1000

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