Hooks

Definition

Hooks are a mechanism provided by EMQX Broker, which modifies or extends system functions by intercepting function calls, message passing, and event passing between modules.

In simple terms, the purpose of this mechanism is to enhance the scalability of the software system, facilitate integration with the three-party systems, or change the original default behavior of its system. Such as:

When the Hooks mechanism does not exist in the system, the entire event processing flow (from the input of the event, to the handler and the result) is invisible and cannot be modified for the external system .

In the process, if a HookPoint where a function can be mounted is added, it will allow external plugins to mount multiple callback functions to form a call chain. Then, the internal event processing can be extended and modified .

The authentication plugin commonly used in the system is implemented according to this logic. Take the simplest plugin of emqx_auth_mnesia (opens new window) as an example:

When only the emqx_auth_mnesia authentication plugin is enabled and anonymous authentication is disabled, according to the processing logic of the event according in the figure above, the logic of the authentication module at this time is:

1. Receive user authentication request (Authenticate)
2. Read the parameter of Whether to allow anonymous login and get deny result
3. Execute the hook of the authentication event , that is, call back to the emqx_auth_mnesia plugin, assume this authentication is valid, and get allow result
4. Return Authentication succeeded, and successfully access the system

It is shown in the following figure:

                     EMQX Core          Hooks & Plugins
                |<---  Scope  --->|<-------  Scope  -------->|
                |                 |                          |
  Authenticate  |     Allow       |   emqx_auth_mnesia       | Authenticate
 =============> > - - - - - - No -> - - - - - - - - - - -Yes->==============> Success
     Request    |    Anonymous?   |     authenticate?        |     Result
                |                 |                          |
                +-----------------+--------------------------+

Therefore, in EMQX Broker, the mechanism of Hooks greatly facilitates the extension of the system. We don’t need to modify the emqx (opens new window) core code, but only need to bury the HookPoint in a specific location to allow external plugins to extend EMQX Broker with various behaviors.

For implementers, it only needs to pay attention to:

1. The location of HookPoint: Including its role, timing of execution, and how to mount and cancel mount.
2. Implementation of callback function: including the number of input parameters, role, data structure of the callback function, and the meaning of the returned value.
3. Understand the mechanism of callback function execution on the chain: including the order in which callback functions are executed, and how to terminate the execution of the chain in advance.

If you used hooks in the development of extension plugin, you should be able to fully understand these three above points, and try not to use blocking functions inside the hooks, which will affect system throughput.

Callback Functions Chain

There may be multiple plugins on a single HookPoint that need to care about the event and perform the corresponding operation, so there may be multiple callback functions on each HookPoint.

We call this chain composed of multiple callback functions executed sequentially Callback Functions Chain.

Callback Functions Chain is Currently implemented according to the concept of Chain-of-Responsibility (opens new window). In order to satisfy the functionality and flexibility of the hook, it must have the following attributes:

• The callback functions on the Callback Functions Chain must be executed in certain order.
• There must be an input and output for the Callback Functions Chain (output is not necessary in notification events, such as “a client has successfully logged in”).
• Callback Functions Chain is transitive, meaning that the chain will input the input parameters of the chain to the first callback function, and the returned value of the first callback function will be passed to the second callback function until the last function, whose returned value is the returned value of the entire chain.
• Callback Functions Chain needs to allow the functions with it to terminate the chain in advance and ignore this operation.
  • Termination in advance: After the execution of this function is completed, the execution of the chain is directly terminated. All subsequent callback functions on the chain are ignored. For example, an authentication plugin believes that such clients do not need to check other authentication plug-ins after they are allowed to log in, so they need to be terminated in advance.
    • Ignore this operation: Do not modify the processing result on the chain, and pass it directly to the next callback function. For example, when there are multiple authentication plug-ins, an authentication plug-in believes that such clients do not belong to its authentication scope, and it does not need to modify the authentication results. This operation should be ignored and the returned value of the previous function should be passed directly to the next function on the chain.

Therefore, we can get a design sketch of the chain:

The meaning of the figure is:

1. The input parameters of the chain are read-only Args and the parameter Acc for function modification on the chain
2. Regardless of how the execution of chain is terminated, its return value is the new Acc
3. A total of three callback functions are registered on the chain in the figure,Fun1 Fun2 Fun3 , which are executed in the order indicated
4. The callback function execution order is determined by the priority, and the same priority is executed in the order of mounting
5. The callback function returns with:
   • ok: ignore this operation, continue the chain execution with read-only Args and Acc returned by the previous function
   • {ok, NewAcc}: perform some operations, modify Acc content, continue chain execution with read-only Args and new NewAcc
6. The callback function also returns with:
   • stop: Stop the transfer of the chain and immediately return the result of Acc from the previous function
   • {stop, NewAcc}: it means to stop the transfer of the chain and immediately return the result of NewAcc from this modification

The above is the main design concept of the callback function chain, which regulates the execution logic of the callback function on the hook.

In the following two sections of HookPoint and callback function, all operations on hooks depend on Erlang code-level API provided by emqx (opens new window). They are the basis for the entire hook logic implementation.

• For hooks and HTTP server applications, Refer to: WebHook
• For hooks and other language applications, Refer to: Modules - Extensions

HookPoint

EMQX Broker is based on a client’s key activities during its life cycle, and presets a large number of HookPoints. The preset mount points in the system are:

TIP

• The session is discarded: When the client logs in with the method of clean session, if the client’s session already exists on the server, the old session will be discarded.
• The Session is taken over: When the client logs in with the method of Reserved Session, if the client’s session already exists on the server, the old session will be taken over by the new connection

Hook and unhook

EMQX Broker provides an API for the operation of hooking and unhooking.

Hook:

%% Name: name of hook(hook point), such as 'client.authenticate'
%% {Module, Function, Args}: Modules, methods, and additional parameters for callback functions
%% Priority：integar, 0 by default
emqx:hook(Name, {Module, Function, Args}, Priority).

After the hook is completed, the callback functions will be executed in the order of priority, or the order of hook for the same priority. All official plugin mount hooks have a priority of 0.

Unhook：

%% Name:name of hook(hook point), such as'client.authenticate'
%% {Module, Function}: Modules and methods for callback functions
emqx:unhook(Name, {Module, Function}).

Callback function

The input parameters and returned value of the callback function are shown in the following table:

(For parameter data structure, see:emqx_types.erl (opens new window))

For the application of these hooks, see:emqx_plugin_template (opens new window)