Erlang/OTP Kernel应用基础：启动与服务详解

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Erlang/OTP中的Kernel应用是启动的第一个应用程序，它是构建最小系统所必需的，其中包括Kernel和STDLIB这两个基础组件。Kernel应用提供了一系列关键服务，这些服务对整个Erlang运行时环境的正常运行至关重要。 1. **应用控制器** (application controller): 应用控制器负责管理其他应用程序的加载、停止和监控，确保它们按照预期的顺序和方式启动。 2. **代码管理** (code): 包括代码加载、存储和更新功能，这是Erlang程序执行的基础，允许模块在运行时被动态加载。 3. **磁盘日志** (disk_log): 提供错误检测和恢复机制，当系统崩溃时，通过检查日志来帮助恢复进程状态。 4. **分布式应用控制器** (dist_ac): 用于协调分布式应用程序的部署、通信和故障恢复，支持Erlang系统在多个节点上的扩展。 5. **erl_boot_server** 和 **erl_ddll**: 分别负责系统启动时的引导过程和动态链接库（DLL）的支持，确保系统模块的正确加载。 6. **错误日志器** (error_logger): 记录和处理程序运行时的错误和警告信息，帮助诊断问题。 7. **文件系统服务** (file): 提供文件操作接口，包括读写、目录管理等，为Erlang程序与外部数据交互提供便利。 8. **全局变量管理** (global): 允许在进程之间共享和同步数据，是实现跨进程通信的重要工具。 9. **心跳检测** (heart): 监控系统中的进程是否存活，确保分布式系统的健康运行。 10. **网络接口** (inet): 提供网络服务，如TCP/IP连接、IP地址管理和网络通信，支持远程进程间通信。 11. **Net Kernel** (net_kernel): 网络核心，它处理网络连接的底层细节，确保数据的可靠传输。 12. **操作系统接口** (os): 提供与操作系统交互的功能，如进程管理、内存管理、信号处理等。 13. **PG2** (Probabilistic Garbage Collection): 是一种优化的垃圾回收机制，用于提高性能和内存管理效率。 14. **RPC** (Remote Procedure Call): 远程过程调用服务，使Erlang进程能够在分布式环境中像调用本地函数一样调用其他节点上的函数。 15. **序列跟踪** (seq_trace): 用于调试和分析系统行为的工具，记录进程之间的消息交换。 16. **用户接口** (user): 提供命令行工具和其他用户界面，使得开发者可以方便地与Erlang环境进行交互。 Kernel应用是Erlang/OTP系统的核心部分，它确保了运行环境的稳定性、扩展性和高效性。理解并熟悉这些服务对于开发、部署和维护基于Erlang的应用至关重要。

application

takeover(Application, Type) -> ok | {error, Reason}

Types:

Application = atom()

Type = restart_type()

Reason = term()

Performs a takeover of the distributed application Application, which executes at another node Node. At

the current node, the application is restarted by calling Module:start({takeover,Node},StartArgs).

Module and StartArgs are retrieved from the loaded application specification. The application at the

other node is not stopped until the startup is completed, i.e. when Module:start/2 and any calls to

Module:start_phase/3 have returned.

Thus two instances of the application will run simultaneously during the takeover, which makes it possible to transfer

data from the old to the new instance. If this is not acceptable behavior, parts of the old instance may be shut down when

the new instance is started. Note that the application may not be stopped entirely however, at least the top supervisor

must remain alive.

See start/1,2 for a description of Type.

unload(Application) -> ok | {error, Reason}

Types:

Application = atom()

Reason = term()

Unloads the application specification for Application from the application controller. It will also unload the

application specifications for any included applications. Note that the function does not purge the actual Erlang object

code.

unset_env(Application, Par) -> ok

unset_env(Application, Par, Opts) -> ok

Types:

Application = Par = atom()

Opts = [{timeout, timeout()} | {persistent, boolean()}]

Removes the configuration parameter Par and its value for Application.

unset_env/2 uses the standard gen_server timeout value (5000 ms). The timeout option can be provided if

another timeout value is useful, for example, in situations where the application controller is heavily loaded.

unset_env/3 also allows the persistent option to be passed (see set_env/4 above).

Warning:

Use this function only if you know what you are doing, that is, on your own applications. It is very application

and configuration parameter dependent when and how often the value is read by the application, and careless use

of this function may put the application in a weird, inconsistent, and malfunctioning state.

which_applications() -> [{Application, Description, Vsn}]

which_applications(Timeout) -> [{Application, Description, Vsn}]

Types:

application

Timeout = timeout()

Application = atom()

Description = Vsn = string()

Returns a list with information about the applications which are currently running. Application is the application

name. Description and Vsn are the values of its description and vsn application specification keys,

respectively.

which_applications/0 uses the standard gen_server timeout value (5000 ms). A Timeout argument can

be provided if another timeout value is useful, for example, in situations where the application controller is heavily

loaded.

CALLBACK MODULE

The following functions should be exported from an application callback module.

Exports

Module:start(StartType, StartArgs) -> {ok, Pid} | {ok, Pid, State} | {error,

Reason}

Types:

StartType = start_type()

StartArgs = term()

Pid = pid()

State = term()

This function is called whenever an application is started using application:start/1,2, and should start the

processes of the application. If the application is structured according to the OTP design principles as a supervision

tree, this means starting the top supervisor of the tree.

StartType defines the type of start:

• normal if it's a normal startup.

• normal also if the application is distributed and started at the current node due to a failover from another node,

and the application specification key start_phases == undefined.

• {takeover,Node} if the application is distributed and started at the current node due to a takeover from

Node, either because application:takeover/2 has been called or because the current node has higher

priority than Node.

• {failover,Node} if the application is distributed and started at the current node due to a failover from

Node, and the application specification key start_phases /= undefined.

StartArgs is the StartArgs argument defined by the application specification key mod.

The function should return {ok,Pid} or {ok,Pid,State} where Pid is the pid of the top supervisor and

State is any term. If omitted, State defaults to []. If later the application is stopped, State is passed to

Module:prep_stop/1.

Module:start_phase(Phase, StartType, PhaseArgs) -> ok | {error, Reason}

Types:

Phase = atom()

StartType = start_type()

PhaseArgs = term()

application

Pid = pid()

State = state()

This function is used to start an application with included applications, when there is a need for synchronization

between processes in the different applications during startup.

The start phases is defined by the application specification key start_phases == [{Phase,PhaseArgs}].

For included applications, the set of phases must be a subset of the set of phases defined for the including application.

The function is called for each start phase (as defined for the primary application) for the primary application and all

included applications, for which the start phase is defined.

See Module:start/2 for a description of StartType.

Module:prep_stop(State) -> NewState

Types:

State = NewState = term()

This function is called when an application is about to be stopped, before shutting down the processes of the application.

State is the state returned from Module:start/2, or [] if no state was returned. NewState is any term and

will be passed to Module:stop/1.

The function is optional. If it is not defined, the processes will be terminated and then Module:stop(State) is

called.

Module:stop(State)

Types:

State = term()

This function is called whenever an application has stopped. It is intended to be the opposite of Module:start/2

and should do any necessary cleaning up. The return value is ignored.

State is the return value of Module:prep_stop/1, if such a function exists. Otherwise State is taken from

the return value of Module:start/2.

Module:config_change(Changed, New, Removed) -> ok

Types:

Changed = [{Par,Val}]

New = [{Par,Val}]

Removed = [Par]

Par = atom()

Val = term()

This function is called by an application after a code replacement, if there are any changes to the configuration

parameters.

Changed is a list of parameter-value tuples with all configuration parameters with changed values, New is a list of

parameter-value tuples with all configuration parameters that have been added, and Removed is a list of all parameters

that have been removed.

Erlang/OTP Kernel应用基础：启动与服务详解

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