Introduction
Erlang, a functional programming language created by Ericsson in the late 1980s, is known for its exceptional reliability and scalability. Originally designed for building telecom systems, Erlang has proven itself in a wide range of applications, from web servers to financial systems. One of its standout features is the ease with which it allows developers to build reliable distributed systems. In this article, we’ll explore some best practices in Erlang development to help you harness its full potential for creating robust distributed systems.
1. Concurrency and Processes
Erlang is designed for concurrent and parallel programming. In Erlang, processes are lightweight, isolated units of execution, making it possible to spawn thousands or even millions of processes without exhausting system resources. To create a new process, you use the spawn function. Here’s a simple example:
-module(my_module).
-export([start/0, worker/1]).spawn(my_module, worker, [“Hello, Erlang!”]).
io:format(“Received message: ~s~n”, [Message]).
In this example, the start/0 function spawns a new process that runs the worker/1 function with the message “Hello, Erlang!”.
2. Fault Tolerance with Supervisors
Fault tolerance is one of Erlang’s most celebrated features. It’s achieved through the use of supervisors, which are processes responsible for monitoring and restarting worker processes in case of failures. Here’s a simple supervisor example:
-module(my_supervisor).
-behaviour(supervisor).-export([init/1]).
supervisor:start_link({local, ?MODULE}, ?MODULE, []).
{ok, {{one_for_one, 5, 10}, []}}.
In this example, my_supervisor is a simple one-for-one supervisor, which means it will restart a failed child process without affecting others. The supervisor is configured to restart a maximum of 5 times within 10 seconds.
3. Message Passing
Erlang’s message-passing mechanism enables communication between processes. This is the foundation of building distributed systems. Here’s an example of sending and receiving messages between processes:
-module(message_example).
-export([start/0, worker/0]).WorkerPid = spawn(message_example, worker, []),
WorkerPid ! {self(), “Hello, worker”},
receive
{WorkerPid, Reply} ->
io:format(“Received reply: ~s~n”, [Reply])
after 5000 ->
io:format(“No reply received~n”)
end.
receive
{From, Message} ->
io:format(“Worker received: ~s~n”, [Message]),
From ! {self(), “Hello, from worker”}
end.
In this example, the start/0 function creates a worker process and sends a message to it. The worker, upon receiving the message, sends a reply back to the sender.
4. Hot Code Swapping
Erlang supports hot code swapping, allowing you to upgrade your system without stopping it. This is invaluable for building highly available systems. Here’s a basic example of code swapping:
-module(my_server).
-export([start/0, loop/0]).spawn(my_server, loop, []).
receive
{upgrade} ->
% Load and apply a new version of the code here
io:format(“Upgraded code~n”),
loop();
{request, Msg} ->
io:format(“Received request: ~s~n”, [Msg]),
loop()
end.
In this example, you can send an {upgrade} message to the server process to apply a new version of the code, all without stopping the server.
5. Distribution with Erlang/OTP
Erlang’s built-in support for distribution makes it easy to build distributed systems. Erlang/OTP (Open Telecom Platform) provides abstractions like gen_server and gen_fsm for creating distributed servers and finite state machines. Here’s a simplified example of a distributed system:
-module(distributed_server).
-behaviour(gen_server).-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3]).
gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
{ok, []}.
Reply = process_request(Request),
{reply, Reply, State}.
{noreply, State}.
{noreply, State}.
ok.
{ok, State}.
% Processing logic here
Request.
In this example, distributed_server is a simple gen_server. By using distribution, you can run multiple instances of this server on different nodes and communicate seamlessly between them.
6. Testing and Test-Driven Development (TDD)
Testing is critical for building reliable systems. Erlang provides a powerful framework for writing unit and integration tests. Using libraries like EUnit and Common Test, you can ensure your code behaves as expected. Here’s a simple EUnit test case:
-module(my_module_tests).
-include_lib("eunit/include/eunit.hrl").?assertEqual(ok, my_module:start()),
?assertEqual(“Hello, from worker”, my_module:worker(“Hello, worker”)).
This test case checks if the start/0 function and worker/1 function behave correctly.
7. Monitoring and Tracing
Erlang/OTP provides tools for monitoring and tracing processes in real-time. You can use observer or programmatic tools like sys to inspect system activity, diagnose performance issues, and debug problems.
Conclusion
Erlang’s unique features make it a standout choice for building reliable distributed systems. By following these best practices, you can harness its power to create robust and fault-tolerant applications. Erlang’s message-passing model, lightweight processes, supervision trees, and hot code swapping capabilities are essential tools for building distributed systems that can handle failures gracefully and provide high availability. When combined with a test-driven development approach and monitoring tools, Erlang becomes a formidable platform for tackling complex distributed problems.