Rebuilding Redis in Ruby - Chapter 1 - A basic TCP server

What we'll cover

This chapter will cover the creation of a TCP server in ruby, as well as how to interact with it with the netcat utility , available as nc on linux and macOS. We will briefly look at concurrency and parallelism and how threads can
impact the behavior of our server.

Introduction

The goal of this book is to implement a Redis-like server, "from scratch". At the time of this writing Redis supports 9 different data types, dozens of commands related to those data types as well as many features in the Administration category, such as Redis Sentinel for High Availability.

I will start with a heavily simplified version of Redis, and slowly add more features, trying to get as close as possible to what Redis supports today.

I'm choosing Ruby for this book for no other reasons that I like it, and I find it fun to play with. Ruby is often characterized as a "slow language" but performance is not a big concern here. While I will make sure to make sound
implementation choices, the goal is to learn more about Redis, Ruby, networking, OS signals and so on and not to produce
a production ready software.

A note about "from scratch"

"From scratch" can be a an ambiguous term, especially with a language like Ruby that provides so many features out of
the box.
My goal with this book is to rely exclusively on the Ruby standard library. At the time of this writing, April 2020, the
latest Ruby version is 2.7.1.

And now, let's write some code

The main Redis component is redis-server, which is an executable that starts a TCP server. When experimenting with
Redis, it is common to use redis-cli, which is an executable that starts a REPL client that connects to a redis server.
By default Redis runs on port 6379 locally.

The official Ruby documentation shows the following example for the TCPServer class:

require 'socket'

server = TCPServer.new 2000 # Server bind to port 2000
loop do
  client = server.accept    # Wait for a client to connect
  client.puts "Hello !"
  client.puts "Time is #{Time.now}"
  client.close
end

Followed by the following example and comment: "A more usable server (serving multiple clients)"

require 'socket'

server = TCPServer.new 2000
loop do
  Thread.start(server.accept) do |client|
    client.puts "Hello !"
    client.puts "Time is #{Time.now}"
    client.close
  end
end

I will ignore the second example for now because concurrency/parallelism is a complicated topic and I want to address it in depth in a later post.
So, for now, let's focus on the first example, line by line:

First, we require 'socket'. Ruby's require syntax has always been weird to me, especially compared to more explicit
ones like Python's for instance.
After a bit of browsing the Ruby source code, my guess is that this require will add all the constants defined in the c
files in the lib/socket/ folder of the ruby source code to $LOAD_PATH, making a bunch of
classes such as Addrinfo, UnixServer,
UNIXSocket, UDPSocket, TCPSocket,
TCPServer & SOCKSocket.
You can try this on your own in an irb shell, requiring any of these constants would fail with a NameError before
calling require 'socket' and would work afterwards:

irb(main):001:0> TCPSocket
Traceback (most recent call last):
# [truncated]
NameError (uninitialized constant TCPSocket)
irb(main):002:0> UNIXServer
Traceback (most recent call last):
# [truncated]
NameError (uninitialized constant UNIXServer)
irb(main):003:0> require 'socket'
=> true
irb(main):004:0> TCPSocket
=> TCPSocket
irb(main):005:0> UNIXServer
=> UNIXServer

The next line creates a new TCPServer instance, listening on port 2000. The documentation for new is the following:

new([hostname], port) => tcpserver

Creates a new server socket bound to port.

If hostname is given, the socket is bound to it.

Internally, ::new calls getaddrinfo() function to obtain addresses. If getaddrinfo() returns multiple addresses, ::new
tries to create a server socket for each address and returns first one that is successful.

What the documentation implies is that the hostname argument is optional and that omitting it runs on localhost by default.
Great, so now we have a TCP server running on localhost:2000, but is it actually doing anything? Let's see.

We can run a server in an irb shell:

irb(main):001:0> require 'socket'
=> true
irb(main):002:0> TCPServer.new 2000
=> #<TCPServer:fd 10, AF_INET6, ::, 2000> # Note that fd value might be different on your system.

We'll spend more time digging into what these values mean, but for now let's just very briefly look at what we have
here:

fd 10

fd stands for File Descriptor, if you're interested you can see all the descriptors used by a process on macOS with the lsof tool: lsof -p <process id>. We can use pgrep to get the process id (often called pid) of the irb session we started the server from. There was only one result on my machine, 86096, but if you have more than irb sessions running, you can use ps aux to get more info about a specific process, like its start timestamp for instance. This is what the last line looked like for me, and we can see that there is an open file with the value 10 for
FD.

[...]
ruby    86096 pierre   10u   IPv6 0x80d76e9380eb855d      0t0     TCP *:callbook (LISTEN)

2000

This is the port value, which we passed as an argument to the constructor

AF_INET6 & ::

I wasn't exactly sure what the two values in the middle were, AF_INET6 & ::, so I first tried to figure out how this
string was built by looking at where the inspect method came from on the TCPServer instance:

irb(main):001:0> TCPServer.new(2000).method(:inspect)
=> #<Method: TCPServer(IPSocket)#inspect()>

This makes sense, TCPServer inherits from TCPSocket, which itself inherits from IPSocket. The inspect method on IPSocket is defined as a C function in ipsocket.c. It looks like the values come from the addr function, which according to the ruby documentation: "Returns the local address as an array which contains address_family, port,
hostname and numeric_address." A bit of Gooling about AF_INET6 confirmed this, AF stands for Address Family, and INET mean Internet Protocol v4, and INET6 means Internet Protocol v6. :: has a special meaning for IPv6 addresses, equivalent to 0.0.0.0 for IPv4.

Back to our server, if you still have a terminal open, where you ran TCPServer.new 2000 in an irb shell, now open a
new terminal and run nc localhost 2000. nc, which is the cli for the netcat utility, is used for, according to its
man page: "[...] just about anything under the sun involving TCP or UDP". telnet is another similar tool, but it
does not come bundled with macOS by default, that being said, it is only a brew install away.

Running nc localhost 2000 should "hang", nothing is happening and you cannot type more commands in the shell. Feel
free to exit with Ctrl-C. You can confirm that it did indeed do something, because if you try it with an unused port,
such as 2001, it should return right away, with an exit code of 1, aka, an error. If it hangs on port 2001 as well, it
might be because you have something running on port 2001 on your machine.

nc has a -w flag, to specify a timeout, you can see all available flags with nc -h. You can experiment yourself by
running the same command, with a value for -w. Running nc -w 5 localhost 2000 will wait for 5 seconds and exit with
a status of 0 after that.

Regardless of the environment, I think it's an overall best practice to set timeouts when dealing with network calls
like HTTP requests. While it is unlikely that a server would have such a blatant bad behavior as the one we currently
have, accept a request and do absolutely nothing with it, it is entirely possible that a server would take a very long
time to return a response, if a spike in incoming traffic is causing the server to become unresponsive for instance. In
this case, it might be beneficial for the client to abort after the timeout, instead of waiting.

Quoting the wikipedia page:

Each process returns an exit status, as an integer when it terminates. 0 represents a success, and every other values.

Some terminals are configured to show the exit status, some do it only for non zero codes, but you can always use echo $? to see the status of the last command. Running ls followed by echo $? should output 0.

If you've seen C code, the main function's signature is int main() because the value returned by main is the value used as the exit status, unless you call exit explicitly.
So this is why you might see return 0 at the end of the main function in a C program, it means "return success". It also seems common to use exit(EXIT_SUCCESS) or exit(EXIT_FAILURE).

Some codes have special meanings, such as 127 meaning "Command not found"

Exit statuses are not specific to POSIX systems as documented on Wikipedia.

Let's dive a bit deeper by passing the verbose flag, -v: nc -v localhost 2000:

found 0 associations
found 1 connections:
     1: flags=82<CONNECTED,PREFERRED>
        outif lo0
        src 127.0.0.1 port 53022
        dst 127.0.0.1 port 2000
        rank info not available
        TCP aux info available

Connection to localhost port 2000 [tcp/callbook] succeeded!

So it did connect, and it does nothing, because our server was just started, but it wasn't instructed to do anything
with the incoming connection.

loop in Ruby starts an infinite loop, nothing special here, it is common for a server to start and never end. Think
about Redis for a minute, once the server is running, we want it to run forever, we don't expect it to stop unless told
to do so. An inifinite loop makes perfect sense for a use case like that.

The next line is really interesting, and probably one of the most interesting ones in this small snippet: server.accept.
Let's go back to irb for a moment, because it makes it easier to experiment with these methods, one at a time.

irb(main):001:0> require 'socket'
=> true
irb(main):002:0> s = TCPServer.new 2000
irb(main):003:0> s.accept

The accept method does not return. The documentation is sadly very succint:

Accepts an incoming connection. It returns a new TCPSocket object.

What it doesn't tell us is that it effectively a "blocking" method. This can technically be inferred by the presence of
another method on TCPServer, accept_nonblock, which, according to the documentation:

Accepts an incoming connection using accept(2) after O_NONBLOCK is set for the underlying file descriptor. It returns
an accepted TCPSocket for the incoming connection.

We'll look closer at accept_nonblock in a later chapter. Let's go back to our second shell, note that the hanging nc
localhost 2000 ended if you closed the first irb shell. What happened is that the server closed the connection, and
the client, nc, stopped as well. Let's re-run the same command, nc localhost 2000. We'll see a very similar output
we saw above, saying that the connection succeeded.
The main difference is that the hanging accept call in irb now returned, with a TCPSocket instance. Let's get a
reference to this socket with socket = _ (_ is a reference to the last value returned in irb) and send something
to our client: socket.puts "Hey!". If you go back to the terminal where you ran nc, you should see "Hey!". Let's now
close the connection with socket.close and we can observer that the nc calls returned, with an exit code of 0, aka,
success.

And that's it, we went through the official example of TCPServer. The example starts a TCP server on port 2000, and
then enters an infinite loop, it first waits for an incoming connection, and does nothing else until a client connects,
once a client connects, it writes "Hello !" first and then the current time and finally closes the connection, and
starts over.

If you remember, at the beginning we briefly looked at the second example in the documentation, using Thread.start. The justification for this example was that it was more usable by being able to serve multiple clients.
There is indeed a major issue with the initial example, it can only serve one client at a time. If you were to put the
example in a file, say, server.rb and run it with ruby server.rb, it would start one process, one thread and start
executing the loop.
The second example improves the situation by passing the result of the blocking operation, server.accept, to a new
thread and letting it handle the client.

There are a few important things to note here. First, ruby does not support lazy arguments, so when we write
Thread.start(server.accept), we first need to evaluate the argument, and only then will start be called, with the
result of the evaluation.
What that means for our example is that the loop starts, then blocks until a client connects, and once the client is
connected, the resulting socket is passed to Thread.start.

Let's illustrate this with an example, we'll simulate a slow server by adding a sleep call. Still in irb, run the
following:

loop { socket = server.accept; socket.write "Hello"; sleep 5; socket.close }

This is almost identical to the first example, except that the main thread sleeps for five seconds before closing the
socket. Back to the other terminal, run nc localhost 2000 again, and you'll see "Hello" being printed almost
instantly, followed by the process hanging for five seconds and then exiting. While it is hanging, open a terminal and
run the same command, nc localhost 2000, if you see "Hello" right away, it might be because the five seconds elapsed
in the previous terminal. Feel free to close the inifinite loop in irb with Ctrl-C and increase the value to 10
seconds or more and experiment with this.

What this shows us is that while the server is busy dealing with a client, even if it doesn't do anything and just
sleeps, all other incoming clients are effectively waiting to be served.
The second example improves on this as can be seen in irb if you run the following instead:

loop {
  Thread.start(server.accept) { |socket|
    socket.write "Hello"
    sleep 5
    socket.close
  }
}

Running the same manual test, we should see that both nc localhost 2000 calls get the "Hello" response, and then they
proceed to both wait for five seconds until the server closes the socket.
This is because each client is being handled by a different thread. The first client will trigger the first
server.accept call to return, resulting in a second thread being started (second thread because the initial program is
itself running in a thread as well). While the second thread is sleeping, the main thread is not blocked anymore, it
passed the socket to the second thread and is back at being blocked on server.accept. When the second client connects,
the same thing happens, a new thread is started, and it's being given the newly created socket.

We have a server that can server multiple clients at one, which is great. That being said, there is a big problem with
this approach. Threads are not unlimited, if we were to create more and more threads, we could be at risk of slowing
down the whole system. Using multiple threads is something that needs to be done very carefully and it can easily lead
to race conditions. As mentioned earlier, concurrency and parallelism are both complicated topics and we'll try to cover
them in future chapters.

Conclusion

We now know how to run a basic TCP server, it can write strings to its clients and then close the connection, that's
it. That being said, as we'll see in future chapters, we can do a lot with that. We also looked at the limitiation of a
single threaded approach, and while we could use threads to improve the situation, we are purposefully not doing it for
now, to keep our example simple and improve it one step at a time.

In the next chapter we'll create a server class and make it read input from clients, and respond to GET and SET, in
their most basic forms, we won't implement things like TTL or other options,

Appendix - A C implementation of the server

I was interested to see what a lower level implementation would be like, so I tried to put together an example in C, of
a server, listening on port 2000, and writing data back to clients when they connect. Here is the code.

Note: If you do not have gcc installed your machine, you can install in on macOS with the following command:
xcode-select --install, if you're on a different OS, I'll let you search, this should be a fairly common questions and
have lots of answers on stackoverflow and the likes.

The code I'm sharing here is a simplified version of the client/server code available on
GeeksforGeeks:

Server:

#include <stdio.h> // For printf
#include <netdb.h> // For bind, listen, AF_INET, SOCK_STREAM, socklen_t, sockaddr_in, INADDR_ANY
#include <stdlib.h> // For exit
#include <string.h> // For bzero
#include <unistd.h> // For close & write
#include <errno.h> // For errno, duh!
#include <arpa/inet.h> // For inet_ntop

#define MAX 80
#define PORT 2000
#define SA struct sockaddr

int main()
{
    socklen_t client_address_length;
    int server_socket_file_descriptor, client_socket_file_descriptor;
    struct sockaddr_in server_address, client_address;

    // socket create and verification
    server_socket_file_descriptor = socket(AF_INET, SOCK_STREAM, 0);
    if (server_socket_file_descriptor == -1) {
        printf("socket creation failed...\n");
        exit(0);
    }
    else {
        printf("Socket successfully created..\n");
    }
    bzero(&server_address, sizeof(server_address));

    // assign IP, PORT
    server_address.sin_family = AF_INET;
    server_address.sin_addr.s_addr = htonl(INADDR_ANY);
    server_address.sin_port = htons(PORT);

    // Binding newly created socket to given IP and verification
    if ((bind(server_socket_file_descriptor, (SA*)&server_address, sizeof(server_address))) != 0) {
        printf("socket bind failed... : %d, %d\n", server_socket_file_descriptor, errno);
        exit(0);
    }
    else {
        printf("Socket successfully binded..\n");
    }

    // Now server is ready to listen and verification
    if ((listen(server_socket_file_descriptor, 5)) != 0) {
        printf("Listen failed...\n");
        exit(0);
    }
    else {
        printf("Server listening..\n");
    }
    client_address_length = sizeof(client_address);

    // Accept the data packet from client and verification
    client_socket_file_descriptor = accept(server_socket_file_descriptor, (SA*)&client_address, &client_address_length);
    if (client_socket_file_descriptor < 0) {
        printf("server acccept failed: %d,%d...\n", client_socket_file_descriptor, errno);
        exit(0);
    }
    else {
        printf("server acccept the client...\n");
        char human_readable_address[INET_ADDRSTRLEN];
        inet_ntop(AF_INET, &client_address.sin_addr, human_readable_address, sizeof(human_readable_address));
        printf("Client address: %s\n", human_readable_address);
    }

    char message_buffer[MAX];
    read(client_socket_file_descriptor, message_buffer, sizeof(message_buffer));
    printf("From Client: %s\n", message_buffer);
    bzero(message_buffer, MAX);

    strcpy(message_buffer, "Hello, this is Server!");
    write(client_socket_file_descriptor, message_buffer, sizeof(message_buffer));

    // After chatting close the socket
    printf("Closing server_socket_file_descriptor\n");
    close(server_socket_file_descriptor);
}

Client:

#include <stdio.h> // For printf
#include <netdb.h> // For AF_INET, SOCK_STREAM, sockaddr_in
#include <stdlib.h> // For exit
#include <string.h> // For bzero
#include <sys/socket.h> // For connect
#include <arpa/inet.h> // For inet_addr
#include <unistd.h> // for close

#define MAX 80
#define PORT 2000
#define SA struct sockaddr

int main() {
    int server_socket_file_descriptor;
    struct sockaddr_in server_address;

    // socket create and varification
    server_socket_file_descriptor = socket(AF_INET, SOCK_STREAM, 0);
    if (server_socket_file_descriptor == -1) {
        printf("socket creation failed...\n");
        exit(0);
    }
    else {
        printf("Socket successfully created..\n");
    }

    bzero(&server_address, sizeof(server_address));

    // assign IP, PORT
    server_address.sin_family = AF_INET;
    server_address.sin_addr.s_addr = inet_addr("127.0.0.1");
    server_address.sin_port = htons(PORT);

    // connect the client socket to server socket
    if (connect(server_socket_file_descriptor, (SA*)&server_address, sizeof(server_address)) != 0) {
        printf("connection with the server failed...\n");
        exit(0);
    }
    else {
        printf("connected to the server..\n");
    }

    char message_buffer[MAX] = "Hello, this is Client";
    write(server_socket_file_descriptor, message_buffer, sizeof(message_buffer));
    bzero(message_buffer, sizeof(message_buffer));
    read(server_socket_file_descriptor, message_buffer, sizeof(message_buffer));
    printf("From Server: %s", message_buffer);

    // close the socket
    close(server_socket_file_descriptor);
}

As usual, let's run a few manual tests, but since this is C, we first need to compile this:

$ gcc server.c -o server
$ gcc client.c -o client

We're going to need to shells, start the server in the first one, ./server. It should log the following:

Socket successfully created..
Socket successfully binded..
Server listening..

Note that, as we saw previously when creating a server from Ruby, it is "hanging" and hasn't returned yet. In the other
shell, run the client: ./client, you should see the following output:

$ ./client
Socket successfully created..
connected to the server..
From Server: Hello, this is Server!

And if you return to the other shell, where the server was running, you can see that it now returned and logged a few
more things before doing so:

server acccept the client...
Client address: 127.0.0.1
From Client: Hello, this is Client
Closing server_socket_file_descriptor

It works! A server, that waits until a client connects, reads what the client sent and writes a message back, and once all of that is done, exits.

Doing a step by step walkthrough of the client and server code is both a little bit out of scope and frankly something that I am not really capable of doing at the moment. That being said, I thought it would be interesting to visualize a similar-ish implementation to get a rough idea of what Ruby does for us under the hood.