DEV Community: Juhee Kang

Control flow: try-catch or if-else?

Juhee Kang — Tue, 06 Apr 2021 12:03:01 +0000

Introduction

Lately, while working on a new project, I had a chance to think about the proper way to handle control flow. As all developers might know well, the most common approaches used are try-catch and if-else. So far, I haven't thought deeply about the difference between these two. From time to time, I think I mainly adopted a more concise way of writing code. Stability was significant on this project, applying the appropriate control flow processing approach was one of the keys. For this reason, I wrote pseudocode based on the scenario for these two approaches and compared them, and I'd like to share the result with this community.

Scenario

Suppose that the signup logic of the virtual Web service 'A' is not allowed to have duplicate nicknames, e-mails, and phones.

The signup verification process is as follows:
1. Check for duplicate nickname.
2. Check duplicate on personal information.
a. Verify email address is unique.
b. Verify phone number is unique.

The code below is an implementation of this logic.(I've excluded error-handling logic at here on purpose.)

// signUp.js 
function signUp(){      // function of signup
     userTable.isUnique(nickname) // Check for duplicate nickname.
     duplicateDetailInfo(email, phone); // Check duplicate on personal information. 
     addUser(user) // Add a user
}  

// duplicateDetailInfo.js
function duplicateDetailInfo(email, phone){
    userTable.isUnique(email);  // Verify email address is unique.
    userTable.isUnique(phone);  // Verify phone number is unique.
}


// userTable.js
class userTable {
    function isUnique(value){
        // Will be implemented according to each approach
    }
}

Although it's not perfect, I purposely wrote the code to explain the difference in control flow.

On this code, depending on how to implement `userTable.isUnique`as Exception or return values(false, null, etc..), I will explain using `try-catch` and `if-else`, respectively.

`try-catch` Exception

On this implementation approach, userTable.isUnique() will raise an error if the value exists.

// signup.js 
function signUp(){
     try {
         userTable.isUnique(nickname);      // Raise Exception if the nickname is not unique.
         duplicateDetailInfo(email, phone); // Check for duplicate personal information.
    } catch (e) {
        console.log("fail")
    }
    addUser(user);
}  

// duplicateDetailInfo.js
function duplicateDetailInfo(email, phone){
    userTable.isUnique(email);  // Raise Exception if the email is not unique.
    userTable.isUnique(phone);  // Raise Exception if the phone is not unique.
}

// userTable.js
class userTable {
    function isUnique(value){
        value = userDB.find(value);
        return !value? true: throw Error(); // Raise Exception if the value is not unique.
    }
}

The problem with this approach is that the flow of processing is not explicitly visible.

signUp()
├── try
│   ├── .isUnique(nickname)
│   │    └── raise Exception
│   │   
│   └── duplicateDetailInfo()
│       ├── .isUnique(email)
│       │    └── raise Exception
│       │   
│       └── .isUnique(phone)
│            └── raise Exception
│           
└── catch

For example, the nickname exception is processed by the parent function (signUp), so you can easily find the order of the control flow. However, for email and phone exceptions, it is difficult to identify the control flow because it is not easy to know where to handle the exceptions.

This code consists of two depths, so it is easy to verify, but it is not known what happens to the control flow after this line code. In case when try-catch is used multiple times, it will become harder to figure out the control flow. Furthermore, code will be less intuitive.

Of course, this try-catch approach has the advantage of being able to handle all exceptions in one place. But, this can be a disadvantage. However, if there are hundreds of exceptions, the code may also be less intuitive because different exception logic is handled in a single location.

if - else

On this implementation approach, userTable.isUnique() will return true if the value exists.

// signup.js 
function signUp(){
    if (!userTable.isUnique(nickname)) {    // Return false if the nickname is not unique.
      return console.log("fail")
    }   
    if(!duplicateDetailInfo(email, phone)) {  // Return false if the details is not unique.
      return console.log("fail")
    };
    addUser(user);
}  

// duplicateDetailInfo.js
function duplicateDetailInfo(email, phone){
    if(!userTable.isUnique(email)) {        // Return false if the email is duplicated.
        return false;
    }
    if(userTable.isUnique(phone)) {         // Return false if the phone is duplicated.
        return false;
    };
    return true
}

// userTable.js
class userTable {
    function isUnique(value){
        value = userDB.find(value);
        return value? true: false;          // Return false if the value is not unique.
    }
}

For this approach, predictable and formatting can safely implement code is an advantage.

The advantage of this approach is that it can predict the flow of code and implement the code by specifying the return type(in this case, boolean). Since try-catch statement is not used, the control flow can be easily to figure out because it is handlded directly from a parent function rather than from another location(exception catch). By this benefit, it is usally possible to check quickly even if something goes wrong.

signUp()
├── .isUnique(nickname)
│    └── return false? => handling error
│   
└── duplicateDetailInfo()
     └── return false? => handling error

In the previous try-catch case shown earlier, it is difficult to identify control flow because it was not easy to determine where exceptions is handled in the case of overlapping email and phone. On the other hand, for the if-else approach, the control flow is intuitively processed according to the return value of function, so it is easy to find out what logic runs next and where the error occurs.

Of course, this approach also has the disadvantage of having to branch out with if statements for every case that occurs.

Conclusion

I looked up a lot of materials about control flow and found that using try-catch was considered an anti-pattern because it is difficult to identify the flow of code. Therefore, I believe that the best way is to treat the control flow as an intuitive if-else approach according to the return value, even if the amount of code is large, rather than a try-catch approach where the code is concise but unclear where an exception is handled.

If you want to get further details about why try-catch exception handling is an anti-pattern, I recommend you refer to this post.

This article is originally published by myself here.

The Best Way To Migrate a RabbitMQ Server

Juhee Kang — Sat, 03 Apr 2021 03:16:37 +0000

Instead of repetitive tasks, I discovered a relationship between RabbitMQ settings and a hostname, which ultimately decreased repetition when migrating servers.

Introduction

As the userbase of my company increased over time, I realized it was necessary to apply auto-scaling to my servers. It required server migration. Migrating RabbitMQ settings meant a lot of repetitive tasks to ensure a smooth migration process; rather than perform this, I discovered a relationship between the RabbitMQ settings and the hostname, which ultimately decreased repetition.

In this article, I'll explain how to migrate RabbitMQ using the hostname.

A Basic Way To Migrate RabbitMQ (Using rabbitmqctl)

In order to migrate RabbitMQ to the same existing server setting, you need to manually set each of the settings (such as vhost, username, permission, etc.) from the existing server. For example, there is a RabbitMQ server consisted of the settings which the Vhost is test and username is Claudia. If you migrate this server, you will go through the following steps.

Check the Existing Server RabbitMQ Setting

1) Check the vhost

$ sudo rabbitmqctl list_vhosts
Listing vhosts ...
name
/
test

2) Check the username

$ sudo rabbitmqctl list_users 
Listing users ... 
user tags 
claudia [test] 
guest [administrator]

Try To Migrate To the RabbitMQ Server

1) Install the RabbitMQ server on a new server
2) Create a vhost using rabbitmqctl on the new server

$ sudo rabbitmqctl add_vhost test

3) Create a user using rabbitmqctl on the new server

$ sudo rabbitmqctl add_user claudia claudia

4) Set user permission using rabbitmqctl on the new server

$ sudo rabbitmqctl set_user_tags claudia administrator 
$ sudo rabbitmqctl set_permissions -p test claudia ".*" ".*" ".*"

It is easy to migrate the RabbitMQ server using this way when the server has some vhost, user, etc… But if the server has many settings, it is inefficient to migrate this way because it will take a lot of time. (Maybe it requires repetition according to the settings.)

Therefore, I will introduce a new migration way without repetition that works.

A New Way To Migrate RabbitMQ Using Copy and Setting Hostname

For using the new way, we need to check the setting files. You can find the setting files on /var/lib/rabbitmq. Thus, let's start a migration using these files.

Check the Existing Server RabbitMQ Setting

$ sudo rabbitmqctl list_vhosts
Listing vhosts ...
name
/
test
$ sudo rabbitmqctl list_users
Listing users ...
user tags
claudia [test]
guest [administrator]
$ hostname
claudia

Try To Migrate the RabbitMQ Server

1) Find the setting files on the existing server
You can find the setting files according to the following command. (Tip: the path of RabbitMQ setting files is /var/lib/rabbitmq.)

$ ls /var/lib/rabbitmq/
mnesia
$ cd /var/lib/rabbitmq/mnesia/
root@claudia:/var/lib/rabbitmq/mnesia# ls
rabbit@claudia rabbit@claudia-feature_flags
rabbit@claudia.pid rabbit@claudia-plugins-expand

2) Install the RabbitMQ server on a new server.

3) Copy the setting file of the existing server to /var/lib/rabbitmq/mnesia on the new server.

$ scp -r /var/lib/rabbitmq/mnesia <new-server-ip>:/var/lib/rabbitmq/mnesia

4) Change the hostname
If the hostname is not equal to the past one, the new server will not work properly.

If the hostname is not equal to the past one, the new server will not work properly.

$ hostnamectl set-hostname claudia 
$ hostname 
claudia

5) Restart the RabbitMQ server
$ sudo systemctl restart rabbitmq-server

Check the RabbitMQ Setting on the New Server

You can see that the migration is completed successfully:

$ rabbitmqctl list_vhosts
Listing vhosts ...
name
/
test
$ rabbitmqctl list_users
Listing users ...
user tags
claudia [test]
guest [administrator]

With this method, no matter how many settings you have, it is easy to complete the migration.

How Is This Possible?

When starting the RabbitMQ server, it reads the environment files from the base directory according to rabbitMQ_NODENAME. Let's check the initialization environment process of the rabbitmq-server code, which is a function(get_prefixed_env_var).

rabbit_env.erl#L464–473

%% -------------------------------------------------------------------
%%
%% RABBITMQ_NODENAME
%%   Erlang node name.
%%   Default: rabbit@
%%
%% RABBITMQ_USE_LONGNAME
%%   Flag indicating if long Erlang node names should be used instead
%%   of short ones.
%%   Default: unset (use short names)

rabbit_env.erl#L744-L836

%% -------------------------------------------------------------------
%%
%% RABBITMQ_MNESIA_BASE
%%   Directory where to create Mnesia directory.
%%   Default: (Unix) ${SYS_PREFIX}/var/lib/rabbitmq/mnesia
%%         (Windows) ${RABBITMQ_BASE}/db
%%
%% RABBITMQ_MNESIA_DIR
%%   Directory where to put Mnesia data.
%%   Default: (Unix) ${RABBITMQ_MNESIA_BASE}/${RABBITMQ_NODENAME}
%%         (Windows) ${RABBITMQ_MNESIA_BASE}\${RABBITMQ_NODENAME}-mnesia

mnesia_base_dir(#{from_remote_node := Remote} = Context) ->
    case get_prefixed_env_var("RABBITMQ_MNESIA_BASE") of
        false when Remote =:= offline ->
            update_context(Context, mnesia_base_dir, undefined, default);
        false ->
            mnesia_base_dir_from_node(Context);
        Value ->
            Dir = normalize_path(Value),
            update_context(Context, mnesia_base_dir, Dir, environment)
    end;
mnesia_base_dir(Context) ->
    mnesia_base_dir_from_env(Context).


mnesia_base_dir_from_env(Context) ->
    case get_prefixed_env_var("RABBITMQ_MNESIA_BASE") of
        false ->
            Dir = get_default_mnesia_base_dir(Context),
            update_context(Context, mnesia_base_dir, Dir, default);
        Value ->
            Dir = normalize_path(Value),
            update_context(Context, mnesia_base_dir, Dir, environment)
    end.
...

This means that if you copy the existing setup file format to the base directory /var/lib/RabbitMQ/mnesia, the setup will be applied as it is. In addition, you can see this type of setup files such as rabbit@<hostname>-<option> because of the default value of RabbitMQ_NODENAME(rabbit@<hostname>).

Therefore, if you have a hostname that is not the same as the existing server, the server will not run normally.

Conclusion

There are many different ways to migrate RabbitMQ, but it is difficult to find a way to do it easily and quickly. If you need to migrate RabbitMQ with complex settings, I recommend you use this hostname and the setup file copy method. Maybe using hostname makes for a more easy time to migrate RabbitMQ.

This article is originally published by myself here.

DEV Community: Juhee Kang

Control flow: try-catch or if-else?

Introduction

Scenario

On this code, depending on how to implement userTable.isUniqueas Exception or return values(false, null, etc..), I will explain using try-catch and if-else, respectively.

try-catch Exception

if - else

Conclusion

The Best Way To Migrate a RabbitMQ Server

Introduction

A Basic Way To Migrate RabbitMQ (Using rabbitmqctl)

Check the Existing Server RabbitMQ Setting

Try To Migrate To the RabbitMQ Server

A New Way To Migrate RabbitMQ Using Copy and Setting Hostname

Check the Existing Server RabbitMQ Setting

Try To Migrate the RabbitMQ Server

Check the RabbitMQ Setting on the New Server

How Is This Possible?

Conclusion

On this code, depending on how to implement `userTable.isUnique`as Exception or return values(false, null, etc..), I will explain using `try-catch` and `if-else`, respectively.

`try-catch` Exception