In the general data serialization refers to a process in which we take an arbitrary data structure (a hash of hashes of arrays...) and convert it to a string in a way that the string can be later converted back to the same complex data structure in a process called de-serialization of data.

Once the data was serialized to a string it can be easily stored on disk (e.g. in a file or in a database), and it can be easily transmitted via some network protocol.

Instead of serialization some people use the word Marshalling or flattening of data.

Use cases

Two major uses of data serialization are data persistence and data communication or transmission

Data Persistence

While a process is running we can keep data in memory, but when the process ends, either naturally, or due to a crash, or due to some external force (someone killed the process or turned off the computer) this data is gone. If we want to keep the data for later usage we need to store it somewhere.

Next time when we launch the application we can then load the data from that storage.

The storage itself can be a file or a database which in turns saves our data in a file.

The easiest way we can store arbitrary complex data structures is to serialize them to a string.

This is called data persistence because the data persist beyond the life of the current process.

Data communication or transmission

If there are two processes running on the same machine that need to send data to each other, or if there are two processes on two different machines that need to communicate we encounter a similar problem. We need to be able to convert the data in the memory of one process to a string, send the string over to the other process and then let the other process convert the string back to the data structure.

Solutions

There are a number of solution in Perl for data serialization with various characteristics. We are going to see some of them here:

Data::Dumper

Data::Dumper can turn any data structure into a string, that when executed will build the same data structure. The main advantage of it is that it comes with every version of Perl. It is a "standard library".

The disadvantage is that only Perl understands it so it cannot be used to communicate between applications written in other languages. As the serialized data is actually an piece of executable perl code, and the de-serialization requires the evaluation of this code, this can lead to security vulnerability. If the data can be changed by an untrusted person then not only will we have incorrect data, but our process might execute an arbitrary piece of code injected by an attacker.

Use this only if you have absolutely no way to use any of the other solution.

(The real use of Data::Dumper is to display arbitrary data-structure for debugging purpose. For that it is quite good and does not have the security issue as we only serialize the data and never try to de-serialize it.)

Storable

Storable creates a binary format using the store and nstore functions and uses the retrieve to convert the binary data saved in a file to a data structure.

It is faster than Data::Dumper and it parses the data instead of executing it. So it is safe. It has been distributed with standard perl since perl 5.8. Those are its advantages.

The disadvantage is that it is only implemented for Perl and thus it cannot be used for inter-process communication if any of the processes is written in another language.

It requires a C-compiler to be installed which makes it a bit harder to install or to distribute in case you'd like to upgrade from the version that came with your distribution of perl.

FreezeThaw

The FreezeThaw module provides the freeze function to serialize data and the thaw function to de-serialize it.

It creates a string that, as far as I know is only implemented for Perl. It has the advantage over Data::Dumper that the string is parsed and not eval-ed during de-serialization and thus it is safer. It has the advantage over Storable that it is pure-perl so you can install it even if you don't have a C compiler.

I have not measured it, but it is probably much slower than Storable.

YAML - YAML Ain't Markup Language

YAML is a language-independent data serialization language suitable for representing arrays, hashes, and of course scalar values.

It was designed to be human readable and to be able to represent the data structures of most modern programming languages.

A YAML file looks like this:

---
name: ingy
age: old
weight: heavy
# I should comment that I also like pink, but don't tell anybody.
favorite colors:
    - red
    - green
    - blue

YAML is excellent for data-serialization even when the applications are written in several programming languages.

In Perl there are several implementations of the YAML generating and parsing tools. Some of them are pure-perl, others require a C compiler to install, but provide faster serialization and de-serialization.

With the advance of JSON it is used less for data serialization, but it is still used quite a lot for configuration files.

A number of modules implementing YAML serialization and de-serialization:

JSON

JSON (JavaScript Object Notation) is a lightweight data interchange (serialization) format inspired by JavaScript object literal syntax. It is very similar to YAML in its format though YAML seems to be easier to understand for humans. JSON also has several implementation in Perl.

The main advantage of JSON over YAML is that probably more programming languages have more implementation for JSON than for YAML, and that JavaScript has native parser for JSON. Therefore communication with a JavaScript based application running in a web browser is much easier with JSON than it would be with YAML. In fact, JSON is the number one data serialization used in Ajax-based web applications.

XML

XML can be also used for serialization and de-serialization. There are many modules for Perl to handle XML data. Some of them are pure perl, others require a C compiler. XML itself is quite readable for humans, though it is much more verbose than YAML or JSON.

In general XML is not really a good choice for data serialization (JSON or Sereal would be much better), but due to a lot of investment in promoting XML a lot of corporations feel require that they cannot live without XML in their systems.

Arguably XML can represent more complex data that JSON or YAML could, and in those cases the use of XML might be justified, on the other hand maybe having such complex data is a sign that you should simplify the data.

While not recommended for general XML handling XML::Simple might be used for serialization and de-serialization of data structures and XML::Dumper was specifically created for that.

Sereal

Sereal was developed at Booking.com to provide fast binary serialization and de-serialization of Perl data structures. According to their own measurements it is even faster than Storable though it has the same drawbacks:

It is implemented for Perl and for a number of other languages. it requires a C compiler to be installed. For more details see the GitHub repository

Data::Serializer

Data::Serializer seems to be a module that provides a generic interface to any of the serializers mentioned above. I am not sure if its use is warranted, but it can provide flexibility to select the data-serialization format later.

Compare the solutions

Format Standard Pure Perl Fast Multi-language Secure
Data::Dumper
Storable
FreezeThaw
YAML [1]
JSON [1]
XML [1]
Sereal

[1] - have both pure-Perl and XS based implementation. The latter is probably much faster than the former.

Recommendation

If speed is important and you only need to communicate with Perl-based applications then used Sereal.

If you need to communicate with applications written in other languages then use JSON. If speed is important pick a C-based module that is fast. If "install-ability" is more important then pick a pure-perl implementation. (JSON::MaybeXS allows you to let the end-user decide which JSON implementation to use.)

Alternatively, you can also implement Sereal for the other languages you need, but that's probably a big task.

Discussion

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