I know that in terms of several distributed techniques (such as RPC), the term "Marshaling" is used but don't understand how it differs from Serialization. Aren't they both transforming objects into series of bits?
13 Answers
Marshaling and serialization are loosely synonymous in the context of remote procedure call, but semantically different as a matter of intent.
In particular, marshaling is about getting parameters from here to there, while serialization is about copying structured data to or from a primitive form such as a byte stream. In this sense, serialization is one means to perform marshaling, usually implementing pass-by-value semantics.
It is also possible for an object to be marshaled by reference, in which case the data "on the wire" is simply location information for the original object. However, such an object may still be amenable to value serialization.
As @Bill mentions, there may be additional metadata such as code base location or even object implementation code.
From the Marshalling (computer science) Wikipedia article:
The term "marshal" is considered to be synonymous with "serialize" in the Python standard library1, but the terms are not synonymous in the Java-related RFC 2713:
To "marshal" an object means to record its state and codebase(s) in such a way that when the marshalled object is "unmarshalled", a copy of the original object is obtained, possibly by automatically loading the class definitions of the object. You can marshal any object that is serializable or remote. Marshalling is like serialization, except marshalling also records codebases. Marshalling is different from serialization in that marshalling treats remote objects specially. (RFC 2713)
To "serialize" an object means to convert its state into a byte stream in such a way that the byte stream can be converted back into a copy of the object.
So, marshalling also saves the codebase of an object in the byte stream in addition to its state.
Basics First
Byte Stream - Stream is a sequence of data. Input stream - reads data from source. Output stream - writes data to destination. Java Byte Streams are used to perform input/output byte by byte (8 bits at a time). A byte stream is suitable for processing raw data like binary files. Java Character Streams are used to perform input/output 2 bytes at a time, because Characters are stored using Unicode conventions in Java with 2 bytes for each character. Character stream is useful when we process (read/write) text files.
RMI (Remote Method Invocation) - an API that provides a mechanism to create distributed application in java. The RMI allows an object to invoke methods on an object running in another JVM.
Both Serialization and Marshalling are loosely used as synonyms. Here are few differences.
Serialization - Data members of an object is written to binary form or Byte Stream (and then can be written in file/memory/database etc). No information about data-types can be retained once object data members are written to binary form.
Marshalling - Object is serialized (to byte stream in binary format) with data-type + Codebase attached and then passed Remote Object (RMI). Marshalling will transform the data-type into a predetermined naming convention so that it can be reconstructed with respect to the initial data-type.
So Serialization is a part of Marshalling.
CodeBase is information that tells the receiver of Object where the implementation of this object can be found. Any program that thinks it might ever pass an object to another program that may not have seen it before must set the codebase, so that the receiver can know where to download the code from, if it doesn't have the code available locally. The receiver will, upon deserializing the object, fetch the codebase from it and load the code from that location. (Copied from @Nasir answer)
Serialization is almost like a stupid memory-dump of the memory used by the object(s), while Marshalling stores information about custom data-types.
In a way, Serialization performs marshalling with implementation of pass-by-value because no information of data-type is passed, just the primitive form is passed to byte stream.
Serialization may have some issues related to big-endian, small-endian if the stream is going from one OS to another if the different OS have different means of representing the same data. On the other hand, marshalling is perfectly fine to migrate between OS because the result is a higher-level representation.
I think that the main difference is that Marshalling supposedly also involves the codebase. In other words, you would not be able to marshal and unmarshal an object into a state-equivalent instance of a different class.
Serialization just means that you can store the object and reobtain an equivalent state, even if it is an instance of another class.
That being said, they are typically synonyms.
Marshalling is usually between relatively closely associated processes; serialization does not necessarily have that expectation. So when marshalling data between processes, for example, you may wish to merely send a REFERENCE to potentially expensive data to recover, whereas with serialization, you would wish to save it all, to properly recreate the object(s) when deserialized.
Serialisation vs Marshalling
Problem: Object belongs to some process(VM) and it's lifetime is the same
Serialisation - transform object state into stream of bytes(JSON, XML...) for saving, sharing, transforming...
Marshalling - contains Serialisation + codebase. Usually it used by Remote procedure call(RPC) -> Java Remote Method Invocation(Java RMI) where you are able to invoke a object's method which is hosted on remote Java processes.
codebase - is a place or URL to class definition where it can be downloaded by ClassLoader. CLASSPATH[About] is as a local codebase
JVM -> Class Loader -> load class definition
java -Djava.rmi.server.codebase="<some_URL>" -jar <some.jar>
Very simple diagram for RMI
Serialisation - state
Marshalling - state + class definition
Marshaling uses Serialization process actually but the major difference is that it in Serialization only data members and object itself get serialized not signatures but in Marshalling Object + code base(its implementation) will also get transformed into bytes.
Marshalling is the process to convert java object to xml objects using JAXB so that it can be used in web services.
Think of them as synonyms, both have a producer that sends stuff over to a consumer... In the end fields of instances are written into a byte stream and the other end foes the reverse ands up with the same instances.
NB - java RMI also contains support for transporting classes that are missing from the recipient...
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