Assignment #3 Remote Method Invocation (RMI)

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Distributed File System (DFS)
COMP6231- Assignment #3
1 Project Objective
The objective of this assignment is to apply the knowledge of client-server communication
and Remote Method Invocation (RMI) to build a distributed file system, which we refer to as
Remote and Distributed File Storage (or simply DFS). RMI involves the creation of stubs and
skeletons at client and server sides, respectively which allow for transparent location, reading
and writing of files maintained at networked computers. In RMI, the underlying details are
generally hidden from users, whereby a calling object can invoke a method in a potentially
remote node as if it is local.
The following subsections will provide an overview of DFS, followed by descriptions of its
inherent functionalities, entities, architecture, communication between entities and the
required RMI library and interfaces.
2.1 Overview
In this project, you will implement DFS, a distributed file system that stores a vast amount of
data (files) which typically do not fit on a single machine. Briefly, the files are physically
stored on a set of servers called Storage Servers. Users, referred to as Clients, can create,
delete, read, write, and list files (among others), all via using Remote Method Invocation
(RMI). As a requisite step, Clients need to identify Storage Servers that host the required
files. They do so with the help of a mediator. Clients contact a Naming Server (in this project
we allow only one centralized Naming Server) which maps every file name to a Storage
Server. The Naming Server is thus a repository of metadata or data about data.
2.2 Functionalities
The operations (or functionalities) that are available to the Clients of DFS are:
1. CreateFile(path): create the file referred to by path1.
2. CreateDirectory(path) : create the directory referred to by path.
3. Read(path, o, n) : read n bytes of data from the file referred to by path starting at an
offset o.
4. Write(path, o, data) : write n bytes of data to the file referred to by path starting at an
offset o.
5. Size(path): return the size, in bytes, of the file referred to by path.
6. IsDirectory(path) : return true if path refers to a directory.
7. List(path): list the contents of the directory referred to by path.
8. Delete(path): delete the file or directory referred to by path.
9. GetStorage(path) : get the Storage Server (or more precisely, a representing stub)
hosting the file referred to by path.
In this project, you will implement all the above-listed functionalities except the Delete(path)
function, which is left as a bonus to implement (NOTE: this is the Naming Server delete).
1Path is a string that refers to an absolute path (with “/” being the root) of a file or a directory in the Naming Server’s
directory tree.
2.3 Entities, Architecture, and Communication
Figure 1: Architecture of DFS
The main entities in DFS are: Clients, Naming Server, and Storage Servers. A Client is the
end-user of DFS who wishes to perform operations on files. We assume that a Client knows
the path of a file it wishes to manipulate. The Naming Server is a critical entity in DFS
because it is the means by which Clients locate files stored at Storage Servers. It runs at a
pre-dened address that is known by both, Clients and Storage Servers. A Storage Server
physically stores files in its local file system. The project assumes that a file cannot be
partitioned across Storage Servers (i.e., no file striping is applied) and that a Storage Server
can host multiple files.
DFS is based on a client-server architecture as shown in Figure 1. In this architecture, a client is
a service requester and a server is a service provider. Servers also behave as clients when
requesting services provided by other servers. For example, the Naming Server behaves as a
client when requesting the services of a Storage Server. In Figure 1, all forward arrows (arrows
2, 3, and 5) represent requests originating from clients to servers, and the corresponding
backward arrows represent the services provided by the servers in response.
It is evident from the arrows in Figure 1 that distributed systems entail a lot of communication between its entities. DFS is no exception. We will now discuss the purpose of
communication in DFS and the communicating entities.
2.3.1 Storage Servers – Naming Server Communication
Upon start-up, each Storage Server sends a list of paths (representing the files that it
currently hosts) to the Naming Server, a process we denote as registration. The Naming
Server then traverses this list and adds the paths to its directory tree where each leaf node
is a (filename, Storage Server) tuple. During the traversal, the Naming Server remembers
paths of existing files and ultimately replies back to the Storage Server with a list of
duplicate paths (see Section 5.3 for details on that). Figure 1 depicts this communication
with arrows 1 and 2. After registration by all Storage Servers, the Naming Server is deemed
to be capable of locating all files stored at each Storage Server.
2.3.2 Client – Naming Server Communication
Arrows 3 and 4 in Figure 1 illustrate this communication. A Client contacts the Naming
Server whenever it needs to perform an operation on a file. While some operations cannot
be directly handled by the Naming Server, in which case it replies back with the Storage
Server that hosts the file, other operations can be directly handled by it.
Operations requiring the content of a file namely read, write and size, cannot be directly
handled by the Naming Server. When a Client wishes to perform any of those operations, it
first contacts the Naming Server to get the Storage Server (or more precisely a representing
stub) that hosts the file. It does so using the getStorage operation and, subsequently,
communicates with the respective Storage Server to handle the operation. In Figure 1,
arrows 5 and 6 mark this communication.
All the other operations, namely: createFile, createDirectory, isDirectory, list, and delete, can be
handled by the Naming Server without involving the Storage Servers. This is because the
Naming Server merely leverages its tree to handle such operations without requiring file
contents. It also ensures the integrity of the Naming Server’s directory tree. Clients cannot
create/delete files/directories on the Storage Servers without the awareness of the Naming
Server. Therefore, the Naming Server first updates its directory tree by adding/deleting a
file/directory and then instructs the respective Storage Server to perform the physical
You have to create a design that supports the delete operation without actually implementing it. The implementation of the delete operation is a bonus-question in this assignment.
2.3.3 Naming Server – Storage Servers Communication
This communication is illustrated in Figure 1 by arrows 1 and 2. Upon start-up, each Storage
Server recursively lists its hosted files and sends the resultant list of paths to the Naming
Server. As described earlier, we denote this process as registration. In response, the
Naming Server replies with a list of duplicate files (if any) which a Storage Server deletes
from its local file system. In addition, it also deletes any directories that are rendered empty,
a process known as pruning.
Besides their communication upon boot-strapping DFS, the Naming Server sends create
and delete operations to Storage Servers on behalf of Clients after changes to its directory
tree have been successfully committed.
2.3.4 Client – Storage Servers Communication
Arrows 5 and 6 illustrate this communication that occurs when a Client wishes to perform a
read, write, or size operation, after retrieving the respective Storage Server from the Naming
Server (see Client – Naming Server Communication above).
2.4 RMI Library
As explained earlier, Clients request the Naming and the Storage Servers to perform
operations (methods) on files. The Naming Server also behaves as a Client when it requests
the Storage Server to perform create/delete operations. The recipient server, in return, fullls
requests by executing the operations’ logic and returning results. This implies that servers
can transiently act as clients when requesting the services of other servers. We use the
terms client (with a lower-case c) and server (with a lower case s) to denote any service
requester and provider respectively. For the rest of the document, we will also use the term ”
invoking a method ” when we actually mean “requesting to perform an operation.”
When a client invokes a method, it essentially invokes a remote method, hence, the name
Remote Method Invocation (since the method’s logic resides on a server). The client is only
aware of the method’s name, not where it actually resides. To enable a client to execute a
remote method, we would require an RMI library. RMI library takes care of initiating client
connections to the appropriate servers and forwarding method invocations to them, thereby,
making them appear to clients as if the methods are implemented locally. At the recipient
servers, the RMI library receives client connections, invokes the requested methods, and
returns results. The servers execute the methods while being totally oblivious to the fact that
the invocations were initiated by remote Clients. Thus, the RMI library helps masking the
client-server communication.
An RMI library is based on the concept of Stubs and Skeletons. When a client needs to perform
an operation, it invokes a corresponding remote method via an object called the “stub.” The stub
object, or simply the stub, is part of the client and is responsible for handling the invoked method,
be it local or remote. If a method is local, the stub merely invokes a helper function that
implements the “logic” of the operation. On the other hand, if the method is remote, the stub
initiates a connection to the appropriate server (more precisely the server’s skeleton), marshalls 2
the method name and arguments, transmits the byte stream over the network, unmarshalls 3 the
result and returns it to the client. Thus, stubs allow clients to invoke local and remote methods
alike, leaving the underlying complexity associated with remote methods to the stub.
Skeletons are counterparts of stubs but reside reversely at servers. Each stub communicates to
a skeleton. A skeleton is an object responsible for listening to multiple client connections,
unmarshalling the byte stream, invoking the method implementing the logic of the requested
operation, marshalling the results, and sending them back to the client.
2 Marshaling is the process of converting a datum (e.g., an object) into a byte stream that can be transmitted
over a network.
3Unmarshaling is the reverse process of marshaling, whereby a datum or an object is reconstructed from a byte
2.5 Interfaces
A server usually declares all the methods it handles in interfaces. An interface contains a
subset of the methods that can be invoked by a particular client. For example, the Naming
Server declares two interfaces, one for methods that can be invoked by Clients and the
other for methods that can be invoked by Storage Servers. Segregating declarations into
multiple interfaces ensures that clients can only invoke their permissible methods. In DFS,
the Naming Server splits its method declarations across two interfaces:
Registration: defines a single method, namely register, invoked by Storage Servers upon
bootstrapping DFS.
Service: defines the methods that can be invoked by Clients and handled directly by the
Naming Server. As described in Section 2.3, the methods are: getStorage, isDirectory, list,
createFile, createDirectory, and delete.
Similarly, Storage Servers split their method declarations into two interfaces:
Command : defines two methods create and delete, which can be invoked by the Naming
Server whenever a Client requests any of the createFile, createDirectory or delete operations. For any of these operations, the Naming Server essentially commands a specic
Storage Server to alter its local file system accordingly.
Storage : defines the methods that can be invoked by Clients and handled only by Storage
Servers. As described in Section 2.3, these methods are: size, read and write.
For each interface, a stub and a corresponding skeleton are required. A stub uses an
interface to determine if an invoked method is remote or local, and subsequently performs
tasks as described in Section 2.3. The corresponding skeleton uses the interface to verify if
an invocation is legitimate (i.e., the invoked method belongs to the interface). If so, the
skeleton acts as described in Section 2.3.
To create a stub, a client requires the following information: an interface and the network
address (IP and port) of the corresponding skeleton (or more precisely, the IP address of
the Storage Server at which the skeleton exists). Similarly, to create a skeleton, a server
requires the following information: an interface, a class that implements the logic of the
methods dened in the interface, the server’s IP address, and a port number. Essentially all
skeletons belonging to a server possess the same IP address but different port numbers.
The Naming Server creates two skeletons, one for each interface. We call these skeletons
RegistrationSkeleton and ServiceSkeleton, which correspond to the Registration and the Service
interfaces respectively. In any distributed system, stubs that communicate with a known server,
such as the Naming Server, are created by clients at boot-up. In other words, since the network
addresses of RegistrationSkeleton and ServiceSkeleton are predefined, each Storage Server
and Client, at boot-up, creates RegistrationStub and ServiceStub respectively.
Likewise, a Storage Server creates two skeletons, one for each interface. We call these
skeletons CommandSkeleton and StorageSkeleton, which correspond to the Command and the
Storage interfaces, respectively. A problem, however, is that a Storage Server may run on any
machine and its address (and, thereby, the addresses of its skeletons) is not predefined. This
implies that neither Clients nor the Naming Server can create the corresponding stubs to
communicate with Storage Servers. To resolve this issue, each Storage Server creates its
CommandStub and StorageStub. During registration, the Storage Servers transmit their stubs to
the Naming Server along with their list of files. Hence, when a Client invokes the getStorage
method, what the Naming Server actually returns is the CommandStub of the Storage Server
which hosts the requested file. Similarly, when the Naming Server needs to communicate with a
Storage Server, it uses the respective StorageStub.
3 Recap
Let us revisit the whole problem at a high level. The main issue is to enable Clients to
perform operations on files stored on remote servers in a distributed file system. Technically,
we refer to the act of performing an operation as invoking a remote method. We denote a
method as remote because the “logic” of the method is implemented on remote servers,
namely the Naming Server or the Storage Servers. In order to trigger a method at a server
and return its results, we need stub and corresponding skeleton objects. A client may either
possess a stub (created at boot-up) or may have acquired it from the Naming Server. A
server creates its skeletons at boot-up using its pre-defined interfaces.
4 A Working Example
Consider the situation in which a Client wishes to perform a read operation on a file named
“abc” (assume the name “abc” is of type String). To do so, the Client requires StorageStub
to communicate with StorageSkeleton of the Storage Server hosting the file “abc.” So as a
first step, it must contact the Naming Server to get StorageStub. As such, it invokes
getStorage(abc) using ServiceStub. ServiceStub at the Client side connects to
ServiceSkeleton at the Naming Server, marshalls the method name (i.e., getStorage),
argument types (i.e., String) and argument values (i.e., “abc”), transmits the byte stream,
unmarshalls the result (after finally received from the Naming Server) and closes the
connection. ServiceSkeleton listens for incoming connections, accepts the new connection,
unmarshalls the incoming stream to identify the requested method, calls the locally defined
getStorage(abc) function, marshalls StorageStub object and returns it to ServiceStub.
Now that the Client has StorageStub, the second step is to contact the Storage Server. The
Client invokes read(abc,0,10) using StorageStub which in return communicates with StorageSkeleton. StorageStub and StorageSkeleton interact in a similar manner as ServiceStub
and ServiceSkeleton, and the Client eventually acquires a buffer containing 10 bytes of data
read from the file “abc.” The entire example is demonstrated in Figure 2 (following page).
Figure 2: An example of a Client performing a read operation on file ‘abc’
5 Implementation
In this project you will be using the Java Programming Language. You are provided with a
starter code that contains five primary packages namely rmi, naming, storage, client, and common, which you must implement to create a fully-functional distributed
file system (i.e., DFS). We recommend that you implement the packages in the following
order, (1) rmi, (2) common, (3) naming, and (4) storage. The client package has been fully
implemented for you.
In this section, we provide you with design considerations for successfully implementing
each package. In addition, it is important that you read the package-info document under
each package prior to implementing the package.
5.1 The RMI Package
Figure 3: Sockets and Multi-threading
The RMI package is your RMI library. The RMI library consists of two generic (parametrized)
Skeleton and Stub. Both, the Skeleton and the Stub classes take a remote interface 4 as a
parameter. They define and implement methods that are common to all skeletons and stubs
in DFS (e.g., constructors to instantiate skeleton/stub objects as well as start() and stop()
methods to start and stop skeletons/stubs, respectively).
The connection and communication between stubs and skeletons are carried out using Java API
for TCP socket5 programming. The skeleton is multi-threaded. When it is started using the start()
method, its main thread creates a listening socket (see Figure 3), which waits for incoming client
connections. Once a client’s request is received, the skeleton accepts the request, creates a new
thread (or what we call a client thread ) to service the request, and instantiates a new service
socket within the client thread to handle further communication with the client. Figure 3 illustrates
the concept of stubs and skeletons using multi-threaded socket programming.
A stub is implemented in Java as a dynamic proxy (java.lang.reflect.Proxy ). A proxy has an
associated invocation handler. When a method is invoked on a proxy (stub) object, the
method name and parameters are encoded and dispatched to the invoke method of the
invocation handler. For instance, in Figure 2, when the client invokes the method getStorage
on the proxy ServiceStub, the method name and arguments (i.e., getStorage and “abc”) are
encoded and dispatched to the invoke method of the ServiceStub’s invocation handler.
The invoke method checks whether the invoked method is local or remote. In the case of a
local method (i.e. a method that the proxy implements), the local method is simply invoked
and passed the arguments. In this project, local methods are equals(), hashCode() and
toString() .The equals method determines if two proxies were created for the same skeleton.
HashCode returns the hash code of a proxy object. toString prints information about a proxy.
In the case of a remote method, the proxy connects to the corresponding skeleton at the
server side, marshalls the method name, parameter types and values, and sends the
entailed byte stream.
4A remote Interface is a Java interface in which each method declared in it throws an exception of type
5A socket is an end-point in a bi-directional communication between two processes running typically on two
separate computers on a network. Each socket is bound to a dierent port number which is utilized by TCP to
identify the data destined to the socket.
You can read more about implementing a dynamic proxy via JavaDocs.
The RMI library is best implemented in two phases:
Phase 1: learn more about the two Java socket APIs (Socket and ServerSocket) and
dynamic proxies (java.lang.reflect.Proxy) to implement a basic stub-skeleton communication.
Use ObjectOuputStream and ObjectInputStream which allow writing and reading primitive
data types of Java objects (or referred interchangeably to as serializing data) to output and
input streams, respectively. At the end of this phase, a stub should be able to connect to a
skeleton, and send and receive objects. Similarly, a skeleton must be able to accept several
client connections, read objects and send back objects (including exceptions).
You should make the server as robust as possible via handling various types of exceptions and
displaying meaningful messages whenever needed, so as to avoid abrupt crashes. For example,
if the stub fails to create an input stream, it should throw an RMIException with a meaningful
message. You should also clean-up (i.e. close sockets and streams) whenever required.
Phase 2 : Now that you are able to send and receive serialized objects (using ObjectInputStreams and ObjectOutputStreams), it is time to send and receive more meaningful data. The
stub should now check whether the invoked method is local or remote using its defined interface.
When the invoked method is local, the stub simply invokes a corresponding locally implemented
method (or what we call an implementor method). On the other hand, when the invoked method
is remote, the stub sends the method name and parameter types and values to the respective
skeleton. The skeleton should receive them, invoke the corresponding method, which
implements the necessary logic, and send back the generated result to the stub. Exceptions that
arise due to unsupported methods during unmarshalling, and the ones that are thrown by the
implementor methods should be communicated back to the client.
5.2 The Naming Package
The naming package contains the Registration and Service interfaces, as well as the NamingServer class that creates the necessary skeletons and stubs and implements the logic of
all the operations handled by the Naming Server. For the create operation, we leave the
strategy of selecting a Storage Server to host a new file or directory up to you.
The Naming Server creates and maintains the DFS directory tree, with the top-level directory being the root represented by the path “/”. While the inner tree nodes represent directories, the leaves represent files (or more precisely file,stubs tuples). The Naming Server
gradually builds its tree during registration. After registration, the Naming Server uses its
tree to handle operations. It is important to design the directory tree in a way that allows the
Naming Server to easily look-up, traverse and alter the tree, as well as detect invalid paths
(e.g., a path that denotes a non-existing file).
5.3 The Storage Package
The storage package contains the Command and Storage interfaces, as well as the
StorageServer class that creates the necessary skeletons and stubs and implements the
logic of all the operations handled by the Storage Server.
Each Storage Server has its own local file system. The files hosted by a Storage Server are
stored in its local file system in a directory denoted as temporary directory. A temporary
directory and all its sub-directories and files are part of DFS i.e. part of the Naming Server’s
directory tree. Each file/directory at a Storage Server has an absolute local path with respect
to the root directory of the Storage Server.
In other words, a local path dictates the location of a file/directory at the local file system
of a Storage Server. Paths we have been referring to so far in the document are relative
paths that dictate the locations of files/directories on DFS. We call these paths DFS paths .
DFS paths refer to the directory tree of the Naming Server and are prefixed with the root
directory of DFS (i.e., “/”) followed by the temporary directories of Storage Servers. To
exemplify, consider a file f1.txt stored in a temporary directory, tmp, at Storage Server, SS1,
with an absolute path of /home/SS1/Public/tmp/sub_dir1/f1.txt . Therefore, the local path at
SS1 will be /home/SS1/Public/tmp/sub_dir1/f1.txt , while the DFS path at the directory tree of
the Naming Server would be /tmp/sub_dir1/f1.txt. Clients and the Naming Server know and
use DFS paths only. The job of resolving (or mapping) DFS paths into local paths (and viceversa) is the responsibility of Storage Servers.
During registration, the Storage Server recursively lists the contents of its temporary directory and sends the list of local paths (of files only) along with its stubs to the Naming Server.
The Naming Server maps the received local paths to DFS paths and sends back a list of
duplicate files for deletion. Duplicate files are files that have been already registered and
therefore exist in the Naming Server’s directory tree. For instance, during the registration of
Storage Server SS1, if the Naming Server encounters f1.txt sent by SS1 in its directory tree,
then f1.txt is deemed a duplicate file.
In Java, directories and files are represented as objects . Peruse through the
documentation of to understand more the File’s constructors and methods.
5.4 The Common Package
This package contains the class Path that defines utility functions which manipulate paths.
These functions are used as helper methods by the Naming Server and the Storage Servers.
6 Test Suite
We have provided four packages. Specically, rmi, common, naming and storage, containing
test files that test the corresponding package. We recommend that you test your packages
as you move forward. The test cases test if your implementation conforms to the design
specications, and checks the correctness of the implementation. Please note that this is a
service offered to help you design and test your code faster. You are solely responsible to
ensure that your implementation is flawless. During grading, we may use other test cases to
make sure that your project works as expected.
7 Tips
Start early!
Read about Java multi-threading and synchronization that is used to synchronize
accesses to shared variables. When multiple threads invoke a method that alters
shared variable(s), you must ensure that the method is declared as synchronized.
Learn more about Java libraries used to access and manipulate a file system and its
constituent files/folders.
Read the package-info provided in each package to understand the package’s
functionalities and how to use or integrate it with the other packages.
For each package of the four packages (i.e., the rmi, naming, storage, and common
packages), it is important to read the respective test files found under the conformance
package. This will give you an idea on how to design and implement a package.
Do not defer testing until the end. Test your packages as you go.
Getting Started
Download the Starter Code. This is the framework that you will be modifying. Unzip the
file. Read the README document. For the complete documentation of the framework,
read the following (preferably in the same order)
The README file under the project archive.
The Implementation section in the project handout.
The javadoc of the starter code. Note: the make docs-all command creates javadoc
of the starter code.
Importing code to Eclipse (if you are using eclipse)
Open StarterCode and delete all .class files under all sub-directories. You can do
this manually or from the command-line, cd to StarterCode and type make clean.
Open Eclipse and create a new Java Project (File -> New -> Java Project). Name
the project.
Under the Project name, right click on src and select Import -> File System ->
Browse. Browse to StarterCode and click Ok.
Check the checkbox next to StarterCode in the left pane and click Finish.
You’ll notice that the class contains an error (undened class
SampleClassUnderTest). This class does not actually exist, it just demonstrates how
you can write unit tests. Therefore, just comment out all the lines that contain errors.
Now you should be able to run under the Conformance
package. Of course, you won’t pass any of the tests because you haven’t
implemented anything yet!


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