Distributed Systems
COMP90015 2017 SM1
Project 1 – EZShare
Resource Sharing Network
Introduction
In Project 1 we will build a resource sharing network that consists of servers, which can communicate with
each other, and clients which can communicate with the servers. The system will be called EZShare.
In typical usage, each user that wants to share files will start an EZShare server on the machine that contains
the files. An EZShare client can be used to instruct the server to share the files.
Servers can be queried for what files they are sharing. Clients can request a shared file be downloaded to
them.
Servers can connect to other servers and queries can propogate throughout all of the servers.
In general, servers can publish resources; a file is just one kind of resource. In EZShare, other resources are
just references (URIs) to e.g. web pages.
Every published resource (including shared files) has an optional owner and channel to which it belongs.
These things allow resources to be controlled, e.g. not all shared resources have to be available to the public.
Architecture
Architecture 1
Communication
All communication will be via TCP.
All messages, apart from file contents, will be in JSON format, one JSON message per line.
The text encoding for messages will be Java Modified UTF-8 Encoding, which is the format used by the
writeUTF() and readUTF() methods in Java.
File contents will be transmitted as exact byte sequences, mixed between JSON messages as required.
Interactions will be synchronous request-reply, with a single request per connection.
Resource
A Resource has the following attributes:
Name: optional user supplied name (String), default is “”.•
Description: optional user supplied description (String), default is “”.•
Tags: optional user supplied list of tags (Array of Strings), default is empty list.•
URI: mandatory user supplied absolute URI, that is unique for each resource on a given EZShare
Server within each Channel on the server (String). The URI must conform to official URI format.
•
Channel: optional user supplied channel name (String), default is “”.•
Owner: optional user supplied owner name (String), default is “”.•
EZserver: system supplied server:port name that lists the Resource (String).•
Resources need to be stored, looked up and transmitted, so it will be wise to develop a robust Resource class.
Some special rules for strings are that they must not contain the null character “\0” and must not start or end
with white space. The server may silently remove these things from receive resource descriptions. As well, the
Owner cannot be the character “*”.
Channels and Owners
Each resource must be stored and processed in a way that respects its Channel and Owner. You may think of
the primary key for a resource being a tuple:
(owner,channel,uri)
The default Owner is “”, the default Channel is “”, and an absolute URI must always be given. The tuple
becomes:
(“”,””,uri)
A Channel may be used without an Owner and vice versa, and they may be used together.
There is no client command that will list used channels or owners. These things are kept secret by the server.
The user needs to remember the owner and channel, if one was used, in order to refer to the resource at a later
Channels and Owners 2
time. The default channel can be thought of as the public channel. Other channels are thought of as private
channels. The default owner means that anyone can update the resource. Otherwise updates require the correct
owner name to work.
Shared File
A shared file is a Resource with a file URI, e.g. file:///path/to/file.doc.
The EZShare Server that lists the shared file can be asked to download it.
Other URIs, such as e.g. http and ftp, will be for informational purposes only. Accessing and downloading
these is not required in this project.
EZShare Server Commands
PUBLISH: create a new Resource and make it available•
REMOVE: remove an existing Resource•
SHARE: create a new Resource with a file URI and make it avialble•
QUERY: list all resources that match a Resource template•
FETCH: download all resources that match a Resource template which includes a file URI•
EXCHANGE: receive a list of EZShare host:port names•
PUBLISH command
{
“command”: “PUBLISH”,
“resource”: {
“name”: “Unimelb website”,
“tags”: [
“web”,
“html”
],
“description”: “The main page for the University of Melbourne”,
“uri”: “http:\/\/www.unimelb.edu.au”,
“channel”: “”,
“owner”: “”,
“ezserver”: null
}
}
This creates a resource on the server with primary key (“”,””,http://www.unimelb.edu.au)
PUBLISH rules enforced by server
The command field is case sensitive (this is the same for all commands).•
A valid resource must be given. Missing fields may be filled in with defaults, which are the empty
sting “” in most case, or the empty array for tags (this is the same for all commands).
•
The URI must be present, must be absolute and cannot be a file scheme.•
Publishing a resource with the same primary key as an existing resource simply overwrites the
existing resource.
•
PUBLISH rules enforced by server 3
Publishing a resource with the same channel and URI but different owner is not allowed. This ensures
that in any given channel, a given URI is only present once.
•
String values must not contain the “\0” character, nor start or end with whitespace. The server may
silently remove such characters or may consider the resource invalid if such things are found (this is
the same for all commands).
•
The Owner field must not be the single character “*”. The resource is invalid in this case. (This is the
same for all commands.)
•
PUBLISH responses from server
For a successful publish:
{ “response” : “success” }
If the publishing rules (other than below) were broken:
{ “response” : “error”,
“errorMessage” : “cannot publish resource”
}
If the resource contained incorrect information that could not be recovered from:
{ “response” : “error”,
“errorMessage” : “invalid resource”
}
If the resource field was not given or not of the correct type:
{ “response” : “error”,
“errorMessage” : “missing resource”
}
Generic responses from server
If the command is invalid (unknown):
{ “response” : “error”,
“errorMessage” : “invalid command”
}
If the command is missing or incorrect type:
{ “response” : “error”,
“errorMessage” : “missing or incorrect type for command”
}
REMOVE command
{
“command”: “REMOVE”,
“resource”: {
“name”: “”,
“tags”: [],
REMOVE command 4
“description”: “”,
“uri”: “http:\/\/www.unimelb.edu.au”,
“channel”: “”,
“owner”: “”,
“ezserver”: null
}
}
This will remove the resource with the primary key (“”,””,http://www.unimelb.edu.au).
The other fields of the resource are not needed, since only the primary key fields are required to remove the
resource. If the other fields are given, they are ignored.
REMOVE responses from server
For a successful remove:
{ “response” : “success” }
If the resource did not exist:
{ “response” : “error”,
“errorMessage” : “cannot remove resource”
}
If the resource contained incorrect information that could not be recovered from:
{ “response” : “error”,
“errorMessage” : “invalid resource”
}
If the resource field was not given or not of the correct type:
{ “response” : “error”,
“errorMessage” : “missing resource”
}
SHARE command
{
“command”: “SHARE”,
“secret”: “2os41f58vkd9e1q4ua6ov5emlv”,
“resource”: {
“name”: “EZShare JAR”,
“tags”: [
“jar”
],
“description”: “The jar file for EZShare. Use with caution.”,
“uri”:”file:\/\/\/\/home\/aaron\/EZShare\/ezshare.jar”,
“channel”: “my_private_channel”,
“owner”: “aaron010”,
“ezserver”: null
}
}
SHARE command 5
The SHARE command works almost identically to the PUBLISH command, with the major difference being
that the URI must be a file scheme, while the PUBLISH command enforces that the URI cannot be a file
scheme.
Another difference is that the server secret is required for the command to be successful.
SHARE rules enforced by the server
The server secret must be present and must equal the value known to the server, for the command to
succeed.
•
The URI must be present, must be absolute, non-authoritative and must be a file scheme. It must point
to a file on the local file system that the server can read as a file.
•
Sharing a resource with the same primary key as an existing resource simply overwrites the existing
resource (same as PUBLISH command).
•
Sharing a resource with the same channel and URI but different owner is not allowed. This ensures
that in any given channel, a given URI is only present once (same as PUBLISH command).
•
SHARE responses from server
For a successful share:
{ “response” : “success” }
If the rules (other than below) are broken:
{ “response” : “error”,
“errorMessage” : “cannot share resource”
}
If the resource contained incorrect information that could not be recovered from:
{ “response” : “error”,
“errorMessage” : “invalid resource”
}
If the secret was incorrect:
{ “response” : “error”,
“errorMessage” : “incorrect secret”
}
If the resource or secret field was not given or not of the correct type:
{ “response” : “error”,
“errorMessage” : “missing resource and\/or secret”
}
QUERY command
{
“command”: “QUERY”,
“relay”: true,
QUERY command 6
“resourceTemplate”: {
“name”: “”,
“tags”: [],
“description”: “”,
“uri”: “”,
“channel”: “”,
“owner”: “”,
“ezserver”: null
}
}
This command also contains a relay field. In usual circumstances this would be set true by the client.
There is no resource, but rather a resourceTemplate. The purpose of the template is to specify the query in
terms of desired fields that must match.
QUERY rules enforced by the server
The purpose of the query is to match the template against existing resources. The template will match a
candidate resource if:
(The template channel equals (case sensitive) the resource channel AND
If the template contains an owner that is not “”, then the candidate owner must equal it (case sensitive) AND
Any tags present in the template also are present in the candidate (case insensitive) AND
If the template contains a URI then the candidate URI matches (case sensitive) AND
(The candidate name contains the template name as a substring (for non “” template name) OR
The candidate description contains the template description as a substring (for non “” template descriptions)
OR
The template description and name are both “”))
QUERY responses from server
The response format is a sequence of messages. For a successful query, e.g. that matched two resources:
{ “response” : “success” }
{ RESOURCE }
{ RESOURCE }
{ “resultSize” : 2 }
An example returned Resource is:
{
“name”: “Unimelb website”,
“tags”: [
“web”,
“html”
],
“description”: “The main page for the University of Melbourne”,
QUERY responses from server 7
“uri”: “http:\/\/www.unimelb.edu.au”,
“channel”: “”,
“owner”: “”,
“ezserver”: “aaron9010:3780”
}
Note that the ezserver field has been filled in by the server, to represent the server’s hostname and port.
QUERY responses from server
The server will never reveal the owner of a resource in a response. If a resource has an owner then it will be
replaced with the “*” character as in the following example:
{
“name”: “EZShare JAR”,
“tags”: [
“jar”
],
“description”: “The jar file for EZShare. Use with caution.”,
“uri”: “file:\/\/\/\/home\/aaron\/EZShare\/ezshare.jar”,
“channel”: “my_private_channel”,
“owner”: “*”,
“ezserver”: “aaron9010:3780”
}
This example also shows a resource that matched a channel, i.e. the user had specified the channel name in
their query template.
QUERY responses from server
Other responses are related to standard errors.
If the resource template contained incorrect information that could not be recovered from:
{ “response” : “error”,
“errorMessage” : “invalid resourceTemplate”
}
If the resource or secret field was not given or not of the correct type:
{ “response” : “error”,
“errorMessage” : “missing resourceTemplate”
}
FETCH command
{
“command”: “FETCH”,
“resourceTemplate”: {
“name”: “”,
“tags”: [],
“description”: “”,
“uri”: “file:\/\/\/\/home\/aaron\/EZShare\/ezshare.jar”,
“channel”: “my_private_channel”,
FETCH command 8
“owner”: “”,
“ezserver”: null
}
}
The role of the fetch command is to download the file resource from the server to the client.
Only the channel and URI fields in the template is relevant as it must be an exact match for the command to
work.
Recall that, in a given channel, a given URI can only be present once, so that this command will only ever
download a single file.
FETCH responses from server
A successful fetch will respond as follows:
{ “response” : “success” }
{ RESOURCE }
exact bytes of resource
{ “resultSize” : 1 }
The resource will have an additional field resourceSize that specifies the number of bytes (i.e. file size), e.g.:
{
“name”: “EZShare JAR”,
“tags”: [
“jar”
],
“description”: “The jar file for EZShare. Use with caution.”,
“uri”: “file:\/\/\/\/home\/aaron\/EZShare\/ezshare.jar”,
“channel”: “my_private_channel”,
“owner”: “*”,
“ezserver”: “aaron9010:3780”,
“resourceSize”: 328515
}
The resourceSize field allows the client to read exactly the bytes of the file that follow.
FETCH responses from server
Other responses are related to standard errors.
If the resource template contained incorrect information that could not be recovered from:
{ “response” : “error”,
“errorMessage” : “invalid resourceTemplate”
}
If the resource template was not given or not of the correct type:
{ “response” : “error”,
“errorMessage” : “missing resourceTemplate”
}
FETCH responses from server 9
EXCHANGE command
{
“command”: “EXCHANGE”,
“serverList”: [
{
“hostname”: “115.146.85.165”,
“port”: 3780
},
{
“hostname”: “115.146.85.24”,
“port”: 3780
}
]
}
The purpose of the exchange command is to tell the server about a list of other servers.
The server is free to process any valid server record that it finds in the list and ignore others.
EXCHANGE responses from server
If the command succeeded:
{ “response” : “success” }
If a server record is found to be invalid:
{ “response” : “error”,
“errorMessage” : “missing resourceTemplate”
}
If the server list was missing or invalid:
{ “response” : “error”,
“errorMessage” : “missing or invalid server list”
}
Server Interactions
Each server maintains a list of Server Records, which are hostname:port strings. To begin, this list is empty.
Every X minutes (10 minutes by default, but configurable on the command line when the server is run), the
server contacts a randomly selected server from the Server Records and initiates an EXCHANGE command
with it. It provides the selected server with a copy of its entire Server Records list.
If the selected server is not reachable or a communication error occurs then the selected server is removed
from the Server Records and no further action is taken in this round.
The receiving server processes the EXCHANGE command as explained earlier, essentially just adding the
servers to its list.
Server Interactions 10
QUERY relay
When a QUERY message is received with relay field set as true then the server sends a QUERY command to
each of the servers in the Server Records list with the following change:
the owner and channel information in the original query are both set to “” in the forwarded query•
relay field is set to false•
Results returned from other servers are forwarded back to the original client on the same connection,
aggregated with the results of the query processed locally. Therefore the response in the successful case is:
{ “response” : “success” }
{ RESOURCE }
{ RESOURCE }
…
{ “resultSize” : X }
where X is the number of hits, taking all of the results from other servers into account.
Connection Interval Limit
The server will ensure that the time between successive connections from any IP address will be no less than a
limit (1 second by default but configurable on the command line).
An incomming request that violates this rule will be closed immediately with no response.
Client command line arguments
The client must work exactly with the following command line options:
-channel
-debug print debug information
-description
-exchange exchange server list with server
-fetch fetch resources from server
-host
-name
-owner
-port
-publish publish resource on server
-query query for resources from server
-remove remove resource from server
-secret
-servers
-share share resource on server
-tags
-uri
Client command line arguments 11
Example command lines
java -cp ezshare.jar EZShare.Client -query -channel myprivatechannel -debug
java -cp ezshare.jar EZShare.Client -exchange -servers 115.146.85.165:3780,115.146.85.24:3780 -debug
java -cp ezshare.jar EZShare.Client -fetch -channel myprivatechannel -uri
file:///home/aaron/EZShare/ezshare.jar -debug
java -cp ezshare.jar EZShare.Client -share -uri file:///home/aaron/EZShare/ezshare.jar -name “EZShare JAR”
-description “The jar file for EZShare. Use with caution.” -tags jar -channel myprivatechannel -owner
aaron010 -secret 2os41f58vkd9e1q4ua6ov5emlv -debug
java -cp ezshare.jar EZShare.Client -publish -name “Unimelb website” -description “The main page for the
University of Melbourne” -uri http://www.unimelb.edu.au -tags web,html -debug
java -cp ezshare.jar EZShare.Client -query
java -cp ezshare.jar EZShare.Client -remove -uri http://www.unimelb.edu.au
Server command line arguments
The server must work exactly with the following command line options:
-advertisedhostname
-connectionintervallimit
-exchangeinterval
-port
-secret
-debug print debug information
The default secret will be a large random string.
The default advertised host name will be the operating system supplied hostname.
The default exchange interval will be 10 minutes (600 seconds).
Example server output when just started
java -cp ezshare.jar EZShare.Server
20/03/2017 01:17:57.953 – [EZShare.Server.main] – [INFO] – Starting the EZShare Server
20/03/2017 01:17:57.979 – [EZShare.ServerControl.] – [INFO] – using secret: 5uv1ii7ec362me7hkch3s7l5c4
20/03/2017 01:17:57.981 – [EZShare.ServerControl.] – [INFO] – using advertised hostname: aaron9010
20/03/2017 01:17:57.984 – [EZShare.ServerIO.] – [INFO] – bound to port 3780
20/03/2017 01:17:57.986 – [EZShare.ServerExchanger.] – [INFO] – started
Example server output when just started 12
Debug command line option
The purpose of the debug option is that your system will print out every message sent or received, as in the
following example for the client. So long as the words “SENT: …msg…” and “RECEIVED: …msg…” are
present on a single line, it does not matter what else is on the same line (in this example the Java Logger is
being used).
java -cp ezshare.jar EZShare.Client -publish -name “Unimelb website” -description “The main page for the
University of Melbourne” -uri http://www.unimelb.edu.au -tags web,html -debug
20/03/2017 01:20:45.807 – [EZShare.Client.main] – [INFO] – setting debug on
20/03/2017 01:20:45.809 – [EZShare.Client.publishCommand] – [FINE] – publishing to localhost:3780
20/03/2017 01:20:45.865 – [EZShare.Client.sendMessage] – [FINE] – SENT: { “command” : “PUBLISH”,
“resource” : { “name” : “Unimelb website”, “tags” : [“web”, “html”], “description” : “The main page for the
University of Melbourne”, “uri” : “http://www.unimelb.edu.au”, “channel” : “”, “owner” : “”, “ezserver” : null
} }
20/03/2017 01:20:45.912 – [EZShare.Client.publishCommand] – [FINE] – RECEIVED: { “response” :
“success” }
Technical aspects
Requires Java 1.8 or above.•
Suggested to make use of the URI class to enforce URI rules.•
Make sure to use a JSON parser/formatter to generate correct JSON messages.•
Apache Commons CLI library is good for parsing command line options.•
Everyone should implement the same protocol, which means that clients and servers from different
groups should interoperate fine.
•
Your Report
Use 10pt font, double column, 1 inch margin all around.•
On the first page, clearly show your group’s name and the names of all members in the group. Clearly
show the login names with university emails as well. The members of the group MUST match the
information entered into LMS.
•
The report is aimed at addressing a number of questions discussed next. Have one section for in the
report for each.
•
Figures in the report, including examples of messages and protocol interaction, and any pseudo-code
(that you may or may not use), are not counted as part of the word length guidelines.
•
Introduction
Write roughly 125 words to briefly describe in your own words:
what the project was about•
Introduction 13
what were the technical challenges that you faced building the system•
what outcomes did you achieve•
Scalability
There are a number of aspects of the system that present a scalability challenge. In roughly 375 words:
identify aspects of the system that present problems for scalability
be specific with why it is not scalable♦
•
suggest revisions to the protocol that may overcome these problems•
Concurrency
For a small system, with only a couple of servers and a few clients, concurrency issues are unlikely to arise.
However consider a system with hundreds of servers and thousands of clients. There are some aspects of the
system that may require further thought to ensure that concurrency issues are properly handled. Concurrency
issues may include things that go technically wrong, but may also include things that do not work as well as
expected.
In roughly 375 words describe:
any concurrency issues that you identify, be specific with examples•
possible revisions to the protocol that may overcome these issues•
Other Distributed System Challenges
Choose a third distributed system challenge that relates to the system and write roughly 375 words explaining
how it relates, how the system currently addresses the challenge (if it does at all), and how you might change
the system to improve it with respect to the challenge.
Submission
You need to submit the following via LMS:
Your report in PDF format only.•
Your ezshare.jar (that contains both Client and Server main classes as exemplified earlier).•
Your source files in a .ZIP or .TAR archive only.•
Submissions will be due at the end of Week 8 via a group submission.
Submission 14