CSCI 5105: Project 2: Bulletin Board Consistency


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In this project, you will implement a simple Bulletin Board (BB) system (like our Moodle forum) in which
clients can post, reply, and read articles stored in the BB. The BB is maintained by a group of replicated
servers that offer sequential consistency, Quorum consistency, or Read-your-Write consistency. You may
reuse any of the code developed in Project 1 or you can start from scratch. However, unlike the PubSub
system, your server(s) will store and remember all articles and not communicate with other student’s servers.
You can use any communication protocol/system (e.g. UDP, TCP, and RPC) as you want. In this lab, you will
learn about how to implement various forms of consistency and their tradeoffs. The desired consistency
mechanism is supplied as a parameter at runtime.
2. Project Details
The project will be comprised of: clients and servers (one of the servers is designed as the primary or
coordinator). The client does not know or care who the coordinator is. All of the other servers are configured
to know who the coordinator is. Clients can connect to any server(s). Thus, the client knows the address of
every server (you may deploy servers on the same machine with different ports if you like or within virtual
machines). To keep things simple, there is only one BB with a single topic stream. The operations that the
clients should be able to perform are below:
1. Post an article
2. Read a list of articles and display the list one item per line (maybe just the first few words or the title of
the article)
3. Choose one of articles and display its contents
4. Reply to an existing article (also posts a new article)
Internally, the client should be able to connect or disconnect to any server (not just the coordinator) to carry
out any operation. Each article has an internal unique ID that is generated by the coordinator. So the
contacted server (on a post or reply) will ask the coordinator for the next unique ID to use. The server will
order the articles in increasing order using the ID (1, 2 …) as this captures time order. For the Read
comment, the client should print the articles in this order using indentation for replies, e.g.
1 Article 1
2 Article 2
3 A reply to Article 2
4 A reply to Article 2
5 A reply to Article 4
Since the number of articles in the BB can be large, you should consider how many articles should be shown
in a page and how to enable subsequent pages to be shown.
The article IDs are returned with the article and can be used both in formatting and in calls to choose or
reply. You can decide on the structure and format of an article. There are multiple clients in this system
and each client will perform operations concurrently. Your clients will have a simple UI to manipulate the
operations on the BB (the look of the interface is up to you). You will need to decide how to represent “reply”
articles so that the client may format things properly. To emulate the propagation delay one might see in a
wide-area network, you can delay the sending a message (from client to server or server to server) by sleeping
a random amount of seconds.
a) Implement sequential consistency
This means that all clients should see the same order of articles on a read from any server even if they were
posted by concurrent clients to any servers. You can use the primary-backup protocol.
b) Implement quorum consistency
Given N replicas, you will to assemble a read quorum (NR) which is an arbitrary collection of servers in
order to read/choose, and a write quorum (NW), an arbitrary collection of servers in order to
post/reply for the client. The values of NR and NW are subject to the following two constraints:
1. NR + NW > N
2. NW > N/2
You may use the coordinator as a control point for your quorum. That is, the client contacts any server, which
in turn, contacts the coordinator to do the operation contacting the other randomly chosen servers needed for
the quorum. Now, vary the values of NR and NW and measure the cost (as seen from the client) to do a write
or read operation. Present data graphs and provide simple analysis.
c) Implement Read-your-Write consistency
For this, suppose a client C posts an article or reply to a specific server S1. Later, if the client C connects
to a different server S2 and does a read or choose, they are guaranteed to see the prior updates.
You can use the local-write protocol. Measure the cost of client operations and compare.
For all consistency policies, measure the cost of client operations, and compare across the policies.
Done Early? Try this for no extra credit:
d) Allow the coordinator to fail and hold a leader election to determine a new coordinator.
e) Pick another consistency policy to implement.
To realize your project goals, you have to define an API for server-server communication to propagate
updates, request new article IDs, and so on. This is up to you to define.
3. Implementation Details
You may borrow code that you like from Project 1. Do not use any code found on-line. To make multiple
servers run easily, your servers may run in a single machine with different port numbers. Note that your
servers are also expected to work when deployed across different machines. In the quorum protocol, replicas
can get out of synch. That is, a reader is always guaranteed to get the most recent article (i.e. the latest ID)
from one of the replicas, but there is no guarantee that the history of updates will be preserved at all replicas.
To fix this problem, implement a synch operation that brings all replicas up to date with each other and can
be called periodically in the background.
4. Project Group
All students should work in groups of size 2. There are some students who have done the Project 1 without a
partner. So if you were doing the project alone but want to find a partner, we encourage you to use the forum
to find your partner.
5. Test Cases
You should also develop your own test cases for all the components of the architecture, and provide
documentation that explains how to run each of your test cases, including the expected results. Also tell us
about any possible deadlocks or race conditions.
There are many failure modes relating to the content of messages, parameters, and system calls. Try to catch
as many of these as possible. Finally, you should run your clients and servers within the CSE firewall –
as outside access particularly through UDP and TCP may be blocked.
6. Deliverables
a. Design document briefly describing each component. Performance graphs and simple analysis. Not to
exceed 3 pages.
b. Instructions explaining how to run each component and how to use the service as a whole, including
command line syntax, configuration file particulars, and user input interface.
c. Testing description, including a list of cases attempted (which must include negative cases) and the
d. Source code, makefiles and/or a script to start the system, and executables.
Note: a, b, and c should be in a single document file.
7. Grading
The grade for this assignment will include the following components:
a. 20% – The document you submit – This includes a detailed description of the system design and
operation along with the test cases used for the system (must include negative cases)
b. 70% – The functionality and correctness of your own server and clients
c. 10% – The quality of the source code, in terms of style and in line documentation
8. Resources
a. D. K. GIFFORD, Weighted voting for replicated data, in Proc. 7th Annual ACM Symp. Oper. Sys.
Principles (SIGOPS), ACM, New York, 1979.
b. S. B. DAVIDSON, H. GARCIA-MOLINA, AND D. SKEEN, Consistency in partitioned networks,
ACM Computing Surveys, 17 (1985).