6.824 - Spring 2012

6.824 Lab 1: Lock Server

Due: Friday, February 17th, 5:00pm


In this series of labs, you will implement a fully functional distributed file server as described in the overview. To work correctly, the yfs servers need a locking service to coordinate updates to the file system structures. In this lab, you'll implement a simple lock service.

The core logic of the lock service consists of two modules, the lock client and the lock server, which communicate via RPCs. A client requests a specific lock from the lock server by sending an acquire request. The lock server grants the requested lock to one client at a time. When a client is done with the granted lock, it sends a release request to the server so the server can grant the lock to another client (if any) waiting to acquire the lock.

In addition to implementing the lock service, you'll also augment the provided RPC library to ensure at-most-once execution by eliminating duplicate RPC requests. Duplicate requests exist because the RPC system must re-transmit lost RPCs in the face of lossy network connections and such re-transmissions often lead to duplicate RPC delivery when the original request turns out not to be lost.

Duplicate RPC delivery, when not handled properly, often violates application semantics. Here's a example of duplicate RPCs causing incorrect lock server behavior. A client sends an acquire request for lock x, the server grants the lock, the client releases the lock with a release request, a duplicate RPC for the original acquire request then arrives at the server, the server grants the lock again, but the client will never release the lock again since the second acquire is just a duplicate. Such behavior is clearly incorrect.


The files you will need for this and subsequent lab assignments in this course are distributed using the Git version control system. To learn more about Git, take a look at the Git user's manual, or, if you are already familiar with other version control systems, you may find this CS-oriented overview of Git useful.

The URL for the course Git repository is http://am.lcs.mit.edu/6.824-2012/yfs-class.git. To install the files in your Athena account, you need to clone the course repository, by running the commands below. You must use an x86 or x86_64 Athena machine; that is, uname -a should mention i386 GNU/Linux or i686 GNU/Linux or x86_64 GNU/Linux. You can log into a public i686 Athena host with ssh -X linerva.mit.edu.

athena% mkdir ~/6.824
athena% cd ~/6.824
athena% add git
athena% git clone http://am.lcs.mit.edu/6.824-2012/yfs-class.git lab
Initialized empty Git repository in ../6.824/lab/.git/
got c9c80a1686710307b99b2c8642311e001920641f
walk c9c80a1686710307b99b2c8642311e001920641f
got 468233b68dff78915966975f5e9aad54beeea84e
got 60e68c3bfa43d5d6d1f2b8a2c7a1d5db5fa0e860
Checking out files: 100% (44/44), done.
athena% cd lab

Git allows you to keep track of the changes you make to the code. For example, if you are finished with one of the exercises, and want to checkpoint your progress, you can commit your changes by running:

athena% git commit -am 'my solution for lab1 exercise9'
Created commit 60d2135: my solution for lab1 exercise9
 1 files changed, 1 insertions(+), 0 deletions(-)

You can keep track of your changes by using the git diff command. Running git diff will display the changes to your code since your last commit, and git diff origin/lab1 will display the changes relative to the initial code supplied for this lab. Here, origin/lab1 is the name of the git branch with the initial code you downloaded from our server for this assignment.

Getting started

We provide you with a skeleton RPC-based lock server, a lock client interface, a sample application that uses the lock client interface, and a tester. Now compile and start up the lock server, giving it a port number on which to listen to RPC requests. You'll need to choose a port number that other programs aren't using. For example:

% cd lab
% make
% ./lock_server 3772
Now open a second terminal on the same machine and run lock_demo, giving it the port number on which the server is listening:
% cd lab
% ./lock_demo 3772
stat returned 0

lock_demo asks the server for the number of times a particular lock has been acquired, using the stat RPC that we have provided. In the skeleton code, this will always return 0. You can use it as an example of how to add RPCs. You don't need to fix stat to report the actual number of acquisitions of the given lock in this lab, but you may if you wish.

The lock client skeleton does not do anything yet for the acquire and release operations; similarly, the lock server does not implement lock granting or releasing. Your job is to implement this functionality in the server, and to arrange for the client to send RPCs to the server.

Your Job

Your first job is to implement a correct lock server assuming a perfect underlying network. Correctness means obeying this invariant: at any point in time, there is at most one client holding a lock with a given identifier.

We will use the program lock_tester to check the correctness invariant, i.e. whether the server grants each lock just once at any given time, under a variety of conditions. You run lock_tester with the same arguments as lock_demo. A successful run of lock_tester (with a correct lock server) will look like this:

% ./lock_tester 3772
simple lock client
acquire a release a acquire a release a
acquire a acquire b release b release a
test2: client 0 acquire a release a
test2: client 2 acquire a release a
. . .
./lock_tester: passed all tests successfully
If your lock server isn't correct, lock_tester will print an error message. For example, if lock_tester complains "error: server granted XXX twice", the problem is probably that lock_tester sent two simultaneous requests for the same lock, and the server granted both requests. A correct server would have granted the lock to just one client, waited for a release, and only then sent granted the lock to the second client.

Your second job is to augment the RPC library to guarantee at-most-once execution. You can tell the RPC library to simulate a lossy network by setting the environment variable RPC_LOSSY. If you can pass both the RPC system tester and the lock_tester, you are done. Here's a successful run of both testers:

% export RPC_LOSSY=0
% ./rpc/rpctest
simple test
. . .
rpctest OK

% killall lock_server
% export RPC_LOSSY=5
% ./lock_server 3722 &
% ./lock_tester 3772
simple lock client
acquire a release a acquire a release a
. . .
./lock_tester: passed all tests successfully

Your code must pass both ./rpc/rpctest and lock_tester; you should ensure it passes several times in a row to guarantee there are no rare bugs. You should only make modifications on files rpc.{cc,h}, lock_client.{cc,h}, lock_server.{cc,h} and lock_smain.cc. We will test your code with with our own copy of the rest of the source files and testers. You are free to add new files to the directory as long as the Makefile compiles them appropriately, but you should not need to.

For this lab, you will not have to worry about server failures or client failures. You also need not be concerned about malicious or buggy applications.

Detailed Guidance

In principle, you can implement whatever design you like as long as it satisfies the requirements in the "Your Job" section and passes the testers. In practice, you should follow the detailed guidance below. You might want to look at the general programming tips in the lab overview page.

Step One: implement the lock_server assuming a perfect network

First, you should get the lock_server running correctly without worrying about duplicate RPCs.

Step two: Implement at-most-once delivery in RPC

The RPC code we provide you has a complete client implementation of at-most-once delivery: the client code times out while waiting for a response, re-sends the request, and accompanies each request with information the server will need for its part of at-most-once delivery. However, the code is missing some of the server at-most-once code, in particular the implementation of the functions rpcs::checkduplicate_and_update and rpcs::add_reply. It is your job to implement those two functions.

After your lock server has passed lock_tester, test it with a simulated lossy network: type "export RPC_LOSSY=5", restart your lock_server, and try lock_tester again. Very likely you will see the lock_tester fail or hang indefinitely. Try to understand exactly why your lock_tester fails when re-transmissions cause duplicate RPC delivery.

Skim the RPC source code in rpc/rpc.{cc,h} and try to grasp the overall structure of the RPC library as much as possible first by yourself before proceeding. rpc.cc already contains some of the code required to cope with duplicate requests; your job will be to complete that code.

The rpcc class manages RPC calls for clients. At its core lies the rpcc::call1 function, which accepts a marshalled RPC request for transmission to the RPC server. call1 attaches additional RPC fields to each marshalled request:

   // add RPC fields before the RPC request data
   req_header h(ca.xid, proc, clt_nonce_, srv_nonce_, xid_rep_window_.front());
What's the purpose for each field in req_header? (Hint: many of them are going to help you implement at-most-once delivery.) After call1 has finished preparing the final RPC request, it sits in a "while(1)" loop to (repeatedly) update the timeout value for the next retransmission and waits for the corresponding RPC reply or timeout to happen. Also, if the underlying (TCP) connection to the server fails, rpcc automatically re-connects to the server again (in function get_refconn) in order to retransmit.

The rpcs class manages RPC calls for the server. When a connection receives an RPC request, it calls rpcs::got_pdu to dispatch the request to a thread from the pool. The thread pool (class ThrPool) consists of a fixed number of threads that call rpcs::dispatch to dispatch an RPC request to the relevant registered RPC handler. rpcs::dispatch extracts RPC fields from the request, including the RPC procedure number which is used to find the corresponding handler. The header fields also provide sufficient information for you to ensure that the server eliminates all duplicate requests.

Question: The partial lock server we provide you uses "blocking" RPC handlers that sometimes wait (for lock releases). How many concurrent "blocking" lock acquire requests can the server handle? (Hint: our implementation of rpcs currently uses a thread pool of 10 threads).

How to ensure at-most-once delivery? A strawman approach is to make the server remember all unique RPCs ever received. Each unique RPC is identified by both its xid (unique across a client instance) and clt_nonce (unique across all client instances). In addition to the RPC ids, the server must also remember the original return value for each RPC so that the server can re-send it in response to a duplicate request. This strawman guarantees at-most-once, but is not ideal since the memory holding the RPC ids and replies grows indefinitely. A better alternative is to use a sliding window of remembered RPCs at the server. Such an approach requires the client to generate xid in a strict sequence, i.e. 0, 1, 2, 3... When can the server safely forget about a received RPC and its response, i.e. slide the window forward? What if a retransmitted request arrives while the server is still processing the original request?

Once you figure out the basic design for at-most-once delivery, go ahead and implement your design in rpc.cc and (if needed) rpc.h; you should not need to modify any other files. You need to add code in two places. First, rpcs::checkduplicate_and_update should 1) check if a request is a duplicate and return information about the remembered reply if it is, 2) remember that a new request has arrived if it is not a duplicate, and 3) trim the window of remembered requests and reply values. Second, rpcs::add_reply should remember the RPC reply values for an RPC call that the server has completed. You should store the remembered RPC reply values in rpcs::reply_window_, declared in rpc.h.

After you are done with step two, test your RPC implementation with ./rpc/rpctest and RPC_LOSSY set to 0 ("export RPC_LOSSY=0"). Make sure ./rpc/rpctest passes all tests. Once your RPC implementation passes all these tests, test your lock server again in a lossy environment by restarting your lock_server and lock_tester after setting RPC_LOSSY to 5 ("export RPC_LOSSY=5"). The RPC_LOSSY environment variable must be set for both lock_server and lock_tester.

Handin procedure

E-mail your code as a gzipped tar file to 6.824-submit@pdos.csail.mit.edu by the deadline at the top of the page. To do this, execute these commands:
% cd ~/6.824
% tar czvf `whoami`-lab1.tgz lab/
% cd ~/6.824/lab
% make handin
That should produce a file called [your_user_name]-lab1.tgz in your lab/ directory. Attach that file to an email and send it to the address above.

You will receive full credit if your software passes the same tests we gave you when we run your software on our machines.

Please post questions or comments on Piazza.
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