Lab1 Lock Server - Due: 09-26 24:00

Introduction

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, or when the server reboots.

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, server grants the lock, client releases the lock with a release request, a duplicate RPC for the original acquire request then arrives at the server, 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.

Software

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.

You can get original source code from code server. After clone the git reposetory ads-lab directory will be found in the your local node. The git repository is also included in this directory. Make sure you have upload your rsa key to access the git server, seen in lab overview 

debian% git clone ads@202.120.40.188:ads-lab.git
debian% cd ./ads-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: 

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

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 ads-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 ads-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 any form of lock granting or releasing. Your job in this lab is to fill in the client and server function and the RPC protocol between the two processes.

Your Job

Your first job is to implement a correct lock server assuming a perfect underlying network. In the context of a lock service, correctness means obeying this invariant: at any instance of time, there is at most one client holding a lock of a given name.

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 a twice!", the problem is probably that lock_tester sent two simultaneous requests for the same lock, and the server granted the lock twice (once for each request). A correct server would have sent one grant, waited for a release, and only then sent a second grant.

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 3772 &
% ./lock_tester 3772
simple lock client
acquire a release a acquire a release a
. . .
./lock_tester: passed all tests successfully

For this lab, your lock server and RPC augmentation must pass the 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 clients doing things like releasing locks that they don't hold.

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 under lossy networks.
  • Using the RPC system:

    The RPC library's source code is in the subdirectory rpc/. A server uses the RPC library by creating an RPC server object (rpcs) listening on a port and registering various RPC handlers (see lock_smain.cc). A client creates a RPC client object (rpcc), asks for it to be connected to the lock_server's address and port, and invokes RPC calls (see lock_client.cc).

    Each RPC procedure is identified by a unique procedure number. We have defined the acquire and release RPC numbers you will need in lock_protocol.h. Other RPC numbers defined there are for use in later labs. You must register handlers for these RPCs with the RPC server object.

    You can learn how to use the RPC system by studying the stat call implementation in lock_client and lock_server. RPC handlers have a standard interface with x+1 (x must be less than 6) arguments and an integer return value. The last argument, a reference to an arbitary type, allows the handler to return results (e.g. lock_server::stat returns the number of acquires for a lock). The handler also returns an integer status code, and the convention is to return zero for success and to return positive numbers for various errors. If the RPC fails in the RPC library (e.g.timeouts), the RPC client gets a negative return value instead. The various reasons for RPC failures in the RPC library are defined in rpc.h under rpc_const.

    The RPC system marshalls objects into a stream of bytes to transmit over the network and unmarshalls them at the other end. Beware: the RPC library does not check that the data in an arriving message have the expected type(s). If a client sends one type and the server is expecting a different type, something bad will happen. You should check that the client's RPC call function sends types that are the same as those expected by the corresponding server handler function.

    The RPC library provides marshall/unmarshall methods for standard C++ objects such as std::string, int, and char (see file rpc.cc). If your RPC call includes different types of objects as arguments, you must provide your own marshalling method. You should be able to complete this lab with existing marshall/unmarshall methods.

  • Implementing the lock server:

    The lock server can manage many distinct locks. Each lock is identified by an integer of type lock_protocol::lockid_t. The set of locks is open-ended: if a client asks for a lock that the server has never seen before, the server should create the lock and grant it to the client. When multiple clients request the same lock, the lock server must grant the lock to one client at a time.

    You will need to modify the lock server skeleton implementation in files lock_server.{cc,h} to accept acquire/release RPCs from the lock client, and to keep track of the state of the locks. Here is our suggested implementation plan.

    On the server, a lock can be in one of two states;

    • free: no clients own the client
    • locked: some client owns the lock

    The RPC handler for acquire should first check if the lock is locked, and if so, the handler should block until the lock is free. When the lock is free, acquire changes its state to locked, then returns to the client, which indicates that the client now has the lock. The value r returned by acquire doesn't matter. The handler for release should change the lock state to free, and notify any threads that are waiting for the lock.

    Consider using the C++ STL (Standard Template Library) std::map class to hold the table of lock states.

  • Implementing the lock client:

    The class lock_client is a client-side interface to the lock server (found in files lock_client.{cc,h}). The interface provides acquire() and release() functions that should send and receive RPCs. Multiple threads in the client program can use the same lock_client object and request the same lock name. See lock_demo.cc for an example of how an application uses the interface. Note that a basic requirement of the client interface is that lock_client::acquire must not return until it has acquired the requested lock.

  • Handling multi-thread concurrency:

    Both lock_client and lock_server's functions will be invoked by multiple threads concurrently. On the lock server side, the RPC library keeps a thread pool and invokes the RPC handler using one of the idle threads in the pool. On the lock client side, many different threads might also call lock_client's acquire() and release() functions concurrently.

    You should use pthread mutexes to guard uses of data that is shared among threads. You should use pthread condition variables so that the lock server acquire handler can wait for a lock. The general tips contain a link to information about pthreads, mutexes, and condition variables.

    Threads should wait on a condition variable inside a loop that checks the boolean condition on which the thread is waiting. This protects the thread from spurious wake-ups from the pthread_cond_wait() and pthread_cond_timedwait() functions.

    Use a simple mutex scheme: a single pthreads mutex for all of lock_server. You don't really need (for example) a mutex per lock, though such a setup can be made to work. Using "coarse-granularity" mutexes will simplify your code.

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

After your lock server has passed lock_tester under a perfect network, enable RPC_LOSSY by typing "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());
   req.pack_req_header(h);
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 (rpc.cc is the only file you should modify). 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

You will need to upload your completed code as a gzipped tar file named by your student number to "/upload/lab1" directory of ftp://public.sjtu.edu.cn/ (username:xiechenny password:public) by the deadline at the top of the page. To do this, execute these commands: 
% cd ~/aos-lab
% handin.sh "your student number"
That should produce a file called [your_student_number].tar.gz in your aos-lab/ directory. Upload that file to the ftp above.

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

Credits: labs and papers from MIT 6.824

Labs

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