In this MP you will be implementing a distributed transaction system. Your goal is to support transactions that read and write to distributed objects while ensuring full ACI(D) properties. (The D is in parentheses because you are not required to store the values in durable storage or implement crash recovery.)
You are (once again) implementing a system that represents a collection of accounts and their balances. The accounts are stored in five different branches (named A, B, C, D, E). An account is named with the identifier of the branch followed by an account name; e.g., A.xyz is account xyz stored at branch A. Account names will be comprised of lowercase english letters.
You will need to implement a server that represents a branch and keeps track of all accounts in that branch, and a client that executes transactions. For each transaction, a client randomly picks one of the five servers to communicate with. The chosen server acts as the coordinator for that transaction, and communicates with the client and all other necessary servers. The coordinator can therefore be different for different transactions. More details on transactions and their properties have been provided later in this specification.
Unlike the previous MPs, you do not have to handle failures and can assume that all the servers remain up for the duration of the demo. Clients may exit but you do not have to deal with clients crashing in the middle of a transaction.
Each server must take two arguments. The first argument identifies the branch that the server must handle. The second argument is a configuration file. The configuration file has 5 lines, each containing the branch, hostname, and the port no. of a server. The configuration file provided to each server is the same. A sample configuration file for a cluster running on group g01 would look like this:
A sp21-cs425-g01-01.cs.illinois.edu 1234
B sp21-cs425-g01-02.cs.illinois.edu 1234
C sp21-cs425-g01-03.cs.illinois.edu 1234
D sp21-cs425-g01-04.cs.illinois.edu 1234
E sp21-cs425-g01-05.cs.illinois.edu 1234
A client takes two arguments – the first argument is a client id (unique for each client), and the second argument is the configuration file (same as above), which provides the required details for connecting to a server when processing a transaction.
At start up, the client starts accepting commands from stdin (typed in by the user). The user will execute the following commands:
BEGIN: Open a new transaction. The client must connect to a randomly selected server which will coordinate the transaction, and reply “OK” to the user.BEGIN OK
DEPOSIT server.account amount: Deposit some amount into an account. The amount will be a positive integer. (You can assume that the value of any account will never exceed 100,000,000.) The account balance should increase by the given amount. If the account does not exist (in other words, if it is the first time a deposit operation on the account has been issued), the account should be created with an initial balance of amount. The client should reply with OK.DEPOSIT A.foo 10 OK DEPOSIT B.bar 30 OK
BALANCE server.account: The client should display the current balance in
the given account:BALANCE A.foo A.foo = 10
If a query is made to an account that has not previously received a deposit, the server must abort the transaction and inform the client. The client should then print NOT FOUND, ABORTED.
BEGIN OK BALANCE C.baz NOT FOUND, ABORTED
WITHDRAW server.account amount: Withdraw some amount from an account. The account balance should
decrease by the withdrawn amount. The client should reply with OK if the operation is successful. If the
account does not exist (i.e, has never received any deposits), the server must abort the transaction and inform the client. The client should then print NOT FOUND, ABORTED.BEGIN OK WITHDRAW B.bar 5 OK WITHDRAW C.baz 5 NOT FOUND, ABORTED
COMMIT: Commit the transaction, making its results visible to other transactions. The client should reply either with COMMIT OK or ABORTED, in the case that the transaction had to be aborted during the commit process.ABORT: Abort the transaction. All updates made during the transaction must be rolled back. The client should reply with ABORTED to confirm that the transaction was aborted.The client will forward each command in a transaction to the randomly selected coordinating server. The coordinating server communicates with the appropriate server (corresponding to the branch in which the account is maintained) by sending it the command and receiving the outcome, which it then forwards to the client. The coordinating server handles ABORT and COMMIT commands by appropriately communicating with all other servers. Each server must process each command it receives in a way that satisfies the properties discussed below.
Notes:
BEGIN).DEPOSIT A.foo -232 or
WITHDRAW B.$#@% 0.DEPOSIT A.foo 10 and then call BALANCE on A.foo in the same transaction, I should see the deposited amounts. Whether updates from other transactions are seen depends on whether those transactions are committed and on the isolation properties, discussed below.DEPOSIT A.foo 10), and a concurrent transaction T2 also issues a deposit on the same account (say by calling DEPOSIT A.foo 30). Also suppose that T1 is before T2 in the serial equivalence order. If T1 gets aborted and T2 is committed, the account A.foo would then effectively be created by T2 with a balance of 30. Moreover, if T2 issues any WITHDRAW or BALANCE commands on A.foo after its deposit, these commands should return successfully irrespective of whether T1 commits or aborts. Note that A.foo’s balance would be different depending on whether T1 commits or aborts, and the success of T2’s commit would be contingent on meeting the consistency requirement specified below.DEPOSIT A.foo 10), and a concurrent transaction T2 calls WITHDRAW A.foo 5 (without making any prior deposits to the account). If T1 is before T2 in the serial equivalence order and T1 commits, then T2’s WITHDRAW command should return successfully. On the other hand, if T1 is before T2 in the serial equivalence order but T1 aborts, then T2’s WITHDRAW command should return NOT FOUND, ABORTED. Likewise, if T2 is before T1 in the serial equivalence order, then T2’s WITHDRAW command should return NOT FOUND, ABORTED. A similar logic would hold if T2 issues BALANCE A.foo instead of WITHDRAW.ABORTED to inform the user. In order to continue issuing commands, the user will need to open a new transaction using BEGIN. E.g.:BEGIN OK DEPOSIT A.foo 10 OK DEPOSIT B.bar 30 ABORTED BEGIN //next transaction begins OK ….
Transactions should execute atomically. In particular, any changes made by a transaction should be rolled back in case of an abort (initiated either by the user or a server) and all account values should be restored to their state before the transaction.
As described above, a transaction should not reference any accounts that have not yet received any deposits in a WITHDRAW or BALANCE command. An additional consistency constraint is that, at the end of a transaction no account balance should be negative. If any balances are negative when a user specifies COMMIT, the transaction should be aborted.
BEGIN OK DEPOSIT B.bar 20 OK WITHDRAW B.bar 30 OK COMMIT ABORTED
However, it is OK for accounts to have negative balances during the transaction, assuming those are eventually resolved:
BEGIN OK DEPOSIT B.bar 20 OK WITHDRAW B.bar 30 OK DEPOSIT B.bar 15 OK COMMIT COMMIT OK
Your system should be able to support concurrent transactions from multiple clients simultaneously (we will runs tests with up to 4 simultaneous clients). Note that each individual client handles sequential commands, only one transaction at a time. Your system should guarantee the serializability of the executed transactions. This means that the results should be equivalent to a serial execution of all committed transactions. (Aborted transactions should have no impact on other transactions.) A natural choice is to use either two-phase locking or timestamped concurrency to achieve this (though these are not strict requirements).
You must support concurrency between transactions that do not interfere with each other. E.g., if T1 on client 1 executes DEPOSIT A.x, BALANCE B.y and then T2 on client 2 executes DEPOSIT A.w, BALANCE B.z, the transactions should both proceed without waiting for each other. In particular, using a single global lock (or one lock per server) will not satisfy the concurrency requirements of this MP. You should support read sharing as well, so BALANCE A.x executed by two transactions should not be considered interfering.
On the other hand, if T1 executes DEPOSIT A.x and T2 executes BALANCE A.x, you may delay the execution
of one of the transactions while waiting for the other to complete; e.g., BALANCE A.x in T2 may wait to return a response until T1 is committed or aborted.
Your system must implement a deadlock resolution strategy. One option is deadlock detection, where the system detects a deadlock and aborts one of the transactions. You can also use concurrency control strategies that avoid deadlocks altogether (e.g. via timestamped concurrency control).
You should not use timeouts as your deadlock detection strategy because transactions will be executed interactively and this will therefore result in too many false positives. Moreover, your deadlock resolution strategy cannot assume that the objects requiring a lock are known apriori (when the transaction begins), as the user will input commands interactively.
As discussed earlier, the server system may choose to spontaneously abort a transaction (e.g. to break a deadlock, or while implementing timestamped concurrency control). When this happens, it must inform the client, and the client must display ABORTED to the user to indicate that the transaction has been aborted.
Your code needs to be in a git repository hosted by the Engineering GitLab. After creating your repository, please add the following NetIDs as members with Reporter access:
You will also need to submit a report through Gradescope. In your report, you must include the URL of your repository and the full revision number (git commit id) that you want us to grade, as shown by git rev-parse HEAD. Make sure that revision is pushed to GitLab before you submit. Only one submission per group – it is set up as a “group submission” on Gradescope, so please make sure you add your partner to your Gradescope submission.
In addition to this, your report should include:
Makefile if you’re using a compiled language! If there are any libraries or packages that need to be installed, please list those, too. Like the other MPs, we will run your code ourselves on our VMs and check that it works as expected, so it is important that your instructions are sufficiently clear.Design document
A detailed explanation of your concurrency control approach. Explain how and where locks are maintained, when they are acquired, and when they are released. If you are using a lock-free strategy, explain the other data structures used in your implementation. If your algorithm implements a strategy that does not directly follow a concurrency strategy described in the lecture or the literature, you will also need to include an argument for why your strategy ensures serial equivalence of transactions.
A description of how transactions are aborted and their actions are rolled back. Be sure to mention how your strategy ensures that other transactions do not use partial results from aborted transactions.
Describe how you detect or prevent deadlocks.
Code guidelines
The client should only print the responses to the commands or the ABORTED message (as described above). It should not print any additional messages.
For testing purposes, the client should support commands entered interactively via stdin (as exemplified above), as well as reading transaction commands one line at a time from a file redirected to stdin. For example, an input test file, in.txt, may contain the following lines:
BEGIN DEPOSIT A.foo 20 COMMIT BEGIN DEPOSIT A.foo 30 WITHDRAW A.foo 10 COMMIT BEGIN BALANCE B.bar DEPOSIT C.zee 5 COMMIT BEGIN BALANCE A.foo COMMIT
Running ./client config.txt < in.txt should print the following output:
OK OK COMMIT OK OK OK OK COMMIT OK OK NOT FOUND, ABORTED OK A.foo = 40 COMMIT OK
Any command entered after a transaction has been aborted, and before the next BEGIN command, must be ignored. In this case, the 10th and 11th lines of the input file (DEPOSIT C.zee and COMMIT) get ignored by the client, as they occur “outside a transaction”.
Every time a server commits any updates to its objects, it should print the balance of all accounts with non-zero values. Other than the above, the servers should not print any additional messages.
Graphs
You do not need to perform any experiment, or plot any graphs for this MP.
We would like to acknowledge Prof. Nikita Borisov for the initial design of this MP.
The source code of the website has been borrowed from Nikita Borisov