Question

Suppose we were to implement remote file system mounting using an unreliable RPC protocol that offers zero-or-more semantics. If a message reply is received, this improves to at-least-once semantics. We define read() to return the specified Nth block, rather than the next block in sequence; this way reading once is the same as reading twice and at-least-once semantics is thus the same as exactly once. (a) For what other file system operations is there no difference between at- leastonce and exactly once semantics? Consider open, create, write, seek, opendir, readdir, mkdir, delete (aka unlink), and rmdir. (b) For the remaining operations, which can have their semantics altered to achieve equivalence of at-least-once and exactly once? What file system operations are irreconcilable with at-least-once semantics? (c) Suppose the semantics of the rmdir system call are now that the given directory is removed if it exists, and nothing is done otherwise. How could you write a program to delete directories that distinguishes between these two cases?

Short answer

Operations such as read(), seek(), and rmdir (with modified semantics) can equate at-least-once to exactly-once. Open, create, write, opendir, readdir, mkdir, and delete have differences. Use directory listings before and after rmdir to distinguish outcomes.

Step-by-step solution

Identify File System Operations with No Difference in Semantics

Examine each file system operation to determine if multiple executions result in the same state as a single execution.

Analyze Open, Create, Write, and Seek Operations

Evaluate each operation to determine if multiple invocations lead to the same file system state.

Analyze Opendir, Readdir, Mkdir, Delete, and Rmdir Operations

Evaluate each operation to determine their behavior under repeated execution and identify operations irreconcilable with at-least-once semantics.

Discuss Alterable Operations for Semantics Equivalence

Identify operations that can have their semantics altered to match exactly-once.

Propose Program for Distinguishing rmdir Outcomes

Design and describe a program to differentiate if a directory existed before being removed.

Key concepts

Unreliable RPC Protocol

RPC stands for Remote Procedure Call. It's a way for a computer program to cause a procedure to execute in another address space, commonly on a remote server. An RPC protocol is what governs this communication. An unreliable RPC protocol can fail to deliver messages, or it might deliver them multiple times.
When using unreliable RPC, we might encounter zero-or-more semantics. This means a request can be delivered zero, one, or more times. Unlike reliable protocols, you can't be certain the request was processed. Nevertheless, certain methods can mitigate this:

File System Consistency

File system consistency is crucial for ensuring data integrity. It means that the file system remains in a valid state, even after crashes. Consistency is achieved through proper synchronization and sequencing of file operations.
For instance:

• Atomic Operations: Either completely succeed or have no effect.
• Journaling: Logs actions before applying them.
• Transactional Filesystems: Rollback operations in case of failure.
• File Locking: Prevents data races and corruption.

Maintaining consistency becomes challenging when using unreliable protocols, but design techniques like at-least-once semantics can help.

At-least-once Semantics

At-least-once semantics mean that every RPC request will be executed at least once, but could be executed multiple times due to retries. This ensures that a request will not be lost, but it does not prevent duplicates.
Certain file system operations can tolerate at-least-once semantics:

• Read: Reading the Nth block doesn't change the state.
• Mkdir: Multiple attempts result in the same directory creation.
• Delete: Attempts to delete an already deleted file have no effect.

However, some operations may inconsistently update the state if executed multiple times, such as write() and seek(), which may require special handling to prevent inconsistencies.

Exactly-once Semantics

Exactly-once semantics ensure that each operation is executed exactly one time. It's the ideal scenario but hard to achieve, particularly with unreliable RPC.
Implementing such semantics usually involves additional mechanisms:

• Request Tracking: Use unique identifiers to track requests and ensure each is processed once.
• Idempotent Operation Design: Ensure operations can safely be applied multiple times without changing the outcome.
• Acknowledgements: Confirm receipt and processing of requests to avoid duplicates.

Although challenging, exactly-once semantics are crucial for operations that must not be repeated, such as billing systems.

Remote File System Mounting

Remote file system mounting allows a client computer to access and manipulate files on a remote server as if they were on its local storage. This is typically done via protocols like NFS (Network File System) or SMB (Server Message Block).
File system mounting involves:

• Setting Mount Points: Directories on the client where the remote file system is accessible.
• Authentication: Ensuring secure access to remote files.
• Path Resolution: Mapping client paths to server paths.

Challenges include maintaining file consistency and handling network errors, particularly with unreliable RPC protocols. Proper protocol design and error handling are keys to seamless integration.