Encapsulate over Arc<Mutex<_>>

help
1

Say we're gonna impl a thread-safe in-memory cache in Rust

I can think of some following code:

use std::{collections::HashMap, sync::Arc, sync::Weak};

pub struct CacheManager {
    storage: HashMap<u8, String>,
}

#[derive(Debug)]
pub enum QueryResult {
    Hit(String),
    None,
}

impl CacheManager {
    pub fn new() -> Self {
        CacheManager {
            storage: HashMap::new(),
        }
    }
    pub fn query(&self, key: u8) -> QueryResult {
        let storage = &self.storage;

        let find = storage.get(&key);

        if let Some(val) = find {
            return QueryResult::Hit(val.clone());
        }

        return QueryResult::None;
    }
    pub fn insert(&mut self, key: u8, val: String) -> bool {
        let storage = &mut self.storage;

        if storage.contains_key(&key) {
            return false;
        }

        let rlt = storage.insert(key, val);
        assert!(rlt.is_none(), "rlt is not none");

        return true;
    }
}

And the user would use this cache like:

let cache_manager = Arc::new(Mutex::new(CacheManager::new()));
// ...

This seems to work fine.

But my question is:

could we encapsulate more so that our user doesn't need to call Arc::new(Mutext::new(...)) by their own? (Since we're providing a thread-safe cache)

Rather, they just call somehing like

let c_m = ThreadSafeCache::new()
c_m.aquery(&15) // atomic query
c_m.ainsert(&15, String::from("nasd")) // atomic insert
// ...

I think this level of encapsulation is necessary for the sake of safety and convenience.

2

To make this work, you just need to write a definition for ThreadSafeCache:

pub struct ThreadSafeCache(Arc<Mutex<CacheManager>>);

impl ThreadSafeCache {
    pub fn new()->Self {
        ThreadSafeCache(Arc::new(Mutex::new(CacheManager::new())))
    }
    
    pub fn aquery(&self, key: u8) -> QueryResult {
        self.0.lock().expect("Mutex poisoned!").query(key)
    }
    
    pub fn ainsert(&self, key:u8, val:String) -> bool {
        self.0.lock().expect("Mutex poisoned!").insert(key, val)
    }
}

NB: I changed your API slightly by replacing some &u8s with u8.

3

You should use lock instead of try_lock. Otherwise you'll get spurious panics when aquery or ainsert races with themselves or each other.

1 Like
4

Thanks; I've corrected my post above.

5

Thanks for the reply!

Sorry to say I'm probably overly-frightened by the thread::spawn stuff :sweat_smile:

At that time, I was just wondering: Oh are there any traits that I should impl to make the compiler happy?

6

@2e71828

Sorry to say, after playing with this ThreadSafeCache a little bit, I've found some inconvience:

Originally, users would write code like

fn main() {
    let cache_manager = Arc::new(Mutex::new(CacheManager::new()));

    cache_manager.lock().unwrap().insert(13, String::from("13"));

    let mut threads = vec![];
    for i in 1..=12 {
        // !! natural & classical way to do this !!
        let cm = Arc::clone(&cache_manager);
        let handle = thread::spawn(move || {
            println!("in thread {}, query: {:?}", i, cm.lock().unwrap().query(15))
        });
        threads.push(handle);
    }

    threads.into_iter().for_each(|h| h.join().unwrap());
}

This is fine

But after changing to ThreadSafeCache, this way doesn't work...

fn main() {
    let cache_manager = ThreadSafeCache::new();

    cache_manager.ainsert(13, String::from("13"));

    let mut threads = vec![];
    for i in 1..=12 {

        // !! We cannot call Arc::clone here !!
        let cm = Arc::clone(&cache_manager);

        let handle = thread::spawn(move || println!("in thread {}, query: {:?}", i, cm.aquery(15)));
        threads.push(handle);
    }

    threads.into_iter().for_each(|h| h.join().unwrap());
}

Would you mind further explaining how you expect users to use this ThreadSafeCache when sticking to std::thread ?

(In last reply, I mistakenly tried with crossbeam::thread::scope and let cm = &cache_manager and passed the compiler)

7

To explain a little bit,

I think we'd better find a way to use ThreadSafeCache in the same old way as we have explicit Arc<Mutex<_>> on it

Specifically, the ownship gets passed into different threads just like what happens when we use Arc::clone

user land's api better to stay same, i.e. Arc::clone(&thread_safe_cache_manager), but a comprimise is also understandable

8

If you make ThreadSafeCacheManager derive Clone:

#[derive(Clone)]
pub struct ThreadSafeCache(Arc<Mutex<CacheManager>>);

then you can do this, which will clone the internal Arc:

        let cm = cache_manager.clone();
4 Likes
9

Thanks for the reply!

Actually, last night after posting above 2 questions and going to bed, I also found this way somehow(on the bed, yes!)

But after some further consideration, I decide to impl a plain rc_clone method which looks like:

impl ThreadSafeCache {
    // ...
    pub fn rc_lcone(&self) -> Self {
        Self(Arc::clone(&self.0))
    }
    //...
}

Reasons:

  1. when impling std::clone::Clone, we're sensing that this object gets a pure clone, yielding an independent object(2 cache in this case!). This may make users confused
  2. This would stop us from impling real Clone in future

by impling a rc_clone method, we explicitly add the rc into method name, which I think would clear users' confuse somehow

10

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