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…ache Remove the thread pool from `future::Cache`
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@@ -33,6 +33,7 @@ | |
| "Hasher", | ||
| "Kawano", | ||
| "mapref", | ||
| "Miri", | ||
| "Moka", | ||
| "mpsc", | ||
| "MSRV", | ||
|
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||
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| # Moka Cache — Migration Guide | ||
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| ## Migrating to v0.12 from a prior version | ||
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| v0.12.0 has major breaking changes on the API and internal behavior. This section | ||
| describes the code changes required to migrate to v0.12.0. | ||
|
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| ### `future::Cache` | ||
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| - The thread pool was removed from `future::Cache`. The background threads are no | ||
| longer spawned. | ||
| - The `notification::DeliveryMode` for eviction listener was changed from `Queued` to | ||
| `Immediate`. | ||
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| To support these changes, the following API changes were made: | ||
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| 1. `future::Cache::get` method is now `async fn`, so you must `await` for the result. | ||
| 2. `future::Cache::blocking` method was removed. | ||
| - Please use async runtime's blocking API instead. | ||
| - See [Replacing the blocking API](#replacing-the-blocking-api) for more details. | ||
| 3. Now `or_insert_with_if` method of the entry API requires `Send` bound for the | ||
| `replace_if` closure. | ||
| 4. `eviction_listener_with_queued_delivery_mode` method of `future::CacheBuilder` was | ||
| removed. | ||
| - Please use one of the new methods `eviction_listener` or | ||
| `async_eviction_listener` instead. | ||
| - See [Updating the eviction listener](#updating-the-eviction-listener) for more | ||
| details. | ||
| 5. `future::ConcurrentCacheExt::sync` method was renamed to | ||
| `future::Cache::run_pending_tasks`. It was also changed to `async fn`. | ||
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| The following internal behavior changes were made: | ||
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| 1. Maintenance tasks such as removing expired entries are not executed periodically | ||
| anymore. | ||
| - See [Maintenance tasks](#maintenance-tasks) for more details. | ||
| 2. Now `future::Cache` only supports `Immediate` delivery mode for eviction listener. | ||
| - In older versions, only `Queued` delivery mode was supported. | ||
| - If you need `Queued` delivery mode back, please file an issue. | ||
|
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| #### Replacing the blocking API | ||
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| `future::Cache::blocking` method was removed. Please use async runtime's blocking API | ||
| instead. | ||
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| **Tokio** | ||
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| 1. Call `tokio::runtime::Handle::current()` in async context to obtain a handle to | ||
| the current Tokio runtime. | ||
| 2. From outside async context, call cache's async function using `block_on` method of | ||
| the runtime. | ||
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| ```rust | ||
| use std::sync::Arc; | ||
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| #[tokio::main] | ||
| async fn main() { | ||
| // Create a future cache. | ||
| let cache = Arc::new(moka::future::Cache::new(100)); | ||
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| // In async context, you can obtain a handle to the current Tokio runtime. | ||
| let rt = tokio::runtime::Handle::current(); | ||
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| // Spawn an OS thread. Pass the handle and cache. | ||
| let thread = { | ||
| let cache = Arc::clone(&cache); | ||
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| std::thread::spawn(move || { | ||
| // Call async function using block_on method of Tokio runtime. | ||
| rt.block_on(cache.insert(0, 'a')); | ||
| }) | ||
| }; | ||
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| // Wait for the threads to complete. | ||
| thread.join().unwrap(); | ||
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| // Check the result. | ||
| assert_eq!(cache.get(&0).await, Some('a')); | ||
| } | ||
| ``` | ||
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| **async-std** | ||
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| - From outside async context, call cache's async function using | ||
| `async_std::task::block_on` method. | ||
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| ```rust | ||
| use std::sync::Arc; | ||
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| #[async_std::main] | ||
| async fn main() { | ||
| // Create a future cache. | ||
| let cache = Arc::new(moka::future::Cache::new(100)); | ||
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| // Spawn an OS thread. Pass the cache. | ||
| let thread = { | ||
| let cache = Arc::clone(&cache); | ||
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| std::thread::spawn(move || { | ||
| use async_std::task::block_on; | ||
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| // Call async function using block_on method of async_std. | ||
| block_on(cache.insert(0, 'a')); | ||
| }) | ||
| }; | ||
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| // Wait for the threads to complete. | ||
| thread.join().unwrap(); | ||
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| // Check the result. | ||
| assert_eq!(cache.get(&0).await, Some('a')); | ||
| } | ||
| ``` | ||
|
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| #### Updating the eviction listener | ||
|
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| - The `notification::DeliveryMode` for eviction listener was changed from `Queued` to | ||
| `Immediate`. | ||
| - `eviction_listener_with_queued_delivery_mode` method of `future::CacheBuilder` was | ||
| removed. Please use one of the new methods `eviction_listener` or | ||
| `async_eviction_listener` instead. | ||
|
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| ##### `eviction_listener` method | ||
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| `eviction_listener` takes the same closure as the old method. If you do not need to | ||
| `.await` anything in the eviction listener, use this method. | ||
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| This code snippet is borrowed from [an example][listener-ex1] in the document of | ||
| `future::Cache`: | ||
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| ```rust | ||
| let eviction_listener = |key, _value, cause| { | ||
| println!("Evicted key {key}. Cause: {cause:?}"); | ||
| }; | ||
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| let cache = Cache::builder() | ||
| .max_capacity(100) | ||
| .expire_after(expiry) | ||
| .eviction_listener(eviction_listener) | ||
| .build(); | ||
| ``` | ||
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| [listener-ex1]: https://docs.rs/moka/latest/moka/future/struct.Cache.html#per-entry-expiration-policy | ||
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| ##### `async_eviction_listener` method | ||
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| `async_eviction_listener` takes a closure that returns a `Future`. If you need to | ||
| `.await` something in the eviction listener, use this method. The actual return type | ||
| of the closure is `future::ListenerFuture`, which is a type alias of | ||
| `Pin<Box<dyn Future<Output = ()> + Send>>`. You can use the `boxed` method of | ||
| `future::FutureExt` trait to convert a regular `Future` into this type. | ||
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| This code snippet is borrowed from [an example][listener-ex2] in the document of | ||
| `future::Cache`: | ||
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| ```rust | ||
| use moka::notification::ListenerFuture; | ||
| // FutureExt trait provides the boxed method. | ||
| use moka::future::FutureExt; | ||
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| let eviction_listener = move |k, v: PathBuf, cause| -> ListenerFuture { | ||
| println!( | ||
| "\n== An entry has been evicted. k: {:?}, v: {:?}, cause: {:?}", | ||
| k, v, cause | ||
| ); | ||
| let file_mgr2 = Arc::clone(&file_mgr1); | ||
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| // Create a Future that removes the data file at the path `v`. | ||
| async move { | ||
| // Acquire the write lock of the DataFileManager. | ||
| let mut mgr = file_mgr2.write().await; | ||
| // Remove the data file. We must handle error cases here to | ||
| // prevent the listener from panicking. | ||
| if let Err(_e) = mgr.remove_data_file(v.as_path()).await { | ||
| eprintln!("Failed to remove a data file at {:?}", v); | ||
| } | ||
| } | ||
| // Convert the regular Future into ListenerFuture. This method is | ||
| // provided by moka::future::FutureExt trait. | ||
| .boxed() | ||
| }; | ||
|
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| // Create the cache. Set time to live for two seconds and set the | ||
| // eviction listener. | ||
| let cache = Cache::builder() | ||
| .max_capacity(100) | ||
| .time_to_live(Duration::from_secs(2)) | ||
| .async_eviction_listener(eviction_listener) | ||
| .build(); | ||
| ``` | ||
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| [listener-ex2]: https://docs.rs/moka/latest/moka/future/struct.Cache.html#example-eviction-listener | ||
|
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| #### Maintenance tasks | ||
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| In older versions, the maintenance tasks needed by the cache were periodically | ||
| executed in background by a global thread pool managed by `moka`. Now `future::Cache` | ||
| does not use the thread pool anymore, so those maintenance tasks are executed | ||
| _sometimes_ in foreground when certain cache methods (`get`, `get_with`, `insert`, | ||
| etc.) are called by user code. | ||
|
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|  | ||
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| Figure 1. The lifecycle of cached entries | ||
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| These maintenance tasks include: | ||
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| 1. Determine whether to admit a "temporary admitted" entry or not. | ||
| 2. Apply the recording of cache reads and writes to the internal data structures, | ||
| such as LFU filter, LRU queues, and timer wheels. | ||
| 3. When cache's max capacity is exceeded, select existing entries to evict and remove | ||
| them from cache. | ||
| 4. Remove expired entries. | ||
| 5. Remove entries that have been invalidated by `invalidate_all` or | ||
| `invalidate_entries_if` methods. | ||
| 6. Deliver removal notifications to the eviction listener. (Call the eviction | ||
| listener closure with the information about evicted entry) | ||
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| They will be executed in the following cache methods when necessary: | ||
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| - All cache write methods: `insert`, `get_with`, `invalidate`, etc. | ||
| - Some of the cache read methods: `get` | ||
| - `run_pending_tasks` method, which executes the pending maintenance tasks | ||
| explicitly. | ||
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| Although expired entries will not be removed until the pending maintenance tasks are | ||
| executed, they will not be returned by cache read methods such as `get`, `get_with` | ||
| and `contains_key`. So unless you need to remove expired entries immediately (e.g. to | ||
| free some memory), you do not need to call `run_pending_tasks` method. | ||
|
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| ### `sync::Cache` and `sync::SegmentedCache` | ||
|
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| 1. (Not in v0.12.0-beta.1) `sync` caches will be no longer enabled by default. Use a | ||
| crate feature `sync` to enable it. | ||
| 2. (Not in v0.12.0-beta.1) The thread pool will be disabled by default. | ||
| - In older versions, the thread pool was used to execute maintenance tasks in | ||
| background. | ||
| - When disabled: | ||
| - those maintenance tasks are executed _sometimes_ in foreground when certain | ||
| cache methods (`get`, `get_with`, `insert`, etc.) are called by user code | ||
| - See [Maintenance tasks](#maintenance-tasks) for more details. | ||
| - To enable it, see [Enabling the thread pool](#enabling-the-thread-pool) for more | ||
| details. | ||
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| #### Enabling the thread pool | ||
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| To enable the thread pool, do the followings: | ||
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| - Specify a crate feature `thread-pool`. | ||
| - At the cache creation time, call the `thread_pool_enabled` method of | ||
| `CacheBuilder`. |
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