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| </pre><pre class="rust"><code><span class="doccomment">//! Common use patterns |
| //! |
| //! Here are some common patterns one can use for inspiration. These are mostly covered by examples |
| //! at the right type in the crate, but this lists them at a single place. |
| //! |
| //! # Sharing of configuration data |
| //! |
| //! We want to share configuration from some source with rare updates to some high performance |
| //! worker threads. It can be configuration in its true sense, or a routing table. |
| //! |
| //! The idea here is, each new version is a newly allocated in its own [`Arc`]. It is then stored |
| //! into a *shared* `ArcSwap` instance. |
| //! |
| //! Each worker then loads the current version before each work chunk. In case a new version is |
| //! stored, the worker keeps using the loaded one until it ends the work chunk and, if it's the |
| //! last one to have the version, deallocates it automatically by dropping the [`Guard`] |
| //! |
| //! Note that the configuration needs to be passed through a *single shared* [`ArcSwap`]. That |
| //! means we need to share that instance and we do so through an [`Arc`] (one could use a global |
| //! variable instead). |
| //! |
| //! Therefore, what we have is `Arc<ArcSwap<Config>>`. |
| //! |
| //! ```rust |
| //! # use std::sync::Arc; |
| //! # use std::sync::atomic::{AtomicBool, Ordering}; |
| //! # use std::thread; |
| //! # use std::time::Duration; |
| //! # |
| //! # use arc_swap::ArcSwap; |
| //! # struct Work; |
| //! # impl Work { fn fetch() -> Self { Work } fn perform(&self, _: &Config) {} } |
| //! # |
| //! #[derive(Debug, Default)] |
| //! struct Config { |
| //! // ... Stuff in here ... |
| //! } |
| //! |
| //! // We wrap the ArcSwap into an Arc, so we can share it between threads. |
| //! let config = Arc::new(ArcSwap::from_pointee(Config::default())); |
| //! |
| //! let terminate = Arc::new(AtomicBool::new(false)); |
| //! let mut threads = Vec::new(); |
| //! |
| //! // The configuration thread |
| //! threads.push(thread::spawn({ |
| //! let config = Arc::clone(&config); |
| //! let terminate = Arc::clone(&terminate); |
| //! move || { |
| //! while !terminate.load(Ordering::Relaxed) { |
| //! thread::sleep(Duration::from_secs(6)); |
| //! // Actually, load it from somewhere |
| //! let new_config = Arc::new(Config::default()); |
| //! config.store(new_config); |
| //! } |
| //! } |
| //! })); |
| //! |
| //! // The worker thread |
| //! for _ in 0..10 { |
| //! threads.push(thread::spawn({ |
| //! let config = Arc::clone(&config); |
| //! let terminate = Arc::clone(&terminate); |
| //! move || { |
| //! while !terminate.load(Ordering::Relaxed) { |
| //! let work = Work::fetch(); |
| //! let config = config.load(); |
| //! work.perform(&config); |
| //! } |
| //! } |
| //! })); |
| //! } |
| //! |
| //! // Terminate gracefully |
| //! terminate.store(true, Ordering::Relaxed); |
| //! for thread in threads { |
| //! thread.join().unwrap(); |
| //! } |
| //! ``` |
| //! |
| //! # Consistent snapshots |
| //! |
| //! While one probably wants to get a fresh instance every time a work chunk is available, |
| //! therefore there would be one [`load`] for each work chunk, it is often also important that the |
| //! configuration doesn't change in the *middle* of processing of one chunk. Therefore, one |
| //! commonly wants *exactly* one [`load`] for the work chunk, not *at least* one. If the processing |
| //! had multiple phases, one would use something like this: |
| //! |
| //! ```rust |
| //! # use std::sync::Arc; |
| //! # |
| //! # use arc_swap::ArcSwap; |
| //! # struct Config; |
| //! # struct Work; |
| //! # impl Work { |
| //! # fn fetch() -> Self { Work } |
| //! # fn phase_1(&self, _: &Config) {} |
| //! # fn phase_2(&self, _: &Config) {} |
| //! # } |
| //! # let config = Arc::new(ArcSwap::from_pointee(Config)); |
| //! let work = Work::fetch(); |
| //! let config = config.load(); |
| //! work.phase_1(&config); |
| //! // We keep the same config value here |
| //! work.phase_2(&config); |
| //! ``` |
| //! |
| //! Over this: |
| //! |
| //! ```rust |
| //! # use std::sync::Arc; |
| //! # |
| //! # use arc_swap::ArcSwap; |
| //! # struct Config; |
| //! # struct Work; |
| //! # impl Work { |
| //! # fn fetch() -> Self { Work } |
| //! # fn phase_1(&self, _: &Config) {} |
| //! # fn phase_2(&self, _: &Config) {} |
| //! # } |
| //! # let config = Arc::new(ArcSwap::from_pointee(Config)); |
| //! let work = Work::fetch(); |
| //! work.phase_1(&config.load()); |
| //! // WARNING!! This is broken, because in between phase_1 and phase_2, the other thread could |
| //! // have replaced the config. Then each phase would be performed with a different one and that |
| //! // could lead to surprises. |
| //! work.phase_2(&config.load()); |
| //! ``` |
| //! |
| //! # Caching of the configuration |
| //! |
| //! Let's say that the work chunks are really small, but there's *a lot* of them to work on. Maybe |
| //! we are routing packets and the configuration is the routing table that can sometimes change, |
| //! but mostly doesn't. |
| //! |
| //! There's an overhead to [`load`]. If the work chunks are small enough, that could be measurable. |
| //! We can reach for [`Cache`]. It makes loads much faster (in the order of accessing local |
| //! variables) in case nothing has changed. It has two costs, it makes the load slightly slower in |
| //! case the thing *did* change (which is rare) and if the worker is inactive, it holds the old |
| //! cached value alive. |
| //! |
| //! This is OK for our use case, because the routing table is usually small enough so some stale |
| //! instances taking a bit of memory isn't an issue. |
| //! |
| //! The part that takes care of updates stays the same as above. |
| //! |
| //! ```rust |
| //! # use std::sync::Arc; |
| //! # use std::thread; |
| //! # use std::sync::atomic::{AtomicBool, Ordering}; |
| //! # use arc_swap::{ArcSwap, Cache}; |
| //! # struct Packet; impl Packet { fn receive() -> Self { Packet } } |
| //! |
| //! #[derive(Debug, Default)] |
| //! struct RoutingTable { |
| //! // ... Stuff in here ... |
| //! } |
| //! |
| //! impl RoutingTable { |
| //! fn route(&self, _: Packet) { |
| //! // ... Interesting things are done here ... |
| //! } |
| //! } |
| //! |
| //! let routing_table = Arc::new(ArcSwap::from_pointee(RoutingTable::default())); |
| //! |
| //! let terminate = Arc::new(AtomicBool::new(false)); |
| //! let mut threads = Vec::new(); |
| //! |
| //! for _ in 0..10 { |
| //! let t = thread::spawn({ |
| //! let routing_table = Arc::clone(&routing_table); |
| //! let terminate = Arc::clone(&terminate); |
| //! move || { |
| //! let mut routing_table = Cache::new(routing_table); |
| //! while !terminate.load(Ordering::Relaxed) { |
| //! let packet = Packet::receive(); |
| //! // This load is cheaper, because we cache in the private Cache thing. |
| //! // But if the above receive takes a long time, the Cache will keep the stale |
| //! // value alive until this time (when it will get replaced by up to date value). |
| //! let current = routing_table.load(); |
| //! current.route(packet); |
| //! } |
| //! } |
| //! }); |
| //! threads.push(t); |
| //! } |
| //! |
| //! // Shut down properly |
| //! terminate.store(true, Ordering::Relaxed); |
| //! for thread in threads { |
| //! thread.join().unwrap(); |
| //! } |
| //! ``` |
| //! |
| //! # Projecting into configuration field |
| //! |
| //! We have a larger application, composed of multiple components. Each component has its own |
| //! `ComponentConfig` structure. Then, the whole application has a `Config` structure that contains |
| //! a component config for each component: |
| //! |
| //! ```rust |
| //! # struct ComponentConfig; |
| //! |
| //! struct Config { |
| //! component: ComponentConfig, |
| //! // ... Some other components and things ... |
| //! } |
| //! # let c = Config { component: ComponentConfig }; |
| //! # let _ = c.component; |
| //! ``` |
| //! |
| //! We would like to use [`ArcSwap`] to push updates to the components. But for various reasons, |
| //! it's not a good idea to put the whole `ArcSwap<Config>` to each component, eg: |
| //! |
| //! * That would make each component depend on the top level config, which feels reversed. |
| //! * It doesn't allow reusing the same component in multiple applications, as these would have |
| //! different `Config` structures. |
| //! * One needs to build the whole `Config` for tests. |
| //! * There's a risk of entanglement, that the component would start looking at configuration of |
| //! different parts of code, which would be hard to debug. |
| //! |
| //! We also could have a separate `ArcSwap<ComponentConfig>` for each component, but that also |
| //! doesn't feel right, as we would have to push updates to multiple places and they could be |
| //! inconsistent for a while and we would have to decompose the `Config` structure into the parts, |
| //! because we need our things in [`Arc`]s to be put into [`ArcSwap`]. |
| //! |
| //! This is where the [`Access`] trait comes into play. The trait abstracts over things that can |
| //! give access to up to date version of specific T. That can be a [`Constant`] (which is useful |
| //! mostly for the tests, where one doesn't care about the updating), it can be an |
| //! [`ArcSwap<T>`][`ArcSwap`] itself, but it also can be an [`ArcSwap`] paired with a closure to |
| //! project into the specific field. The [`DynAccess`] is similar, but allows type erasure. That's |
| //! more convenient, but a little bit slower. |
| //! |
| //! ```rust |
| //! # use std::sync::Arc; |
| //! # use arc_swap::ArcSwap; |
| //! # use arc_swap::access::{DynAccess, Map}; |
| //! |
| //! #[derive(Debug, Default)] |
| //! struct ComponentConfig; |
| //! |
| //! struct Component { |
| //! config: Box<dyn DynAccess<ComponentConfig>>, |
| //! } |
| //! |
| //! #[derive(Debug, Default)] |
| //! struct Config { |
| //! component: ComponentConfig, |
| //! } |
| //! |
| //! let config = Arc::new(ArcSwap::from_pointee(Config::default())); |
| //! |
| //! let component = Component { |
| //! config: Box::new(Map::new(Arc::clone(&config), |config: &Config| &config.component)), |
| //! }; |
| //! # let _ = component.config; |
| //! ``` |
| //! |
| //! One would use `Box::new(Constant(ComponentConfig))` in unittests instead as the `config` field. |
| //! |
| //! The [`Cache`] has its own [`Access`][crate::cache::Access] trait for similar purposes. |
| //! |
| //! [`Arc`]: std::sync::Arc |
| //! [`Guard`]: crate::Guard |
| //! [`load`]: crate::ArcSwapAny::load |
| //! [`ArcSwap`]: crate::ArcSwap |
| //! [`Cache`]: crate::cache::Cache |
| //! [`Access`]: crate::access::Access |
| //! [`DynAccess`]: crate::access::DynAccess |
| //! [`Constant`]: crate::access::Constant |
| </span></code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../../" data-current-crate="arc_swap" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
