Consistency level in projector and store (#122)

* Add consistency level in Projector and change store persist function

* Update changelog

* Add PR number in Changelog

* Format

CHANGELOG.md

@@ -13,6 +13,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   in the `AggregateManager` the `EventStore` associated type as `Box<dyn EventStore<Manager = >>` (with `Send` + `Sync` 
   bounds).
 - [[#115]]: Added `apply_events` to `Aggregate` with default implementation.
+- [[#122]]: 
+  - `Consistency` level enum.
+  - `consistency` function to `Projector` to instruct the `EventStore` on the persistence guarantees with default 
+    implementation returning `Consistency::Strong`.
 - [[#123]]: Added `postgres` documentation in docs.rs with `package.metadata.docs.rs` in `Cargo.toml`. Improved 
   modules documentation.
 
@@ -136,6 +140,7 @@ Refer to: [#107], [#108] and [#109]
 [0.6.2]: https://github.com/primait/event_sourcing.rs/compare/0.6.1...0.6.2
 
 [#123]: https://github.com/primait/event_sourcing.rs/pull/123
+[#122]: https://github.com/primait/event_sourcing.rs/pull/122
 [#117]: https://github.com/primait/event_sourcing.rs/pull/117
 [#115]: https://github.com/primait/event_sourcing.rs/pull/115
 [#114]: https://github.com/primait/event_sourcing.rs/pull/114

src/esrs/postgres/projector.rs

@@ -4,6 +4,21 @@ use uuid::Uuid;
 
 use crate::{AggregateManager, StoreEvent};
 
+/// This enum is used to instruct via [`Projector::consistency`] function which guarantees to have
+/// while projecting an event in the read side.
+/// - [`Consistency::Strong`] means that the projected data will be always available in the read
+///   side. In the actual default store implementation it implies that if a strong consistent
+///   projector fails the event will not be stored in the event store and the transaction rollbacks.
+/// - [`Consistency::Eventual`] means that there are no guarantees for the projected data to be
+///   persisted in the read side. In the actual default store implementation it implies that if an
+///   eventual consistent projector fails that event is stored anyway but nothing will be persisted
+///   in the read side. If no other projector fails the event will be stored in the event store with
+///   all the other projections and the transaction will be committed.
+pub enum Consistency {
+    Strong,
+    Eventual,
+}
+
 /// This trait is used to implement a `Projector`. A projector is intended to be an entity where to
 /// create, update and delete a read side. Every projector should be responsible to update a single
 /// read model.
@@ -12,6 +27,14 @@ pub trait Projector<Manager>: Sync
 where
     Manager: AggregateManager,
 {
+    /// This function could be used to instruct the [`Projector`] about its the [`Consistency`] level.
+    ///
+    /// It has a default implementation that returns [`Consistency::Strong`]. Override this function
+    /// to change its [`Consistency`] level.
+    fn consistency(&self) -> Consistency {
+        Consistency::Strong
+    }
+
     /// This function projects one event in each read model that implements this trait.
     /// The result is meant to catch generic errors.
     ///

src/esrs/postgres/store.rs

@@ -13,6 +13,7 @@ use sqlx::{Executor, Pool, Postgres, Transaction};
 use uuid::Uuid;
 
 use crate::esrs::policy;
+use crate::esrs::postgres::projector::Consistency;
 use crate::types::SequenceNumber;
 use crate::{Aggregate, AggregateManager, EventStore, StoreEvent};
 
@@ -238,7 +239,12 @@ where
 
         for store_event in &store_events {
             for projector in self.projectors().iter() {
-                projector.project(store_event, &mut transaction).await?;
+                match projector.consistency() {
+                    Consistency::Strong => projector.project(store_event, &mut transaction).await?,
+                    Consistency::Eventual => {
+                        let _ = projector.project(store_event, &mut transaction).await;
+                    }
+                }
             }
         }
 

src/lib.rs

@@ -19,7 +19,7 @@ mod esrs;
 #[cfg(feature = "postgres")]
 pub mod postgres {
     //! Provides implementation of the [`EventStore`] for Postgres.
-    pub use crate::esrs::postgres::projector::Projector;
+    pub use crate::esrs::postgres::projector::{Consistency, Projector};
     pub use crate::esrs::postgres::store::PgStore;
 }