feat: add Storage trait and JSONL journal backend

Replace the internal Vec<CompletedTrial<V>> with a pluggable Storage<V>
trait. MemoryStorage is the default (no behavior change for existing
users). Behind the `journal` feature flag, JournalStorage persists
trials to a JSONL file with fs2 file locking for multi-process safety.

- Storage<V> trait with push(), trials_arc(), refresh() methods
- MemoryStorage<V> wraps Arc<RwLock<Vec<CompletedTrial<V>>>>
- JournalStorage<V> appends JSON lines with exclusive file locks
- Study::with_sampler_and_storage() general constructor
- Study::with_journal() convenience constructor (journal feature)
- Refresh from storage on create_trial() for multi-process discovery
- MSRV bumped to 1.89

.github/workflows/ci.yml

@@ -139,14 +139,14 @@ jobs:
           fail_ci_if_error: true
 
   msrv:
-    name: MSRV (1.88)
+    name: MSRV (1.89)
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - name: Install Rust 1.88
+      - name: Install Rust 1.89
         run: |
-          rustup override set 1.88
+          rustup override set 1.89
-          rustup update 1.88
+          rustup update 1.89
       - uses: Swatinem/rust-cache@v2
       - name: Check compilation
         run: cargo check --all-features

Cargo.toml

@@ -5,7 +5,7 @@ members = ["optimizer-derive"]
 name = "optimizer"
 version = "0.8.1"
 edition = "2024"
-rust-version = "1.88"
+rust-version = "1.89"
 license = "MIT"
 authors = ["Manuel Raimann <raimannma@outlook.de"]
 description = "A Rust library for optimization algorithms."
@@ -26,12 +26,14 @@ serde_json = { version = "1", optional = true }
 tracing = { version = "0.1.29", optional = true }
 sobol_burley = { version = "0.5", optional = true }
 nalgebra = { version = "0.34", optional = true }
+fs2 = { version = "0.4", optional = true }
 
 [features]
 default = []
 async = ["dep:tokio"]
 derive = ["dep:optimizer-derive"]
 serde = ["dep:serde", "dep:serde_json"]
+journal = ["dep:fs2", "serde"]
 tracing = ["dep:tracing"]
 sobol = ["dep:sobol_burley"]
 cma-es = ["dep:nalgebra"]
@@ -71,3 +73,7 @@ required-features = ["derive"]
 name = "visualization_demo"
 path = "examples/visualization_demo.rs"
 required-features = ["visualization"]
+
+[[test]]
+name = "journal_tests"
+required-features = ["journal"]

optimizer-derive/Cargo.toml

@@ -2,7 +2,7 @@
 name = "optimizer-derive"
 version = "0.1.0"
 edition = "2024"
-rust-version = "1.88"
+rust-version = "1.89"
 license = "MIT"
 description = "Derive macros for the optimizer crate"
 repository = "https://github.com/raimannma/rust-optimizer"

src/error.rs

@@ -93,6 +93,11 @@ pub enum Error {
     #[cfg(feature = "async")]
     #[error("async task error: {0}")]
     TaskError(String),
+
+    /// Returned when a storage operation fails.
+    #[cfg(feature = "journal")]
+    #[error("storage error: {0}")]
+    Storage(String),
 }
 
 pub type Result<T> = core::result::Result<T, Error>;

src/lib.rs

@@ -233,6 +233,7 @@ pub mod pareto;
 pub mod pruner;
 mod rng_util;
 pub mod sampler;
+pub mod storage;
 mod study;
 mod trial;
 mod types;
@@ -269,6 +270,9 @@ pub use sampler::random::RandomSampler;
 #[cfg(feature = "sobol")]
 pub use sampler::sobol::SobolSampler;
 pub use sampler::tpe::TpeSampler;
+#[cfg(feature = "journal")]
+pub use storage::JournalStorage;
+pub use storage::{MemoryStorage, Storage};
 pub use study::Study;
 #[cfg(feature = "serde")]
 pub use study::StudySnapshot;
@@ -314,6 +318,9 @@ pub mod prelude {
     #[cfg(feature = "sobol")]
     pub use crate::sampler::sobol::SobolSampler;
     pub use crate::sampler::tpe::TpeSampler;
+    #[cfg(feature = "journal")]
+    pub use crate::storage::JournalStorage;
+    pub use crate::storage::{MemoryStorage, Storage};
     pub use crate::study::Study;
     #[cfg(feature = "serde")]
     pub use crate::study::StudySnapshot;

src/storage/journal.rs

@@ -0,0 +1,164 @@
+//! JSONL-based journal storage backend.
+
+use core::marker::PhantomData;
+use std::fs::{File, OpenOptions};
+use std::io::{BufRead, BufReader, Write};
+use std::path::{Path, PathBuf};
+use std::sync::Arc;
+
+use fs2::FileExt;
+use parking_lot::{Mutex, RwLock};
+use serde::Serialize;
+use serde::de::DeserializeOwned;
+
+use super::{MemoryStorage, Storage};
+use crate::sampler::CompletedTrial;
+
+/// A storage backend that appends completed trials as JSON lines to a file.
+///
+/// Trials are kept in memory for fast read access and simultaneously
+/// persisted to a JSONL file.  Multiple processes can safely share
+/// the same file: writes use an exclusive file lock, reads use a
+/// shared file lock.
+///
+/// The type parameter `V` is the objective value type (typically `f64`).
+/// It must be serializable so that trials can be written to disk.
+///
+/// # Examples
+///
+/// ```no_run
+/// use optimizer::storage::JournalStorage;
+///
+/// let storage: JournalStorage<f64> = JournalStorage::new("trials.jsonl");
+/// ```
+pub struct JournalStorage<V = f64> {
+    memory: MemoryStorage<V>,
+    path: PathBuf,
+    /// Serialise in-process writes so we only hold the file lock briefly.
+    write_lock: Mutex<()>,
+    _marker: PhantomData<V>,
+}
+
+impl<V: Serialize + DeserializeOwned + Send + Sync> JournalStorage<V> {
+    /// Creates a new journal storage that writes to the given path.
+    ///
+    /// The file does not need to exist yet — it will be created on the
+    /// first write.  Existing trials in the file are **not** loaded
+    /// until [`refresh`](Storage::refresh) is called (which happens
+    /// automatically at the start of each trial via the [`Study`](crate::Study)).
+    ///
+    /// To pre-load existing trials, use [`JournalStorage::open`].
+    #[must_use]
+    pub fn new(path: impl AsRef<Path>) -> Self {
+        Self {
+            memory: MemoryStorage::new(),
+            path: path.as_ref().to_path_buf(),
+            write_lock: Mutex::new(()),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Opens an existing journal file and loads all stored trials.
+    ///
+    /// If the file does not exist, returns an empty storage (no error).
+    ///
+    /// # Errors
+    ///
+    /// Returns a [`Storage`](crate::Error::Storage) error if the file
+    /// exists but cannot be read or parsed.
+    pub fn open(path: impl AsRef<Path>) -> crate::Result<Self> {
+        let path = path.as_ref().to_path_buf();
+        let trials = load_trials_from_file(&path)?;
+        Ok(Self {
+            memory: MemoryStorage::with_trials(trials),
+            path,
+            write_lock: Mutex::new(()),
+            _marker: PhantomData,
+        })
+    }
+
+    /// Append a single trial to the JSONL file (best-effort).
+    fn write_to_file(&self, trial: &CompletedTrial<V>) -> crate::Result<()> {
+        let _guard = self.write_lock.lock();
+
+        let mut file = OpenOptions::new()
+            .create(true)
+            .append(true)
+            .open(&self.path)
+            .map_err(|e| crate::Error::Storage(e.to_string()))?;
+
+        file.lock_exclusive()
+            .map_err(|e| crate::Error::Storage(e.to_string()))?;
+
+        let line =
+            serde_json::to_string(trial).map_err(|e| crate::Error::Storage(e.to_string()))?;
+
+        writeln!(file, "{line}").map_err(|e| crate::Error::Storage(e.to_string()))?;
+        file.flush()
+            .map_err(|e| crate::Error::Storage(e.to_string()))?;
+
+        file.unlock()
+            .map_err(|e| crate::Error::Storage(e.to_string()))?;
+
+        Ok(())
+    }
+}
+
+impl<V: Serialize + DeserializeOwned + Send + Sync> Storage<V> for JournalStorage<V> {
+    fn push(&self, trial: CompletedTrial<V>) {
+        // Best-effort persist; the trial stays in memory regardless.
+        let _ = self.write_to_file(&trial);
+        self.memory.push(trial);
+    }
+
+    fn trials_arc(&self) -> &Arc<RwLock<Vec<CompletedTrial<V>>>> {
+        self.memory.trials_arc()
+    }
+
+    fn refresh(&self) -> bool {
+        let Ok(loaded) = load_trials_from_file::<V>(&self.path) else {
+            return false;
+        };
+        let mut guard = self.memory.trials_arc().write();
+        if loaded.len() > guard.len() {
+            *guard = loaded;
+            true
+        } else {
+            false
+        }
+    }
+}
+
+/// Read all trials from a JSONL file.  Returns an empty vec if the
+/// file does not exist.
+fn load_trials_from_file<V: DeserializeOwned>(
+    path: &Path,
+) -> crate::Result<Vec<CompletedTrial<V>>> {
+    let file = match File::open(path) {
+        Ok(f) => f,
+        Err(e) if e.kind() == std::io::ErrorKind::NotFound => return Ok(Vec::new()),
+        Err(e) => return Err(crate::Error::Storage(e.to_string())),
+    };
+
+    file.lock_shared()
+        .map_err(|e| crate::Error::Storage(e.to_string()))?;
+
+    let reader = BufReader::new(&file);
+    let mut trials = Vec::new();
+
+    for line in reader.lines() {
+        let line = line.map_err(|e| crate::Error::Storage(e.to_string()))?;
+        let line = line.trim();
+        if line.is_empty() {
+            continue;
+        }
+        let trial: CompletedTrial<V> =
+            serde_json::from_str(line).map_err(|e| crate::Error::Storage(e.to_string()))?;
+        trials.push(trial);
+    }
+
+    file.unlock()
+        .map_err(|e| crate::Error::Storage(e.to_string()))?;
+
+    Ok(trials)
+}

src/storage/memory.rs

@@ -0,0 +1,47 @@
+use std::sync::Arc;
+
+use parking_lot::RwLock;
+
+use super::Storage;
+use crate::sampler::CompletedTrial;
+
+/// In-memory trial storage (the default).
+///
+/// This is a thin wrapper around `Arc<RwLock<Vec<CompletedTrial<V>>>>`.
+pub struct MemoryStorage<V> {
+    trials: Arc<RwLock<Vec<CompletedTrial<V>>>>,
+}
+
+impl<V> MemoryStorage<V> {
+    /// Creates a new, empty in-memory store.
+    #[must_use]
+    pub fn new() -> Self {
+        Self {
+            trials: Arc::new(RwLock::new(Vec::new())),
+        }
+    }
+
+    /// Creates an in-memory store pre-populated with `trials`.
+    #[must_use]
+    pub fn with_trials(trials: Vec<CompletedTrial<V>>) -> Self {
+        Self {
+            trials: Arc::new(RwLock::new(trials)),
+        }
+    }
+}
+
+impl<V> Default for MemoryStorage<V> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<V: Send + Sync> Storage<V> for MemoryStorage<V> {
+    fn push(&self, trial: CompletedTrial<V>) {
+        self.trials.write().push(trial);
+    }
+
+    fn trials_arc(&self) -> &Arc<RwLock<Vec<CompletedTrial<V>>>> {
+        &self.trials
+    }
+}

src/storage/mod.rs

@@ -0,0 +1,48 @@
+//! Trial storage backends.
+//!
+//! The [`Storage`] trait defines how completed trials are stored and
+//! accessed.  [`MemoryStorage`] keeps trials in memory (the default).
+//! With the `journal` feature enabled, [`JournalStorage`] appends
+//! trials to a JSONL file with file-level locking so multiple
+//! processes can safely share state.
+
+#[cfg(feature = "journal")]
+mod journal;
+
+use std::sync::Arc;
+
+#[cfg(feature = "journal")]
+pub use journal::JournalStorage;
+use parking_lot::RwLock;
+
+mod memory;
+pub use memory::MemoryStorage;
+
+use crate::sampler::CompletedTrial;
+
+/// Trait for storing and retrieving completed trials.
+///
+/// Every [`Study`](crate::Study) owns an `Arc<dyn Storage<V>>`.  The
+/// default implementation is [`MemoryStorage`], which keeps trials in
+/// a plain `Vec` behind a read-write lock.
+///
+/// Implementations must be safe to use from multiple threads.
+pub trait Storage<V>: Send + Sync {
+    /// Append a completed trial to the store.
+    fn push(&self, trial: CompletedTrial<V>);
+
+    /// Return a reference to the in-memory trial buffer.
+    ///
+    /// All implementations must maintain an `Arc<RwLock<Vec<…>>>` that
+    /// reflects the current set of trials.  Callers may acquire a read
+    /// lock for efficient, allocation-free access.
+    fn trials_arc(&self) -> &Arc<RwLock<Vec<CompletedTrial<V>>>>;
+
+    /// Reload from an external source (e.g. a file written by another
+    /// process).  Returns `true` if the in-memory buffer was updated.
+    ///
+    /// The default implementation is a no-op that returns `false`.
+    fn refresh(&self) -> bool {
+        false
+    }
+}

src/study.rs

@@ -49,8 +49,8 @@ where
     sampler: Arc<dyn Sampler>,
     /// The pruner used to decide whether to stop trials early.
     pruner: Arc<dyn Pruner>,
-    /// Completed trials (wrapped in Arc for sharing with Trial).
+    /// Trial storage backend (default: [`MemoryStorage`](crate::storage::MemoryStorage)).
-    completed_trials: Arc<RwLock<Vec<CompletedTrial<V>>>>,
+    storage: Arc<dyn crate::storage::Storage<V>>,
     /// Counter for generating unique trial IDs.
     next_trial_id: AtomicU64,
     /// Optional factory for creating sampler-aware trials.
@@ -84,7 +84,7 @@ where
     #[must_use]
     pub fn new(direction: Direction) -> Self
     where
-        V: 'static,
+        V: Send + Sync + 'static,
     {
         Self::with_sampler(direction, RandomSampler::new())
     }
@@ -109,7 +109,7 @@ where
     #[must_use]
     pub fn minimize(sampler: impl Sampler + 'static) -> Self
     where
-        V: 'static,
+        V: Send + Sync + 'static,
     {
         Self::with_sampler(Direction::Minimize, sampler)
     }
@@ -134,7 +134,7 @@ where
     #[must_use]
     pub fn maximize(sampler: impl Sampler + 'static) -> Self
     where
-        V: 'static,
+        V: Send + Sync + 'static,
     {
         Self::with_sampler(Direction::Maximize, sampler)
     }
@@ -158,40 +158,28 @@ where
     /// ```
     pub fn with_sampler(direction: Direction, sampler: impl Sampler + 'static) -> Self
     where
-        V: 'static,
+        V: Send + Sync + 'static,
     {
-        let sampler: Arc<dyn Sampler> = Arc::new(sampler);
+        Self::with_sampler_and_storage(
-        let completed_trials = Arc::new(RwLock::new(Vec::new()));
-
-        let pruner: Arc<dyn Pruner> = Arc::new(NopPruner);
-
-        // For Study<f64>, set up a trial factory that provides sampler integration.
-        // This uses Any downcasting to check at runtime whether V = f64.
-        let trial_factory = Self::make_trial_factory(&sampler, &completed_trials, &pruner);
-
-        Self {
             direction,
             sampler,
-            pruner,
+            crate::storage::MemoryStorage::<V>::new(),
-            completed_trials,
+        )
-            next_trial_id: AtomicU64::new(0),
-            trial_factory,
-            enqueued_params: Arc::new(Mutex::new(VecDeque::new())),
-        }
     }
 
     /// Builds a trial factory for sampler integration when `V = f64`.
     fn make_trial_factory(
         sampler: &Arc<dyn Sampler>,
-        completed_trials: &Arc<RwLock<Vec<CompletedTrial<V>>>>,
+        storage: &Arc<dyn crate::storage::Storage<V>>,
         pruner: &Arc<dyn Pruner>,
     ) -> Option<Arc<dyn Fn(u64) -> Trial + Send + Sync>>
     where
         V: 'static,
     {
-        // Try to downcast the completed_trials Arc to the f64 specialization.
+        // Try to downcast the storage's trial buffer to the f64 specialization.
         // This succeeds only when V = f64, enabling automatic sampler integration.
-        let any_ref: &dyn Any = completed_trials;
+        let trials_arc = storage.trials_arc();
+        let any_ref: &dyn Any = trials_arc;
         let f64_trials: Option<&Arc<RwLock<Vec<CompletedTrial<f64>>>>> = any_ref.downcast_ref();
 
         f64_trials.map(|trials| {
@@ -210,6 +198,42 @@ where
         })
     }
 
+    /// Creates a study with a custom sampler and storage backend.
+    ///
+    /// This is the most general constructor — all other constructors
+    /// delegate to this one.
+    pub fn with_sampler_and_storage(
+        direction: Direction,
+        sampler: impl Sampler + 'static,
+        storage: impl crate::storage::Storage<V> + 'static,
+    ) -> Self
+    where
+        V: 'static,
+    {
+        let sampler: Arc<dyn Sampler> = Arc::new(sampler);
+        let pruner: Arc<dyn Pruner> = Arc::new(NopPruner);
+        let storage: Arc<dyn crate::storage::Storage<V>> = Arc::new(storage);
+        let trial_factory = Self::make_trial_factory(&sampler, &storage, &pruner);
+
+        let next_id = storage
+            .trials_arc()
+            .read()
+            .iter()
+            .map(|t| t.id)
+            .max()
+            .map_or(0, |id| id + 1);
+
+        Self {
+            direction,
+            sampler,
+            pruner,
+            storage,
+            next_trial_id: AtomicU64::new(next_id),
+            trial_factory,
+            enqueued_params: Arc::new(Mutex::new(VecDeque::new())),
+        }
+    }
+
     /// Returns the optimization direction.
     pub fn direction(&self) -> Direction {
         self.direction
@@ -254,18 +278,19 @@ where
         pruner: impl Pruner + 'static,
     ) -> Self
     where
-        V: 'static,
+        V: Send + Sync + 'static,
     {
         let sampler: Arc<dyn Sampler> = Arc::new(sampler);
         let pruner: Arc<dyn Pruner> = Arc::new(pruner);
-        let completed_trials = Arc::new(RwLock::new(Vec::new()));
+        let storage: Arc<dyn crate::storage::Storage<V>> =
-        let trial_factory = Self::make_trial_factory(&sampler, &completed_trials, &pruner);
+            Arc::new(crate::storage::MemoryStorage::<V>::new());
+        let trial_factory = Self::make_trial_factory(&sampler, &storage, &pruner);
 
         Self {
             direction,
             sampler,
             pruner,
-            completed_trials,
+            storage,
             next_trial_id: AtomicU64::new(0),
             trial_factory,
             enqueued_params: Arc::new(Mutex::new(VecDeque::new())),
@@ -277,8 +302,7 @@ where
         V: 'static,
     {
         self.sampler = Arc::new(sampler);
-        self.trial_factory =
+        self.trial_factory = Self::make_trial_factory(&self.sampler, &self.storage, &self.pruner);
-            Self::make_trial_factory(&self.sampler, &self.completed_trials, &self.pruner);
     }
 
     /// Sets a new pruner for the study.
@@ -291,8 +315,7 @@ where
         V: 'static,
     {
         self.pruner = Arc::new(pruner);
-        self.trial_factory =
+        self.trial_factory = Self::make_trial_factory(&self.sampler, &self.storage, &self.pruner);
-            Self::make_trial_factory(&self.sampler, &self.completed_trials, &self.pruner);
     }
 
     /// Returns a reference to the study's pruner.
@@ -401,6 +424,13 @@ where
     /// assert_eq!(trial2.id(), 1);
     /// ```
     pub fn create_trial(&self) -> Trial {
+        if self.storage.refresh() {
+            let trials = self.storage.trials_arc().read();
+            if let Some(max_id) = trials.iter().map(|t| t.id).max() {
+                self.next_trial_id.fetch_max(max_id + 1, Ordering::SeqCst);
+            }
+        }
+
         let id = self.next_trial_id();
         let mut trial = if let Some(factory) = &self.trial_factory {
             factory(id)
@@ -455,7 +485,8 @@ where
         );
         completed.state = TrialState::Complete;
         completed.constraints = trial.constraint_values().to_vec();
-        self.completed_trials.write().push(completed);
+
+        self.storage.push(completed);
     }
 
     /// Records a failed trial with an error message.
@@ -565,7 +596,8 @@ where
         );
         completed.state = TrialState::Pruned;
         completed.constraints = trial.constraint_values().to_vec();
-        self.completed_trials.write().push(completed);
+
+        self.storage.push(completed);
     }
 
     /// Returns an iterator over all completed trials.
@@ -596,7 +628,7 @@ where
     where
         V: Clone,
     {
-        self.completed_trials.read().clone()
+        self.storage.trials_arc().read().clone()
     }
 
     /// Returns the number of completed trials.
@@ -619,12 +651,13 @@ where
     /// assert_eq!(study.n_trials(), 1);
     /// ```
     pub fn n_trials(&self) -> usize {
-        self.completed_trials.read().len()
+        self.storage.trials_arc().read().len()
     }
 
     /// Returns the number of pruned trials.
     pub fn n_pruned_trials(&self) -> usize {
-        self.completed_trials
+        self.storage
+            .trials_arc()
             .read()
             .iter()
             .filter(|t| t.state == TrialState::Pruned)
@@ -703,7 +736,7 @@ where
     where
         V: Clone,
     {
-        let trials = self.completed_trials.read();
+        let trials = self.storage.trials_arc().read();
         let direction = self.direction;
 
         let best = trials
@@ -767,7 +800,7 @@ where
     where
         V: Clone,
     {
-        let trials = self.completed_trials.read();
+        let trials = self.storage.trials_arc().read();
         let direction = self.direction;
         let mut completed: Vec<_> = trials
             .iter()
@@ -848,7 +881,7 @@ where
                     #[cfg(feature = "tracing")]
                     {
                         tracing::info!(trial_id, "trial completed");
-                        let trials = self.completed_trials.read();
+                        let trials = self.storage.trials_arc().read();
                         if trials
                             .iter()
                             .filter(|t| t.state == TrialState::Complete)
@@ -876,7 +909,8 @@ where
 
         // Return error if no trials completed successfully
         let has_complete = self
-            .completed_trials
+            .storage
+            .trials_arc()
             .read()
             .iter()
             .any(|t| t.state == TrialState::Complete);
@@ -975,7 +1009,8 @@ where
 
         // Return error if no trials completed successfully
         let has_complete = self
-            .completed_trials
+            .storage
+            .trials_arc()
             .read()
             .iter()
             .any(|t| t.state == TrialState::Complete);
@@ -1101,7 +1136,8 @@ where
 
         // Return error if no trials completed successfully
         let has_complete = self
-            .completed_trials
+            .storage
+            .trials_arc()
             .read()
             .iter()
             .any(|t| t.state == TrialState::Complete);
@@ -1197,7 +1233,7 @@ where
                     #[cfg(feature = "tracing")]
                     {
                         tracing::info!(trial_id, "trial completed");
-                        let trials = self.completed_trials.read();
+                        let trials = self.storage.trials_arc().read();
                         if trials
                             .iter()
                             .filter(|t| t.state == TrialState::Complete)
@@ -1210,7 +1246,7 @@ where
                     }
 
                     // Get the just-completed trial for the callback
-                    let trials = self.completed_trials.read();
+                    let trials = self.storage.trials_arc().read();
                     let Some(completed) = trials.last() else {
                         return Err(crate::Error::Internal(
                             "completed trial not found after adding",
@@ -1243,7 +1279,8 @@ where
 
         // Return error if no trials completed successfully
         let has_complete = self
-            .completed_trials
+            .storage
+            .trials_arc()
             .read()
             .iter()
             .any(|t| t.state == TrialState::Complete);
@@ -1328,7 +1365,8 @@ where
         }
 
         let has_complete = self
-            .completed_trials
+            .storage
+            .trials_arc()
             .read()
             .iter()
             .any(|t| t.state == TrialState::Complete);
@@ -1420,7 +1458,7 @@ where
                     #[cfg(feature = "tracing")]
                     {
                         tracing::info!(trial_id, "trial completed");
-                        let trials = self.completed_trials.read();
+                        let trials = self.storage.trials_arc().read();
                         if trials
                             .iter()
                             .filter(|t| t.state == TrialState::Complete)
@@ -1432,7 +1470,7 @@ where
                         }
                     }
 
-                    let trials = self.completed_trials.read();
+                    let trials = self.storage.trials_arc().read();
                     let Some(completed) = trials.last() else {
                         return Err(crate::Error::Internal(
                             "completed trial not found after adding",
@@ -1461,7 +1499,8 @@ where
         }
 
         let has_complete = self
-            .completed_trials
+            .storage
+            .trials_arc()
             .read()
             .iter()
             .any(|t| t.state == TrialState::Complete);
@@ -1519,7 +1558,8 @@ where
         }
 
         let has_complete = self
-            .completed_trials
+            .storage
+            .trials_arc()
             .read()
             .iter()
             .any(|t| t.state == TrialState::Complete);
@@ -1608,7 +1648,8 @@ where
         }
 
         let has_complete = self
-            .completed_trials
+            .storage
+            .trials_arc()
             .read()
             .iter()
             .any(|t| t.state == TrialState::Complete);
@@ -1716,7 +1757,8 @@ where
 
         // Return error if no trials completed successfully
         let has_complete = self
-            .completed_trials
+            .storage
+            .trials_arc()
             .read()
             .iter()
             .any(|t| t.state == TrialState::Complete);
@@ -1746,7 +1788,7 @@ where
     pub fn to_csv(&self, mut writer: impl std::io::Write) -> std::io::Result<()> {
         use std::collections::BTreeMap;
 
-        let trials = self.completed_trials.read();
+        let trials = self.storage.trials_arc().read();
 
         // Collect all unique parameter labels (sorted for deterministic column order).
         let mut param_columns: BTreeMap<ParamId, String> = BTreeMap::new();
@@ -1873,7 +1915,7 @@ where
     pub fn summary(&self) -> String {
         use fmt::Write;
 
-        let trials = self.completed_trials.read();
+        let trials = self.storage.trials_arc().read();
         let n_complete = trials
             .iter()
             .filter(|t| t.state == TrialState::Complete)
@@ -1967,7 +2009,7 @@ where
         use crate::param::ParamValue;
         use crate::types::TrialState;
 
-        let trials = self.completed_trials.read();
+        let trials = self.storage.trials_arc().read();
         let complete: Vec<_> = trials
             .iter()
             .filter(|t| t.state == TrialState::Complete)
@@ -2064,7 +2106,7 @@ where
         use crate::param::ParamValue;
         use crate::types::TrialState;
 
-        let trials = self.completed_trials.read();
+        let trials = self.storage.trials_arc().read();
         let complete: Vec<_> = trials
             .iter()
             .filter(|t| t.state == TrialState::Complete)
@@ -2258,6 +2300,63 @@ impl Study<f64> {
     }
 }
 
+impl<V: PartialOrd + Send + Sync + 'static> Study<V> {
+    /// Creates a study with a custom sampler, pruner, and storage backend.
+    pub fn with_sampler_pruner_and_storage(
+        direction: Direction,
+        sampler: impl Sampler + 'static,
+        pruner: impl Pruner + 'static,
+        storage: impl crate::storage::Storage<V> + 'static,
+    ) -> Self {
+        let sampler: Arc<dyn Sampler> = Arc::new(sampler);
+        let pruner: Arc<dyn Pruner> = Arc::new(pruner);
+        let storage: Arc<dyn crate::storage::Storage<V>> = Arc::new(storage);
+        let trial_factory = Self::make_trial_factory(&sampler, &storage, &pruner);
+
+        let next_id = storage
+            .trials_arc()
+            .read()
+            .iter()
+            .map(|t| t.id)
+            .max()
+            .map_or(0, |id| id + 1);
+
+        Self {
+            direction,
+            sampler,
+            pruner,
+            storage,
+            next_trial_id: AtomicU64::new(next_id),
+            trial_factory,
+            enqueued_params: Arc::new(Mutex::new(VecDeque::new())),
+        }
+    }
+}
+
+#[cfg(feature = "journal")]
+impl<V> Study<V>
+where
+    V: PartialOrd + Send + Sync + serde::Serialize + serde::de::DeserializeOwned + 'static,
+{
+    /// Creates a study backed by a JSONL journal file.
+    ///
+    /// Any existing trials in the file are loaded into memory and the
+    /// trial ID counter is set to one past the highest stored ID. New
+    /// trials are written through to the file on completion.
+    ///
+    /// # Errors
+    ///
+    /// Returns a [`Storage`](crate::Error::Storage) error if loading fails.
+    pub fn with_journal(
+        direction: Direction,
+        sampler: impl Sampler + 'static,
+        path: impl AsRef<std::path::Path>,
+    ) -> crate::Result<Self> {
+        let storage = crate::storage::JournalStorage::<V>::open(path)?;
+        Ok(Self::with_sampler_and_storage(direction, sampler, storage))
+    }
+}
+
 #[cfg(feature = "visualization")]
 impl Study<f64> {
     /// Generates an HTML report with interactive Plotly.js charts.
@@ -2382,7 +2481,7 @@ impl<V: PartialOrd + Clone + Default + serde::Serialize> Study<V> {
 }
 
 #[cfg(feature = "serde")]
-impl<V: PartialOrd + Clone + serde::de::DeserializeOwned + 'static> Study<V> {
+impl<V: PartialOrd + Send + Sync + Clone + serde::de::DeserializeOwned + 'static> Study<V> {
     /// Loads a study from a JSON file.
     ///
     /// The loaded study uses a `RandomSampler` by default. Call
@@ -2397,7 +2496,7 @@ impl<V: PartialOrd + Clone + serde::de::DeserializeOwned + 'static> Study<V> {
         let snapshot: StudySnapshot<V> = serde_json::from_reader(file)
             .map_err(|e| std::io::Error::new(std::io::ErrorKind::InvalidData, e))?;
         let study = Study::new(snapshot.direction);
-        *study.completed_trials.write() = snapshot.trials;
+        *study.storage.trials_arc().write() = snapshot.trials;
         study
             .next_trial_id
             .store(snapshot.next_trial_id, Ordering::Relaxed);

tests/journal_tests.rs

@@ -0,0 +1,215 @@
+//! Integration tests for the journal storage backend.
+
+use std::collections::HashMap;
+use std::sync::Arc;
+
+use optimizer::parameter::{FloatParam, Parameter};
+use optimizer::sampler::CompletedTrial;
+use optimizer::sampler::random::RandomSampler;
+use optimizer::storage::{JournalStorage, Storage};
+use optimizer::{Direction, Study};
+
+fn temp_path() -> std::path::PathBuf {
+    let mut path = std::env::temp_dir();
+    path.push(format!(
+        "optimizer_journal_test_{}.jsonl",
+        std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .unwrap()
+            .as_nanos()
+    ));
+    path
+}
+
+fn sample_trial(id: u64, value: f64) -> CompletedTrial<f64> {
+    CompletedTrial::new(id, HashMap::new(), HashMap::new(), HashMap::new(), value)
+}
+
+#[test]
+fn roundtrip_single_trial() {
+    let path = temp_path();
+    let storage = JournalStorage::new(&path);
+
+    storage.push(sample_trial(0, 42.0));
+
+    let loaded = storage.trials_arc().read().clone();
+    assert_eq!(loaded.len(), 1);
+    assert_eq!(loaded[0].id, 0);
+    assert_eq!(loaded[0].value, 42.0);
+
+    // Also verify via a fresh open from disk
+    let storage2 = JournalStorage::<f64>::open(&path).unwrap();
+    let loaded2 = storage2.trials_arc().read().clone();
+    assert_eq!(loaded2.len(), 1);
+    assert_eq!(loaded2[0].value, 42.0);
+
+    std::fs::remove_file(&path).ok();
+}
+
+#[test]
+fn append_multiple_trials() {
+    let path = temp_path();
+    let storage = JournalStorage::new(&path);
+
+    for i in 0..5 {
+        storage.push(sample_trial(i, i as f64));
+    }
+
+    // Reload from disk
+    let storage2 = JournalStorage::<f64>::open(&path).unwrap();
+    let loaded = storage2.trials_arc().read().clone();
+    assert_eq!(loaded.len(), 5);
+    for (i, trial) in loaded.iter().enumerate() {
+        assert_eq!(trial.id, i as u64);
+        assert_eq!(trial.value, i as f64);
+    }
+
+    std::fs::remove_file(&path).ok();
+}
+
+#[test]
+fn missing_file_returns_empty() {
+    let path = temp_path();
+    let storage = JournalStorage::<f64>::open(&path).unwrap();
+
+    let loaded = storage.trials_arc().read().clone();
+    assert!(loaded.is_empty());
+}
+
+#[test]
+fn concurrent_writes() {
+    let path = temp_path();
+    let storage = Arc::new(JournalStorage::new(&path));
+
+    let mut handles = Vec::new();
+    for thread_id in 0..4u64 {
+        let s = Arc::clone(&storage);
+        handles.push(std::thread::spawn(move || {
+            for i in 0..25u64 {
+                let id = thread_id * 25 + i;
+                s.push(sample_trial(id, id as f64));
+            }
+        }));
+    }
+    for h in handles {
+        h.join().unwrap();
+    }
+
+    // Reload from disk to verify persistence
+    let storage2 = JournalStorage::<f64>::open(&path).unwrap();
+    let loaded = storage2.trials_arc().read().clone();
+    assert_eq!(loaded.len(), 100);
+
+    // Verify all IDs are present (order may vary)
+    let mut ids: Vec<u64> = loaded.iter().map(|t| t.id).collect();
+    ids.sort();
+    assert_eq!(ids, (0..100).collect::<Vec<_>>());
+
+    std::fs::remove_file(&path).ok();
+}
+
+#[test]
+fn study_with_journal_integration() {
+    let path = temp_path();
+    let x = FloatParam::new(-10.0, 10.0);
+
+    // First "process": run some trials
+    {
+        let study =
+            Study::with_journal(Direction::Minimize, RandomSampler::with_seed(1), &path).unwrap();
+        study
+            .optimize(5, |trial| {
+                let val = x.suggest(trial)?;
+                Ok::<_, optimizer::Error>(val * val)
+            })
+            .unwrap();
+        assert_eq!(study.n_trials(), 5);
+    }
+
+    // Second "process": loads the same file, sees existing trials
+    let study2 =
+        Study::with_journal(Direction::Minimize, RandomSampler::with_seed(2), &path).unwrap();
+    assert_eq!(study2.n_trials(), 5);
+
+    // Continue optimizing
+    study2
+        .optimize(5, |trial| {
+            let val = x.suggest(trial)?;
+            Ok::<_, optimizer::Error>(val * val)
+        })
+        .unwrap();
+    assert_eq!(study2.n_trials(), 10);
+
+    // Verify all 10 written to disk
+    let storage3 = JournalStorage::<f64>::open(&path).unwrap();
+    let loaded = storage3.trials_arc().read().clone();
+    assert_eq!(loaded.len(), 10);
+
+    std::fs::remove_file(&path).ok();
+}
+
+#[test]
+fn ids_are_unique_after_reload() {
+    let path = temp_path();
+
+    // First batch
+    {
+        let study =
+            Study::with_journal(Direction::Minimize, RandomSampler::with_seed(1), &path).unwrap();
+        study
+            .optimize(3, |trial| {
+                let _ = FloatParam::new(0.0, 1.0).suggest(trial)?;
+                Ok::<_, optimizer::Error>(1.0)
+            })
+            .unwrap();
+    }
+
+    // Second batch — IDs should continue from 3
+    let study =
+        Study::with_journal(Direction::Minimize, RandomSampler::with_seed(2), &path).unwrap();
+    study
+        .optimize(3, |trial| {
+            let _ = FloatParam::new(0.0, 1.0).suggest(trial)?;
+            Ok::<_, optimizer::Error>(1.0)
+        })
+        .unwrap();
+
+    let all = study.trials();
+    let mut ids: Vec<u64> = all.iter().map(|t| t.id).collect();
+    ids.sort();
+    // All 6 IDs should be unique
+    ids.dedup();
+    assert_eq!(ids.len(), 6);
+
+    std::fs::remove_file(&path).ok();
+}
+
+#[test]
+fn pruned_trials_are_stored() {
+    let path = temp_path();
+    let study =
+        Study::with_journal(Direction::Minimize, RandomSampler::with_seed(1), &path).unwrap();
+
+    // Complete one, prune one
+    let x = FloatParam::new(0.0, 1.0);
+    study
+        .optimize(3, |trial| {
+            let _ = x.suggest(trial)?;
+            if trial.id() == 1 {
+                Err(optimizer::TrialPruned)?;
+            }
+            Ok::<_, optimizer::Error>(1.0)
+        })
+        .unwrap();
+
+    let storage2 = JournalStorage::<f64>::open(&path).unwrap();
+    let loaded = storage2.trials_arc().read().clone();
+    assert_eq!(loaded.len(), 3);
+    assert!(
+        loaded
+            .iter()
+            .any(|t| t.state == optimizer::TrialState::Pruned)
+    );
+
+    std::fs::remove_file(&path).ok();
+}