Configure file storage

Goal: give your app a place to put uploaded files — on disk, in memory (for tests), or in a cloud bucket — through one consistent Storage API, without hand-rolling an OpenDAL client yourself.

Loco's storage layer is a thin abstraction over Apache OpenDAL. Every driver ends up implementing the same StoreDriver trait, so your controller code doesn't change when you swap local disk for S3.

Prerequisites

Local, in-memory, and null storage work with no extra Cargo features. Cloud drivers need one of:

loco-rs = { version = "...", features = ["storage_aws_s3"] } # or storage_azure, storage_gcp, all_storage

See the feature flags reference for the full matrix.

1. Wire up a single driver

Storage isn't configured in YAML — it's wired in code, in the after_context hook (src/app.rs), and lands on ctx.storage: Arc<Storage>.

use loco_rs::storage::{self, drivers};

async fn after_context(ctx: AppContext) -> Result<AppContext> {
    Ok(ctx
        .into_builder()
        .storage(storage::Storage::single(drivers::local::new()).into())
        .build())
}

AppContext is #[non_exhaustive], so AppContext { storage, ..ctx } won't compile in your app — that's what keeps a new field in a future Loco release from breaking your build. into_builder() is the replacement: it carries every component the boot sequence already attached (mailer, queue, cache, shared store) across, and you override just the one you care about. Building from AppContext::builder(..) instead would compile and silently drop the rest.

If you don't override after_context at all, Loco defaults to the Null driver — every storage operation returns StorageError::Any("Operation not supported by null storage"). That's a deliberate fail-fast default, not a bug: it means "you haven't wired storage yet."

2. Pick a driver

Every driver is built by a plain constructor function under loco_rs::storage::drivers::* — no trait object wrangling required.

All the cloud constructors return StorageResult<Box<dyn StoreDriver>> (they can fail to build the underlying OpenDAL operator), so propagate the error with ?:

use loco_rs::storage::{self, drivers};

async fn after_context(ctx: AppContext) -> Result<AppContext> {
    let store = drivers::aws::new("my-app-uploads", "us-east-1")?;
    Ok(ctx
        .into_builder()
        .storage(storage::Storage::single(store).into())
        .build())
}

For credentials that aren't in the environment/instance profile, pass them explicitly:

use loco_rs::storage::drivers::aws::{self, Credential};

let credential = Credential {
    key_id: std::env::var("AWS_ACCESS_KEY_ID")?,
    secret_key: std::env::var("AWS_SECRET_ACCESS_KEY")?,
    token: None,
};
let store = aws::with_credentials("my-app-uploads", "us-east-1", credential)?;

The storage driver trait is StoreDriver (not StorageDriver) — you'll see it in error messages and if you implement your own driver.

3. Use multiple drivers with a strategy (optional)

For redundancy across providers, set up several named stores and a StorageStrategy that decides how operations fan out across them.

Mirror — replicates uploads/deletes/renames/copies to every store; download tries the primary, then falls through to secondaries on failure. This is ReplicatedStrategy::mirror.

use std::collections::BTreeMap;
use loco_rs::storage::{
    drivers, Storage,
    strategies::{replicated::{ReplicatedStrategy, FailurePolicy}, StorageStrategy},
};

let primary = drivers::aws::new("bucket-primary", "us-east-1")?;
let mirror = drivers::azure::new("container", "account", "access-key", "https://account.blob.core.windows.net")?;

let strategy: Box<dyn StorageStrategy> = Box::new(ReplicatedStrategy::mirror(
    "primary",
    Some(vec!["mirror".to_string()]),
    FailurePolicy::FailIfAny, // or AllowAll
));

let storage = Storage::new(
    BTreeMap::from([
        ("primary".to_string(), primary),
        ("mirror".to_string(), mirror),
    ]),
    strategy,
);

FailurePolicy::FailIfAny requires every secondary to succeed (errors bubble up as StorageError::Multi); AllowAll swallows secondary failures.

Backup — the primary must always succeed for writes; secondary failures are governed by a separate failure policy, and downloads always come from the primary only. This is ReplicatedStrategy::backup.

use loco_rs::storage::strategies::replicated::{ReplicatedStrategy, FailurePolicy};

let strategy: Box<dyn StorageStrategy> = Box::new(ReplicatedStrategy::backup(
    "primary",
    Some(vec!["backup_store".to_string()]),
    FailurePolicy::AllowAll, // also: FailIfAny, AllowSingleFailure, FailAtFailures(n)
));

Mirror and backup are both ReplicatedStrategy, differing only in the constructor used (mirror vs backup) and the FailurePolicy you pick. It exposes a _with_policy/_with_strategy variant on every Storage method (upload_with_strategy, download_with_policy, ...) if you need to override the strategy for a single call.

4. Upload and download in a controller

use loco_rs::prelude::*;
use std::path::PathBuf;

async fn upload_file(
    State(ctx): State<AppContext>,
    mut multipart: Multipart,
) -> Result<Response> {
    while let Some(field) = multipart.next_field().await.map_err(|_| {
        Error::BadRequest("could not read multipart".into())
    })? {
        let file_name = field
            .file_name()
            .map(str::to_string)
            .ok_or_else(|| Error::BadRequest("file name not found".into()))?;

        let content = field
            .bytes()
            .await
            .map_err(|_| Error::BadRequest("could not read bytes".into()))?;

        let path = PathBuf::from("uploads").join(file_name);
        ctx.storage.as_ref().upload(&path, &content).await?;

        return format::json(serde_json::json!({ "path": path }));
    }
    not_found()
}

(Requires the multipart feature on the axum crate.)

5. Stream large files instead of buffering them

For files too large to comfortably hold in memory, use the streaming API — download_stream/upload_stream return/accept a BytesStream, which converts directly to/from an axum Body. This is undocumented in earlier Loco releases but is a stable, full public feature.

Streaming a download straight into an HTTP response, with zero extra buffering:

use axum::response::IntoResponse;
use std::path::Path;

async fn download_video(State(ctx): State<AppContext>) -> Result<impl IntoResponse> {
    let stream = ctx.storage.download_stream(Path::new("videos/demo.mp4")).await?;
    Ok(stream.into_body())
}

Streaming an upload from an incoming request body (axum's Body stream yields axum::Error, so map it to std::io::Error first — that's the error type BytesStream expects):

use loco_rs::storage::stream::BytesStream;
use futures_util::StreamExt;
use std::path::Path;

async fn upload_video(State(ctx): State<AppContext>, body: axum::body::Body) -> Result<Response> {
    let mapped = body
        .into_data_stream()
        .map(|chunk| chunk.map_err(std::io::Error::other));
    let stream = BytesStream::from_body_stream(mapped);
    ctx.storage.upload_stream(Path::new("videos/demo.mp4"), stream).await?;
    format::empty()
}

If you need the whole payload as one Bytes buffer anyway, BytesStream::collect() gives you that — but at that point you've given up the memory benefit of streaming.

Strategy caveat: streaming isn't uniformly "true streaming" once a strategy other than SingleStrategy is involved. ReplicatedStrategy unifies the former mirror/backup behavior: reads (both the buffered download and download_stream) fall back to secondaries when read_from_secondaries is set — i.e. constructed via ReplicatedStrategy::mirror — and are served from the primary only when constructed via ReplicatedStrategy::backup. Either way, upload_stream buffers the whole payload once via collect() and then fans out concurrently to secondaries. If you need guaranteed zero-buffering streaming to a single store, stick to SingleStrategy (the default).

6. Check existence, list, and stat

Storage also exposes exists, list, and stat, each going through the selected strategy the same way upload/download do (with exists_with_policy/list_with_policy/stat_with_policy siblings for overriding the strategy per call).

use std::path::Path;

// Does a key exist?
let found = ctx.storage.exists(Path::new("uploads/report.pdf")).await?;

// List everything under a prefix, recursively.
let all_entries = ctx.storage.list(Path::new("uploads"), true).await?;

// List one level deep — child prefixes come back as directory entries.
let top_level = ctx.storage.list(Path::new("uploads"), false).await?;

// Metadata for a single key, without downloading its content.
let meta = ctx.storage.stat(Path::new("uploads/report.pdf")).await?;
println!("{} bytes, is_dir={}", meta.content_length.unwrap_or(0), meta.is_dir);

Each entry returned by list/stat is a storage::drivers::ListEntry (prefer ListEntry::new(...) over struct literals).

On ReplicatedStrategy (mirror / read_from_secondaries):

• stat falls back to secondaries on primary error (same as download)
• exists falls back when the primary reports false or errors (a miss is not itself an error)
• list falls back when the primary errors or returns an empty listing; empty secondaries are skipped so a later secondary with data still wins

Backup mode (read_from_secondaries: false) keeps all three primary-only.

7. Verify

use axum_test::multipart::{MultipartForm, Part}; // not re-exported by the testing prelude
use loco_rs::testing::prelude::*;

#[tokio::test]
#[serial]
async fn can_upload_and_download() {
    request::<App, _, _>(|request, ctx| async move {
        let file_content = "loco file upload";
        let file_part = Part::bytes(file_content.as_bytes()).file_name("loco.txt");
        let multipart_form = MultipartForm::new().add_part("file", file_part);

        let response = request.post("/upload/file").multipart(multipart_form).await;
        response.assert_status_ok();

        let res: serde_json::Value = serde_json::from_str(&response.text()).unwrap();
        let path = res["path"].as_str().unwrap();

        let stored: String = ctx.storage.as_ref().download(&std::path::Path::new(path)).await.unwrap();
        assert_eq!(stored, file_content);
    })
    .await;
}

Reference

• storage_aws_s3 / storage_azure / storage_gcp / all_storage feature flags: Feature flags reference
• Storage has no YAML configuration surface — everything above is the complete configuration story; there is no storage: key to look up in the Configuration reference