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// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements. See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership. The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License. You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied. See the License for the
// specific language governing permissions and limitations
// under the License.
//! The internal implementation details of OpenDAL.
//!
//! OpenDAL provides unified abstraction via two-level API sets:
//!
//! - Public API like [`Operator`] provides user-level API.
//! - Raw API like [`Service`], [`Layer`] provides developer-level API.
//!
//! OpenDAL tries its best to keep the public API stable. But raw APIs
//! may change between minor releases from time to time. So most users
//! should only use the public API. Developers use raw APIs to implement
//! storage services or their own [`Layer`].
//!
//! In this section, we will talk about the following components:
//!
//! - [`Service`][accessor]: to connect underlying storage services.
//! - [`Layer`][layer]: middleware/interceptor between storage services.
//!
//! The relation between [`Service`], [`Layer`], [`Servicer`], [`OperationContext`],
//! and [`Operator`] looks like the following:
//!
//! ```text
//! ┌──────────────┐ ┌──────────────────────────────┐ ┌──────────────┐
//! │ Service │ │ Servicer │ │ Operator │
//! │ builders ├────►│ + Layer::apply_* replay ├────►│ srv + ctx │
//! └──────────────┘ │ + OperationContext resources │ └──────────────┘
//! └──────────────────────────────┘
//! ```
//!
//! [`Builder`]: crate::Builder
//! [`Operator`]: crate::Operator
//! [`Service`]: crate::raw::Service
//! [`Servicer`]: crate::raw::Servicer
//! [`OperationContext`]: crate::raw::OperationContext
//! [`Layer`]: crate::raw::Layer