Merge pull request #510 from jiangliu/dependency

Add documentation for overall architecture

docs/images/crate-dependency.drawio

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docs/images/nydus-architecture-overview.drawio

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docs/images/nydus-storage-architecture.drawio

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docs/nydus-dev.md

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+## Architecture Overview
+
+![overview](images/nydus-architecture-overview.svg)
+
+### Crate Dependency
+
+The dependency among Nydus crates are shown below: 
+
+![dependency](images/crate-dependency.svg)
+
+To ease crate publishing process and avoid frequent dependency failures, please follow the rules below to specify dependencies:
+- Library crates only specify major and minor version numbers, such as `nydus-error = "0.2"`.
+- Binary crates specify major, minor and patch version numbers, such as `nydus-error = "0.2.1"`.
+
+
+## Storage Subsystem
+The storage subsystem is the core of [Nydus Image Service](https://nydus.dev/) project, which caches data fetched from storage backends and provides data services to rafs, blobfs and fscache etc.
+The storage subsystem contains several sub-modules:
+- API Layer: define external APIs for clients.
+  - [BlobFactory](https://docs.rs/nydus-storage/latest/nydus_storage/factory/struct.BlobFactory.html): create and garbage-collect cached blob objects.
+  - [BlobDevice](https://docs.rs/nydus-storage/latest/nydus_storage/device/struct.BlobDevice.html) and [BlobIoVec](https://docs.rs/nydus-storage/latest/nydus_storage/device/struct.BlobIoVec.html): provide data services for rafs and blobfs in unit of data chunks.
+  - [BlobObject](https://docs.rs/nydus-storage/latest/nydus_storage/device/trait.BlobObject.html): provide data services in unit of address ranges.
+- Cache Layer: decompress, decrypt and cache blob data fetched from storage backends.
+  - `PrefetchWorker`: workers to prefetch blob data from storage backend in background.
+  - [BlobCache](https://docs.rs/nydus-storage/latest/nydus_storage/cache/trait.BlobCache.html): define the core interface to access cached blob object.
+  - [DummyCacheMgr](https://docs.rs/nydus-storage/latest/nydus_storage/cache/struct.DummyCacheMgr.html): a dummy cache manager to fetch data from storage backend on demand and never caches data locally.
+  - [FileCacheMgr](https://docs.rs/nydus-storage/latest/nydus_storage/cache/struct.FileCacheMgr.html): a generic cache manager to cache data fetched from storage backend by using local files.
+  - [FsCacheMgr](https://docs.rs/nydus-storage/latest/nydus_storage/cache/struct.FsCacheMgr.html): a special cache manager to cooperate Linux fscache driver to cache data from storage backend by using local files.
+  - [BlobMetaInfo](https://docs.rs/nydus-storage/latest/nydus_storage/meta/struct.BlobMetaInfo.html): blob metadata to support fetch, decompress and decrypt blob data on demand.
+- Cache State Layer: track status of local cached blob data.
+  - [ChunkMap](https://docs.rs/nydus-storage/latest/nydus_storage/cache/state/trait.ChunkMap.html): maintain data cache status for data chunks.
+  - [RangeMap](https://docs.rs/nydus-storage/latest/nydus_storage/cache/state/trait.RangeMap.html): maintain data cache status for data ranges.
+- Backend Layer: fetch blob data from storage backends.
+  - [BlobBackend](https://docs.rs/nydus-storage/latest/nydus_storage/backend/trait.BlobBackend.html): manager to get access to data blobs from storage backend.
+  - [BlobReader](https://docs.rs/nydus-storage/latest/nydus_storage/backend/trait.BlobReader.html): reader to access blob data from storage backend.
+  - [LocalFs](https://docs.rs/nydus-storage/latest/nydus_storage/backend/localfs/struct.LocalFs.html): storage backend to access blobs stored in local accessible filesystems, such ext4, xfs, NFS, CephFS, GlusterFS etc.
+  - [Oss](https://docs.rs/nydus-storage/latest/nydus_storage/backend/oss/index.html): storage backend to access blobs from object storage systems.
+  - [Registry](https://docs.rs/nydus-storage/latest/nydus_storage/backend/registry/index.html): storage backend to access blobs from container image registries.
+
+![storage](images/nydus-storage-architecture.svg)
+
+### IO Mode
+
+When the Nydus Image Service is created in 2019, the rust async io ecosystem is still under development.
+So the Nydus storage subsystem has used multi-threaded, synchronous IO mode for IO operations.
+This mode has several advantages, especially easy to develop and maintain.
+But it also has drawbacks too, too many worker threads under heavy loads, hard to use AIO/io uring asynchronous IO framework.
+
+Now the rust async IO ecosystem is much more mature, and is suitable for IO intensive workloads like Nydus.
+It uses the most popular tokio async io framework. Tokio supports two types of runtime engines: multi-threaded and current thread.
+The tokio multi-threaded Runtime will automatically dispatch work items among worker threads. 
+And the tokio current thread Runtime runs pending work items by current thread, and doesn't dispatch work items to different worker threads.
+
+On the other hand, tokio-uring to do asynchronous IO by using io-uring only supports tokio current thread Runtime engine.
+Thus tokio current thread Runtime is adopted to support tokio-uring in future. 
+And an asynchronous Multi-Producer, Multi-Consumer channel is used to dispatch work items among different tokio current thread Runtimes.
+
+TODO: enable io-uring for asynchronous IO. It's a challenging task due to data buffer lifetime management.
+
+### Blob Data Format (TODO: nydus data blob, stargz data blob, compression algorithms, encryption algorithms, digest algorithms)