This document describes the registry storage driver model, implementation, and explains how to contribute new storage drivers.
Provided Drivers
================
This storage driver package comes bundled with three default drivers.
1. filesystem: A local storage driver configured to use a directory tree in the local filesystem.
2. s3: A driver storing objects in an Amazon Simple Storage Solution (S3) bucket.
3. inmemory: A temporary storage driver using a local inmemory map. This exists solely for reference and testing.
Storage Driver API
==================
The storage driver API is designed to model a filesystem-like key/value storage in a manner abstract enough to support a range of drivers from the local filesystem to Amazon S3 or other distributed object storage systems.
Storage drivers are required to implement the `storagedriver.StorageDriver` interface provided in `storagedriver.go`, which includes methods for reading, writing, and deleting content, as well as listing child objects of a specified prefix key.
Storage drivers are intended (but not required) to be written in go, providing compile-time validation of the `storagedriver.StorageDriver` interface, although an IPC driver wrapper means that it is not required for drivers to be included in the compiled registry. The `storagedriver/ipc` package provides a client/server protocol for running storage drivers provided in external executables as a managed child server process.
Driver Selection and Configuration
==================================
The preferred method of selecting a storage driver is using the `StorageDriverFactory` interface in the `storagedriver/factory` package. These factories provide a common interface for constructing storage drivers with a parameters map. The factory model is based off of the [Register](http://golang.org/pkg/database/sql/#Register) and [Open](http://golang.org/pkg/database/sql/#Open) methods in the builtin [database/sql](http://golang.org/pkg/database/sql) package.
Storage driver factories may be registered by name using the `factory.Register` method, and then later invoked by calling `factory.Create` with a driver name and parameters map. If no driver is registered with the given name, this factory will attempt to find an executable storage driver with the same name and return an IPC storage driver wrapper managing the driver subprocess. If no such storage driver can be found, `factory.Create` will return an `InvalidStorageDriverError`.
Driver Contribution
===================
## Writing new storage drivers
To create a valid storage driver, one must implement the `storagedriver.StorageDriver` interface and make sure to expose this driver via the factory system and as a distributable IPC server executable.
### In-process drivers
Storage drivers should call `factory.Register` with their driver name in an `init` method, allowing callers of `factory.New` to construct instances of this driver without requiring modification of imports throughout the codebase.
### Out-of-process drivers
As many users will run the registry as a pre-constructed docker container, storage drivers should also be distributable as IPC server executables. Drivers written in go should model the main method provided in `main/storagedriver/filesystem/filesystem.go`. Parameters to IPC drivers will be provided as a JSON-serialized map in the first argument to the process. These parameters should be validated and then a blocking call to `ipc.StorageDriverServer` should be made with a new storage driver.
Out-of-process drivers must also implement the `ipc.IPCStorageDriver` interface, which exposes a `Version` check for the storage driver. This is used to validate storage driver api compatibility at driver load-time.
Storage driver test suites are provided in `storagedriver/testsuites/testsuites.go` and may be used for any storage driver written in go. Two methods are provided for registering test suites, `RegisterInProcessSuite` and `RegisterIPCSuite`, which run the same set of tests for the driver imported or managed over IPC respectively.
## Drivers written in other languages
Although storage drivers are strongly recommended to be written in go for consistency, compile-time validation, and support, the IPC framework allows for a level of language-agnosticism. Non-go drivers must implement the storage driver protocol by mimicing StorageDriverServer in `storagedriver/ipc/server.go`. As the IPC framework is a layer on top of [docker/libchan](https://github.com/docker/libchan), this currently limits language support to Java via [ndeloof/chan](https://github.com/ndeloof/jchan) and Javascript via [GraftJS/jschan](https://github.com/GraftJS/jschan), although contributions to the libchan project are welcome.
