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Switch to using Go modules. This migrates our vendor.json to use Go 1.11's modules system, and replaces the vendor folder with the output of go mod vendor. The vendored code should remain basically the same; I believe some tree shaking of packages and support scripts/licenses/READMEs/etc. happened. This also fixes Travis and our Makefile to no longer use govendor.
169 lines
7.8 KiB
Markdown
169 lines
7.8 KiB
Markdown
# Go Plugin System over RPC
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`go-plugin` is a Go (golang) plugin system over RPC. It is the plugin system
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that has been in use by HashiCorp tooling for over 4 years. While initially
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created for [Packer](https://www.packer.io), it is additionally in use by
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[Terraform](https://www.terraform.io), [Nomad](https://www.nomadproject.io), and
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[Vault](https://www.vaultproject.io).
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While the plugin system is over RPC, it is currently only designed to work
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over a local [reliable] network. Plugins over a real network are not supported
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and will lead to unexpected behavior.
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This plugin system has been used on millions of machines across many different
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projects and has proven to be battle hardened and ready for production use.
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## Features
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The HashiCorp plugin system supports a number of features:
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**Plugins are Go interface implementations.** This makes writing and consuming
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plugins feel very natural. To a plugin author: you just implement an
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interface as if it were going to run in the same process. For a plugin user:
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you just use and call functions on an interface as if it were in the same
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process. This plugin system handles the communication in between.
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**Cross-language support.** Plugins can be written (and consumed) by
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almost every major language. This library supports serving plugins via
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[gRPC](http://www.grpc.io). gRPC-based plugins enable plugins to be written
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in any language.
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**Complex arguments and return values are supported.** This library
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provides APIs for handling complex arguments and return values such
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as interfaces, `io.Reader/Writer`, etc. We do this by giving you a library
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(`MuxBroker`) for creating new connections between the client/server to
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serve additional interfaces or transfer raw data.
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**Bidirectional communication.** Because the plugin system supports
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complex arguments, the host process can send it interface implementations
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and the plugin can call back into the host process.
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**Built-in Logging.** Any plugins that use the `log` standard library
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will have log data automatically sent to the host process. The host
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process will mirror this output prefixed with the path to the plugin
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binary. This makes debugging with plugins simple. If the host system
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uses [hclog](https://github.com/hashicorp/go-hclog) then the log data
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will be structured. If the plugin also uses hclog, logs from the plugin
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will be sent to the host hclog and be structured.
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**Protocol Versioning.** A very basic "protocol version" is supported that
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can be incremented to invalidate any previous plugins. This is useful when
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interface signatures are changing, protocol level changes are necessary,
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etc. When a protocol version is incompatible, a human friendly error
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message is shown to the end user.
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**Stdout/Stderr Syncing.** While plugins are subprocesses, they can continue
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to use stdout/stderr as usual and the output will get mirrored back to
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the host process. The host process can control what `io.Writer` these
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streams go to to prevent this from happening.
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**TTY Preservation.** Plugin subprocesses are connected to the identical
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stdin file descriptor as the host process, allowing software that requires
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a TTY to work. For example, a plugin can execute `ssh` and even though there
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are multiple subprocesses and RPC happening, it will look and act perfectly
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to the end user.
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**Host upgrade while a plugin is running.** Plugins can be "reattached"
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so that the host process can be upgraded while the plugin is still running.
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This requires the host/plugin to know this is possible and daemonize
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properly. `NewClient` takes a `ReattachConfig` to determine if and how to
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reattach.
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**Cryptographically Secure Plugins.** Plugins can be verified with an expected
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checksum and RPC communications can be configured to use TLS. The host process
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must be properly secured to protect this configuration.
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## Architecture
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The HashiCorp plugin system works by launching subprocesses and communicating
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over RPC (using standard `net/rpc` or [gRPC](http://www.grpc.io)). A single
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connection is made between any plugin and the host process. For net/rpc-based
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plugins, we use a [connection multiplexing](https://github.com/hashicorp/yamux)
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library to multiplex any other connections on top. For gRPC-based plugins,
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the HTTP2 protocol handles multiplexing.
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This architecture has a number of benefits:
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* Plugins can't crash your host process: A panic in a plugin doesn't
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panic the plugin user.
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* Plugins are very easy to write: just write a Go application and `go build`.
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Or use any other language to write a gRPC server with a tiny amount of
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boilerplate to support go-plugin.
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* Plugins are very easy to install: just put the binary in a location where
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the host will find it (depends on the host but this library also provides
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helpers), and the plugin host handles the rest.
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* Plugins can be relatively secure: The plugin only has access to the
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interfaces and args given to it, not to the entire memory space of the
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process. Additionally, go-plugin can communicate with the plugin over
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TLS.
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## Usage
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To use the plugin system, you must take the following steps. These are
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high-level steps that must be done. Examples are available in the
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`examples/` directory.
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1. Choose the interface(s) you want to expose for plugins.
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2. For each interface, implement an implementation of that interface
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that communicates over a `net/rpc` connection or over a
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[gRPC](http://www.grpc.io) connection or both. You'll have to implement
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both a client and server implementation.
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3. Create a `Plugin` implementation that knows how to create the RPC
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client/server for a given plugin type.
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4. Plugin authors call `plugin.Serve` to serve a plugin from the
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`main` function.
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5. Plugin users use `plugin.Client` to launch a subprocess and request
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an interface implementation over RPC.
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That's it! In practice, step 2 is the most tedious and time consuming step.
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Even so, it isn't very difficult and you can see examples in the `examples/`
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directory as well as throughout our various open source projects.
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For complete API documentation, see [GoDoc](https://godoc.org/github.com/hashicorp/go-plugin).
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## Roadmap
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Our plugin system is constantly evolving. As we use the plugin system for
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new projects or for new features in existing projects, we constantly find
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improvements we can make.
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At this point in time, the roadmap for the plugin system is:
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**Semantic Versioning.** Plugins will be able to implement a semantic version.
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This plugin system will give host processes a system for constraining
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versions. This is in addition to the protocol versioning already present
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which is more for larger underlying changes.
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**Plugin fetching.** We will integrate with [go-getter](https://github.com/hashicorp/go-getter)
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to support automatic download + install of plugins. Paired with cryptographically
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secure plugins (above), we can make this a safe operation for an amazing
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user experience.
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## What About Shared Libraries?
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When we started using plugins (late 2012, early 2013), plugins over RPC
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were the only option since Go didn't support dynamic library loading. Today,
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Go supports the [plugin](https://golang.org/pkg/plugin/) standard library with
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a number of limitations. Since 2012, our plugin system has stabilized
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from tens of millions of users using it, and has many benefits we've come to
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value greatly.
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For example, we use this plugin system in
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[Vault](https://www.vaultproject.io) where dynamic library loading is
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not acceptable for security reasons. That is an extreme
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example, but we believe our library system has more upsides than downsides
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over dynamic library loading and since we've had it built and tested for years,
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we'll continue to use it.
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Shared libraries have one major advantage over our system which is much
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higher performance. In real world scenarios across our various tools,
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we've never required any more performance out of our plugin system and it
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has seen very high throughput, so this isn't a concern for us at the moment.
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