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chvck and yurishkuro Update storage grpc plugin README (#1581)
Signed-off-by: Charles Dixon <chvckd@gmail.com>
Latest commit 5b36e3b Jun 4, 2019

README.md

gRPC Storage Plugins

gRPC Storage Plugins currently use the Hashicorp go-plugin. This requires the implementer of a plugin to develop the "server" side of the go-plugin system. At a high level this looks like:

+----------------------------------+                  +-----------------------------+
|                                  |                  |                             |
|                  +-------------+ |   unix-socket    | +-------------+             |
|                  |             | |                  | |             |             |
| jaeger-component | grpc-client +----------------------> grpc-server | plugin-impl |
|                  |             | |                  | |             |             |
|                  +-------------+ |                  | +-------------+             |
|                                  |                  |                             |
+----------------------------------+                  +-----------------------------+

       parent process                                        child sub-process

Implementing a plugin

Although the instructions below are limited to Go, plugins can be implemented any language. Languages other than Go would implement a gRPC server using the storage_v1.proto interfaces. The proto file can be found in plugin/storage/grpc/proto/. To generate the bindings for your language you would use protoc with the appropriate xx_out= flag. This is detailed in the protobuf documentation and you can see an example of how it is done for Go in the top level Jaeger Makefile.

There are instructions on implementing a go-plugin server for non-Go languages in the go-plugin non-go guide. Take note of the required health check service.

A Go plugin is a standalone application which calls grpc.Serve(&plugin) in its main function, where the grpc package is github.com/jaegertracing/jaeger/plugin/storage/grpc.

    package main

    import (
        "flag"
        "github.com/jaegertracing/jaeger/plugin/storage/grpc"
    )
    
    func main() {
        var configPath string
        flag.StringVar(&configPath, "config", "", "A path to the plugin's configuration file")
        flag.Parse()

        plugin := myStoragePlugin{}
        
        grpc.Serve(&plugin)
    }

Note that grpc.Serve is called as the final part of the main. This should be called after you have carried out any necessary setup for your plugin, as once running Jaeger may start calling to read/write spans straight away. You could defer setup until the first read/write but that could make the first operation slow and also lead to racing behaviours.

A plugin must implement the StoragePlugin interface of:

type StoragePlugin interface {
   	SpanReader() spanstore.Reader
   	SpanWriter() spanstore.Writer
   	DependencyReader() dependencystore.Reader
}

As your plugin will be dependent on the protobuf implementation within Jaeger you will likely need to vendor your dependencies, you can also use go.mod to achieve the same goal of pinning your plugin to a Jaeger point in time.

A simple plugin which uses the memstore storage implementation can be found in the examples directory of the top level of the Jaeger project.

Running with a plugin

A plugin can be run using the all-in-one application within the top level cmd package of the Jaeger project. To do this an environment variable must be set to tell the all-in-one application to use the gRPC plugin storage: export SPAN_STORAGE_TYPE="grpc-plugin"

Once this has been set then there are two command line flags that can be used to configure the plugin. The first is --grpc-storage-plugin.binary which is required and is the path to the plugin binary. The second is --grpc-storage-plugin.configuration-file which is optional and is the path to the configuration file which will be provided to your plugin as a command line flag. This command line flag is config, as can be seen in the code sample above. An example invocation would be:

./all-in-one --grpc-storage-plugin.binary=/path/to/my/plugin --grpc-storage-plugin.configuration-file=/path/to/my/config

As well as passing configuration values via the command line through the configuration file it is also possible to use environment variables. When you invoke all-in-one any environment variables that have been set will also be accessible from within your plugin, this is useful if using Docker.

Logging

In order for Jaeger to include the log output from your plugin you need to use hclog ("github.com/hashicorp/go-hclog"). The plugin framework will only include any log output created at the the WARN or above levels. If you log output in this way before calling grpc.Serve then it will still be included in the Jaeger output.

An example logger instantiation could look like:

logger := hclog.New(&hclog.LoggerOptions{
   Level:      hclog.Warn,
   Name:       "my-jaeger-plugin",
   JSONFormat: true,
})

There are more logger options that can be used with hclog listed on godoc.

Note: Setting the Output option to os.Stdout can confuse the go-plugin framework and lead it to consider the plugin errored.

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