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+# OpenCensus Libraries for Go
+
+[![Build Status][travis-image]][travis-url]
+[![Windows Build Status][appveyor-image]][appveyor-url]
+[![GoDoc][godoc-image]][godoc-url]
+[![Gitter chat][gitter-image]][gitter-url]
+
+OpenCensus Go is a Go implementation of OpenCensus, a toolkit for
+collecting application performance and behavior monitoring data.
+Currently it consists of three major components: tags, stats and tracing.
+
+## Installation
+
+```
+$ go get -u go.opencensus.io
+```
+
+The API of this project is still evolving, see: [Deprecation Policy](#deprecation-policy).
+The use of vendoring or a dependency management tool is recommended.
+
+## Prerequisites
+
+OpenCensus Go libraries require Go 1.8 or later.
+
+## Getting Started
+
+The easiest way to get started using OpenCensus in your application is to use an existing
+integration with your RPC framework:
+
+* [net/http](https://godoc.org/go.opencensus.io/plugin/ochttp)
+* [gRPC](https://godoc.org/go.opencensus.io/plugin/ocgrpc)
+* [database/sql](https://godoc.org/github.com/opencensus-integrations/ocsql)
+* [Go kit](https://godoc.org/github.com/go-kit/kit/tracing/opencensus)
+* [Groupcache](https://godoc.org/github.com/orijtech/groupcache)
+* [Caddy webserver](https://godoc.org/github.com/orijtech/caddy)
+* [MongoDB](https://godoc.org/github.com/orijtech/mongo-go-driver)
+* [Redis gomodule/redigo](https://godoc.org/github.com/orijtech/redigo)
+* [Redis goredis/redis](https://godoc.org/github.com/orijtech/redis)
+* [Memcache](https://godoc.org/github.com/orijtech/gomemcache)
+
+If you're using a framework not listed here, you could either implement your own middleware for your
+framework or use [custom stats](#stats) and [spans](#spans) directly in your application.
+
+## Exporters
+
+OpenCensus can export instrumentation data to various backends.
+OpenCensus has exporter implementations for the following, users
+can implement their own exporters by implementing the exporter interfaces
+([stats](https://godoc.org/go.opencensus.io/stats/view#Exporter),
+[trace](https://godoc.org/go.opencensus.io/trace#Exporter)):
+
+* [Prometheus][exporter-prom] for stats
+* [OpenZipkin][exporter-zipkin] for traces
+* [Stackdriver][exporter-stackdriver] Monitoring for stats and Trace for traces
+* [Jaeger][exporter-jaeger] for traces
+* [AWS X-Ray][exporter-xray] for traces
+* [Datadog][exporter-datadog] for stats and traces
+* [Graphite][exporter-graphite] for stats
+* [Honeycomb][exporter-honeycomb] for traces
+
+## Overview
+
+![OpenCensus Overview](https://i.imgur.com/cf4ElHE.jpg)
+
+In a microservices environment, a user request may go through
+multiple services until there is a response. OpenCensus allows
+you to instrument your services and collect diagnostics data all
+through your services end-to-end.
+
+## Tags
+
+Tags represent propagated key-value pairs. They are propagated using `context.Context`
+in the same process or can be encoded to be transmitted on the wire. Usually, this will
+be handled by an integration plugin, e.g. `ocgrpc.ServerHandler` and `ocgrpc.ClientHandler`
+for gRPC.
+
+Package `tag` allows adding or modifying tags in the current context.
+
+[embedmd]:# (internal/readme/tags.go new)
+```go
+ctx, err = tag.New(ctx,
+ tag.Insert(osKey, "macOS-10.12.5"),
+ tag.Upsert(userIDKey, "cde36753ed"),
+)
+if err != nil {
+ log.Fatal(err)
+}
+```
+
+## Stats
+
+OpenCensus is a low-overhead framework even if instrumentation is always enabled.
+In order to be so, it is optimized to make recording of data points fast
+and separate from the data aggregation.
+
+OpenCensus stats collection happens in two stages:
+
+* Definition of measures and recording of data points
+* Definition of views and aggregation of the recorded data
+
+### Recording
+
+Measurements are data points associated with a measure.
+Recording implicitly tags the set of Measurements with the tags from the
+provided context:
+
+[embedmd]:# (internal/readme/stats.go record)
+```go
+stats.Record(ctx, videoSize.M(102478))
+```
+
+### Views
+
+Views are how Measures are aggregated. You can think of them as queries over the
+set of recorded data points (measurements).
+
+Views have two parts: the tags to group by and the aggregation type used.
+
+Currently three types of aggregations are supported:
+* CountAggregation is used to count the number of times a sample was recorded.
+* DistributionAggregation is used to provide a histogram of the values of the samples.
+* SumAggregation is used to sum up all sample values.
+
+[embedmd]:# (internal/readme/stats.go aggs)
+```go
+distAgg := view.Distribution(1<<32, 2<<32, 3<<32)
+countAgg := view.Count()
+sumAgg := view.Sum()
+```
+
+Here we create a view with the DistributionAggregation over our measure.
+
+[embedmd]:# (internal/readme/stats.go view)
+```go
+if err := view.Register(&view.View{
+ Name: "example.com/video_size_distribution",
+ Description: "distribution of processed video size over time",
+ Measure: videoSize,
+ Aggregation: view.Distribution(1<<32, 2<<32, 3<<32),
+}); err != nil {
+ log.Fatalf("Failed to register view: %v", err)
+}
+```
+
+Register begins collecting data for the view. Registered views' data will be
+exported via the registered exporters.
+
+## Traces
+
+A distributed trace tracks the progression of a single user request as
+it is handled by the services and processes that make up an application.
+Each step is called a span in the trace. Spans include metadata about the step,
+including especially the time spent in the step, called the span’s latency.
+
+Below you see a trace and several spans underneath it.
+
+![Traces and spans](https://i.imgur.com/7hZwRVj.png)
+
+### Spans
+
+Span is the unit step in a trace. Each span has a name, latency, status and
+additional metadata.
+
+Below we are starting a span for a cache read and ending it
+when we are done:
+
+[embedmd]:# (internal/readme/trace.go startend)
+```go
+ctx, span := trace.StartSpan(ctx, "cache.Get")
+defer span.End()
+
+// Do work to get from cache.
+```
+
+### Propagation
+
+Spans can have parents or can be root spans if they don't have any parents.
+The current span is propagated in-process and across the network to allow associating
+new child spans with the parent.
+
+In the same process, `context.Context` is used to propagate spans.
+`trace.StartSpan` creates a new span as a root if the current context
+doesn't contain a span. Or, it creates a child of the span that is
+already in current context. The returned context can be used to keep
+propagating the newly created span in the current context.
+
+[embedmd]:# (internal/readme/trace.go startend)
+```go
+ctx, span := trace.StartSpan(ctx, "cache.Get")
+defer span.End()
+
+// Do work to get from cache.
+```
+
+Across the network, OpenCensus provides different propagation
+methods for different protocols.
+
+* gRPC integrations use the OpenCensus' [binary propagation format](https://godoc.org/go.opencensus.io/trace/propagation).
+* HTTP integrations use Zipkin's [B3](https://github.com/openzipkin/b3-propagation)
+ by default but can be configured to use a custom propagation method by setting another
+ [propagation.HTTPFormat](https://godoc.org/go.opencensus.io/trace/propagation#HTTPFormat).
+
+## Execution Tracer
+
+With Go 1.11, OpenCensus Go will support integration with the Go execution tracer.
+See [Debugging Latency in Go](https://medium.com/observability/debugging-latency-in-go-1-11-9f97a7910d68)
+for an example of their mutual use.
+
+## Profiles
+
+OpenCensus tags can be applied as profiler labels
+for users who are on Go 1.9 and above.
+
+[embedmd]:# (internal/readme/tags.go profiler)
+```go
+ctx, err = tag.New(ctx,
+ tag.Insert(osKey, "macOS-10.12.5"),
+ tag.Insert(userIDKey, "fff0989878"),
+)
+if err != nil {
+ log.Fatal(err)
+}
+tag.Do(ctx, func(ctx context.Context) {
+ // Do work.
+ // When profiling is on, samples will be
+ // recorded with the key/values from the tag map.
+})
+```
+
+A screenshot of the CPU profile from the program above:
+
+![CPU profile](https://i.imgur.com/jBKjlkw.png)
+
+## Deprecation Policy
+
+Before version 1.0.0, the following deprecation policy will be observed:
+
+No backwards-incompatible changes will be made except for the removal of symbols that have
+been marked as *Deprecated* for at least one minor release (e.g. 0.9.0 to 0.10.0). A release
+removing the *Deprecated* functionality will be made no sooner than 28 days after the first
+release in which the functionality was marked *Deprecated*.
+
+[travis-image]: https://travis-ci.org/census-instrumentation/opencensus-go.svg?branch=master
+[travis-url]: https://travis-ci.org/census-instrumentation/opencensus-go
+[appveyor-image]: https://ci.appveyor.com/api/projects/status/vgtt29ps1783ig38?svg=true
+[appveyor-url]: https://ci.appveyor.com/project/opencensusgoteam/opencensus-go/branch/master
+[godoc-image]: https://godoc.org/go.opencensus.io?status.svg
+[godoc-url]: https://godoc.org/go.opencensus.io
+[gitter-image]: https://badges.gitter.im/census-instrumentation/lobby.svg
+[gitter-url]: https://gitter.im/census-instrumentation/lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge
+
+
+[new-ex]: https://godoc.org/go.opencensus.io/tag#example-NewMap
+[new-replace-ex]: https://godoc.org/go.opencensus.io/tag#example-NewMap--Replace
+
+[exporter-prom]: https://godoc.org/contrib.go.opencensus.io/exporter/prometheus
+[exporter-stackdriver]: https://godoc.org/contrib.go.opencensus.io/exporter/stackdriver
+[exporter-zipkin]: https://godoc.org/contrib.go.opencensus.io/exporter/zipkin
+[exporter-jaeger]: https://godoc.org/contrib.go.opencensus.io/exporter/jaeger
+[exporter-xray]: https://github.com/census-ecosystem/opencensus-go-exporter-aws
+[exporter-datadog]: https://github.com/DataDog/opencensus-go-exporter-datadog
+[exporter-graphite]: https://github.com/census-ecosystem/opencensus-go-exporter-graphite
+[exporter-honeycomb]: https://github.com/honeycombio/opencensus-exporter