title: “Go Concurrency Patterns: Context” date: 2014-07-29 by:
In Go servers, each incoming request is handled in its own goroutine. Request handlers often start additional goroutines to access backends such as databases and RPC services. The set of goroutines working on a request typically needs access to request-specific values such as the identity of the end user, authorization tokens, and the request's deadline. When a request is canceled or times out, all the goroutines working on that request should exit quickly so the system can reclaim any resources they are using.
At Google, we developed a context package that makes it easy to pass request-scoped values, cancellation signals, and deadlines across API boundaries to all the goroutines involved in handling a request. The package is publicly available as context. This article describes how to use the package and provides a complete working example.
The core of the context package is the Context type:
{{code “context/interface.go” /A Context/ /^}/}}
(This description is condensed; the godoc is authoritative.)
The Done method returns a channel that acts as a cancellation signal to functions running on behalf of the Context: when the channel is closed, the functions should abandon their work and return. The Err method returns an error indicating why the Context was canceled. The Pipelines and Cancellation article discusses the Done channel idiom in more detail.
A Context does not have a Cancel method for the same reason the Done channel is receive-only: the function receiving a cancellation signal is usually not the one that sends the signal. In particular, when a parent operation starts goroutines for sub-operations, those sub-operations should not be able to cancel the parent. Instead, the WithCancel function (described below) provides a way to cancel a new Context value.
A Context is safe for simultaneous use by multiple goroutines. Code can pass a single Context to any number of goroutines and cancel that Context to signal all of them.
The Deadline method allows functions to determine whether they should start work at all; if too little time is left, it may not be worthwhile. Code may also use a deadline to set timeouts for I/O operations.
Value allows a Context to carry request-scoped data. That data must be safe for simultaneous use by multiple goroutines.
The context package provides functions to derive new Context values from existing ones. These values form a tree: when a Context is canceled, all Contexts derived from it are also canceled.
Background is the root of any Context tree; it is never canceled:
{{code “context/interface.go” /Background returns/ /func Background/}}
WithCancel and WithTimeout return derived Context values that can be canceled sooner than the parent Context. The Context associated with an incoming request is typically canceled when the request handler returns. WithCancel is also useful for canceling redundant requests when using multiple replicas. WithTimeout is useful for setting a deadline on requests to backend servers:
{{code “context/interface.go” /WithCancel/ /func WithTimeout/}}
WithValue provides a way to associate request-scoped values with a Context:
{{code “context/interface.go” /WithValue/ /func WithValue/}}
The best way to see how to use the context package is through a worked example.
Our example is an HTTP server that handles URLs like /search?q=golang&timeout=1s by forwarding the query “golang” to the Google Web Search API and rendering the results. The timeout parameter tells the server to cancel the request after that duration elapses.
The code is split across three packages:
main function and the handler for /search.Context.Search function for sending a query to Google.The server program handles requests like /search?q=golang by serving the first few Google search results for golang. It registers handleSearch to handle the /search endpoint. The handler creates an initial Context called ctx and arranges for it to be canceled when the handler returns. If the request includes the timeout URL parameter, the Context is canceled automatically when the timeout elapses:
{{code “context/server/server.go” /func handleSearch/ /defer cancel/}}
The handler extracts the query from the request and extracts the client‘s IP address by calling on the userip package. The client’s IP address is needed for backend requests, so handleSearch attaches it to ctx:
{{code “context/server/server.go” /Check the search query/ /userip.NewContext/}}
The handler calls google.Search with ctx and the query:
{{code “context/server/server.go” /Run the Google search/ /elapsed/}}
If the search succeeds, the handler renders the results:
{{code “context/server/server.go” /resultsTemplate/ /(?m)}$/}}
The userip package provides functions for extracting a user IP address from a request and associating it with a Context. A Context provides a key-value mapping, where the keys and values are both of type interface{}. Key types must support equality, and values must be safe for simultaneous use by multiple goroutines. Packages like userip hide the details of this mapping and provide strongly-typed access to a specific Context value.
To avoid key collisions, userip defines an unexported type key and uses a value of this type as the context key:
{{code “context/userip/userip.go” /The key type/ /const userIPKey/}}
FromRequest extracts a userIP value from an http.Request:
{{code “context/userip/userip.go” /func FromRequest/ /}/}}
NewContext returns a new Context that carries a provided userIP value:
{{code “context/userip/userip.go” /func NewContext/ /}/}}
FromContext extracts a userIP from a Context:
{{code “context/userip/userip.go” /func FromContext/ /}/}}
The google.Search function makes an HTTP request to the Google Web Search API and parses the JSON-encoded result. It accepts a Context parameter ctx and returns immediately if ctx.Done is closed while the request is in flight.
The Google Web Search API request includes the search query and the user IP as query parameters:
{{code “context/google/google.go” /func Search/ /q.Encode/}}
Search uses a helper function, httpDo, to issue the HTTP request and cancel it if ctx.Done is closed while the request or response is being processed. Search passes a closure to httpDo handle the HTTP response:
{{code “context/google/google.go” /var results/ /return results/}}
The httpDo function runs the HTTP request and processes its response in a new goroutine. It cancels the request if ctx.Done is closed before the goroutine exits:
{{code “context/google/google.go” /func httpDo/ /^}/}}
Many server frameworks provide packages and types for carrying request-scoped values. We can define new implementations of the Context interface to bridge between code using existing frameworks and code that expects a Context parameter.
For example, Gorilla's github.com/gorilla/context package allows handlers to associate data with incoming requests by providing a mapping from HTTP requests to key-value pairs. In gorilla.go, we provide a Context implementation whose Value method returns the values associated with a specific HTTP request in the Gorilla package.
Other packages have provided cancellation support similar to Context. For example, Tomb provides a Kill method that signals cancellation by closing a Dying channel. Tomb also provides methods to wait for those goroutines to exit, similar to sync.WaitGroup. In tomb.go, we provide a Context implementation that is canceled when either its parent Context is canceled or a provided Tomb is killed.
At Google, we require that Go programmers pass a Context parameter as the first argument to every function on the call path between incoming and outgoing requests. This allows Go code developed by many different teams to interoperate well. It provides simple control over timeouts and cancellation and ensures that critical values like security credentials transit Go programs properly.
Server frameworks that want to build on Context should provide implementations of Context to bridge between their packages and those that expect a Context parameter. Their client libraries would then accept a Context from the calling code. By establishing a common interface for request-scoped data and cancellation, Context makes it easier for package developers to share code for creating scalable services.