#Gophercon Talk by Smita Vijayakumar - Go's Context Library
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This document discusses Go's context library, which allows request-scoped values and cancellation signals to propagate across API boundaries. The context library defines a Context interface that carries deadlines, cancellation signals, and arbitrary key-value pairs. This enables use cases like distributed tracing, cancellable requests, and propagating request context in distributed systems. The document provides examples of creating context nodes with different properties and using context values and cancellation signals when processing asynchronous requests across multiple packages. It notes that while the context library eases handling concurrent requests and flow traceability, its use can increase code complexity, especially across process boundaries, and care must be taken to avoid memory leaks from storing context in structures.
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#Gophercon Talk by Smita Vijayakumar - Go's Context Library
- 1. 1 Go’s Context Library By: Smita Vijayakumar
- 2. Agenda Introduction -Context Management in Distributed Systems Go’s Context Library Example Use case Points to Watch Out For 2 1 2 3 4
- 5. Request Processing A B C 5 D E RPC RPC RPC RPC
- 6. 6 Go Context • Defines *Context* type • Carries request-scoped values: - Deadlines - Cancellation signals - Others • Works across API boundaries - Also between processes
- 7. 7 Details type Context interface { Done() <-chan struct{} Err() error Deadline() (deadline time.Time, ok bool) Value(key interface{}) interface{} }
- 8. 8 1. Distributed Tracing 2. Request Cancellation 3. Context value Primary Use Cases
- 9. 9 struct ContextValue { requestID string } Example 1 - Distributed Tracing
- 10. 10 Types of Context Nodes 1. Background Node 2. Value Node 3. Cancellable Node 4. TODO() Node
- 11. Example - A = context.Background() B, cancelB = context.WithCancel(A) C = context.WithValue(A, “C Key”, “C”) D = context.WithValue(A, “D Key”, “D”) E, cancelE = context.WithTimeout(B, timeout) .. Background Context A With Value C With Cancel B With Value D With Timeout E With Value F With Value G With Value 11 H With Value I With Value J Types - A Context Tree
- 12. 12 package userid const ctxKey string = “UserID” func SetContextValue(ctx context.Context, id int) context.Context { return context.WithValue(ctx, ctxKey, id) } func GetContextValue(ctx context.Context) (int, bool) { id, ok := ctx.Value(ctxKey).(int) return id, ok } Example 2 - UTIL Package
- 13. 13 package request func startRequest(event *event.Event, timeout time.Duration) error { ctx, cancel := context.WithTimeout(context.Background(), timeout) defer cancel() //… ctx = util.SetContextValue(ctx, id) status, err := processor.Server(ctx, event) //… } Example 2 - Cancellable Request - Set the Context
- 14. 14 package request func startRequest(event *event.Event, timeout time.Duration) error { ctx, cancel := context.WithTimeout(context.Background(), timeout) defer cancel() //… ctx = util.SetContextValue(ctx, id) status, err := processor.Server(ctx, event) //… } Example 2 - Cancellable Request - Set the Context
- 15. 15 package request func startRequest(event *event.Event, timeout time.Duration) error { ctx, cancel := context.WithTimeout(context.Background(), timeout) defer cancel() //… ctx = util.SetContextValue(ctx, id) status, err := processor.Server(ctx, event) //… } Example 2 - Cancellable Request - Set the Context
- 16. 16 package processor func Server(ctx context.Context, event *event.Event) error { if id, ok := util.FromContext(ctx); !ok { return errors.New(“Not a valid ID to process”) } p := make(chan error, 1) go func(ctx context.Context, id int, event *event.Event) { p <- processEvent(ctx, id, event) }() Example 2 - Cancellable Request – Get and Handle Context
- 17. 17 package processor func Server(ctx context.Context, event *event.Event) error { if id, ok := util.FromContext(ctx); !ok { return errors.New(“Not a valid ID to process”) } p := make(chan error, 1) go func(ctx context.Context, id int, event *event.Event) { p <- processEvent(ctx, id, event) }() Example 2 - Cancellable Request – Get and Handle Context
- 18. 18 //continued select { case <-ctx.Done(): //… return ctx.Err() case err := <-p return err } } Example 2 - Cancellable Request – Get and Handle Context
- 19. 19 1. Ease of handling multiple, concurrent requests Summary - Use Cases In Distributed System 2. Flow Traceability and Fingerprinting 3. Time Sensitive and Cancellable Request Processing 4. Ease of sending context information
- 20. 20 Remember!
- 21. 21 Remember… For larger systems, complexity is the downside Code Complexity:
- 22. 22 Difficult to actually implement passing cancellable signals downstream Inter-Process Boundaries: Remember…
- 23. 23 Don’t store context variables inside structures Garbage Collection: Remember…
- 24. 24 Holding the right context node Querying: Remember…
- 25. 25 Thank you! For any queries: Smita Vijayakumar smita@exotel.in