Institute of Perception, Action and Behaviour (IPAB) Director: Prof. Sethu Vijayakumar
#Gophercon Talk by Smita Vijayakumar - Go's Context Library
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1 Go’s Context Library By: Smita Vijayakumar
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Transcript of #Gophercon Talk by Smita Vijayakumar - Go's Context Library
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Go’s Context LibraryBy:
Smita Vijayakumar
Agenda
Introduction -Context Management in Distributed Systems
Go’s Context Library
Example Use case
Points to Watch Out For
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Introduction
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Distributed Data Flow
Request Processing
A B C
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D ERPC RPC RPC RPC
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Go Context
• Defines *Context* type • Carries request-scoped
values: - Deadlines - Cancellation signals - Others • Works across API
boundaries - Also between processes
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Details
type Context interface { Done() <-chan struct{} Err() error Deadline() (deadline time.Time, ok bool) Value(key interface{}) interface{} }
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1. Distributed Tracing 2. Request Cancellation 3. Context value
Primary Use Cases
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struct ContextValue { requestID string}
Example 1 - Distributed Tracing
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Types of Context Nodes
1. Background Node 2. Value Node 3. Cancellable Node 4. TODO() Node
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
CWith
Cancel
BWith Value
D
With Timeout
EWith Value
FWith Value
GWith Value
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HWith Value
IWith Value
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Types - A Context Tree
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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
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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
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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
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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
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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
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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
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//continued select { case <-ctx.Done(): //… return ctx.Err() case err := <-p return err }}
Example 2 - Cancellable Request – Get and Handle Context
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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
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Remember!
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Remember… For larger systems, complexity is the downside
Code Complexity:
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Difficult to actually implement passing cancellable signals downstream
Inter-Process Boundaries: Remember…
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Don’t store context variables inside structures
Garbage Collection: Remember…
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Holding the right context nodeQuerying:
Remember…
