#Gophercon Talk by Smita Vijayakumar - Go's Context Library

1
Go’s Context Library
By:
Smita Vijayakumar

Agenda
Introduction
-Context Management in
Distributed Systems
Go’s Context Library
Example Use case
Points to Watch Out For
2
1
2
3
4

Request Processing
A B C
5
D E
RPC RPC RPC RPC

6
Go Context
• Defines *Context* type
• Carries request-scoped
values:
- Deadlines
- Cancellation signals
- Others
• Works across API
boundaries
- Also between processes

7
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

10
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
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

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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
ctx, cancel :=
defer cancel()
//…
//…
}

15
package request
ctx, cancel :=
defer cancel()
//…
//…
}

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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)
}()

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//continued
select {
case <-ctx.Done():
//…
return ctx.Err()
case err := <-p
return err
}
}

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

21
Remember… For larger systems, complexity is
the downside
Code Complexity:

22
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…

24
Holding the right context node
Querying:
Remember…

25
Thank you!
For any queries:
Smita Vijayakumar smita@exotel.in

#Gophercon Talk by Smita Vijayakumar - Go's Context Library

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#Gophercon Talk by Smita Vijayakumar - Go's Context Library