What is sync.Pool and when should you use it?

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Accepted Answer

// Pool of reusable byte buffers
var bufPool = sync.Pool{
    New: func() interface{} {
        // Called when pool is empty
        return bytes.NewBuffer(make([]byte, 0, 1024))
    },
}

func processRequest(data []byte) []byte {
    // Get buffer from pool
    buf := bufPool.Get().(*bytes.Buffer)
    buf.Reset()        // ALWAYS reset before use — pool doesn't clean objects
    defer bufPool.Put(buf) // return to pool when done

// Use the buffer
    buf.Write(data)
    json.NewEncoder(buf).Encode(map[string]int{"len": len(data)})

result := make([]byte, buf.Len())
    copy(result, buf.Bytes())
    return result
}

// Pool for JSON encoders
type encoderPool struct{ pool sync.Pool }

func newEncoderPool(w io.Writer) *encoderPool {
    return &encoderPool{
        pool: sync.Pool{
            New: func() interface{} {
                return json.NewEncoder(w)
            },
        },
    }
} Objects CAN be evicted at any GC cycle — never rely on persistence across requests Always Reset/Clear objects before returning them to the pool — stale state causes bugs Best for: byte buffers, encoder/decoder instances, temporary structs in hot paths The standard library uses Pool internally: fmt package reuses printer objects, json uses encoder pools Measure impact with benchmarks: sync.Pool is only beneficial when allocation cost is proven high

What is sync.Pool and when should you use it?

sync.Pool is a per-processor cache of reusable objects that reduces GC pressure by allowing frequently allocated short-lived objects to be reused rather than garbage collected.

What is sync.Pool and when should you use it?

sync.Pool is a per-processor cache of reusable objects that reduces GC pressure by allowing frequently allocated short-lived objects to be reused rather than garbage collected.

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