Add default value to query frontend memcached config (#3398)

* add default value to query frontend memcached config

Signed-off-by: Ben Ye <yb532204897@gmail.com>

* add changelog

Signed-off-by: Ben Ye <yb532204897@gmail.com>

Co-authored-by: Bartlomiej Plotka <bwplotka@gmail.com>

CHANGELOG.md

@@ -24,6 +24,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 - [#3378](https://github.com/thanos-io/thanos/pull/3378) Ruler: added the ability to send queries via the HTTP method POST. Helps when alerting/recording rules are extra long because it encodes the actual parameters inside of the body instead of the URI. Thanos Ruler now uses POST by default unless `--query.http-method` is set `GET`.
 - [#3381](https://github.com/thanos-io/thanos/pull/3381) Querier UI: Add ability to enable or disable metric autocomplete functionality.
 - [#2979](https://github.com/thanos-io/thanos/pull/2979) Replicator: Add the ability to replicate blocks within a time frame by passing --min-time and --max-time
+- [#3398](https://github.com/thanos-io/thanos/pull/3398) Query Frontend: Add default config for query frontend memcached config.
 - [#3277](https://github.com/thanos-io/thanos/pull/3277) Thanos Query: Introduce dynamic lookback interval. This allows queries with large step to make use of downsampled data.
 
 ### Fixed

docs/components/query-frontend.md

@@ -89,6 +89,21 @@ config:
 
 Other cache configuration parameters, you can refer to [memcached-index-cache]( https://thanos.io/tip/components/store.md/#memcached-index-cache).
 
+The default memcached config is:
+
+```yaml
+type: MEMCACHED
+config:
+  addresses: [your-memcached-addresses]
+  timeout: 500ms
+  max_idle_connections: 100
+  max_async_concurrency: 10
+  max_async_buffer_size: 10000
+  max_get_multi_concurrency: 100
+  max_get_multi_batch_size: 0
+  dns_provider_update_interval: 10s
+  expiration: 24h
+```
 
 ### Slow Query Log
 

pkg/queryfrontend/config.go

@@ -27,6 +27,21 @@ const (
 	MEMCACHED ResponseCacheProvider = "MEMCACHED"
 )
 
+var (
+	defaultMemcachedConfig = MemcachedResponseCacheConfig{
+		Memcached: cacheutil.MemcachedClientConfig{
+			Timeout:                   500 * time.Millisecond,
+			MaxIdleConnections:        100,
+			MaxAsyncConcurrency:       10,
+			MaxAsyncBufferSize:        10000,
+			MaxGetMultiConcurrency:    100,
+			MaxGetMultiBatchSize:      0,
+			DNSProviderUpdateInterval: 10 * time.Second,
+		},
+		Expiration: 24 * time.Hour,
+	}
+)
+
 // InMemoryResponseCacheConfig holds the configs for the in-memory cache provider.
 type InMemoryResponseCacheConfig struct {
 	// MaxSize represents overall maximum number of bytes cache can contain.
@@ -79,7 +94,7 @@ func NewCacheConfig(logger log.Logger, confContentYaml []byte) (*cortexcache.Con
 			},
 		}, nil
 	case string(MEMCACHED):
-		var config MemcachedResponseCacheConfig
+		config := defaultMemcachedConfig
 		if err := yaml.UnmarshalStrict(backendConfig, &config); err != nil {
 			return nil, err
 		}
@@ -98,6 +113,11 @@ func NewCacheConfig(logger log.Logger, confContentYaml []byte) (*cortexcache.Con
 			config.Memcached.DNSProviderUpdateInterval = 10 * time.Second
 		}
 
+		if config.Memcached.MaxAsyncConcurrency <= 0 {
+			level.Warn(logger).Log("msg", "memcached max async concurrency must be positive, defaulting to 10")
+			config.Memcached.MaxAsyncConcurrency = 10
+		}
+
 		return &cortexcache.Config{
 			Memcache: cortexcache.MemcachedConfig{
 				Expiration:  config.Expiration,