cache: enable protection against thundering herd problem by default…

…. The protection may be disabled by setting negative `grace_time`

README.md

@@ -13,7 +13,7 @@ Chproxy, is an http proxy for [ClickHouse](https://ClickHouse.yandex) database.
 - All the limits may be independently set for each input user and for each per-cluster user.
 - May delay request execution until it fits per-user limits.
 - Per-user [response caching](#caching) may be configured.
-- Protection from [thundering herd](https://en.wikipedia.org/wiki/Cache_stampede) problem may be enabled on response caches.
+- Response caches have built-in protection against [thundering herd](https://en.wikipedia.org/wiki/Cache_stampede) aka `dogpile effect`.
 - Evenly spreads requests among cluster nodes using `least loaded` + `round robin` technique.
 - Monitors node health and prevents from sending requests to unhealthy nodes.
 - Supports automatic HTTPS certificate issuing and renewal via [Let’s Encrypt](https://letsencrypt.org/).
@@ -353,7 +353,8 @@ caches:
     # for the cached response during this grace duration.
     # This is known as protection from `thundering herd` problem.
     #
-    # By default `thundering herd` protection is disabled.
+    # By default `grace_time` is 5s. Negative value disables the protection
+    # from `thundering herd` problem.
     grace_time: 20s
 
   - name: "shortterm"

cache/cache.go

@@ -107,7 +107,12 @@ func New(cfg config.Cache) (*Cache, error) {
 	}
 
 	graceTime := cfg.GraceTime
-	if graceTime <= 0 {
+	if graceTime == 0 {
+		// Default grace time.
+		graceTime = 5*time.Second
+	}
+	if graceTime < 0 {
+		// Disable protection from `dogpile effect`.
 		graceTime = 0
 	}
 

config/README.md

@@ -67,7 +67,8 @@ expire: <duration>
 # for the cached response during this grace duration.
 # This is known as protection from `thundering herd` problem.
 #
-# By default `thundering herd` protection is disabled.
+# By default `grace_time` is 5s. Negative value disables the protection
+# from `thundering herd` problem.
 grace_time: <duration>
 ```
 

config/config.go

@@ -467,9 +467,6 @@ func (c *Cache) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	if c.MaxSize <= 0 {
 		return fmt.Errorf("`cache.max_size` must be specified for %q", c.Name)
 	}
-	if c.GraceTime < 0 {
-		return fmt.Errorf("`cache.grace_time` cannot be negative for %q", c.Name)
-	}
 	return checkOverflow(c.XXX, fmt.Sprintf("cache %q", c.Name))
 }
 

config/testdata/full.yml

@@ -33,7 +33,8 @@ caches:
     # for the cached response during this grace duration.
     # This is known as protection from `thundering herd` problem.
     #
-    # By default `thundering herd` protection is disabled.
+    # By default `grace_time` is 5s. Negative value disables the protection
+    # from `thundering herd` problem.
     grace_time: 20s
 
   - name: "shortterm"