BlueThrottle (Apache ZooKeeper

java.lang.Object
- org.apache.zookeeper.server.BlueThrottle

```
public class BlueThrottle
extends Object
```
Implements a token-bucket based rate limiting mechanism with optional probabilistic dropping inspired by the BLUE queue management algorithm [1]. The throttle provides the checkLimit(int) method which provides a binary yes/no decision. The core token bucket algorithm starts with an initial set of tokens based on the maxTokens setting. Tokens are dispensed each checkLimit(int) call, which fails if there are not enough tokens to satisfy a given request. The token bucket refills over time, providing fillCount tokens every fillTime milliseconds, capping at maxTokens. This design allows the throttle to allow short bursts to pass, while still capping the total number of requests per time interval. One issue with a pure token bucket approach for something like request or connection throttling is that the wall clock arrival time of requests affects the probability of a request being allowed to pass or not. Under constant load this can lead to request starvation for requests that constantly arrive later than the majority. In an attempt to combat this, this throttle can also provide probabilistic dropping. This is enabled anytime freezeTime is set to a value other than -1. The probabilistic algorithm starts with an initial drop probability of 0, and adjusts this probability roughly every freezeTime milliseconds. The first request after freezeTime, the algorithm checks the token bucket. If the token bucket is empty, the drop probability is increased by dropIncrease up to a maximum of 1. Otherwise, if the bucket has a token deficit less than decreasePoint * maxTokens, the probability is decreased by dropDecrease. Given a call to checkLimit(int), requests are first dropped randomly based on the current drop probability, and only surviving requests are then checked against the token bucket. When under constant load, the probabilistic algorithm will adapt to a drop frequency that should keep requests within the token limit. When load drops, the drop probability will decrease, eventually returning to zero if possible. [1] "BLUE: A New Class of Active Queue Management Algorithms"

Field Summary

Fields
Modifier and Type	Field and Description
`static String`	`CONNECTION_THROTTLE_DECREASE_RATIO`
`static String`	`CONNECTION_THROTTLE_DROP_DECREASE`
`static String`	`CONNECTION_THROTTLE_DROP_INCREASE`
`static String`	`CONNECTION_THROTTLE_FILL_COUNT`
`static String`	`CONNECTION_THROTTLE_FILL_TIME`
`static String`	`CONNECTION_THROTTLE_FREEZE_TIME`
`static String`	`CONNECTION_THROTTLE_TOKENS`
`static String`	`GLOBAL_SESSION_WEIGHT`
`static String`	`LOCAL_SESSION_WEIGHT`
`static String`	`RENEW_SESSION_WEIGHT`
`static String`	`WEIGHED_CONNECTION_THROTTLE`

Constructor Summary

Constructors
Constructor and Description

BlueThrottle()

Constructors
Constructor and Description
`BlueThrottle()`

Method Summary

All Methods Static Methods Instance Methods Concrete Methods
Modifier and Type	Method and Description
`boolean`	`checkBlue(long now)`
`boolean`	`checkLimit(int need)`
`double`	`getDecreasePoint()`
`int`	`getDeficit()`
`double`	`getDropChance()`
`double`	`getDropDecrease()`
`double`	`getDropIncrease()`
`int`	`getFillCount()`
`int`	`getFillTime()`
`int`	`getFreezeTime()`
`int`	`getMaxTokens()`
`int`	`getRequiredTokensForGlobal()`
`int`	`getRequiredTokensForLocal()`
`int`	`getRequiredTokensForRenew()`
`boolean`	`isConnectionWeightEnabled()`
`protected static void`	`setConnectionWeightEnabled(boolean enabled)`
`void`	`setDecreasePoint(double ratio)`
`void`	`setDropDecrease(double decrease)`
`void`	`setDropIncrease(double increase)`
`void`	`setFillCount(int count)`
`void`	`setFillTime(int time)`
`void`	`setFreezeTime(int time)`
`void`	`setMaxTokens(int max)`

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail
- CONNECTION_THROTTLE_TOKENS
```
public static final String CONNECTION_THROTTLE_TOKENS
```
  See Also:
  
  Constant Field Values
- CONNECTION_THROTTLE_FILL_TIME
```
public static final String CONNECTION_THROTTLE_FILL_TIME
```
  See Also:
  
  Constant Field Values
- CONNECTION_THROTTLE_FILL_COUNT
```
public static final String CONNECTION_THROTTLE_FILL_COUNT
```
  See Also:
  
  Constant Field Values
- CONNECTION_THROTTLE_FREEZE_TIME
```
public static final String CONNECTION_THROTTLE_FREEZE_TIME
```
  See Also:
  
  Constant Field Values
- CONNECTION_THROTTLE_DROP_INCREASE
```
public static final String CONNECTION_THROTTLE_DROP_INCREASE
```
  See Also:
  
  Constant Field Values
- CONNECTION_THROTTLE_DROP_DECREASE
```
public static final String CONNECTION_THROTTLE_DROP_DECREASE
```
  See Also:
  
  Constant Field Values
- CONNECTION_THROTTLE_DECREASE_RATIO
```
public static final String CONNECTION_THROTTLE_DECREASE_RATIO
```
  See Also:
  
  Constant Field Values
- WEIGHED_CONNECTION_THROTTLE
```
public static final String WEIGHED_CONNECTION_THROTTLE
```
  See Also:
  
  Constant Field Values
- GLOBAL_SESSION_WEIGHT
```
public static final String GLOBAL_SESSION_WEIGHT
```
  See Also:
  
  Constant Field Values
- LOCAL_SESSION_WEIGHT
```
public static final String LOCAL_SESSION_WEIGHT
```
  See Also:
  
  Constant Field Values
- RENEW_SESSION_WEIGHT
```
public static final String RENEW_SESSION_WEIGHT
```
  See Also:
  
  Constant Field Values

Constructor Detail
- BlueThrottle
```
public BlueThrottle()
```

Method Detail

setConnectionWeightEnabled

protected static void setConnectionWeightEnabled(boolean enabled)

setMaxTokens
```
public void setMaxTokens(int max)
```

setFillTime
```
public void setFillTime(int time)
```

setFillCount
```
public void setFillCount(int count)
```

setFreezeTime
```
public void setFreezeTime(int time)
```

setDropIncrease

public void setDropIncrease(double increase)

setDropDecrease

public void setDropDecrease(double decrease)

setDecreasePoint

public void setDecreasePoint(double ratio)

getMaxTokens
```
public int getMaxTokens()
```

getFillTime
```
public int getFillTime()
```

getFillCount
```
public int getFillCount()
```

getFreezeTime
```
public int getFreezeTime()
```

getDropIncrease
```
public double getDropIncrease()
```

getDropDecrease
```
public double getDropDecrease()
```

getDecreasePoint
```
public double getDecreasePoint()
```

getDropChance
```
public double getDropChance()
```

getDeficit
```
public int getDeficit()
```

getRequiredTokensForGlobal

public int getRequiredTokensForGlobal()

getRequiredTokensForLocal

public int getRequiredTokensForLocal()

getRequiredTokensForRenew

public int getRequiredTokensForRenew()

isConnectionWeightEnabled

public boolean isConnectionWeightEnabled()

checkLimit
```
public boolean checkLimit(int need)
```

checkBlue
```
public boolean checkBlue(long now)
```

Class BlueThrottle

Field Summary

Constructor Summary

Method Summary

Methods inherited from class java.lang.Object

Field Detail

CONNECTION_THROTTLE_TOKENS

CONNECTION_THROTTLE_FILL_TIME

CONNECTION_THROTTLE_FILL_COUNT

CONNECTION_THROTTLE_FREEZE_TIME

CONNECTION_THROTTLE_DROP_INCREASE

CONNECTION_THROTTLE_DROP_DECREASE

CONNECTION_THROTTLE_DECREASE_RATIO

WEIGHED_CONNECTION_THROTTLE

GLOBAL_SESSION_WEIGHT

LOCAL_SESSION_WEIGHT

RENEW_SESSION_WEIGHT

Constructor Detail

BlueThrottle

Method Detail

setConnectionWeightEnabled

setMaxTokens

setFillTime

setFillCount

setFreezeTime

setDropIncrease

setDropDecrease

setDecreasePoint

getMaxTokens

getFillTime

getFillCount

getFreezeTime

getDropIncrease

getDropDecrease

getDecreasePoint

getDropChance

getDeficit

getRequiredTokensForGlobal

getRequiredTokensForLocal

getRequiredTokensForRenew

isConnectionWeightEnabled

checkLimit

checkBlue