com.alibaba.csp.sentinel.slots.block.flow

Class FlowSlot

java.lang.Object

com.alibaba.csp.sentinel.slotchain.AbstractLinkedProcessorSlot<DefaultNode>

com.alibaba.csp.sentinel.slots.block.flow.FlowSlot

All Implemented Interfaces:

ProcessorSlot<DefaultNode>

public class FlowSlot
extends AbstractLinkedProcessorSlot<DefaultNode>

Combined the runtime statistics collected from the previous slots (NodeSelectorSlot, ClusterNodeBuilderSlot, and StatisticSlot), FlowSlot will use pre-set rules to decide whether the incoming requests should be blocked.

SphU.entry(resourceName) will throw FlowException if any rule is triggered. Users can customize their own logic by catching FlowException.

One resource can have multiple flow rules. FlowSlot traverses these rules until one of them is triggered or all rules have been traversed.

Each FlowRule is mainly composed of these factors: grade, strategy, path. We can combine these factors to achieve different effects.

The grade is defined by the grade field in FlowRule. Here, 0 for thread isolation and 1 for request count shaping (QPS). Both thread count and request count are collected in real runtime, and we can view these statistics by following command:

 curl http://localhost:8719/tree

 idx id    thread pass  blocked   success total aRt   1m-pass   1m-block   1m-all   exception
 2   abc647 0      460    46          46   1    27      630       276        897      0
 

• thread for the count of threads that is currently processing the resource
• pass for the count of incoming request within one second
• blocked for the count of requests blocked within one second
• success for the count of the requests successfully handled by Sentinel within one second
• RT for the average response time of the requests within a second
• total for the sum of incoming requests and blocked requests within one second
• 1m-pass is for the count of incoming requests within one minute
• 1m-block is for the count of a request blocked within one minute
• 1m-all is the total of incoming and blocked requests within one minute
• exception is for the count of business (customized) exceptions in one second

This stage is usually used to protect resources from occupying. If a resource takes long time to finish, threads will begin to occupy. The longer the response takes, the more threads occupy. Besides counter, thread pool or semaphore can also be used to achieve this. - Thread pool: Allocate a thread pool to handle these resource. When there is no more idle thread in the pool, the request is rejected without affecting other resources. - Semaphore: Use semaphore to control the concurrent count of the threads in this resource. The benefit of using thread pool is that, it can walk away gracefully when time out. But it also bring us the cost of context switch and additional threads. If the incoming requests is already served in a separated thread, for instance, a Servlet HTTP request, it will almost double the threads count if using thread pool.

Traffic Shaping

When QPS exceeds the threshold, Sentinel will take actions to control the incoming request, and is configured by controlBehavior field in flow rules.

1. Immediately reject (RuleConstant.CONTROL_BEHAVIOR_DEFAULT)

This is the default behavior. The exceeded request is rejected immediately and the FlowException is thrown

2. Warmup (RuleConstant.CONTROL_BEHAVIOR_WARM_UP)

If the load of system has been low for a while, and a large amount of requests comes, the system might not be able to handle all these requests at once. However if we steady increase the incoming request, the system can warm up and finally be able to handle all the requests. This warmup period can be configured by setting the field warmUpPeriodSec in flow rules.

3. Uniform Rate Limiting (RuleConstant.CONTROL_BEHAVIOR_RATE_LIMITER)

This strategy strictly controls the interval between requests. In other words, it allows requests to pass at a stable, uniform rate.



This strategy is an implement of leaky bucket. It is used to handle the request at a stable rate and is often used in burst traffic (e.g. message handling). When a large number of requests beyond the system’s capacity arrive at the same time, the system using this strategy will handle requests and its fixed rate until all the requests have been processed or time out.

Author:

jialiang.linjl, Eric Zhao

Constructor Summary

Constructors

Constructor and Description
FlowSlot()

Method Summary

Modifier and Type	Method and Description
void	entry(Context context, ResourceWrapper resourceWrapper, DefaultNode node, int count, boolean prioritized, Object... args) Entrance of this slot.
void	exit(Context context, ResourceWrapper resourceWrapper, int count, Object... args) Exit of this slot.

Methods inherited from class com.alibaba.csp.sentinel.slotchain.AbstractLinkedProcessorSlot

fireEntry, fireExit, getNext, setNext

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

FlowSlot

public FlowSlot()

Method Detail

entry

public void entry(Context context,
                  ResourceWrapper resourceWrapper,
                  DefaultNode node,
                  int count,
                  boolean prioritized,
                  Object... args)
           throws Throwable

Description copied from interface: ProcessorSlot

Entrance of this slot.

Parameters:

context - current Context

resourceWrapper - current resource

node - generics parameter, usually is a Node

count - tokens needed

prioritized - whether the entry is prioritized

args - parameters of the original call

Throws:

Throwable - blocked exception or unexpected error

exit

public void exit(Context context,
                 ResourceWrapper resourceWrapper,
                 int count,
                 Object... args)

Description copied from interface: ProcessorSlot

Exit of this slot.

Parameters:

context - current Context

resourceWrapper - current resource

count - tokens needed

args - parameters of the original call