SpikeArrest policy

Spike Arrest icon from the Edge UI

The Spike Arrest policy protects against traffic surges with the <Rate> element. This element throttles the number of requests processed by an API proxy and sent to a backend, protecting against performance lags and downtime.

<SpikeArrest> element

Defines the Spike Arrest policy.

Default Value See Default Policy tab, below
Required? Optional
Type Complex object
Parent Element n/a
Child Elements <Identifier>
<MessageWeight>
<Rate> (Required)
<UseEffectiveCount>

Syntax

The <SpikeArrest> element uses the following syntax:

<SpikeArrest
  continueOnError="[false|true]"
  enabled="[true|false]"
  name="policy_name"
>
  <DisplayName>display_name</DisplayName>
  <Properties/>
  <Identifier ref="flow_variable"/>
  <MessageWeight ref="flow_variable"/>
  <Rate ref="flow_variable">rate[pm|ps]</Rate>
  <UseEffectiveCount>[false|true]</UseEffectiveCount>
</SpikeArrest>

Default Policy

The following example shows the default settings when you add a Spike Arrest policy to your flow in the Edge UI:

<SpikeArrest async="false" continueOnError="false" enabled="true" name="Spike-Arrest-1">
  <DisplayName>Spike Arrest-1</DisplayName>
  <Properties/>
  <Identifier ref="request.header.some-header-name"/>
  <MessageWeight ref="request.header.weight"/>
  <Rate>30ps</Rate>
</SpikeArrest>

Note that the <UseEffectiveCount> element is not defined in the default policy. To use that, you must add it manually.

This element has the following attributes that are common to all policies:

Attribute Default Required? Description
name N/A Required

The internal name of the policy. The value of the name attribute can contain letters, numbers, spaces, hyphens, underscores, and periods. This value cannot exceed 255 characters.

Optionally, use the <DisplayName> element to label the policy in the management UI proxy editor with a different, natural-language name.

continueOnError false Optional Set to "false" to return an error when a policy fails. This is expected behavior for most policies. Set to "true" to have flow execution continue even after a policy fails.
enabled true Optional Set to "true" to enforce the policy. Set to "false" to "turn off" the policy. The policy will not be enforced even if it remains attached to a flow.
async  not_interested false Deprecated This attribute is deprecated.

Examples

The following examples show some of the ways in which you can use the Spike Arrest policy:

Example 1

The following example sets the rate to five per second:

<SpikeArrest name="Spike-Arrest-1">
  <Rate>5ps</Rate>
</SpikeArrest>

The policy smoothes the rate to one request allowed every 200 milliseconds (1000/5).

Example 2

The following example sets the rate to 12 per minute:

<SpikeArrest name="Spike-Arrest-1">
  <Rate>12pm</Rate>
</SpikeArrest>

This example policy smoothes the rate to one request allowed every five seconds (60/12).

Example 3

The following example restricts requests to 12 per minute (one request allowed every five seconds, or 60/12):

<SpikeArrest name="Spike-Arrest-1">
  <Rate>12pm</Rate>
  <Identifier ref="client_id" />
  <MessageWeight ref="request.header.weight" />
</SpikeArrest>

In addition, the <MessageWeight> element accepts a custom value (the weight header) that adjusts message weights for specific apps or clients. This provides additional control over throttling for entities that are identified with the <Identifier> element.

Example 4

The following example instructs Spike Arrest to look for a runtime value set via the request that is passed in as the request.header.runtime_rate flow variable:

<SpikeArrest name="Spike-Arrest-1">
  <Rate ref="request.header.runtime_rate" />
</SpikeArrest>

The value of the flow variable must be in the form of intpm or intps.

To try this example, execute a request like the following:

curl http://myorg-myenv.apigee.net/price -H 'runtime_rate:30ps'

Child element reference

This section describes the child elements of <SpikeArrest>.

<DisplayName>

Use in addition to the name attribute to label the policy in the management UI proxy editor with a different, more natural-sounding name.

The <DisplayName> element is common to all policies.

Default Value n/a
Required? Optional. If you omit <DisplayName>, the value of the policy's name attribute is used
Type String
Parent Element <PolicyElement>
Child Elements None

The <DisplayName> element uses the following syntax:

Syntax

<PolicyElement>
  <DisplayName>policy_display_name</DisplayName>
  ...
</PolicyElement>

Example

<PolicyElement>
  <DisplayName>My Validation Policy</DisplayName>
</PolicyElement>

The <DisplayName> element has no attributes or child elements.

<Identifier>

Lets you choose how to group the requests so that the Spike Arrest policy can be applied based on the client. For example, you can group requests by developer ID, in which case each developer's requests will count towards their own Spike Arrest rate and not all requests to the proxy.

Use in conjunction with <MessageWeight> element for more fine-grained control over request throttling.

If you leave the <Identifier> element empty, one rate limit is enforced for all requests into that API proxy.

Default Value n/a
Required? Optional
Type String
Parent Element <SpikeArrest>
Child Elements None

Syntax

<SpikeArrest
  continueOnError="[false|true]"
  enabled="[true|false]"
  name="policy_name"
>
  <Identifier ref="flow_variable"/>
</SpikeArrest>
        

Example 1

The following example applies the Spike Arrest policy per developer ID:

<SpikeArrest name="Spike-Arrest-1">
  <Identifier ref="developer.id"/>
  <Rate>42pm</Rate/>
</SpikeArrest>

The following table describes the attributes of <Identifier>:

Attribute Description Default Presence
ref Identifies the variable by which Spike Arrest groups incoming requests. You can use any flow variable to indicate a unique client, such those available with the VerifyAPIKey policy. You can also set custom variables using the JavaScript policy or the AssignMessage policy. n/a Required

This element is also discussed in the following Apigee Community post: http://community.apigee.com/questions/2807/how-does-the-edge-quota-policy-work-when-no-identi.html.

<MessageWeight>

Specifies the weighting defined for each message. Message weight modifies the impact of a single request on the calculation of the Spike Arrest rate. Message weight can be any flow variable, such as an HTTP header, query parameter, form parameter, or message body content. You can also use custom variables using the JavaScript policy or the AssignMessage policy.

Use in conjunction with <Identifier> to further throttle requests by specific clients or apps.

For example, if the Spike Arrest <Rate> is 10pm, and an app submits requests with a weight of 2, then only five messages per minute are permitted from that client because each request counts as 2.

Default Value n/a
Required? Optional
Type Integer
Parent Element <SpikeArrest>
Child Elements None

Syntax

<SpikeArrest
  continueOnError="[false|true]"
  enabled="[true|false]"
  name="policy_name"
>
  <MessageWeight ref="flow_variable"/>
</SpikeArrest>

Example 1

The following example restricts requests to 12 per minute (one request allowed every five seconds, or 60/12):

<SpikeArrest name="Spike-Arrest-1">
  <Rate>12pm</Rate>
  <Identifier ref="client_id" />
  <MessageWeight ref="request.header.weight" />
</SpikeArrest>

In this example, <MessageWeight> accepts a custom value (the weight header in the request) that adjusts message weights for specific clients. This provides additional control over throttling for entities that are identified with the <Identifier> element.

The following table describes the attributes of <MessageWeight>:

Attribute Description Presence Default
ref Identifies the flow variable that contains the message weight for the specific client. This can be any flow variable, such as an HTTP query param, header, or message body content. For more information, see Flow variables reference. You can also set custom variables using the JavaScript policy or the AssignMessage policy. Required N/A

<Rate>

Specifies the rate at which to limit traffic spikes (or bursts) by setting the number of requests that are allowed in per minute or per second intervals. You can also use this element in conjunction with <Identifier> and <MessageWeight> to smoothly throttle traffic at runtime by accepting values from the client.

Default Value n/a
Required? Required
Type Integer
Parent Element <SpikeArrest>
Child Elements None

Syntax

You can specify rates in one of the following ways:

  • A static rate that you specify as the body of the <Rate> element
  • A variable value, which can be passed by the client; identify the name of the flow variable using the ref attribute
<SpikeArrest
  continueOnError="[false|true]"
  enabled="[true|false]"
  name="policy_name"
>
  <Rate ref="flow_variable">rate[pm|ps]</Rate>
</SpikeArrest>

Valid rate values (either defined as a variable value or in the body of the element) must conform to the following format:

  • intps (number of requests per second, smoothed into intervals of milliseconds)
  • intpm (number of requests per minute, smoothed into intervals of seconds)

The value of int must be a positive, non-zero integer.

Example 1

The following example sets the rate to five requests per second:

<SpikeArrest name="Spike-Arrest-1">
  <Rate>5ps</Rate>
</SpikeArrest>

The policy smoothes the rate to one request allowed every 200 milliseconds (1000/5).

Example 2

The following example sets the rate to 12 requests per minute:

<SpikeArrest name="Spike-Arrest-1">
  <Rate>12pm</Rate>
</SpikeArrest>

This example policy smoothes the rate to one request allowed every five seconds (60/12).

The following table describes the attributes of <Rate>:

Attribute Description Presence Default
ref Identifies a flow variable that specifies the rate. This can be any flow variable, such as an HTTP query parameter, header, or message body content, or a value such as a KVM. For more information, see Flow variables reference.

You can also use custom variables using the JavaScript policy or the AssignMessage policy.

If you define both ref and the body of this element, the value of ref is applied and takes precedence when the flow variable is set in the request. (The reverse is true when the variable identified in ref is not set in the request.)

For example:

<Rate ref="request.header.custom_rate">1pm</Rate>

In this example, if the client does not pass a "custom_rate" header, then the rate for the API proxy is 1 request per minute for all clients. If the client passes a "custom_rate" header, then the rate limit becomes 10 requests per second for all clients on the proxy — until a request without the "custom_rate" header is sent.

You can use <Identifier> to group requests to enforce custom rates for different type of client.

If you specify a value for ref but do not set the rate in the body of the <Rate> element and the client does not pass a value, then the Spike Arrest policy throws an error.

Optional n/a

<UseEffectiveCount>

Distributes your Spike Arrest counts across Message Processors (MPs) when using auto-scaling groups.

Syntax

<SpikeArrest
  continueOnError="[false|true]"
  enabled="[true|false]"
  name="policy_name"
>
  <UseEffectiveCount>[false|true]</UseEffectiveCount>
</SpikeArrest>

Example 1

The following example sets <UseEffectiveCount> to true:

<SpikeArrest name='Spike-Arrest-1'>
  <Rate>40ps</Rate>
  <UseEffectiveCount>true</UseEffectiveCount>
</SpikeArrest>

The <UseEffectiveCount> element is optional. The default value is false when the element is omitted from your Spike Arrest policy.

Default Value False
Required? Optional
Type Boolean
Parent Element <SpikeArrest>
Child Elements None

The following table describes the attributes of the <UseEffectiveCount> element:

Attribute Description Default Presence
ref Identifies the variable that contains the value of <UseEffectiveCount>. This can be any flow variable, such as an HTTP query param, header, or message body content. For more information, see Flow variables reference. You can also set custom variables using the JavaScript policy or the AssignMessage policy. n/a Optional

The effect of <UseEffectiveCount> depends on it's value:

  • true: An MP's spike rate limit is the <Rate> divided by the current number of MPs. The aggregate limit is the value of <Rate>. When MPs are dynamically added (or removed), their individual spike rate limits will increase (or decrease), but the aggregate limit will stay the same.
  • false (or omitted, as this is the default value): Each MP's spike rate limit is simply the value of its <Rate>. The aggregate limit is the sum of the rates of all the MPs. When MPs are added (or removed), their individual spike rate limits will stay the same, but the aggregate limit will increase (or decrease).

The following table shows the effect of <UseEffectiveCount> on the effective rate limit of each MP:

Value of <UseEffectiveCount>
false false false true true true
# of MPs 8 4 2 8 4 2
Value of <Rate> 10 10 10 40 40 40
Effective Rate per MP 10 10 10 5 10 20
Aggregate Limit 80 40 20 40* 40* 40*
* Same as <Rate>.

In this example, notice that when the number of MPs is decreased from 4 to 2, and <UseEffectiveCount> is false, the effective rate per MP stays the same (at 10). But when <UseEffectiveCount> is true, the effective rate per MP goes from 10 to 20 when the number of MPs is decreased from 4 to 2.

Flow variables

When a Spike Arrest policy executes, the following flow variable is populated:

Variable Type Permission Description
ratelimit.policy_name.failed Boolean Read-Only Indicates whether or not the policy failed (true or false).

For more information, see Flow variables reference.

Error reference

This section describes the fault codes and error messages that are returned and fault variables that are set by Edge when this policy triggers an error. This information is important to know if you are developing fault rules to handle faults. To learn more, see What you need to know about policy errors and Handling faults.

Runtime errors

These errors can occur when the policy executes.

Fault code HTTP status Cause Fix
policies.ratelimit.FailedToResolveSpikeArrestRate 500 This error occurs if the reference to the variable containing the rate setting within the <Rate> element cannot be resolved to a value within the Spike Arrest policy. This element is mandatory and used to specify the spike arrest rate in the form of intpm or intps. build
policies.ratelimit.InvalidMessageWeight 500 This error occurs if the value specified for the <MessageWeight> element through a flow variable is invalid (a non-integer value). build
policies.ratelimit.SpikeArrestViolation 429 The rate limit is exceeded.

Deployment errors

These errors can occur when you deploy a proxy containing this policy.

Error name Cause Fix
InvalidAllowedRate If the spike arrest rate specified in the <Rate> element of the Spike Arrest Policy is not an integer or if the rate does not have ps or pm as a suffix, then the deployment of the API proxy fails. build

Fault variables

These variables are set when a runtime error occurs. For more information, see What you need to know about policy errors.

Variables Where Example
fault.name="fault_name" fault_name is the name of the fault, as listed in the Runtime errors table above. The fault name is the last part of the fault code. fault.name Matches "SpikeArrestViolation"
ratelimit.policy_name.failed policy_name is the user-specified name of the policy that threw the fault. ratelimit.SA-SpikeArrestPolicy.failed = true

Example error response

Shown below is an example error response:

{  
   "fault":{  
      "detail":{  
         "errorcode":"policies.ratelimit.SpikeArrestViolation"
      },
      "faultstring":"Spike arrest violation. Allowed rate : 10ps"
   }
}

Example fault rule

Shown below is an example fault rule to handle a SpikeArrestViolation fault:

<FaultRules>
    <FaultRule name="Spike Arrest Errors">
        <Step>
            <Name>JavaScript-1</Name>
            <Condition>(fault.name Matches "SpikeArrestViolation") </Condition>
        </Step>
        <Condition>ratelimit.Spike-Arrest-1.failed=true</Condition>
    </FaultRule>
</FaultRules>

The current HTTP status code for exceeding the rate limit is 429. If you want the status code to be different, a property needs to be set on your organization. If you're a Cloud customer, contact Apigee Support.

Schemas

Each policy type is defined by an XML schema (.xsd). For reference, policy schemas are available on GitHub.

Related topics