Microsoft.AspNetCore.RateLimiting metrics

[JamesNK]

Background and Motivation

Addresses the rate limiting part of dotnet/runtime#79459

Microsoft.AspNetCore.RateLimiting is new in .NET 7. It currently doesn't have event counters or metrics counters. This PR adds metrics counters, making rate limiting in ASP.NET Core apps more observable.

Notes:

• Microsoft.AspNetCore.RateLimiting meter is created by metrics DI integration.
• These counters are in the middleware layer and focus on how requests are impacted by rate limiting. They don't provide low-level counter information.
• Rate-limiting middleware supports partitioning. The partition isn't added to rate-limiting counter tags. This is for a couple of reasons:
  • Primary reason is the partition is defined by the app could be high-cardinality. For example, a common partition is the authenticated user name. The system could have thousands of users, creating a metrics tag cardinality explosion.
  • It's possible to have a global and endpoint policy for a request. They could have different partition values. Which value to use? Or change counters to measure policies separately? Simpler not to include it.

Proposed API

Microsoft.AspNetCore.RateLimiting

current-lease-requests

Name	Instrument Type	Unit	Description
current-lease-requests	UpDownCounter	{request}	Number of HTTP requests that are currently active on the server that hold a rate limiting lease.

Attribute	Type	Description	Examples	Presence
policy	string	Rate limiting policy name for this request.	MyPolicyName	Added if the matched route has a rate limiting policy name.
method	string	HTTP request method.	GET; POST; HEAD	Added if route endpoint set
route	string	The matched route	{controller}/{action}/{id?}	Added if route endpoint set

lease-request-duration

Name	Instrument Type	Unit	Description
lease-request-duration	Histogram	s	The duration of rate limiting leases held by HTTP requests on the server.

Attribute	Type	Description	Examples	Presence
policy	string	Rate limiting policy name for this request.	MyPolicyName	Added if the matched route has a rate limiting policy name.
method	string	HTTP request method.	GET; POST; HEAD	Added if route endpoint set
route	string	The matched route	{controller}/{action}/{id?}	Added if route endpoint set

current-requests-queued

Name	Instrument Type	Unit	Description
current-requests-queued	UpDownCounter	{request}	Number of HTTP requests that are currently queued, waiting to acquire a rate limiting lease.

Attribute	Type	Description	Examples	Presence
policy	string	Rate limiting policy name for this request.	MyPolicyName	Added if the matched route has a rate limiting policy name.
method	string	HTTP request method.	GET; POST; HEAD	Added if route endpoint set
route	string	The matched route	{controller}/{action}/{id?}	Added if route endpoint set

queued-request-duration

Name	Instrument Type	Unit	Description
queued-request-duration	Histogram	s	The duration of requests in a queue, waiting to acquire a rate limiting lease.

Attribute	Type	Description	Examples	Presence
policy	string	Rate limiting policy name for this request.	MyPolicyName	Added if the matched route has a rate limiting policy name.
method	string	HTTP request method.	GET; POST; HEAD	Added if route endpoint set
route	string	The matched route	{controller}/{action}/{id?}	Added if route endpoint set

lease-failed-requests

Name	Instrument Type	Unit	Description
lease-failed-requests	Counter	{request}	Number of HTTP requests that failed to acquire a rate limiting lease. Requests could be rejected by global or endpoint rate limiting policies. Or the request could be canceled while waiting for the lease.

Attribute	Type	Description	Examples	Presence
reason	string	Reason why acquiring the lease failed.	GlobalLimiter; EndpointLimiter; RequestCanceled	Always.
policy	string	Rate limiting policy name for this request.	MyPolicyName	Added if the matched route has a rate limiting policy name.
method	string	HTTP request method.	GET; POST; HEAD	Added if route endpoint set
route	string	The matched route	{controller}/{action}/{id?}	Added if route endpoint set

Usage Examples

Alternative Designs

Risks

[JamesNK]

@Tratcher @davidfowl @noahfalk @tarekgh @samsp-msft @joperezr

Metrics counters for Microsoft.AspNetCore.RateLimiting. This is ASP.NET Core middleware that makes it easy to apply rate limiting to HTTP requests.

[davidfowl]

Shouldn't the route template be a dimension?

[JamesNK]

If an endpoint has a policy name then that could be used to identify the endpoint.

But yes, route could be included here. If the route-limiting middleware is before UseRouting in the pipeline then it will be null (along with policy name).

[davidfowl]

Feels like that would be a useful thing to add for any meter in the pipeline that is endpoint aware.

[BrennanConroy]

If we add the ability for multiple leases to be acquired by a single request, how will that be displayed by the metrics?
Should current-lease-requests be, current-acquired-leases instead? And not mention HTTP?
Or would we add another counter?

Similarly with current-requests-queued, that could be current-queued-leases.

[JamesNK]

A single request can already acquire multiple leases because of global + endpoint limiters both being acquired. Also, PartitionedRateLimiter.CreateChained can merge multiple limiters together, but we have no visibility of that. From the middleware's perspective, it's one limiter and lease.

The counters specifically focus on requests rather than leases. Perhaps if lower-level rate limiting adds metrics then it could record that level of information.

One potential area of confusion is current-requests-queued. If a request is waiting on the global limiter then it is in the current-requests-queued. However, it's recorded with the endpoint's policy name. The policy name might make someone think the queue is waiting on the endpoint limiter instead of the global limiter.

Maybe the current-requests-queued could be incremented and decremented for each limiter (global and endpoint) and include a tag to say what the queue reason is (i.e. GlobalLimiter, EndpointLimiter) rather than treating them together as the queue.

[BrennanConroy]

A single request can already acquire multiple leases because of global + endpoint limiters both being acquired

That's not what I meant. A potential future feature in the middleware could be to add "costs" to endpoints, so instead of AcquireAsync(1) you could call AcquireAsync(5).

[JamesNK]

Is that permitCount? https://learn.microsoft.com/en-us/dotnet/api/system.threading.ratelimiting.partitionedratelimiter-1.attemptacquire?view=aspnetcore-7.0#definition

Do you imagine that an endpoint could specify a number? e.g. [EnableRateLimiting("myPolicy", PermitCount = 5)]

It could get added as a tag to all the counters like the policy name.

[BrennanConroy]

Do you imagine that an endpoint could specify a number? e.g. [EnableRateLimiting("myPolicy", PermitCount = 5)]

Yeah, something like that. Although, I think there is someone asking to add additional permit cost after the request is done being processed. Which would be problematic since the tags need to match for start and end?

It could get added as a tag to all the counters like the policy name.

Would that start getting close to cardinality explosion? Now you can have any combination of policy and permit count cost.

[JamesNK]

We're adding the route as tag, so there is a separate value per route already. Each route only has one policy and permit count.

"route + policy + permit count" will almost always line up so including the policy and permit count doesn't make it any worse.