The next task was wiring it up to the platform-api gem. I used those two outputs to write what I thought was a pretty good rate throttling algorithm.
I found my simulation took too long to run and so I added a mechanism to speed up the simulated time. I initially just output values to the CLI as the simulation ran, but found it challenging to make sense of them all, so I added charting. I would simulate the server's behavior, and then boot up several processes and threads and hit the simulated server with requests to observe the system's behavior.
REQUEST THROTTLED SIMULATOR
The solution that I came up with was to write a simulator in addition to tests. I also found that checking for quality "this rate throttle strategy is better than others" could not be checked quite as easily. I quickly realized that while testing the behavior "retries a request after a 429 response," it is easy to check. I initially started wanting to write tests for my rate throttling strategy. Making client throttling maintainableīefore we can get into what logic goes into a quality rate throttling algorithm, I want to talk about the process that I used as I think the journey is just as fascinating as the destination.
REQUEST THROTTLED CODE
Since the code needed to be contained entirely in the client library, it needed to be able to function without distributed coordination between multiple clients on multiple machines except for whatever information the Heroku API returned.
That is, if the client makes 100 requests, and 10 of them are a 429 response that its retry rate is 10%. I made it a goal for the rate throttling client also to minimize its retry rate. A "simple" solution would be to add a retry to all requests when they see a 429, but that would effectively DDoS the API. I needed to write an algorithm that never errored as a result of a 429 response. If no tokens remain, further calls will return 429 Too Many Requests until more tokens become available. Tokens are added to the account pool at a rate of roughly 75 per minute (or 4500 per hour), up to a maximum of 4500. Each API call removes one token from the pool. Each account has a pool of request tokens that can hold at most 4500 tokens. The API limits the number of requests each user can make per hour to protect against abuse and buggy code. The Heroku API uses Genetic Cell Rate Algorithm (GCRA) as described by Brandur in this post on the server-side. If the term "rate throttling" is new to you, read Rate limiting, rate throttling, and how they work together That tweet spawned a discussion that generated a quest to add rate throttling logic to the platform-api gem that Heroku maintains for talking to its API in Ruby.
REQUEST THROTTLED GENERATOR
You've got an exception generator with a remote timer.- Richard Schneeman 🤠 Stay Inside June 12, 2019 If you provide an API client that doesn't include rate limiting, you don't really have an API client. The number of attempts to try before rejecting a request.When API requests are made one-after-the-other they'll quickly hit rate limits and when that happens: Otherwise, it will be rejected with a 429 Too Many Requests error. A request can be queued and retried if the limit will be refreshed within a certain time. These settings control how requests that are over the limit are handled. For example, if a service tier is set to allow 5,000 requests per hour, it would be undesirable for all of those requests to be allowed through in a single minute. This field determines the client’s quota over the long-term with a separate short-term quota used to prevent bursts of requests from overloading a Snaplex. The number of requests to allow for the defined period. To match POST requests from users with the admin role: thod = ‘POST’ & request.isUserInRole('admin') The expression has access to the request object, so the condition can match on the method, request headers, URI, and the user name or roles. If no matches are found, the limits defined in Default Tier are used.Ĭhoose the expression that defines this service tier. Enter each Condition as an expression that is evaluated against a request and used with the specified limits to restrict the request rate. An expression enabled field that determines the condition to be fulfilled for the API policy to execute.Įnter levels of a service tier to apply to clients as a condition.
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