hikari.impl.buckets

Rate-limit extensions for RESTful bucketed endpoints.

Provides implementations for the complex rate limiting mechanisms that Discord requires for rate limit handling that conforms to the passed bucket headers correctly.

This was initially a bit of a headache for me to understand, personally, since there is a lot of "implicit detail" that is easy to miss from the documentation.

In an attempt to make this somewhat understandable by anyone else, I have tried to document the theory of how this is handled here.

What is the theory behind this implementation?

In this module, we refer to a CompiledRoute as a definition of a route with specific major parameter values included (e.g. POST /channels/123/messages), and a Route as a definition of a route without specific parameter values included (e.g. POST /channels/{channel}/messages). We can create a CompiledRoute from a Route by providing the corresponding parameters as kwargs, as you may already know.

In this module, a "bucket" is an internal data structure that tracks and enforces the rate limit state for a specific CompiledRoute, and can manage delaying tasks in the event that we begin to get rate limited. It also supports providing in-order execution of queued tasks.

Discord allocates types of buckets to routes. If you are making a request and there is a valid rate limit on the route you hit, you should receive an X-RateLimit-Bucket header from the server in your response. This is a hash that identifies a route based on internal criteria that does not include major parameters. This X-RateLimitBucket is known in this module as an "bucket hash".

This means that generally, the route POST /channels/123/messages and POST /channels/456/messages will usually sit in the same bucket, but GET /channels/123/messages/789 and PATCH /channels/123/messages/789 will usually not share the same bucket. Discord may or may not change this at any time, so hard coding this logic is not a useful thing to be doing.

Rate limits, on the other hand, apply to a bucket and are specific to the major parameters of the compiled route. This means that POST /channels/123/messages and POST /channels/456/messages do not share the same real bucket, despite Discord providing the same bucket hash. A real bucket hash is the string hash of the bucket that Discord sends us in a response concatenated to the corresponding major parameters. This is used for quick bucket indexing internally in this module.

One issue that occurs from this is that we cannot effectively hash a CompiledRoute that has not yet been hit, meaning that until we receive a response from this endpoint, we have no idea what our rate limits could be, nor the bucket that they sit in. This is usually not problematic, as the first request to an endpoint should never be rate limited unless you are hitting it from elsewhere in the same time window outside your hikari.applications. To manage this situation, unknown endpoints are allocated to a special unlimited bucket until they have an initial bucket hash code allocated from a response. Once this happens, the route is reallocated a dedicated bucket. Unknown buckets have a hardcoded initial hash code internally.

Initially acquiring time on a bucket

Each time you call hikari.impl.buckets.RESTBucket.acquire a request timeslice for a given Route, several things happen. The first is that we attempt to find the existing bucket for that route, if there is one, or get an unknown bucket otherwise. This is done by creating a real bucket hash from the compiled route. The initial hash is calculated using a lookup table that maps CompiledRoute objects to their corresponding initial hash codes, or to the unknown bucket hash code if not yet known. This initial hash is processed by the CompiledRoute to provide the real bucket hash we need to get the route's bucket object internally.

The hikari.impl.buckets.RESTBucket.acquire method will take the bucket and acquire a new timeslice on it. This takes the form of a asyncio.Future that is awaited and will complete once the caller is allowed to make a request. Most of the time, this is done instantly, but if the bucket has an active rate limit preventing requests being sent, then the future will be paused until the rate limit is over. This may be longer than the rate limit period if you have queued a large number of requests during this limit, as it is first-come-first-served.

Acquiring a rate limited bucket will start a bucket-wide task (if not already running) that will wait until the rate limit has completed before allowing more futures to complete. This is done while observing the rate limits again, so can easily begin to re-ratelimit itself if needed. Once the task is complete, it tidies itself up and disposes of itself. This task will complete once the queue becomes empty.

The result of hikari.impl.buckets.RESTBucketManager.acquire_bucket is an async context manager that must be acquired during the entirety of the request and released once it is done (in reality, it is just a hikari.impl.buckets.RESTBucket, but we want the ratelimit update to be forced through hikari.impl.buckets.RESTBucketManager.update_rate_limits to keep proper state)

Handling the rate limit headers of a response

Once you have received your response, you are expected to extract the values of the vital rate limit headers manually and parse them to the correct data types. These headers are:

• X-RateLimit-Limit: an int describing the max requests in the bucket from empty to being rate limited.
• X-RateLimit-Remaining: an int describing the remaining number of requests before rate limiting occurs in the current window.
• X-RateLimit-Bucket: a str containing the initial bucket hash.
• X-RateLimit-Reset-After: a float containing the number of seconds when the current rate limit bucket will reset with decimal millisecond precision.

Each of the above values should be passed to the hikari.impl.buckets.RESTBucketManager.update_rate_limits method to ensure that the bucket you acquired time from is correctly updated should Discord decide to alter their ratelimits on the fly without warning (including timings and the bucket).

This method will manage creating new buckets as needed and resetting vital information in each bucket you use.

Tidying up

To prevent unused buckets cluttering up memory, each hikari.impl.buckets.RESTBucketManager instance spins up a asyncio.Task that periodically locks the bucket list (not threadsafe, only using the concept of asyncio not yielding in regular functions) and disposes of any clearly stale buckets that are no longer needed. These will be recreated again in the future if they are needed.

When shutting down an application, one must remember to call hikari.impl.buckets.RESTBucketManager.close. This will ensure the garbage collection task is stopped, and will also ensure any remaining futures in any bucket queues have an asyncio.CancelledError set on them to prevent deadlocking ratelimited calls that may be waiting to be unlocked.

Body-field-specific rate limiting

As of the start of June, 2020, Discord appears to be enforcing another layer of rate limiting logic to their HTTP APIs which is field-specific. This means that special rate limits will also exist on some endpoints that limit based on what attributes you send in a JSON or form data payload.

No information is sent in headers about these specific limits. You will only be made aware that they exist once you get ratelimited. In the 429 ratelimited response, you will have the "global" attribute set to False, and a "reset_after" attribute that differs entirely to the X-RateLimit-Reset-After header. Thus, it is important to not assume the value in the 429 response for the reset time is the same as the one in the bucket headers. hikari's hikari.api.rest.RESTClient implementation specifically uses the value furthest in the future when working out which bucket to adhere to.

It is worth remembering that there is an API limit to the number of 401s, 403s, and 429s you receive, which is around 10,000 per 15 minutes. Passing this limit results in a soft ban of your account.

The true nature of these limits are not known and Discord staff have repeatedly pointed to them never being documented for the sake of system integrity. These special ratelimits are not something a normal user should encounter unless they are calling a single route multiple times with the end goal of editing a single attribute in quick succession. It is up to Discord's discretion on what is considered as "spammy" behaviour and one they would not like to allow on their API.

These ratelimits should not be "properly" handled and instead be avoided completely by the end developer (similar to Cloudflare 429s).

UNKNOWN_HASH module-attribute

UNKNOWN_HASH: Final[str] = 'UNKNOWN'

The hash used for an unknown bucket that has not yet been resolved.

RESTBucket

RESTBucket(name: str, compiled_route: CompiledRoute, global_ratelimit: ManualRateLimiter, max_rate_limit: float)

Bases: WindowedBurstRateLimiter

Represents a rate limit for an HTTP endpoint.

Component to represent an active rate limit bucket on a specific HTTP route with a specific major parameter combo.

This is somewhat similar to the hikari.impl.rate_limits.WindowedBurstRateLimiter in how it works.

This algorithm will use fixed-period time windows that have a given limit (capacity). Each time a task requests processing time, it will drip another unit into the bucket. Once the bucket has reached its limit, nothing can drip and new tasks will be queued until the time window finishes.

Once the time window finishes, the bucket will empty, returning the current capacity to zero, and tasks that are queued will start being able to drip again.

Additional logic is provided by the hikari.impl.buckets.RESTBucket.update_rate_limit call which allows dynamically changing the enforced rate limits at any time.

is_unknown property

is_unknown: bool

Whether it represents an UNKNOWN bucket.

acquire async

acquire() -> None

Acquire time and the lock on this bucket.

Note

You should afterwards invoke hikari.impl.buckets.RESTBucket.update_rate_limit to update any rate limit information you are made aware of and hikari.impl.buckets.RESTBucket.release to release the lock.

RAISES	DESCRIPTION
RateLimitTooLongError	If the rate limit is longer than max_rate_limit.

release

release() -> None

Release the lock on the bucket.

resolve

resolve(real_bucket_hash: str) -> None

Resolve an unknown bucket.

PARAMETER	DESCRIPTION
real_bucket_hash	The real bucket hash for this bucket. TYPE: str

RAISES	DESCRIPTION
RuntimeError	If the hash of the bucket is already known.

update_rate_limit

update_rate_limit(remaining: int, limit: int, reset_at: float) -> None

Update the rate limit information.

Note

The reset_at epoch is expected to be a time.monotonic monotonic epoch, rather than a time.time date-based epoch.

PARAMETER	DESCRIPTION
remaining	The calls remaining in this time window. TYPE: int
limit	The total calls allowed in this time window. TYPE: int
reset_at	The epoch at which to reset the limit. TYPE: float

RESTBucketManager

RESTBucketManager(max_rate_limit: float)

The main rate limiter implementation for HTTP clients.

This is designed to provide bucketed rate limiting for Discord HTTP endpoints that respects the X-RateLimit-Bucket rate limit header. To do this, it makes the assumption that any limit can change at any time.

PARAMETER	DESCRIPTION
max_rate_limit	The max number of seconds to backoff for when rate limited. Anything greater than this will instead raise an error. TYPE: float

is_alive property

is_alive: bool

Whether the component is alive.

acquire_bucket

acquire_bucket(compiled_route: CompiledRoute, authentication: Optional[str]) -> AsyncContextManager[None]

Acquire a bucket for the given route.

Note

You MUST keep the context manager acquired during the full duration of the request: from making the request until calling hikari.impl.buckets.RESTBucket.update_rate_limit.

PARAMETER	DESCRIPTION
compiled_route	The route to get the bucket for. TYPE: CompiledRoute
authentication	The authentication that will be used in the request. TYPE: Optional[str]

RETURNS	DESCRIPTION
AsyncContextManager	The context manager to use during the duration of the request.

close async

close() -> None

Close the garbage collector and kill any tasks waiting on ratelimits.

start

start(poll_period: float = 20.0, expire_after: float = 10.0) -> None

Start this ratelimiter up.

This spins up internal garbage collection logic in the background to keep memory usage to an optimal level as old routes and bucket hashes get discarded and replaced.

PARAMETER	DESCRIPTION
poll_period	Period to poll the garbage collector at in seconds. TYPE: float DEFAULT: 20.0
expire_after	Time after which the last hikari.impl.buckets.RESTBucket.reset_at was hit for a bucket to remove it. Higher values will retain unneeded ratelimit info for longer, but may produce more effective rate-limiting logic as a result. Using 0 will make the bucket get garbage collected as soon as the rate limit has reset. TYPE: float DEFAULT: 10.0

throttle

throttle(retry_after: float) -> None

Throttle the global ratelimit for the buckets.

PARAMETER	DESCRIPTION
retry_after	How long to throttle for. TYPE: float

update_rate_limits

update_rate_limits(compiled_route: CompiledRoute, authentication: Optional[str], bucket_header: str, remaining_header: int, limit_header: int, reset_after: float) -> None

Update the rate limits for a bucket using info from a response.

PARAMETER	DESCRIPTION
compiled_route	The compiled route to get the bucket for. TYPE: CompiledRoute
authentication	The authentication that was used in the request. TYPE: Optional[str]
bucket_header	The X-RateLimit-Bucket header that was provided in the response. TYPE: str
remaining_header	The X-RateLimit-Remaining header cast to an int. TYPE: int
limit_header	The X-RateLimit-Limit header cast to an int. TYPE: int
reset_after	The X-RateLimit-Reset-After header cast to a float. TYPE: float