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 hikari.internal.routes.CompiledRoute
as a definition
of a route with specific major parameter values included (e.g.
POST /channels/123/messages
), and a hikari.internal.routes.Route
as a
definition of a route without specific parameter values included (e.g.
POST /channels/{channel}/messages
). We can compile a
hikari.internal.routes.CompiledRoute
from a hikari.internal.routes.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 hikari.internal.routes.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 str
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
hikari.internal.routes.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 BaseRateLimiter.acquire()
a request timeslice for a given
hikari.internal.routes.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
hikari.internal.routes.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 hikari.internal.routes.CompiledRoute
to provide the real bucket
hash we need to get the route’s bucket object internally.
The BaseRateLimiter.acquire()
method will take the bucket and acquire a new
timeslice on it. This takes the form of a asyncio.Future
which should be
awaited by the caller 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 RESTBucketManager.acquire()
is a tuple of a asyncio.Future
to
await on which completes when you are allowed to proceed with making a request,
and a real bucket hash which should be stored temporarily. This will be
explained in the next section.
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 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 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 close()
the
RESTBucketManager
that has been used. 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.
At the time of writing, the only example of this appears to be on the
PATCH /channels/{channel_id}
endpoint. This has a limit of two changes per
10 minutes. More details about how this is implemented have yet to be
released or documented…
Module Contents#
- class hikari.impl.buckets.RESTBucket(name, compiled_route, global_ratelimit, max_rate_limit)[source]#
Bases:
hikari.impl.rate_limits.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
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
RESTBucket.update_rate_limit
call which allows dynamically changing the enforced rate limits at any time.- async acquire()[source]#
Acquire time and the lock on this bucket.
Note
You should afterwards invoke
RESTBucket.update_rate_limit
to update any rate limit information you are made aware of andRESTBucket.release
to release the lock.- Raises:
hikari.errors.RateLimitTooLongError
If the rate limit is longer than
max_rate_limit
.
- resolve(real_bucket_hash)[source]#
Resolve an unknown bucket.
- Parameters:
- real_bucket_hash
str
The real bucket hash for this bucket.
- real_bucket_hash
- Raises:
RuntimeError
If the hash of the bucket is already known.
- update_rate_limit(remaining, limit, reset_at)[source]#
Update the rate limit information.
Note
The
reset_at
epoch is expected to be atime.monotonic
monotonic epoch, rather than atime.time
date-based epoch.
- class hikari.impl.buckets.RESTBucketManager(max_rate_limit)[source]#
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.- Parameters:
- max_rate_limit
float
The max number of seconds to backoff for when rate limited. Anything greater than this will instead raise an error.
- max_rate_limit
- acquire_bucket(compiled_route, authentication)[source]#
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
update_rate_limits
.- Parameters:
- compiled_route
hikari.internal.routes.CompiledRoute
The route to get the bucket for.
- authentication
typing.Optional
[str
] The authentication that will be used in the request.
- compiled_route
- Returns:
typing.AsyncContextManager
The context manager to use during the duration of the request.
- start(poll_period=20.0, expire_after=10.0)[source]#
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.
- Parameters:
- poll_period
float
Period to poll the garbage collector at in seconds. Defaults to
20
seconds.- expire_after
float
Time after which the last
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. Using0
will make the bucket get garbage collected as soon as the rate limit has reset. Defaults to10
seconds.
- poll_period
- throttle(retry_after)[source]#
Throttle the global ratelimit for the buckets.
- Parameters:
- retry_after
float
How long to throttle for.
- retry_after
- update_rate_limits(compiled_route, authentication, bucket_header, remaining_header, limit_header, reset_after)[source]#
Update the rate limits for a bucket using info from a response.
- Parameters:
- compiled_route
hikari.internal.routes.CompiledRoute
The compiled route to get the bucket for.
- authentication
typing.Optional
[str
] The authentication that was used in the request.
- bucket_header
str
The
X-RateLimit-Bucket
header that was provided in the response.- remaining_header
int
The
X-RateLimit-Remaining
header cast to anint
.- limit_header
int
The
X-RateLimit-Limit
header cast to anint
.- reset_after
float
The
X-RateLimit-Reset-After
header cast to afloat
.
- compiled_route