The `Limiter` API has been adjusted significantly to both improve both
safety and ergonomics and two `Limit` types have been corrected to match
the legacy implementations.
**Safety**
Previously, the key used for looking up limit overrides and for fetching
individual buckets from the key-value store was constructed within the
WFE. This posed a risk: if the key was malformed, the default limit
would still be enforced, but individual overrides would fail to function
properly. This has been addressed by the introduction of a new
`BucketId` type along with a `BucketId` constructor for each `Limit`
type. Each constructor is responsible for producing a well-formed bucket
key which undergoes the very same validation as any potentially matching
override key.
**Ergonomics**
Previously, each of the `Limiter` methods took a `Limit` name, a bucket
identifier, and a cost to be spent/ refunded. To simplify this, each
method now accepts a new `Transaction` type which provides a cost, and
wraps a `BucketId` identifying the specific bucket.
The two changes above, when taken together, make the implementation of
batched rate limit transactions considerably easier, as a batch method
can accept a slice of `Transaction`.
**Limit Corrections**
PR #6947 added all of the existing rate limits which could be made
compatible with the key-value approach. Two of these were improperly
implemented;
- `CertificatesPerDomain` and `CertificatesPerFQDNSet`, were implemented
as
- `CertificatesPerDomainPerAccount` and
`CertificatesPerFQDNSetPerAccount`.
Since we do not actually associate these limits with a particular ACME
account, the `regID` portion of each of their bucket keys has been
removed.
This design seeks to reduce read-pressure on our DB by moving rate limit
tabulation to a key-value datastore. This PR provides the following:
- (README.md) a short guide to the schemas, formats, and concepts
introduced in this PR
- (source.go) an interface for storing, retrieving, and resetting a
subscriber bucket
- (name.go) an enumeration of all defined rate limits
- (limit.go) a schema for defining default limits and per-subscriber
overrides
- (limiter.go) a high-level API for interacting with key-value rate
limits
- (gcra.go) an implementation of the Generic Cell Rate Algorithm, a
leaky bucket-style scheduling algorithm, used to calculate the present
or future capacity of a subscriber bucket using spend and refund
operations
Note: the included source implementation is test-only and currently
accomplished using a simple in-memory map protected by a mutex,
implementations using Redis and potentially other data stores will
follow.
Part of #5545
Samantha