Applicability
As of July 21, 2024, packages downloaded from www.bioconductor.org are subject to a rate limit. Specifically, no more than 3,000 packages can be downloaded directly in any 10-minute period from a single IP address. If this limit is reached, the server will return an HTTP 429 Too Many Requests client error response status code.
Most 429 response codes will arise from systems that have a very high-speed link (cloud-to-cloud) and that (a) perform bulk downloads of the complete Bioconductor library for more than one version or (b) loop on requesting the same package or set of packages repeatedly with faulty retry count logic.
If you have encountered a 429 while downloading packages from www.bioconductor.org, then read on.
Background
As more Bioconductor users have cloud-based systems with extremely high-speed connections, it has become increasingly common for a runaway downloading process to generate a huge volume of traffic. We have seen a single IP address generate over a million HTTP requests in one day. This can happen so quickly and silently that the data consumer is hardly aware that anything is amiss. Historically, we have blocked abusing IP addresses that have come to our attention. But as the number and capabilities of our downloaders have increased, this has become a frequent occurrence.
Understanding Rate-Limiting (Status Code 429)
The historical way to handle this condition is to declare the server unavailable (HTTP status 503), which is both too severe and uninformative to the data consumer. IETF RFC 8516 proposes a solution to this problem in the form of HTTP status code 429 that signals the client that it has issued too many requests over some period of time and optionally advises the client how long it should wait before resuming requests. The implementation of status code 429 processing is becoming more common to address the issue of very high demand from anonymous data consumers. It is also a useful adjunct to the protection of sites from hostile use. Some newer HTTP clients automatically support 429 processing.
Our Implementation Details
On July 21, 2024, we activated rate limiting on www.bioconductor.org. This rule only applies to URLs that are in the form of a package download. After modeling the expected behavior of this implementation over a month of traffic from April 2024, we chose parameters that would almost never affect typical users. Our parameters are as follows:
Look-back window: 10 minutes
Rate limit: 3,000 operations
This means any specific IP address that makes 3,000 or fewer HTTP requests within a 10- minute window will not receive a 429 response. Since rate limit violations are checked asynchronously, the 429 responses might be returned when the count exceeds 3,000.
Handling 429 Responses
When a 429 status code is returned, we also include the Retry-After: 600 header. This standard header indicates the number of seconds (600 seconds in this case) the client should wait before retrying the request. To handle 429 responses effectively:
- Trap the 429 responses.
- Wait for the duration specified in the Retry-After header before retrying the operation. This approach ensures functionality with only a minor increase in duration.
Simple Workaround
To avoid hitting the rate limit, consider adding a 200-millisecond wait between operations (600 seconds / 3,000 operations = 0.2 sec per operation).
This implementation and handling strategy should ensure smooth operation while adhering to rate limits.
