Avoiding "length" errors in Apache Bench with the -l option

I was using the Apache Bench ab command to exercise some new code I'm writing in Datasette and I noticed I was getting a lot of errors:

ab -n 1000 -c 10 \

Truncated output:

Benchmarking (be patient)
Completed 100 requests
Completed 200 requests
Complete requests:      1000
Failed requests:        953
   (Connect: 0, Receive: 0, Length: 953, Exceptions: 0)

What's with the 953 failed requests? My server logged a 200 status code for each of them, so it seemed to think they were being returned without errors.

Thanks to Stack Overflow I learned that by default Apache Bench considers a request a "length" error if it does not match exactly the length of the first recieved page.

This particular Datasette response includes millisecond timing information in the JSON (thanks to the ?_trace=1 option), and since those timings differ the overall length differs between requests.

The solution is to use the -l (lower case L) option, as documented in ab --help like this:

Accept variable document length (use this for dynamic pages)

So the following command runs without showing those errors:

ab -l -n 1000 -c 10 \

Truncated output:

Concurrency Level:      10
Time taken for tests:   1.407 seconds
Complete requests:      1000
Failed requests:        0
Total transferred:      2962382 bytes
HTML transferred:       2823382 bytes
Requests per second:    710.53 [#/sec] (mean)
Time per request:       14.074 [ms] (mean)
Time per request:       1.407 [ms] (mean, across all concurrent requests)
Transfer rate:          2055.52 [Kbytes/sec] received


Created 2023-02-08T11:26:45-08:00 · Edit