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:

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ab -n 1000 -c 10 \
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  'http://127.0.0.1:8002/fixtures/compound_primary_key?_extra=count&_size=3&_extra=columns&_extra=primary_keys&_trace=1&_extra=debug'

Truncated output:

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Benchmarking 127.0.0.1 (be patient)
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Completed 100 requests
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Completed 200 requests
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...
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Complete requests:      1000
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Failed requests:        953
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   (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:

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ab -l -n 1000 -c 10 \
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  'http://127.0.0.1:8002/fixtures/compound_primary_key?_extra=count&_size=3&_extra=columns&_extra=primary_keys&_trace=1&_extra=debug'

Truncated output:

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Concurrency Level:      10
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Time taken for tests:   1.407 seconds
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Complete requests:      1000
4
Failed requests:        0
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Total transferred:      2962382 bytes
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HTML transferred:       2823382 bytes
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Requests per second:    710.53 [#/sec] (mean)
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Time per request:       14.074 [ms] (mean)
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Time per request:       1.407 [ms] (mean, across all concurrent requests)
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Transfer rate:          2055.52 [Kbytes/sec] received