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adding note about propagation from apache reverse proxy #44

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adding note about propagation from apache reverse proxy
jorgheymans Oct 24, 2020
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jorgheymans Oct 24, 2020
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Update README.md
jorgheymans Oct 26, 2020
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Expand Up @@ -347,3 +347,17 @@ recent and largely contained to Envoy, which already supports B3 single format.
Sending a larger second header would increase the pain to others who don't have
this problem anyway. Put another way, the few proxies missing B3 single header
support should update instead of burdening more people from lack thereof.

# Showcase: propagation from an Apache reverse proxy

Zipkin users were <a href="https://github.com/openzipkin/zipkin/issues/1479">wondering</a>
if it is possible to start B3 propagation from an Apache reverse proxy. @adammichalik
came up with a <a href="https://github.com/openzipkin/zipkin/issues/1479#issuecomment-682639979">clever</a>
way to accomplish this, using Apache modules only:

```
RequestHeader setifempty X-B3-TraceId "expr=%{md5:%{reqenv:UNIQUE_ID}}"
RequestHeader edit X-B3-TraceId "^(.{0,16}).*$" "$1"
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@codefromthecrypt codefromthecrypt Oct 25, 2020

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this is incorrect, it should be the last 16 chars, not the first. Also X-B3-SpanId should be set to the last 16 chars.

Doing this without setting the sampling flag will result in the first hop sampling, which is probably ok as it could be tricky to put sampling logic here :D.

OTOH an example of b3: 0 for /health path would be nice as it shows how to stop things from tracing that.

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@adammichalik adammichalik Oct 25, 2020

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@adriancole Can you please explain a bit on why "this is incorrect, it should be the last 16 chars, not the first"? The way I was thinking (but I wasn't thinking too much of it) is that MD5 generates a "uniformly random" hex string of 32 chars, so it doesn't matter if we take the first, middle or last 16 chars - as a matter of fact this trick was simply a way to generate a pseudorandom 16 char hex string.

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@codefromthecrypt codefromthecrypt Oct 26, 2020

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sure it is here. https://github.com/openzipkin/b3-propagation/blob/master/STATUS.md and also explained here https://github.com/openzipkin/zipkin/blob/master/zipkin/src/main/java/zipkin2/storage/StorageComponent.java#L106-L144

Seems yeah this should be in the top-level readme :D Offers welcome to fix that!

The main thing is that even if at your site, you feel truncating to 64bit is a constant, we shouldn't hint here picking the left hand side as that will conflict with other practice. I think many would use this with 128-bit IDs as almost all libraries at least truncate back on unsupported for the last several years. It isn't necessary to do that here, but it is interesting to show the option. When showing the option, we should be specific on how.

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@adammichalik adammichalik Oct 26, 2020

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Fair points, thanks for the explanation.

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@codefromthecrypt codefromthecrypt Oct 26, 2020

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no prob!

```

>%{reqenv:UNIQUE_ID} is a unique string generated by mod_unique_id. Then, it gets MD5-hashed and assigned to the X-B3-TraceId header. This gives a 32-character hex string = 128 bit traceId. Since my downstream libraries require the traceIds to be 64-bit, the RequestHeader edit extracts the first 16 characters (64 bit) from the value and reassigns it back to the X-B3-TraceId header which is sent downstream.