feat: Add retries and gzip compression

[rafaeelaudibert]

This will hopefully bring us up to standard, meaning we pass all the compliance tests. It's also cool since it saves us some bytess (at the cost of CPU of course)

[github-actions]

posthog-elixir Compliance Report

Date: 2026-02-05 13:42:36 UTC
Duration: 127981ms

⚠️ Some Tests Failed

27/29 tests passed, 2 failed

---

Capture Tests

⚠️ 27/29 tests passed, 2 failed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	258ms
Format Validation.Event Has Uuid	✅	259ms
Format Validation.Event Has Lib Properties	✅	258ms
Format Validation.Distinct Id Is String	✅	259ms
Format Validation.Token Is Present	✅	259ms
Format Validation.Custom Properties Preserved	✅	258ms
Format Validation.Event Has Timestamp	✅	259ms
Retry Behavior.Retries On 503	✅	8070ms
Retry Behavior.Does Not Retry On 400	✅	2262ms
Retry Behavior.Does Not Retry On 401	✅	2261ms
Retry Behavior.Respects Retry After Header	❌	5264ms
Retry Behavior.Implements Backoff	✅	22109ms
Retry Behavior.Retries On 500	✅	6080ms
Retry Behavior.Retries On 502	✅	6081ms
Retry Behavior.Retries On 504	✅	6081ms
Retry Behavior.Max Retries Respected	✅	30108ms
Deduplication.Generates Unique Uuids	✅	269ms
Deduplication.Preserves Uuid On Retry	✅	6081ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	13075ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	6185ms
Deduplication.No Duplicate Events In Batch	✅	264ms
Deduplication.Different Events Have Different Uuids	✅	261ms
Compression.Sends Gzip When Enabled	❌	259ms
Batch Format.Uses Proper Batch Structure	✅	259ms
Batch Format.Flush With No Events Sends Nothing	✅	256ms
Batch Format.Multiple Events Batched Together	✅	264ms
Error Handling.Does Not Retry On 403	✅	2262ms
Error Handling.Does Not Retry On 413	✅	2261ms
Error Handling.Retries On 408	✅	6080ms

Failures

retry_behavior.respects_retry_after_header

Expected at least 2 requests, got 1

compression.sends_gzip_when_enabled

Header 'Content-Encoding' with value 'gzip' not found in requests

[rafaeelaudibert]

Apparently not working 100% yet, will work on it!

[martosaur]

Hmm, why make it configurable?

In general, the idea behind api_client_module option was to provide an avenue to override the whole HTTP client and therefore any option. Otherwise the top-level configuration is doomed to eventually include most options that usually come with HTTP: retries, timeouts, logging, telemetry, middleware, proxies.

If the only reason to configure gzip is to let cpu-sensitive folks disable it, I imagine they would be motivated enough to just define their own client!

[rafaeelaudibert]

100% agree. This is me trying to get to 100% on this arbitrary set of "assumptions" we made and that all clients should fulfill. This was my first dumb way to implement it, but I think I'm tending towards just implementing a client (on the compliance adapter folder) that demonstrates that this can be supported by the implementation (by implementing your own client like one would in Elixir) rather than enabling it as a config.

I won't be merging this soon, I'll iterate on it slowly.

Question for you: how do you feel with gzip being turned on by default in something like Elixir?

[martosaur]

Cool, let me know if you want me to take a stab at any of this!

Answering your question, I don't know much about gzip to be honest. Are there any trade offs? Looking at Req code, it calls built-in :zlib module, which is probably a C NIF under the hood.

[rafaeelaudibert]

I would never reject a PR from you, but I can also work on this eventually, don't worry.

For gzip, there's always the fear that some people already using the SDK could be running very close to their max CPU and turning gzip on could bring them to 100% which isn't really ideal - gzip is notoriously for using CPU. We do need to turn it on to make it compliant and improve our ingestion pipeline, so I probably will do it anyway even if in the end I reach the conclusion that it needs a v3.

[martosaur]

Req itself comes with retries plugin and I think it's even enabled by default, although maybe not for POST requests. We should be able to just configure Req and change tracked client to be nice and not cut it off.

[rafaeelaudibert]

Lovely, I'll do more research, thank you 😄

[rafaeelaudibert]

#79 solves most of this, so let's close this in favor of that one, and then close the remaining bit - automatic gzip compression - in a separate PR with a different approach (as mentioned above)