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Comment by jazzdan | Hacker News Reader

(Replay engineer): we basically record all the inputs and outputs to a program. In the example of an HTTP request: when recording we'd record that a request was made, and the response. When replaying, rather than make the HTTP request, we return the response that was recorded.

If you're interested you can learn more about how Replay works here: https://medium.com/replay-io/how-replay-works-5c9c29580c58

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> Likewise, the raw pointer values don’t have an effect on the behavior of the JavaScript

There are actually some cases where raw pointer values can effect user-visible browser behavior indirectly. Notably if the pointer values affect iteration order in an associative data structure which the browser uses in some way that tickles JS differently depending on the order.

There are bunches of other bizarre edge cases you'd never think of as well that never come up (until inevitably one day they do). Another example: In old versions of Blink I've seen ligatures fail to form depending on what was in the font cache when the page was loaded.

(Replay employee) Yeah, later in the post (https://medium.com/replay-io/effective-determinism-54cc91f56...) we mention needing mitigations to ensure that hashtable iteration is deterministic in cases where it affects how the browser interacts with the JS engine. There are others needed as well to ensure that the browser behaves the same across a large range of websites, my favorite is handling sites that sniff information about the system's math libraries by e.g. calling Math.sin() on specific values and testing the results --- the values when replaying need to be bitwise identical with what happened while recording.

Ah I shouldn't have stopped reading. Then as one of the rare browser replay experts (there's gotta be at least like... 7 of us right?) I can certify your project as the real deal.

Thanks for the pointer. Will take some time to digest the content. Have you considered in making a library, so whoever wants a session replay can manually initiate a recording? So you don't have to maintain a browser fork. I am asking without fully understanding the technical detail. Please forgive my ignorance

From my reading of it it looks like it is language agnostic but only browsers right now. python and ruby support are in the works per their homepage

Thanks!

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