src: add Realm document in the src README.md #47932

legendecas · 2023-05-09T09:39:26Z

Document the current Environment/Realm definitions.

src/README.md

legendecas · 2023-05-10T08:20:58Z

@cjihrig @VoltrexKeyva Updated. Thank you for the suggestions!

src/README.md

joyeecheung · 2023-05-09T11:22:33Z

src/README.md

-It also provides [cleanup hooks][] and maintains a list of [`BaseObject`][]
-instances.
+different built-in modules that can be shared across different `Realm`
+instances, for example a libuv timer for `setTimeout()`.


Should they actually share timers? I think the timer callbacks are tied to specific contexts too? Though I guess the event loop could be shared.

Yeah, it would actually be very helpful to spell out what properties are being shared between different Realms for the same Environment and which not. I've seen a bunch of PRs that move properties from the Environment to the individual Realms, but it's not really clear where the line is and whether we're not essentially just introducing the ability to have multiple Environments similar to #47855.

Thanks for pointing this out! I've added a list of examples that can be shared across realms with an Environment.

It is still yet to be done to share the async hook info between the realms -- it is necessary to link the async continuation between realm boundaries for AsyncLocalStorage to propagate correctly. This is essential to allow JS object access between multiple execution "environments" on the same thread.

I’m not really sure that this answers the question. How is the end state of Realm going to be different from what Environment is (or used to be before Realms were introduced)?

The difference between the end state of the Realm and the Environment would be:

An Environment is associated with an Isolate and provides the per-isolate hooks and APIs, and per-thread states, for instance, inspector agents, profilers, and APIs like RequestInterrupt() and can_call_into_js().

A Realm is associated with a Context and consists of a global object and can be extended as a principal realm or a synthetic realm. Each Environment has a single principal realm as its entry.

As realms share the Environment instance, we don't have to create inspector IO threads, or profiler connections for each realm. The async local storage needs to be propagated across async boundaries in another Realm of an Environment too.

Additionally, a Realm must be able to be repetitively created on the same Isolate and weakly referenced to properly support the ECMAScript ShadowRealm API.

As a conclusion, it is necessary to split the Realm and Environment to distinguish per-isolate states and per-context states.

An Environment is associated with an Isolate and provides the per-isolate hooks and APIs,

You’re describing IsolateData here, though.

You’re describing IsolateData here, though.

Yes, it's true that IsolateData is also associated with an Isolate. However, as documented, IsolateData acts like a string table and contains information (e.g. startup snapshot data) about the isolate. It has its distinct properties compared to an Environment.

Compared to IsolateData, Environment at the end state contains the necessary handles (bootstrapped with the principal realm) to propagate events across realm boundaries, for instance, propagating async local storages and promise rejection events from ShadowRealm.

I think the current state we have is:

There is IsolateData which is meant to hold per-isolate data

There are Realms with are meant to hold per-context data

There is Environment which has been a hotch-potch structure that people dump everything on since forever, so it contains both per-isolate data and per-context data (for the main context) in practice.

What we've been trying to do is to move per-isolate data in Environment to IsolateData and per-context data to Realm, it's happening but it's really not there yet.

@addaleax do you think your question has been answered?

src/README.md

legendecas · 2023-05-22T16:43:01Z

@joyeecheung @addaleax would you mind taking a look at this again? Thank you! <3

nodejs-github-bot · 2023-05-31T15:15:31Z

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----------------------------------- PR info ------------------------------------
Title      src: add Realm document in the src README.md (#47932)
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Branch     legendecas:shadow-realm/src-readme -> nodejs:main
Labels     c++, doc, needs-ci, realm
Commits    3
 - src: add Realm document in the src README.md
 - fixup! src: add Realm document in the src README.md
 - fixup! src: add Realm document in the src README.md
Committers 1
 - Chengzhong Wu 
PR-URL: https://github.com/nodejs/node/pull/47932
Reviewed-By: Colin Ihrig 
Reviewed-By: James M Snell 
Reviewed-By: Joyee Cheung 
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/47932
Reviewed-By: Colin Ihrig 
Reviewed-By: James M Snell 
Reviewed-By: Joyee Cheung 
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   ℹ  This PR was created on Tue, 09 May 2023 09:39:26 GMT
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   ✔  - Joyee Cheung (@joyeecheung) (TSC): https://github.com/nodejs/node/pull/47932#pullrequestreview-1446049565
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nodejs-github-bot · 2023-05-31T15:20:37Z

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nodejs-github-bot · 2023-06-01T07:11:41Z

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PR-URL: #47932 Reviewed-By: Colin Ihrig <cjihrig@gmail.com> Reviewed-By: James M Snell <jasnell@gmail.com> Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

PR-URL: nodejs#47932 Reviewed-By: Colin Ihrig <cjihrig@gmail.com> Reviewed-By: James M Snell <jasnell@gmail.com> Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

PR-URL: #47932 Reviewed-By: Colin Ihrig <cjihrig@gmail.com> Reviewed-By: James M Snell <jasnell@gmail.com> Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

PR-URL: nodejs#47932 Reviewed-By: Colin Ihrig <cjihrig@gmail.com> Reviewed-By: James M Snell <jasnell@gmail.com> Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

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src: add Realm document in the src README.md #47932

src: add Realm document in the src README.md #47932

legendecas commented May 9, 2023

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src: add Realm document in the src README.md #47932

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