Question about ephemera usage

[tgrecojs]

Throughout the code for IST Vault's, I've noticed the use of provideEphemera throughout various files. If I understand correctly, we can use provideEphemera to set new key-value pairs within a Map. Once set, it can be used to look up values associated with a given.

While I believe to have a decent grasp on the use of ephemeral objects, I'm confused about the use of Object.assign shown below.

agoric-sdk/packages/inter-protocol/src/vaultFactory/vaultHolder.js

Lines 45 to 58 in c680b92

	const initState = (vault, storageNode, marshaller) => {
	/** @type {StoredPublishKit<VaultNotification>} */
	const { subscriber, publisher } = makeStoredPublishKit(
	storageNode,
	marshaller,
	);
	holderId += 1n;
	const ephemera = provideEphemera(holderId);
	// UNTIL https://github.com/Agoric/agoric-sdk/issues/4567
	Object.assign(ephemera, { subscriber, publisher });
	return { holderId, vault };
	};

I can't find any references to the ephemera identifier defined on line 53, so I am opening up this issue to get some context into its usage and why it is necessary to achieve the desired functionality.

[turadg]

provideEphemera is the function returned by makeEphemeraProvider which holds the "ephemeras" in a closure

agoric-sdk/packages/inter-protocol/src/contractSupport.js

Line 150 in d7652cb

const ephemeras = new Map();

Each ephemera object holds references that can be lost and the object is keyed by whatever was passed to provideEphemera. In this case it's holderId. (Though in master at the moment it's subscriber which is 1:1 with the holder object).

The object on line 53 isn't strictly necessary. Those lines could be,

Object.assign(provideEphemera(holderId), { subscriber, publisher }); 

The values written into the ephemera object are read without binding an ephemera const. E.g.

agoric-sdk/packages/inter-protocol/src/vaultFactory/vaultHolder.js

Line 78 in c680b92

getSubscriber: ({ state }) => provideEphemera(state.holderId).subscriber,

[tgrecojs]

Ok, thanks!

Seeing this pattern you linked to [here](at the moment it's subscriber helped clear that up for me. 🙂

[tgrecojs]

I'm noticing that in vaultDirector the ephemera object is assigned values and then is accessed directly throughout the file.

This is an instance where I do see the ephemera object being used without any use of provideEphemera so I'm wondering if there is a specific reason as to why this is the case? Thanks again for the help! @turadg

[turadg]

The reason vaultDirector can avoid the provider is it's a singleton, so there's no need to distinguish instances. It can keep the value in module scope because that module is guaranteed to load once per vat. It's still a bit of a hack and we're cleaning it up before Vaults Release.

[tgrecojs]

Awesome. Thanks for the info.