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Light client friendly events #2491

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May 13, 2019
Merged

Light client friendly events #2491

merged 20 commits into from
May 13, 2019

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pepyakin
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@pepyakin pepyakin commented May 6, 2019

This PR proposes a solution for light client friendly SRML events.

Each deposited event can specify zero or more topics. A topic is a fixed sized value (atm, is just a hash). When an event is deposited with some topic, the runtime adds the index of this event in the <Events<T>> list to a mapping that maps from topics to lists of indexes.

Light clients can "subscribe" to changes of specific storage values (see #628). The idea is that light clients would subscribe to changes of the storage values for a topic they are interested in. When the light client detects a change, it fetches the contents of the indexes list for the specific topic and with this the light client can query events' data by the indexes.

Marking this as draft, since I don't know if this is the way that we want to go, I didn't test this with light-client nor inspected the storage values and there are some questions left

@pepyakin pepyakin added the A3-in_progress Pull request is in progress. No review needed at this stage. label May 6, 2019
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Be good to get an opinion on this @svyatonik

@svyatonik svyatonik self-assigned this May 8, 2019
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The idea seems OK to me. I'll submit some issues soon to not-to-forget what needs to be changed with changes tries. We need (at least) to extend changes tries API (and network messages) to allow fetching changes for several keys (here: topics) at once.

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pepyakin commented May 8, 2019

I've checked the storage entries and add a test. This should be ready for review then.

@pepyakin pepyakin marked this pull request as ready for review May 8, 2019 14:53
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Merge 'origin/master' into ser-topic-events
7d97836 
# Conflicts:
#	node/runtime/src/lib.rs
@pepyakin pepyakin added A0-please_review Pull request needs code review. and removed A3-in_progress Pull request is in progress. No review needed at this stage. labels May 8, 2019
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@pepyakin pepyakin added A3-in_progress Pull request is in progress. No review needed at this stage. and removed A0-please_review Pull request needs code review. labels May 8, 2019
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pepyakin commented May 8, 2019

There might be a problem with this approach, marking it inprogress until we figure it out.

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what might the problem be?

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pepyakin commented May 9, 2019 •
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Yes, there is indeed a problem, although not a significant one.

Consider the case when an event E was deposited with a topic A.

So at the end of the block N the field <EventTopics<T>> will be like:

A => vec![EventIndex(0)] // 0 is the index of `A` in `<Events<T>>`

then, let's suppose that the same event E with the same topic A was deposited at the block N+1. Therefore, <EventTopics<T>> will look the same at the end of the N+1 block:

A => vec![EventIndex(0)]

Because the value at the storage location of A doesn't change there will be no notification of change between for the N+1.

I am still not entirely sure, but it seems to be a problem. If the light-client doesn't follow each block, and say, it "subscribes" to changes at the block N+1 it won't get the update.

To circumvent this problem we could store Vec<(EventIndex, BlockNumber)> in EventTopics instead of just Vec<EventIndex>. Or maybe even something like Option<(BlockNumber, Vec<EventIndex>)>.

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pepyakin commented May 9, 2019

Ok, I put up a commit which fixes this. However, I went with Vec<(EventIndex, BlockNumber)> instead of Option<(BlockNumber, Vec<EventIndex>)>. The reason for that is ::append is not available for this type. See the commit for details:
5f9ca06

Apparently it starts asking for a hand-rolled structure tailored for this particular use-case.

@pepyakin
Fix srml-system test.
24b944f
@pepyakin
Merge 'origin/master' into ser-topic-events
f57e32b 
# Conflicts:
#	core/test-runtime/wasm/Cargo.lock
#	node-template/runtime/wasm/Cargo.lock
#	node/executor/src/lib.rs
#	node/runtime/src/lib.rs
#	node/runtime/wasm/Cargo.lock
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Post merge fixes.
40b296e
@pepyakin pepyakin added A0-please_review Pull request needs code review. and removed A3-in_progress Pull request is in progress. No review needed at this stage. labels May 10, 2019
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ok, but please add a TODO and issue explaining that this is highly inefficient and needs to be altered to Option<(BlockNumber, Vec<EventIndex>)> once we can implement ::append for it. please also assign it 1.x milestone

@pepyakin pepyakin modified the milestone: 1.x series May 13, 2019
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@svyatonik svyatonik added A8-looksgood and removed A0-please_review Pull request needs code review. labels May 13, 2019
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@gavofyork gavofyork merged commit 318d2e0 into master May 13, 2019
@gavofyork gavofyork deleted the ser-topic-events branch May 13, 2019 18:56
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xlc commented May 13, 2019

This seems like a breaking change and requires JS SDK side change as well.
@jacogr I don't think it is easy to have a JS SDK that compatible with Substrate versions that with and without this change.

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@svyatonik svyatonik mentioned this pull request Jun 14, 2019
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MTDK1 pushed a commit to bdevux/substrate that referenced this pull request Jul 10, 2019
@pepyakin @MTDK1
Light client friendly events (paritytech#2491)
df4fba5 
* Sketch of indexed events.

* Get EventIndex by holding another variable.

* Add some docs.

* Use DoubleMap to store reverse topic index

* Implement StorageDoubleMap::append

* Use append for EventTopics.

* Refactor.

* Avoid `mutate`

* Docs.

* Add topics to EventRecord

* Update tests.

* Rebuild.

* Bump version.

* Event topics test.

* Mix in BlockNumber to distinguish updates

* Fix srml-system test.

* Post merge fixes.

* Comments/TODO.
@shawntabrizi shawntabrizi mentioned this pull request Jan 8, 2020
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@dvdplm dvdplm mentioned this pull request Feb 12, 2022
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