Make adjustment to cache config of with-apollo example

[glenngijsberts]

Description

This year we implemented the new Apollo config using this example. We recently moved to getServerSideProps as well, however, this was giving us some cache problems. The issue was that the cache was older than the actual data that was coming from the server side query.

Because the merge of the cache in apolloClient.js is merging the existingCache in the initialState it will overwrite the "fresh" initialState with properties that already exists. This means that if you have something in your cache from previous client render, for example user with the value null, and you go to a new page and do a new query on the server which returns a value for the user field, it will still return null because of that merge function.

Since this was weird in our opinion, we've changed this in our own environment by always overwriting the existing cache with the new initialState, so that the initialState that is coming from a fresh server side query is overwriting. We thought it was a good idea to reflect this also in this example, because we couldn't think of a reason why the existing cache should overwrite fresh queries.

I've added a reproduction that shows what this is exactly about. I hope my description makes sense, let me know what you think!

Reproduction of old scenario

Created a reproduction branch here: https://github.com/glenngijsberts/with-apollo-example. Using a different API than in the example, because of #32731.

1. checkout the example
2. yarn
3. yarn dev
4. visit http://localhost:3000/client-only
5. Hit "Update name". This will run a mutation that will update the cache automatically. Because I use a mocked API, the actual value on the API won't be updated, but this is actually the perfect scenario for the problem because in reality data can already change in the meantime when you're doing a new request.
6. Go to the SSR page
7. This will display two values: one is coming from the server side request (which is the latest data, because no cache is used in getServerSideProps) and the other is using the cache, which is outdated at that point, yet it's still returned because the old way of merging the cache was picking the existing cache over the initialState that was coming from the fresh server side query.

Documentation / Examples

• Make sure the linting passes by running yarn lint

[exipnus]

Hi,

This does not solve the main problem though. A new API request is made in getServerSideProps each time you visit the SSR page.

Am I wrong?

[glenngijsberts]

Hi,

This does not solve the main problem though. A new API request is made in getServerSideProps each time you visit the SSR page.

Am I wrong?

@exipnus What's the main problem according to you? You're correct, in the sense that getServerSideProps is always doing a request. The reason for this, as how I understand it, is because you need to initialise a new Apollo client on each server side request, which doesn't have a cache (because it was just initialised), so it will query a new result.

Anyway, this PR is fixing a different problem, so i'm not sure how exactly related your comment is.

[ijjk]

Good catch this sounds like a good change, thanks for the PR!