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Title
s

Stylianos Gakis

05/17/2022, 1:41 PM
The 3.0 migration doc mentions: “Watchers now default to a
CacheOnly
refetchPolicy instead
CACHE_FIRST
. To keep behavior unchanged, set a
refetchPolicy
on your watchers:” But the snippet directly under it shows
subscribe
instead of
watch()
. Is this wrong in the sense that it should be
.watch()
instead, or should the title of the section be
subscriptions
? I am kinda confused at that part of the document.
m

mbonnin

05/17/2022, 6:32 PM
I have no idea what happened in my brain when I typed this... Sorry about this. Watchers are for queries, not subscriptions. It should be
.query(query).watch()
... Fix there: https://github.com/apollographql/apollo-kotlin/pull/4118/files
s

Stylianos Gakis

05/18/2022, 6:31 AM
Hey it’s a small mistake, don’t worry about it 🤗 I do have a question regarding this though then. Looking at the code, watch internally does a toFlow() and then emits the response directly. And only after that it does the copy() with the cache_only policy. Which part am I missing that makes the watch do as the docs suggest aka only fetch from the cache? Also as a side curiosity question, if we had not configured any cache (http or normalized) would a
watch
just do nothing?
m

mbonnin

05/18/2022, 6:58 AM
Looking at the code, watch internally does a toFlow() and then emits the response directly.
That's for the initial response only. If you follow the code, you'll see another lower level
watch
that watches for changed keys in the store
if we had not configured any cache (http or normalized) would a
watch
just do nothing?
It would return the
NetworkOnly
response once and that's it.
And only after that it does the copy() with the cache_only policy.
Ah, sorry I missed that part. There are 2 things:
fetchPolicy
and
refetchPolicy
The migration guide talks about how
refetchPolicy
is CacheOnly by default
I'm not super happy about this API TBH. The fetch vs refetch is a lot of cognitive load. Plus error handling is tough. I'm hoping we can simplify it at some point.
s

Stylianos Gakis

05/18/2022, 7:03 AM
Aha does that mean that before, the refetch was CACHE_FIRST aka if there was a cache miss it would try and hit the network?
👍 1
m

mbonnin

05/18/2022, 7:03 AM
Yes, exactly
Something I missed during the v3 development
s

Stylianos Gakis

05/18/2022, 7:05 AM
Wow yeah very interesting I had no idea! Yeah I missed the part where it said “refetchPolicy” so I didn’t piece the two things together. Thanks a lot for the clarification! I think I have the correct understanding to move forward with this 🙌
m

mbonnin

05/18/2022, 7:07 AM
There's another lower-level watch() in this file that is not advertized as much because it's a lot newer but has less "logic" inside and exposes what is (I believe) a simpler API
The hard part is that you still need an initial set of keys to watch (hence the initial fetch vs refetch in the initial API)
(or you can watch everything but that's a lot of events... But maybe it's okay...)
s

Stylianos Gakis

05/18/2022, 7:15 AM
Yeah I noticed that one too, but I am not sure I understood it exactly. What are you referring to when you say “initial set of keys”? And how is the
data: D?
used in that function? Just looking at the function signature I assume you pass what the first emission of watch will return, and from then on it will return all cache changes, but I am not sure. And as a side note, In all the times I had to use a watch myself, I did in fact want the behavior of fetch once, and listen after, but I imagine there are other use-cases that don’t have the same requirements. But we’re not relying super much on caching in general, so there’s that.
m

mbonnin

05/18/2022, 7:41 AM
data
is used to get the initial set of keys to watch for
If you pass null, you will watch for all changed keys
Meaning everytime something changes in the cache you'll try to run the query against it and try if it succeeds
So that's potentially a lot of querying