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Feature/graphql download queue subscription send only updates (#1011)

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* Emit only download changes instead of full status

The download subscription emitted the full download status, which, depending on how big the queue was, took forever because the graphql subscription does not support data loader batching, causing it to run into the n+1 problem

* Rename "DownloadManager#status" to "DownloadManager#updates"

* Add initial queue to download subscription type

Adds the current queue at the time of sending the initial message.
This field is null for all following messages after the initial one

* Optionally limit and omit download updates

To prevent the n+1 dataloader issue, the max number of updates included in the download subscription can be limited.
This way, the problem will be circumvented and instead, the latest download status should be (re-)fetched via the download status query, which does not run into this problem.

* Formatting
This commit is contained in:
schroda
2024-11-15 00:07:14 +01:00
committed by GitHub
parent f5680c6d69
commit 168b76cb0c
9 changed files with 207 additions and 30 deletions
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40
server/src/main/kotlin/suwayomi/tachidesk/graphql/subscriptions/DownloadSubscription.kt
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@@ -7,14 +7,54 @@
package suwayomi.tachidesk.graphql.subscriptions
import com.expediagroup.graphql.generator.annotations.GraphQLDeprecated
import com.expediagroup.graphql.generator.annotations.GraphQLDescription
import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.map
import suwayomi.tachidesk.graphql.types.DownloadStatus
import suwayomi.tachidesk.graphql.types.DownloadUpdates
import suwayomi.tachidesk.manga.impl.download.DownloadManager
class DownloadSubscription {
@GraphQLDeprecated("Replaced width downloadStatusChanged", ReplaceWith("downloadStatusChanged(input)"))
fun downloadChanged(): Flow<DownloadStatus> =
DownloadManager.status.map { downloadStatus ->
DownloadStatus(downloadStatus)
}
data class DownloadChangedInput(
@GraphQLDescription(
"Sets a max number of updates that can be contained in a download update message." +
"Everything above this limit will be omitted and the \"downloadStatus\" should be re-fetched via the " +
"corresponding query. Due to the graphql subscription execution strategy not supporting batching for data loaders, " +
"the data loaders run into the n+1 problem, which can cause the server to get unresponsive until the status " +
"update has been handled. This is an issue e.g. when mass en- or dequeuing downloads.",
)
val maxUpdates: Int?,
)
fun downloadStatusChanged(input: DownloadChangedInput): Flow<DownloadUpdates> {
val omitUpdates = input.maxUpdates != null
val maxUpdates = input.maxUpdates ?: 50
return DownloadManager.updates.map { downloadUpdates ->
val omittedUpdates = omitUpdates && downloadUpdates.updates.size > maxUpdates
// the graphql subscription execution strategy does not support data loader batching which causes the n+1 problem,
// thus, too many updates (e.g. on mass enqueue or dequeue) causes unresponsiveness of the server until the
// update has been handled
val actualDownloadUpdates =
if (omittedUpdates) {
suwayomi.tachidesk.manga.impl.download.model.DownloadUpdates(
downloadUpdates.status,
downloadUpdates.updates.subList(0, maxUpdates),
downloadUpdates.initial,
)
} else {
downloadUpdates
}
DownloadUpdates(actualDownloadUpdates, omittedUpdates)
}
}
}