This note is in Hasura.GraphQL.Execute.Subscription.Poll.StreamingQuery.
Every cohort of a streaming subscription is associated with a mutable latest cursor value reference, which contains the current value of the cursor.
After a poll, the mutable cursor value gets updated if its value is a non-null one, null value means that there were no rows to get the min/max value from.
After this, the cohort map associated with the poller is also rebuilt, which will make sure that the cohort map’s key contains the latest cursor values. So, any new subscriber will get added to an existing cohort if the new subscriber’s cohort key matches with any of the existing cohorts. We need to rebuild the cohort map, because, say if we don’t rebuild the cohort map then a new subscriber may get added to a cohort which has been streaming for a while now, then the new subscriber will get the responses according to the cursor value stored in the cohort, instead of the initial value specified by the client. For example:
Client 1 started streaming a query at t1, say:
subscription {
log_stream(cursor: {initial_value: {created_at: "2020-01-01"}}, batch_size: 1000) {
id
level
detail
}
}
Client 2 starts streaming at t2 with the same query (where t2 > t1), if the cohort map is not rebuilt (to reflect cursor value in the cohort key), then client 2 and client 1 will both start getting the same responses, which is wrong because client 2 should start streaming from the initial value it provided.