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CockroachDB: Optimize quota manager by using follower reads #2853

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Nov 25, 2022
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CockroachDB: Optimize quota manager by using follower reads #2853

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4 changes: 3 additions & 1 deletion quota/crdbqm/crdb_quota.go
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Original file line number Diff line number Diff line change
Expand Up @@ -30,7 +30,9 @@ const (

// TODO(jaosorior): Come up with a more optimal solution for CRDB, as this is
// linear and too costly.
countFromUnsequencedTable = "SELECT COUNT(*) FROM Unsequenced"
// Using a follower read here to reduce latency on the query. While this will
// slightly less accurate than a read from the leader, it should be good enough.
countFromUnsequencedTable = "SELECT COUNT(*) FROM Unsequenced AS OF SYSTEM TIME follower_read_timestamp()"
)

// ErrTooManyUnsequencedRows is returned when tokens are requested but Unsequenced has grown
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9 changes: 9 additions & 0 deletions quota/crdbqm/crdb_quota_test.go
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Expand Up @@ -36,6 +36,13 @@ func TestQuotaManager_GetTokens(t *testing.T) {
testdb.SkipIfNoCockroachDB(t)
ctx := context.Background()

// This allows for CockroachDB to persist the transaction for the record.
// We're mostly just interested in testing that the quota is managed at all,
// but running this in production will need to tolerate that
// quota management won't be exact.
// The 5 second time was gotten from https://www.cockroachlabs.com/docs/stable/follower-reads.html#when-to-use-exact-staleness-reads
const transactionBackoff = 5 * time.Second

db, done, err := testdb.NewTrillianDB(ctx, testdb.DriverCockroachDB)
if err != nil {
t.Fatalf("GetTestDB() returned err = %v", err)
Expand Down Expand Up @@ -103,6 +110,8 @@ func TestQuotaManager_GetTokens(t *testing.T) {
return
}

time.Sleep(transactionBackoff)

qm := &QuotaManager{DB: db, MaxUnsequencedRows: test.maxUnsequencedRows}

err = qm.GetTokens(ctx, test.numTokens, test.specs)
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