Use MaxAssigned for ReadOnly TS (#3071)

* worker/mutation.go: use max assigned value from delta in readonly ts

We store the max assigned value when processing deltas, we can use
that to save an extra network call to zero when getting read-only ts.

* worker/draft.go: fixed broken error

* worker/mutation.go: ts off by one

The real readTs is MaxTxnTs + 1 not nextTxnTs - 1.

* edgraph/server.go: add support for best effort request query

* worker/mutation.go: read only ts was correct before as maxTxnTs.

* worker/mutation.go: expand Timestamps() to support best effort flag

When the best effort flag is enabled, worker.Timestamps() will try to get the ts
from memory.

* edgraph/server.go: add best effort to ts request and enable if needed.

Add support to the best effort request flag and update the worker.Timestamps() call to
use it.

* update for change in worker.Timestamps() func signature, using safe defaults.

* edgraph/server.go: revert change to getTimestamp(), use best effort from doQuery()

* revert change to worker.Timestamps()

* edgraph/server.go: simplify logic a bit

* edgraph/server.go: make it simpler

Author: srfrog

edgraph/server.go

@@ -36,6 +36,7 @@ import (
 	"github.com/dgraph-io/dgraph/chunker/rdf"
 	"github.com/dgraph-io/dgraph/conn"
 	"github.com/dgraph-io/dgraph/gql"
+	"github.com/dgraph-io/dgraph/posting"
 	"github.com/dgraph-io/dgraph/protos/pb"
 	"github.com/dgraph-io/dgraph/query"
 	"github.com/dgraph-io/dgraph/schema"
@@ -569,6 +570,16 @@ func (s *Server) doQuery(ctx context.Context, req *api.Request) (resp *api.Respo
 		return resp, err
 	}
 
+	// Here we try our best effort to not contact Zero for a timestamp. If we succeed,
+	// then we use the max known transaction ts value (from ProcessDelta) for a read-only query.
+	// If we haven't processed any updates yet then fall back to getting TS from Zero.
+	if req.BestEffort {
+		// Sanity: check that request is read-only too.
+		if !req.ReadOnly {
+			return resp, x.Errorf("A best effort query must be read-only.")
+		}
+		req.StartTs = posting.Oracle().MaxAssigned()
+	}
 	if req.StartTs == 0 {
 		req.StartTs = State.getTimestamp(req.ReadOnly)
 	}

worker/draft.go

@@ -520,8 +520,7 @@ func (n *node) commitOrAbort(pkey string, delta *pb.OracleDelta) error {
 		n.lastCommitTs = status.CommitTs
 	}
 	if err := writer.Flush(); err != nil {
-		x.Errorf("Error while flushing to disk: %v", err)
-		return err
+		return x.Errorf("Error while flushing to disk: %v", err)
 	}
 
 	g := groups()