*: optimize DDL history http API, reduce memory (#36859)

close pingcap/tidb#35838

ddl/ddl.go

@@ -1685,6 +1685,26 @@ func GetAllHistoryDDLJobs(m *meta.Meta) ([]*model.Job, error) {
 	return allJobs, nil
 }
 
+// ScanHistoryDDLJobs get some of the done DDL jobs.
+// When the DDL history is quite large, GetAllHistoryDDLJobs() API can't work well, because it makes the server OOM.
+// The result is in descending order by job ID.
+func ScanHistoryDDLJobs(m *meta.Meta, startJobID int64, limit int) ([]*model.Job, error) {
+	var iter meta.LastJobIterator
+	var err error
+	if startJobID == 0 {
+		iter, err = m.GetLastHistoryDDLJobsIterator()
+	} else {
+		if limit == 0 {
+			return nil, errors.New("when 'start_job_id' is specified, it must work with a 'limit'")
+		}
+		iter, err = m.GetHistoryDDLJobsIterator(startJobID)
+	}
+	if err != nil {
+		return nil, errors.Trace(err)
+	}
+	return iter.GetLastJobs(limit, nil)
+}
+
 // GetHistoryJobByID return history DDL job by ID.
 func GetHistoryJobByID(sess sessionctx.Context, id int64) (*model.Job, error) {
 	err := sessiontxn.NewTxn(context.Background(), sess)

docs/tidb_http_api.md

@@ -463,6 +463,7 @@ timezone.*
     ```shell
     curl http://{TiDBIP}:10080/ddl/history
     ```
+	**Note**: When the DDL history is very very long, it may consume a lot memory and even cause OOM. Consider adding `start_job_id` and `limit`.
 
 1. Get count {number} TiDB DDL job history information.
 
@@ -470,6 +471,12 @@ timezone.*
     curl http://{TiDBIP}:10080/ddl/history?limit={number}
     ```
 
+1. Get count {number} TiDB DDL job history information, start with job {id}
+
+	```shell
+	curl http://{TIDBIP}:10080/ddl/history?start_job_id={id}&limit={number}
+	```
+
 1. Download TiDB debug info
 
     ```shell

meta/meta.go

@@ -1165,6 +1165,18 @@ func (m *Meta) GetLastHistoryDDLJobsIterator() (LastJobIterator, error) {
 	}, nil
 }
 
+// GetHistoryDDLJobsIterator gets the jobs iterator begin with startJobID.
+func (m *Meta) GetHistoryDDLJobsIterator(startJobID int64) (LastJobIterator, error) {
+	field := m.jobIDKey(startJobID)
+	iter, err := structure.NewHashReverseIterBeginWithField(m.txn, mDDLJobHistoryKey, field)
+	if err != nil {
+		return nil, err
+	}
+	return &HLastJobIterator{
+		iter: iter,
+	}, nil
+}
+
 // HLastJobIterator is the iterator for gets the latest history.
 type HLastJobIterator struct {
 	iter *structure.ReverseHashIterator

server/http_handler.go

@@ -93,6 +93,7 @@ const (
 const (
 	qTableID   = "table_id"
 	qLimit     = "limit"
+	qJobID     = "start_job_id"
 	qOperation = "op"
 	qSeconds   = "seconds"
 )
@@ -1251,50 +1252,51 @@ func (h tableHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 
 // ServeHTTP handles request of ddl jobs history.
 func (h ddlHistoryJobHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
-	if limitID := req.FormValue(qLimit); len(limitID) > 0 {
-		lid, err := strconv.Atoi(limitID)
-
+	var jobID, limitID int
+	var err error
+	if jobValue := req.FormValue(qJobID); len(jobValue) > 0 {
+		jobID, err = strconv.Atoi(jobValue)
 		if err != nil {
 			writeError(w, err)
 			return
 		}
-
-		if lid < 1 {
-			writeError(w, errors.New("ddl history limit must be greater than 1"))
+		if jobID < 1 {
+			writeError(w, errors.New("ddl history start_job_id must be greater than 0"))
 			return
 		}
-
-		jobs, err := h.getAllHistoryDDL()
-		if err != nil {
-			writeError(w, errors.New("ddl history not found"))
-			return
-		}
-
-		jobsLen := len(jobs)
-		if jobsLen > lid {
-			start := jobsLen - lid
-			jobs = jobs[start:]
-		}
-
-		writeData(w, jobs)
-		return
 	}
-	jobs, err := h.getAllHistoryDDL()
+	if limitValue := req.FormValue(qLimit); len(limitValue) > 0 {
+		limitID, err = strconv.Atoi(limitValue)
+		if err != nil {
+			writeError(w, err)
+			return
+		}
+		if limitID < 1 {
+			writeError(w, errors.New("ddl history limit must be greater than 0"))
+			return
+		}
+	}
+
+	jobs, err := h.getHistoryDDL(jobID, limitID)
 	if err != nil {
-		writeError(w, errors.New("ddl history not found"))
+		writeError(w, err)
 		return
 	}
 	writeData(w, jobs)
 }
 
-func (h ddlHistoryJobHandler) getAllHistoryDDL() ([]*model.Job, error) {
+func (h ddlHistoryJobHandler) getHistoryDDL(jobID, limit int) (jobs []*model.Job, err error) {
 	txn, err := h.Store.Begin()
 	if err != nil {
 		return nil, errors.Trace(err)
 	}
 	txnMeta := meta.NewMeta(txn)
 
-	jobs, err := ddl.GetAllHistoryDDLJobs(txnMeta)
+	if jobID == 0 && limit == 0 {
+		jobs, err = ddl.GetAllHistoryDDLJobs(txnMeta)
+	} else {
+		jobs, err = ddl.ScanHistoryDDLJobs(txnMeta, int64(jobID), limit)
+	}
 	if err != nil {
 		return nil, errors.Trace(err)
 	}

server/http_handler_test.go

@@ -977,6 +977,25 @@ func TestAllHistory(t *testing.T) {
 	require.NoError(t, err)
 	require.NoError(t, resp.Body.Close())
 	require.Equal(t, data, jobs)
+
+	// Cover the start_job_id parameter.
+	resp, err = ts.fetchStatus("/ddl/history?start_job_id=41")
+	require.NoError(t, err)
+	require.NoError(t, resp.Body.Close())
+
+	resp, err = ts.fetchStatus("/ddl/history?start_job_id=41&limit=3")
+	require.NoError(t, err)
+	decoder = json.NewDecoder(resp.Body)
+	err = decoder.Decode(&jobs)
+	require.NoError(t, err)
+
+	// The result is in descending order
+	lastID := int64(42)
+	for _, job := range jobs {
+		require.Less(t, job.ID, lastID)
+		lastID = job.ID
+	}
+	require.NoError(t, resp.Body.Close())
 }
 
 func dummyRecord() *deadlockhistory.DeadlockRecord {

structure/hash.go

@@ -288,8 +288,24 @@ func (*ReverseHashIterator) Close() {}
 
 // NewHashReverseIter creates a reverse hash iterator.
 func NewHashReverseIter(t *TxStructure, key []byte) (*ReverseHashIterator, error) {
+	return newHashReverseIter(t, key, nil)
+}
+
+// NewHashReverseIterBeginWithField creates a reverse hash iterator, begin with field.
+func NewHashReverseIterBeginWithField(t *TxStructure, key []byte, field []byte) (*ReverseHashIterator, error) {
+	return newHashReverseIter(t, key, field)
+}
+
+func newHashReverseIter(t *TxStructure, key []byte, field []byte) (*ReverseHashIterator, error) {
+	var iterStart kv.Key
 	dataPrefix := t.hashDataKeyPrefix(key)
-	it, err := t.reader.IterReverse(dataPrefix.PrefixNext())
+	if len(field) == 0 {
+		iterStart = dataPrefix.PrefixNext()
+	} else {
+		iterStart = t.encodeHashDataKey(key, field).PrefixNext()
+	}
+
+	it, err := t.reader.IterReverse(iterStart)
 	if err != nil {
 		return nil, errors.Trace(err)
 	}