# Data Migration Error Message Description

This document introduces the error system of TiDB Data Migration and describes the meaning of various error messages.

## DM error system

A new error system has been introduced in DM 1.0.0-GA, which has the following features:

• Add the error code mechanism
• Add the error fields such as class, scope or level
• Improve the error description, error call chain information and stack trace information

For the design and implementation of this error system, refer to Proposal: Improve Error System.

## Error message reference

The following is an actual error message in DM. Taking this message as a sample, this document explains each field of an error message in detail.

[code=38008:class=dm-master:scope=internal:level=high] grpc request error: rpc error: code = Unavailable desc = all SubConns are in TransientFailure, latest connection error: connection error: desc = "transport: Error while dialing dial tcp 172.17.0.2:8262: connect: connection refused"
github.com/pingcap/dm/pkg/terror.(*Error).Delegate
/root/code/gopath/src/github.com/pingcap/dm/pkg/terror/terror.go:267
github.com/pingcap/dm/dm/master/workerrpc.callRPC
/root/code/gopath/src/github.com/pingcap/dm/dm/master/workerrpc/rawgrpc.go:124
github.com/pingcap/dm/dm/master/workerrpc.(*GRPCClient).SendRequest
/root/code/gopath/src/github.com/pingcap/dm/dm/master/workerrpc/rawgrpc.go:64
github.com/pingcap/dm/dm/master.(*Server).getStatusFromWorkers.func2
/root/code/gopath/src/github.com/pingcap/dm/dm/master/server.go:1125
github.com/pingcap/dm/dm/master.(*AgentPool).Emit
/root/code/gopath/src/github.com/pingcap/dm/dm/master/agent_pool.go:117
runtime.goexit
/root/.gvm/gos/go1.12/src/runtime/asm_amd64.s:1337


All error messages in DM have the following three components:

• [basic error information]
• Error message description
• Error stack information (optional)

### Basic error information

• code: error code, which is unique for each error type.

DM uses the same error code for the same error type. An error code does not change as DM version changes.

Some errors might be removed during the DM iteration, but the error code will not be removed. DM uses a new error code instead of an existing one for a new error.

• class: error type

It is used to mark the component where an error occurs (error source).

The table below displays all error types, the corresponding sources and error samples.

Error type Error source Error sample
database Database operations [code=10003:class=database:scope=downstream:level=medium] database driver: invalid connection
functional Underlying functions of DM [code=11005:class=functional:scope=internal:level=high] not allowed operation: alter multiple tables in one statement
config Incorrect configuration [code=20005:class=config:scope=internal:level=medium] empty source-id not valid
binlog-op Binlog operations [code=22001:class=binlog-op:scope=internal:level=high] empty UUIDs not valid
checkpoint Checkpoint operations [code=24002:class=checkpoint:scope=internal:level=high] save point bin.1234 is older than current pos bin.1371
task-check Performing task check [code=26003:class=task-check:scope=internal:level=medium] new table router error
relay-event-lib| Executing the basic functions of the relay module [code=28001:class=relay-event-lib:scope=internal:level=high] parse server-uuid.index
relay-unit Relay processing unit [code=30015:class=relay-unit:scope=upstream:level=high] TCPReader get event: ERROR 1236 (HY000): Could not open log file
dump-unit Dump processing unit [code=32001:class=dump-unit:scope=internal:level=high] mydumper runs with error: CRITICAL **: 15:12:17.559: Error connecting to database: Access denied for user 'root'@'172.17.0.1' (using password: NO)
load-unit Load processing unit [code=34002:class=load-unit:scope=internal:level=high] corresponding ending of sql: ')' not found
sync-unit sync processing unit [code=36027:class=sync-unit:scope=internal:level=high] Column count doesn't match value count: 9 (columns) vs 10 (values)
dm-master DM-master service [code=38008:class=dm-master:scope=internal:level=high] grpc request error: rpc error: code = Unavailable desc = all SubConns are in TransientFailure, latest connection error: connection error: desc = "transport: Error while dialing dial tcp 172.17.0.2:8262: connect: connection refused"
dm-worker DM-worker service [code=40066:class=dm-worker:scope=internal:level=high] ExecuteDDL timeout, try use query-status to query whether the DDL is still blocking
dm-tracer DM-tracer service [code=42004:class=dm-tracer:scope=internal:level=medium] trace event test.1 not found
• scope: Error scope

It is used to identify the scope and source of DM objects when an error occurs, including these four types: not-set, upstream, downstream, and internal.

If the logic of the error directly involves requests between upstream and downstream databases, the scope is set to upstream or downstream. Other error scenarios are currently set to internal.

• level: Error level

The severity level of the error, which includes low, medium, and high.

The low-level error usually relates to user operation and incorrect input, which does not affect normal replication tasks. The medium-level error usually relates to user configuration, which affects some newly started services but does not affect the existing DM replication status. The high-level error usually needs your solution, otherwise there might be such risks as interrupting replication tasks.

For the above error sample:

• code=38008 is the error code indicating that the error occurs in the gRPC communication.
• class=dm-master indicates that the error occurs when DM-master sends gRPC requests to DM-worker.
• scope=interal indicates that the error occurs in DM.
• level=high indicates that it is a high-level error that needs your solution. Find out more details of it according to the error message and error stack.

### Error message description

DM uses the descriptive language to indicate the error details in an error message. The errors.Wrap mode is adopted to wrap and store every additional layer of error message description on the error call chain. The message description wrapped at the outermost layer indicates the error in DM and the message description wrapped at the innermost layer indicates the error from the bottommost error location.

Taking the above error message as an example:

• The error message description of the outermost layer is grpc request error, which describes the error in DM.
• The error message description of the innermost layer is connection error: desc = "transport: Error while dialing dial tcp 172.17.0.2:8262: connect: connection refused". It is the error returned when DM-master fails to established the gRPC connection at the bottom layer.

After analyzing the basic error information and the error message descriptions, you can determine that this error occurs when DM-master sends gRPC requests to DM-worker but it fails to establish the gRPC connection. This error occurs often because DM-worker is not working normally.

### Error stack information

DM decides whether to output the error stack information according to the severity of the error. The error stack records the complete stack trace information when the error occurs. If you cannot figure out the error cause based on the basic information and the error message descriptions, use the error stack information to further check the running path of the error code.

## Published error code

You can find out a complete list of error codes from the published error codes in the DM code warehouse.