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add new and optimize FAQ.md #1115

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226 changes: 116 additions & 110 deletions docs-2.0/20.appendix/0.FAQ.md
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Original file line number Diff line number Diff line change
Expand Up @@ -32,18 +32,19 @@ Nebula Graph is still under development. Its behavior changes from time to time.
Data formats of Neubla Graph {{ nebula.release }} are compatible with Nebula Graph {{ nebula.base20 }}, while their clients are incompatible.
-->

## About executions
<!--
### Why is there no line separating each row in the returned result of Nebula Graph 2.6.0?
## About execution errors

This is caused by the release of Nebula Console 2.6.0, not the change of Nebula Graph core. And it will not affect the content of the returned data itself.
-->
### About dangling edges
### "How to resolve the error `SemanticError: Missing yield clause.`?"

A dangling edge is an edge that only connects to a single vertex and only one part of the edge connects to the vertex.
Starting with Nebula Graph 3.0.0, the statements `LOOKUP`, `GO`, and `FETCH` must output results with the `YIELD` clause. For more information, see [YIELD](../3.ngql-guide/8.clauses-and-options/yield.md).

Dangling edges may appear in Nebula Graph {{ nebula.release }} as the design. And there is no `MERGE` statements of openCypher. The guarantee for dangling edges depends entirely on the application level. For more information, see [INSERT VERTEX](../3.ngql-guide/12.vertex-statements/1.insert-vertex.md), [DELETE VERTEX](../3.ngql-guide/12.vertex-statements/4.delete-vertex.md), [INSERT EDGE](../3.ngql-guide/13.edge-statements/1.insert-edge.md), [DELETE EDGE](../3.ngql-guide/13.edge-statements/4.delete-edge.md).
### "How to resolve the error `Zone not enough!`?"

From Nebula Graph version 3.0.0, the Storage services added in the configuration files **CANNOT** be read or written directly. The configuration files only register the Storage services into the Meta services. You must run the `ADD HOSTS` command to read and write data on Storage servers. For more information, see [Manage Storage hosts](../4.deployment-and-installation/manage-storage-host.md)。

### "How to resolve the error `To get the property of the vertex in 'v.age', should use the format 'var.tag.prop'`?"

From Nebula Graph version 3.0.0, patterns support matching multiple tags at the same time, so you need to specify a tag name when querying properties. The original statement `RETURN variable_name.property_name` is changed to `RETURN variable_name.<tag_name>.property_name`.

### "How to resolve `[ERROR (-1005)]: Used memory hits the high watermark(0.800000) of total system memory.`?"

Expand All @@ -68,17 +69,104 @@ The reason for this error is usually that the storaged process returns too many

It is a known issue. Just retry 1 to N times, where N is the partition number. The reason is that the meta client needs some heartbeats to update or errors to trigger the new leader information.

### "How to resolve the error `SemanticError: Missing yield clause.`?"
### "How to resolve `[ERROR (-1005)]: Schema not exist: xxx`?"

Starting with Nebula Graph 3.0.0, the statements `LOOKUP`, `GO`, and `FETCH` must output results with the `YIELD` clause. For more information, see [YIELD](../3.ngql-guide/8.clauses-and-options/yield.md).
If the system returns `Schema not exist` when querying, make sure that:

### "How to resolve the error `Zone not enough!`?"
- Whether there is a tag or an edge type in the Schema.

From Nebula Graph version 3.0.0, the Storage services added in the configuration files **CANNOT** be read or written directly. The configuration files only register the Storage services into the Meta services. You must run the `ADD HOSTS` command to read and write data on Storage servers. For more information, see [Manage Storage hosts](../4.deployment-and-installation/manage-storage-host.md)。
- Whether the name of the tag or the edge type is a keyword. If it is a keyword, enclose them with backquotes (\`). For more information, see [Keywords](../3.ngql-guide/1.nGQL-overview/keywords-and-reserved-words.md).

### "How to resolve the error `To get the property of the vertex in 'v.age', should use the format 'var.tag.prop'`?"
### Unable to download SNAPSHOT packages when compiling Exchange, Connectors, or Algorithm

From Nebula Graph version 3.0.0, patterns support matching multiple tags at the same time, so you need to specify a tag name when querying properties. The original statement `RETURN variable_name.property_name` is changed to `RETURN variable_name.<tag_name>.property_name`.
Problem description: The system reports `Could not find artifact com.vesoft:client:jar:xxx-SNAPSHOT` when compiling.

Cause: There is no local Maven repository for storing or downloading SNAPSHOT packages. The default central repository in Maven only stores official releases, not development versions (SNAPSHOTs).

Solution: Add the following configuration in the `profiles` scope of Maven's `setting.xml` file:

```xml
<profile>
<activation>
<activeByDefault>true</activeByDefault>
</activation>
<repositories>
<repository>
<id>snapshots</id>
<url>https://oss.sonatype.org/content/repositories/snapshots/</url>
<snapshots>
<enabled>true</enabled>
</snapshots>
</repository>
</repositories>
</profile>
```

### "How to resolve `[ERROR (-7)]: SyntaxError: syntax error near`?"

In most cases, a query statement requires a `YIELD` or a `RETURN`. Check your query statement to see if `YIELD` or `RETURN` is provided.

### "How to resolve the error `can’t solve the start vids from the sentence`?"

The graphd process requires `start vids` to begin a graph traversal. The `start vids` can be specified by the user. For example:

```ngql
> GO FROM ${vids} ...
> MATCH (src) WHERE id(src) == ${vids}
# The "start vids" are explicitly given by ${vids}.
```

It can also be found from a property index. For example:

```ngql
# CREATE TAG INDEX IF NOT EXISTS i_player ON player(name(20));
# REBUILD TAG INDEX i_player;

> LOOKUP ON player WHERE player.name == "abc" | ... YIELD ...
> MATCH (src) WHERE src.name == "abc" ...
# The "start vids" are found from the property index "name".
```

Otherwise, an error like `can’t solve the start vids from the sentence` will be returned.

### "How to resolve the error `Wrong vertex id type: 1001`?"

Check whether the VID is `INT64` or `FIXED_STRING(N)` set by `create space`. For more information, see [create space](../3.ngql-guide/9.space-statements/1.create-space.md).

### "How to resolve the error `The VID must be a 64-bit integer or a string fitting space vertex id length limit.`?"

Check whether the length of the VID exceeds the limitation. For more information, see [create space](../3.ngql-guide/9.space-statements/1.create-space.md).

### "How to resolve the error `edge conflict` or `vertex conflict`?"

Nebula Graph may return such errors when the Storage service receives multiple requests to insert or update the same vertex or edge within milliseconds. Try the failed requests again later.

### "How to resolve the error `RPC failure in MetaClient: Connection refused`?"

The reason for this error is usually that the metad service status is unusual, or the network of the machine where the metad and graphd services are located is disconnected. Possible solutions are as follows:

- Check the metad service status on the server where the metad is located. If the service status is unusual, restart the metad service.

- Use `telnet meta-ip:port` to check the network status under the server that returns an error.

- Check the port information in the configuration file. If the port is different from the one used when connecting, use the port in the configuration file or modify the configuration.

### "How to resolve the error `StorageClientBase.inl:214] Request to "x.x.x.x":9779 failed: N6apache6thrift9transport19TTransportExceptionE: Timed Out` in `nebula-graph.INFO`?"

The reason for this error may be that the amount of data to be queried is too large, and the storaged process has timed out. Possible solutions are as follows:

- When importing data, set [Compaction](../8.service-tuning/compaction.md) manually to make read faster.

- Extend the RPC connection timeout of the Graph service and the Storage service. Modify the value of `--storage_client_timeout_ms` in the `nebula-storaged.conf` file. This configuration is measured in milliseconds (ms). The default value is 60000ms.
<!--Tip: Add `--local_config=true` at the beginning of the configuration file and restart the service.-->

### "How to resolve the error `MetaClient.cpp:65] Heartbeat failed, status:Wrong cluster!` in `nebula-storaged.INFO`, or `HBProcessor.cpp:54] Reject wrong cluster host "x.x.x.x":9771!` in `nebula-metad.INFO`?

The reason for this error may be that the user has modified the IP or the port information of the metad process, or the storage service has joined other clusters before. Possible solutions are as follows:

Delete the `cluster.id` file in the installation directory where the storage machine is deployed (the default installation directory is `/usr/local/nebula`), and restart the storaged service.

## About design and functions

### "How is the `time spent` value at the end of each return message calculated?"

Expand All @@ -98,6 +186,20 @@ Got 1 rows (time spent 1235/1934 us)

- The second number `1934` shows the time spent from the client's perspective, that is, the time it takes for the client from sending a request, receiving a response, and displaying the result on the screen.

### Why does the port number of the `nebula-storaged` process keep showing red after connecting Nebula Graph?

Because the `nebula-storaged` process waits for `nebula-metad` to add the current Storage service during the startup process. The Storage works after it receives the ready signal. Starting from Nebula Graph 3.0.0, the Meta service cannot directly read or write data in the Storage service that you add in the configuration file. The configuration file only registers the Storage service to the Meta service. You must run the `ADD HOSTS` command to enable the Meta to read and write data in the Storage service. For more information, see [Manage Storage hosts](../4.deployment-and-installation/manage-storage-host.md).

### Why is there no line separating each row in the returned result of Nebula Graph 2.6.0?

This is caused by the release of Nebula Console 2.6.0, not the change of Nebula Graph core. And it will not affect the content of the returned data itself.

### About dangling edges

A dangling edge is an edge that only connects to a single vertex and only one part of the edge connects to the vertex.

Dangling edges may appear in Nebula Graph {{ nebula.release }} as the design. And there is no `MERGE` statements of openCypher. The guarantee for dangling edges depends entirely on the application level. For more information, see [INSERT VERTEX](../3.ngql-guide/12.vertex-statements/1.insert-vertex.md), [DELETE VERTEX](../3.ngql-guide/12.vertex-statements/4.delete-vertex.md), [INSERT EDGE](../3.ngql-guide/13.edge-statements/1.insert-edge.md), [DELETE EDGE](../3.ngql-guide/13.edge-statements/4.delete-edge.md).

### "Can I set `replica_factor` as an even number in `CREATE SPACE` statements, e.g., `replica_factor = 2`?"

NO.
Expand Down Expand Up @@ -140,10 +242,6 @@ Therefore, using `GO` and `MATCH` to execute the same semantic query may cause d

For more information, see [Wikipedia](https://en.wikipedia.org/wiki/Path_(graph_theory)#Walk,_trail,_path).

### "How to resolve `[ERROR (-7)]: SyntaxError: syntax error near`?"

In most cases, a query statement requires a `YIELD` or a `RETURN`. Check your query statement to see if `YIELD` or `RETURN` is provided.

### "How to count the vertices/edges number of each tag/edge type?"

See [show-stats](../3.ngql-guide/7.general-query-statements/6.show/14.show-stats.md).
Expand Down Expand Up @@ -176,65 +274,6 @@ You can use [Nebula Algorithm](../nebula-algorithm.md).

Or get vertices by each tag, and then group them by yourself.

### "How to resolve the error `can’t solve the start vids from the sentence`?"

The graphd process requires `start vids` to begin a graph traversal. The `start vids` can be specified by the user. For example:

```ngql
> GO FROM ${vids} ...
> MATCH (src) WHERE id(src) == ${vids}
# The "start vids" are explicitly given by ${vids}.
```

It can also be found from a property index. For example:

```ngql
# CREATE TAG INDEX IF NOT EXISTS i_player ON player(name(20));
# REBUILD TAG INDEX i_player;

> LOOKUP ON player WHERE player.name == "abc" | ... YIELD ...
> MATCH (src) WHERE src.name == "abc" ...
# The "start vids" are found from the property index "name".
```

Otherwise, an error like `can’t solve the start vids from the sentence` will be returned.

### "How to resolve the error `Wrong vertex id type: 1001`?"

Check whether the VID is `INT64` or `FIXED_STRING(N)` set by `create space`. For more information, see [create space](../3.ngql-guide/9.space-statements/1.create-space.md).

### "How to resolve the error `The VID must be a 64-bit integer or a string fitting space vertex id length limit.`?"

Check whether the length of the VID exceeds the limitation. For more information, see [create space](../3.ngql-guide/9.space-statements/1.create-space.md).

### "How to resolve the error `edge conflict` or `vertex conflict`?"

Nebula Graph may return such errors when the Storage service receives multiple requests to insert or update the same vertex or edge within milliseconds. Try the failed requests again later.

### "How to resolve the error `RPC failure in MetaClient: Connection refused`?"

The reason for this error is usually that the metad service status is unusual, or the network of the machine where the metad and graphd services are located is disconnected. Possible solutions are as follows:

- Check the metad service status on the server where the metad is located. If the service status is unusual, restart the metad service.

- Use `telnet meta-ip:port` to check the network status under the server that returns an error.

- Check the port information in the configuration file. If the port is different from the one used when connecting, use the port in the configuration file or modify the configuration.

### "How to resolve the error `StorageClientBase.inl:214] Request to "x.x.x.x":9779 failed: N6apache6thrift9transport19TTransportExceptionE: Timed Out` in `nebula-graph.INFO`?"

The reason for this error may be that the amount of data to be queried is too large, and the storaged process has timed out. Possible solutions are as follows:

- When importing data, set [Compaction](../8.service-tuning/compaction.md) manually to make read faster.

- Extend the RPC connection timeout of the Graph service and the Storage service. Modify the value of `--storage_client_timeout_ms` in the `nebula-storaged.conf` file. This configuration is measured in milliseconds (ms). The default value is 60000ms.
<!--Tip: Add `--local_config=true` at the beginning of the configuration file and restart the service.-->

### "How to resolve the error `MetaClient.cpp:65] Heartbeat failed, status:Wrong cluster!` in `nebula-storaged.INFO`, or `HBProcessor.cpp:54] Reject wrong cluster host "x.x.x.x":9771!` in `nebula-metad.INFO`?

The reason for this error may be that the user has modified the IP or the port information of the metad process, or the storage service has joined other clusters before. Possible solutions are as follows:

Delete the `cluster.id` file in the installation directory where the storage machine is deployed (the default installation directory is `/usr/local/nebula`), and restart the storaged service.

### Can non-English characters be used as identifiers, such as the names of graph spaces, tags, edge types, properties, and indexes?

Expand All @@ -255,39 +294,6 @@ There is no such command.

You can use [Nebula Algorithm](../nebula-algorithm.md).

### "How to resolve `[ERROR (-1005)]: Schema not exist: xxx`?"

If the system returns `Schema not exist` when querying, make sure that:

- Whether there is a tag or an edge type in the Schema.

- Whether the name of the tag or the edge type is a keyword. If it is a keyword, enclose them with backquotes (\`). For more information, see [Keywords](../3.ngql-guide/1.nGQL-overview/keywords-and-reserved-words.md).

### Unable to download SNAPSHOT packages when compiling Exchange, Connectors, or Algorithm

Problem description: The system reports `Could not find artifact com.vesoft:client:jar:xxx-SNAPSHOT` when compiling.

Cause: There is no local Maven repository for storing or downloading SNAPSHOT packages. The default central repository in Maven only stores official releases, not development versions (SNAPSHOTs).

Solution: Add the following configuration in the `profiles` scope of Maven's `setting.xml` file:

```xml
<profile>
<activation>
<activeByDefault>true</activeByDefault>
</activation>
<repositories>
<repository>
<id>snapshots</id>
<url>https://oss.sonatype.org/content/repositories/snapshots/</url>
<snapshots>
<enabled>true</enabled>
</snapshots>
</repository>
</repositories>
</profile>
```

## About operation and maintenance

### "The log files are too large. How to recycle the logs?"
Expand Down