Error handling
This section describes how to manage errors that you may encounter while managing databases.
When running the database management queries, such as CREATE DATABASE
, it is possible to encounter errors.
1. Observing errors
Because database management operations are performed asynchronously, these errors may not returned immediately upon query execution.
Instead, you must monitor the output of SHOW DATABASE
; particularly the error
and currentStatus
columns.
neo4j@system> CREATE DATABASE foo;
0 rows available after 108 ms, consumed after another 0 ms
neo4j@system> SHOW DATABASE foo;
In standalone mode:
+------------------------------------------------------------------------------------------------------------------+ | name | address | role | requestedStatus | currentStatus | error | default | +------------------------------------------------------------------------------------------------------------------+ | "foo" | "localhost:7687" | "standalone" | "online" | "dirty" | "File system permissions" | FALSE | +------------------------------------------------------------------------------------------------------------------+ 1 rows available after 4 ms, consumed after another 1 ms
In a Causal Cluster:
+----------------------------------------------------------------------------------------------------------------+ | name | address | role | requestedStatus | currentStatus | error | default | +----------------------------------------------------------------------------------------------------------------+ | "foo" | "localhost:7687" | "leader" | "online" | "online" | "" | FALSE | | "foo" | "localhost:7688" | "follower" | "online" | "online" | "" | FALSE | | "foo" | "localhost:7689" | "follower" | "online" | "dirty" | "File system permissions" | FALSE | +----------------------------------------------------------------------------------------------------------------+ 3 row available after 100 ms, consumed after another 6 ms
2. Database states
A database management operation may fail for a number of reasons.
For example, if the file system instance has incorrect permissions, or Neo4j itself is misconfigured.
As a result, the contents of the error
column in the SHOW DATABASE
query results may vary significantly.
However, databases may only be in one of a select number of states:
Current state | Description |
---|---|
|
The database has not yet been created. |
|
The database is running. |
|
The database is not running. |
|
The database is currently being updated from another instance of Neo4j. |
|
The database has been deleted. |
|
This state implies an error has occurred. The database’s underlying store files may be invalid. For more information, consult the server’s logs. |
|
The database is effectively stopped and its state may not be changed until no longer quarantined. |
|
This instance of Neo4j doesn’t know the state of this database. |
Most often, when a database management operation fails, Neo4j attempts to transition the database in question to the offline
state.
If the system is certain that no store files have yet been created, it transitions the database to initial
instead.
Similarly, if the system suspects that the store files underlying the database are invalid (incomplete, partially deleted, or corrupt), then it transitions the database to dirty
.
Whilst dropped is a valid database state, it is only transiently observable, as database records are removed from SHOW DATABASE results once the DROP operation is complete.
|
3. Retrying failed operations
Database management operations may be safely retried in the event of failure. However, these retries are not guaranteed to succeed, and errors may persist through several attempts.
neo4j@system> START DATABASE foo;
0 rows available after 108 ms, consumed after another 0 ms
neo4j@system> SHOW DATABASE foo;
+-------------------------------------------------------------------------------------------------------------+ | name | address | role | requestedStatus | currentStatus | error | default | +-------------------------------------------------------------------------------------------------------------+ | "foo" | "localhost:7687" | "standalone" | "online" | "offline" | "Some error message" | FALSE | +-------------------------------------------------------------------------------------------------------------+ 1 rows available after 4 ms, consumed after another 1 ms
After investigating and addressing the underlying issue, you can start the database again and verify that it is running properly:
neo4j@system> START DATABASE foo;
0 rows available after 108 ms, consumed after another 0 ms
neo4j@system> SHOW DATABASE foo;
+------------------------------------------------------------------------------------------------+ | name | address | role | requestedStatus | currentStatus | error | default | +------------------------------------------------------------------------------------------------+ | "foo" | "localhost:7687" | "standalone" | "online" | "online" | "" | FALSE | +------------------------------------------------------------------------------------------------+ 1 rows available after 4 ms, consumed after another 1 ms
If repeated retries of a command have no effect, or if a database is in a dirty
state, you may drop and recreate the database, as detailed in Cypher manual → Administration.
When running |
4. Using quarantine in a cluster for fixing errors
You can use the dbms.cluster.quarantineDatabase
procedure locally (only on the cluster member where it is executed) to isolate a specific database.
For example, when a database is unable to start on a given member due to a file system permissions issue with the volume where the database is located, or when a recently started database begins to log errors.
The quarantine state renders the database inaccessible on that cluster member and prevents its state from being changed, for example, via the START DATABASE
command.
After lifting the quarantine, the cluster member tries to bring the database to the desired state.
It is recommended to run the quarantine procedure over the bolt:// protocol rather than neo4j:// , which may route requests to unexpected instances.
|
Syntax:
CALL dbms.cluster.quarantineDatabase(databaseName,setStatus,reason)
Arguments:
Name | Type | Description |
---|---|---|
|
String |
The name of the database that will be put into or removed from quarantine. |
|
Boolean |
|
|
String |
(Optional) The reason for placing the database in quarantine. |
Returns:
Name | Type | Description |
---|---|---|
|
String |
The name of the database. |
|
String |
Actual state. |
|
String |
Result of the last operation. The result contains the user, the time, and the reason for the quarantine. |
neo4j@system> CALL dbms.cluster.quarantineDatabase("foo",true);
+--------------------------------------------------------------------------------------+ | databaseName | quarantined | result | +--------------------------------------------------------------------------------------+ | "foo" | TRUE | "By neo4j at 2020-10-15T15:10:41.348Z: No reason given" | +--------------------------------------------------------------------------------------+ 3 row available after 100 ms, consumed after another 6 ms
neo4j@system> SHOW DATABASE foo;
+---------------------------------------------------------------------------------------------------------------------------------------------+ | name | address | role | requestedStatus | currentStatus | error | default | +---------------------------------------------------------------------------------------------------------------------------------------------+ | "foo" | "localhost:7688" | "unknown" | "online" | "quarantined" | "By neo4j at 2020-10-15T15:10:41.348Z: No reason given" | FALSE | | "foo" | "localhost:7689" | "follower" | "online" | "online" | "" | FALSE | | "foo" | "localhost:7687" | "leader" | "online" | "online" | "" | FALSE | +---------------------------------------------------------------------------------------------------------------------------------------------+ 3 row available after 100 ms, consumed after another 6 ms
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