CONSISTENCY
Sets and displays the consistency level. The consistency level determines the number of replica nodes that must respond for the coordinator node to successfully process a non-lightweight transaction (non-LWT).
The CQL shell supports only read requests (SELECT statements) when the consistency level is set to SERIAL or LOCAL_SERIAL.
|
To set the consistency level of a lightweight transaction (LWT), use the SERIAL CONSISTENCY command. When using a LWT, you must have both a CONSISTENCY and a SERIAL CONSISTENCY level set. CONSISTENCY cannot be set to SERIAL or LOCAL_SERIAL, only SERIAL CONSISTENCY can be set to SERIAL or LOCAL_SERIAL. |
Synopsis
CONSISTENCY [ <consistency_level> ]| Syntax conventions | Description |
|---|---|
UPPERCASE |
Literal keyword. |
Lowercase |
Not literal. |
|
Variable value. Replace with a user-defined value. |
|
Optional.
Square brackets ( |
|
Group.
Parentheses ( |
|
Or.
A vertical bar ( |
|
Repeatable.
An ellipsis ( |
|
Single quotation ( |
|
Map collection.
Braces ( |
|
Set, list, map, or tuple.
Angle brackets ( |
|
End CQL statement.
A semicolon ( |
|
Separate the command line options from the command arguments with two hyphens ( |
|
Search CQL only: Single quotation marks ( |
|
Search CQL only: Identify the entity and literal value to overwrite the XML element in the schema and solrConfig files. |
- <consistency_level>
-
The consistency level. See the documentation for the list of valid levels and their description, which is extensive.
Display the current consistency level
To show the current consistency level, run the CONSISTENCY command with no options:
CONSISTENCY;The output reflects the current consistency level:
Current consistency level is ONE.Current consistency level is QUORUM.Current consistency level is ALL.Set the consistency level
The consistency level determines that balance of data availability to data accuracy for transactions during the CQL shell session.
Some consistency levels can impact the performance of concurrent transactions occurring in the cluster, such as ALL and SERIAL.
The CQL shell setting overrides the global consistency level setting.
For information about consistency levels, performance, and use cases, see the following:
|
The cluster handles requests by electing a coordinator node for each transaction.
When you initiate a transaction from the CQL shell ( |
Examples
Set the CONSISTENCY QUORUM level to force the majority of the nodes to respond:
CONSISTENCY QUORUMConsistency level set to QUORUM.Set the level to serial for LWT read requests:
CONSISTENCY SERIALConsistency level set to SERIAL.Query that returns all cycling race winners:
SELECT *
FROM cycling.race_winners;The query results are as follows:
@ Row 1
---------------+--------------------------------------------------
race_name | National Championships South Africa WJ-ITT (CN)
race_position | 1
cyclist_name | {firstname: 'Frances', lastname: 'DU TOUT'}
@ Row 2
---------------+--------------------------------------------------
race_name | National Championships South Africa WJ-ITT (CN)
race_position | 2
cyclist_name | {firstname: 'Lynette', lastname: 'BENSON'}
@ Row 3
---------------+--------------------------------------------------
race_name | National Championships South Africa WJ-ITT (CN)
race_position | 3
cyclist_name | {firstname: 'Anja', lastname: 'GERBER'}
@ Row 4
---------------+--------------------------------------------------
race_name | National Championships South Africa WJ-ITT (CN)
race_position | 4
cyclist_name | {firstname: 'Ame', lastname: 'VENTER'}
@ Row 5
---------------+--------------------------------------------------
race_name | National Championships South Africa WJ-ITT (CN)
race_position | 5
cyclist_name | {firstname: 'Danielle', lastname: 'VAN NIEKERK'}
(5 rows)|
The previous query format uses |
A LWT is a write request that contains IF EXISTS or IF NOT EXISTS statements.
To set the consistency level of LWTs, see SERIAL CONSISTENCY.
When using a LWT, you must have both a CONSISTENCY and a SERIAL CONSISTENCY level set.
CONSISTENCY cannot be set to SERIAL or LOCAL_SERIAL, only SERIAL CONSISTENCY can be set to SERIAL or LOCAL_SERIAL.
The following examples show LWT failure scenarios. Inserts for a LWT with CONSISTENCY SERIAL fail:
CONSISTENCY SERIAL
INSERT INTO cycling.race_winners (
race_name,
race_position,
cyclist_name
) VALUES (
'National Championships South Africa WJ-ITT (CN)',
7,
{ firstname: 'Joe', lastname: 'Anderson' }
)
IF NOT EXISTS;InvalidRequest: Error from server: code=2200 [Invalid query]
message="SERIAL is not supported as conditional update commit
consistency. Use ANY if you mean "make sure it is accepted but I don't
care how many replicas commit it for non-SERIAL reads""Updates for a LWT with CONSISTENCY SERIAL also fail:
CONSISTENCY SERIAL
UPDATE cycling.race_winners SET
cyclist_name = { firstname: 'JOHN', lastname: 'DOE' }
WHERE
race_name='National Championships South Africa WJ-ITT (CN)'
AND race_position = 6
IF EXISTS;InvalidRequest: Error from server: code=2200 [Invalid query]
message="SERIAL is not supported as conditional update commit
consistency. Use ANY if you mean "make sure it is accepted but I don't
care how many replicas commit it for non-SERIAL reads""Omitting the IF clause generates errors. Also, using CONSISTENCY LOCAL_SERIAL generates the errors as that consistency level results in an invalid request.
