Class DowngradingConsistencyRetryPolicy

A retry policy that sometimes retry with a lower consistency level than the one initially requested.

BEWARE: This policy may retry queries using a lower consistency level than the one initially requested. By doing so, it may break consistency guarantees. In other words, if you use this retry policy, there are cases (documented below) where a read at Quorum may not see a preceding write at Quorum. Do not use this policy unless you have understood the cases where this can happen and are OK with that. It is also highly recommended to always wrap this policy into LoggingRetryPolicy to log the occurrences of such consistency break. This policy behaves in the same way as the DefaultRetryPolicy policy, except for the following cases:

• On a read timeout: if the number of replica that responded is greater than one but lower than is required by the requested consistency level, the operation is retried at a lower consistency level.
• On a write timeout: if the operation is an UNLOGGED_BATCH and at least one replica acknowledged the write, the operation is retried at a lower consistency level. Furthermore, for other operation, if at least one replica acknowledged the write, the timeout is ignored.
• On an unavailable exception: if at least one replica is alive, the operation is retried at a lower consistency level.

The reasoning behind this retry policy is the following one. If, based on the information the Cassandra coordinator node returns, retrying the operation with the initially requested consistency has a change to succeed, do it. Otherwise, if based on these informations we know the initially requested consistency level cannot be achieve currently, then:

• For writes, ignore the exception (thus silently failing the consistency requirement) if we know the write has been persisted on at least one replica.
• For reads, try reading at a lower consistency level (thus silently failing the consistency requirement).

In other words, this policy : the idea that if the requested consistency level cannot be achieved, the next best thing for writes is to make sure the data is persisted, and that reading something is better than reading nothing, even if there is a risk of reading stale data.

Inheritance

System.Object

DowngradingConsistencyRetryPolicy

Implements

IRetryPolicy

Inherited Members

System.Object.Equals(System.Object)

System.Object.Equals(System.Object, System.Object)

System.Object.GetHashCode()

System.Object.GetType()

System.Object.MemberwiseClone()

System.Object.ReferenceEquals(System.Object, System.Object)

System.Object.ToString()

Namespace: Dse

Assembly: Dse.dll

Syntax

[Obsolete("This retry policy has been deprecated and will be removed in future versions. See the upgrade guide (Docs » Upgrade Guide) for more information")]
public class DowngradingConsistencyRetryPolicy : IRetryPolicy

Fields

Instance

Declaration

public static DowngradingConsistencyRetryPolicy Instance

Field Value

Type	Description
DowngradingConsistencyRetryPolicy

Methods

OnReadTimeout(IStatement, ConsistencyLevel, Int32, Int32, Boolean, Int32)

Defines whether to retry and at which consistency level on a read timeout.

This method triggers a maximum of one retry. If less replica responsed than required by the consistency level (but at least one replica did respond), the operation is retried at a lower consistency level. If enough replica responded but data was not retrieve, the operation is retried with the initial consistency level. Otherwise, an exception is thrown.

Declaration

public RetryDecision OnReadTimeout(IStatement query, ConsistencyLevel cl, int requiredResponses, int receivedResponses, bool dataRetrieved, int nbRetry)

Parameters

Type	Name	Description
IStatement	query	the original query that timeouted.
ConsistencyLevel	cl	the original consistency level of the read that timeouted.
System.Int32	requiredResponses	the number of responses that were required to achieve the requested consistency level.
System.Int32	receivedResponses	the number of responses that had been received by the time the timeout exception was raised.
System.Boolean	dataRetrieved	whether actual data (by opposition to data checksum) was present in the received responses.
System.Int32	nbRetry	the number of retry already performed for this operation.

Returns

Type	Description
RetryDecision	a RetryDecision as defined above.

OnUnavailable(IStatement, ConsistencyLevel, Int32, Int32, Int32)

Defines whether to retry and at which consistency level on an unavailable exception.

This method triggers a maximum of one retry. If at least one replica is know to be alive, the operation is retried at a lower consistency level.

Declaration

public RetryDecision OnUnavailable(IStatement query, ConsistencyLevel cl, int requiredReplica, int aliveReplica, int nbRetry)

Parameters

Type	Name	Description
IStatement	query	the original query for which the consistency level cannot be achieved.
ConsistencyLevel	cl	the original consistency level for the operation.
System.Int32	requiredReplica	the number of replica that should have been (known) alive for the operation to be attempted.
System.Int32	aliveReplica	the number of replica that were know to be alive by the coordinator of the operation.
System.Int32	nbRetry	the number of retry already performed for this operation.

Returns

Type	Description
RetryDecision	a RetryDecision as defined above.

OnWriteTimeout(IStatement, ConsistencyLevel, String, Int32, Int32, Int32)

Defines whether to retry and at which consistency level on a write timeout.

This method triggers a maximum of one retry. If writeType == WriteType.BATCH_LOG, the write is retried with the initial consistency level. If writeType == WriteType.UNLOGGED_BATCH and at least one replica acknowleged, the write is retried with a lower consistency level (with unlogged batch, a write timeout can always mean that part of the batch haven't been persisted at' all, even if receivedAcks > 0). For other writeType, if we know the write has been persisted on at least one replica, we ignore the exception. Otherwise, an exception is thrown.

Declaration

public RetryDecision OnWriteTimeout(IStatement query, ConsistencyLevel cl, string writeType, int requiredAcks, int receivedAcks, int nbRetry)

Parameters

Type	Name	Description
IStatement	query	the original query that timeouted.
ConsistencyLevel	cl	the original consistency level of the write that timeouted.
System.String	writeType	the type of the write that timeouted.
System.Int32	requiredAcks	the number of acknowledgments that were required to achieve the requested consistency level.
System.Int32	receivedAcks	the number of acknowledgments that had been received by the time the timeout exception was raised.
System.Int32	nbRetry	the number of retry already performed for this operation.

Returns

Type	Description
RetryDecision	a RetryDecision as defined above.

Implements

IRetryPolicy