org.apache.cassandra.service.pager

Class MultiPartitionPager

java.lang.Object

org.apache.cassandra.service.pager.MultiPartitionPager

All Implemented Interfaces:

QueryPager

public class MultiPartitionPager
extends java.lang.Object
implements QueryPager

Pager over a list of ReadCommand. Note that this is not easy to make efficient. Indeed, we need to page the first command fully before returning results from the next one, but if the result returned by each command is small (compared to pageSize), paging the commands one at a time under-performs compared to parallelizing. On the other, if we parallelize and each command raised pageSize results, we'll end up with commands.size() * pageSize results in memory, which defeats the purpose of paging. For now, we keep it simple (somewhat) and just do one command at a time. Provided that we make sure to not create a pager unless we need to, this is probably fine. Though if we later want to get fancy, we could use the cfs meanPartitionSize to decide if parallelizing some of the command might be worth it while being confident we don't blow out memory.

Field Summary

Fields inherited from interface org.apache.cassandra.service.pager.QueryPager

EMPTY

Constructor Summary

Constructors

Constructor and Description
MultiPartitionPager(SinglePartitionReadCommand.Group group, PagingState state, int protocolVersion)

Method Summary

Modifier and Type	Method and Description
PartitionIterator	fetchPage(int pageSize, ConsistencyLevel consistency, ClientState clientState) Fetches the next page.
PartitionIterator	fetchPageInternal(int pageSize, ReadOrderGroup orderGroup) Fetches the next page internally (in other, this does a local query).
boolean	isExhausted() Whether or not this pager is exhausted, i.e.
int	maxRemaining() The maximum number of cells/CQL3 row that we may still have to return.
ReadOrderGroup	startOrderGroup() Starts a new read operation.
PagingState	state() Get the current state of the pager.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

MultiPartitionPager

public MultiPartitionPager(SinglePartitionReadCommand.Group group,
                           PagingState state,
                           int protocolVersion)

Method Detail

state

public PagingState state()

Description copied from interface: QueryPager

Get the current state of the pager. The state can allow to restart the paging on another host from where we are at this point.

Specified by:

state in interface QueryPager

Returns:

the current paging state. Will return null if paging is at the beginning. If the pager is exhausted, the result is undefined.

isExhausted

public boolean isExhausted()

Description copied from interface: QueryPager

Whether or not this pager is exhausted, i.e. whether or not a call to fetchPage may return more result.

Specified by:

isExhausted in interface QueryPager

Returns:

whether the pager is exhausted.

startOrderGroup

public ReadOrderGroup startOrderGroup()

Description copied from interface: QueryPager

Starts a new read operation.

This must be called before fetchPageInternal and passed to it to protect the read. The returned object must be closed on all path and it is thus strongly advised to use it in a try-with-ressource construction.

Specified by:

startOrderGroup in interface QueryPager

Returns:

a newly started order group for this QueryPager.

fetchPage

public PartitionIterator fetchPage(int pageSize,
                                   ConsistencyLevel consistency,
                                   ClientState clientState)
                            throws RequestValidationException,
                                   RequestExecutionException

Description copied from interface: QueryPager

Fetches the next page.

Specified by:

fetchPage in interface QueryPager

Parameters:

pageSize - the maximum number of elements to return in the next page.

consistency - the consistency level to achieve for the query.

clientState - the ClientState for the query. In practice, this can be null unless consistency is a serial consistency.

Returns:

the page of result.

Throws:

RequestValidationException

RequestExecutionException

fetchPageInternal

public PartitionIterator fetchPageInternal(int pageSize,
                                           ReadOrderGroup orderGroup)
                                    throws RequestValidationException,
                                           RequestExecutionException

Description copied from interface: QueryPager

Fetches the next page internally (in other, this does a local query).

Specified by:

fetchPageInternal in interface QueryPager

Parameters:

pageSize - the maximum number of elements to return in the next page.

orderGroup - the ReadOrderGroup protecting the read.

Returns:

the page of result.

Throws:

RequestValidationException

RequestExecutionException

maxRemaining

public int maxRemaining()

Description copied from interface: QueryPager

The maximum number of cells/CQL3 row that we may still have to return. In other words, that's the initial user limit minus what we've already returned (note that it's not how many we *will* return, just the upper limit on it).

Specified by:

maxRemaining in interface QueryPager