RAP Latency Parameters

Some RAP parameters are related to latency, and some parameters directly affect latency statistics.

RAP Parameters Related to Latency

The following parameters are related to latency, but do not increase or decrease the amount of time a message takes to travel across the network from publisher to subscriber.

Parameter	Component	Description
LatencyCheckInterval	All RAP components	Determines how often, in seconds (1 to 655535), RAP calculates latency.
ResendPort	All RAP publishers and subscribers	The publisher port number used by subscribers to: Request resends of dropped network packets Calculate network latency between publisher and subscriber
MonitorLatencyRefreshRate	Operations Console	Determines how often the Operations Console receives latency statistics.

RAP Parameters That Affect Latency

The following parameters can directly affect latency calculations; that is, increase or decrease the elapsed time a message takes to travel across the network from a publisher to a subscriber.

Parameter	Component	Description
TargetFileSizeMB	RAPStore	Determines the size of each binary file that is loaded into the RAPStore. Larger values increase latency. Small values require the writing out and loading of many individual files, which can affect system performance.
IdleBufferWriteDelaySec	RAPStore	Determines how long, in seconds, the system waits when not receiving data from the publisher before writing data out to file. Larger values increase latency in situations where data flow is not steady. Very small values increase file overhead costs in the system and will reduce performance
PacketWindowSize	RAP subscriber	Subscribers may receive network packets in an order other than the order the packets were sent. If a packet is received out-of-sequence, this parameter determines how many additional packets can be received before a resend request is sent for a missing packet. A very small value can result in requesting resends for packets that will arrive out of order due to network latencies. A very large value increases the latency that occurs when packets are actually lost over the network. The packet window size is related to the number of packets cached by the publishers sending data to this subscriber (NumPacketBuffers). The packet window size of a subscriber should be 70-80% of the number of packets cached by the publisher to ensure that the publisher has the ability to send missing packets in response to resend requests. This setting should never be set to a value that exceeds the number of packet buffers allocated to its publisher as this will result in data loss in situations where resends are required.
BulkBatchSize	RAPCache	Information loaded into the RAPCache is committed to the database periodically and made available to readers. This parameter controls how much information can be loaded into the RAPCache before it is committed. Higher values increase the latency of the system and require the number of locks in the RAPCache to be increased to accommodate larger database transactions. Smaller values introduce the extra overhead of frequently committing transactions to the database and result in slower load performance.
IdleBufferWriteDelayMSec	RAPCache	Determines the number of milliseconds to wait before data is transferred to the RAPCache when the system is not receiving data from the publisher. Larger values increase latency in situations where data flow is not steady. Smaller values ensure lower system latency in periods where the information flow is not steady.

Note: See "RAP - The Trading Edition Configuration Files" in the Installation and Configuration Guide RAP - The Trading Edition for more information.