Ultra Messaging Knowledge Base
TCP vs RU
Compare UM transports TCP and LBT-RU.
• TCP vs RU
• Introduction
• Pacing
• Source-Paced TCP
Introduction
This article compares and contrasts the UM transport types TCP and LBT-RU. It assumes you are familiar with the basics of Ultra Messaging's messaging paradigm and the basics of network data communication.
Pacing
LBT-RU is "source-paced", which means that the publisher has total control over when messages are sent. When your application calls "send", it will go out.
TCP, as a protocol, can be thought of as "receiver-paced", which means that a slow receiver can push back on a publisher. If your subscribers can't keep up with your publisher, when the intervening buffers fill, the publisher's next call to "send" can block to allow the receives an opportunity to catch up.
An advantage of TCP is reduced chance of message loss. Note that Persistence can be used to almost eliminate message loss, even with LBT-RU.
A disadvantage of TCP is the introduction of latency, which can sometimes be significant. For example, suppose that you have three subscribers and only one is slow. The two fast subscribers will have to wait for the slow receiver. In the most extreme case, a subscriber might encounter a hang or deadlock condition that halts the publisher until the hung subscriber is killed.
In contrast, a source-paced protocol will provide the fast subscribers with the lowest-possible latency at the cost of introducing loss to the slow receiver. Note that this scenario is usually short-lived during an intense burst of traffic, and the slow receiver will typically be able to recover the lost data later. So the slow receiver will frequently eventually get all the data, albeit with a high latency. However, if the burst lasts a long time, or if the receiver is consistently slow, that loss will eventually lead to unrecoverable loss. (Again, Persistence can be used to almost eliminate message loss, even with consistently slow LBT-RU receivers.)
The issue is more nuanced; see Transport Pacing for more depth.
Source-Paced TCP
The configuration option transport_tcp_multiple_receiver_behavior (source) can set to "source_paced". In this mode of operation, if a receiver is slow, the source will simply drop outgoing messages intended for that receiver. This makes the behavior similar to LBT-RU, except that there is no transport-level mechanism to recover the lost data. OTR might be able to recover the data, especially for persisted data. But for non-persisted, OTR is generally less successful than LBT-RU's NAK-based recovery.
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See Notices for important information.