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# Slow Receivers in a Distributed Data Management System

By Sudhir Menon

### Slow receivers explained

A slow receiver is a node in the distributed system that cannot process incoming messages due to limited network bandwidth, CPU, I/O, or a combination of these issues. In all cases, the slow receiver either fails to pick up data from its incoming network buffers causing the system to bottleneck or fails to send application or protocol level acknowledgements which would allow the sender to proceed.

Slow receivers represent a performance problem in a distributed system. When using TCP or multicast, the presence of a slow receiver causes other members of the distributed system to slow down, and in extreme cases bring system throughput to a complete standstill.

In connection-oriented protocols like TCP, the sender has to copy data into its kernel buffers and send it out to each receiver individually. The send completes only when the data has been delivered to the receiver's kernel socket buffers. If the receiver's socket buffers are full, the send blocks until the buffers become available, slowing down the performance of other receivers who cannot receive messages from this sender because the sender is blocked trying to send a message to this slow receiver.

In connectionless protocols like reliable multicast, the sender sends the data out onto the multicast network by copying it out once onto the Ethernet card and then broadcasting it out on the network with the appropriate time-to-live parameters. The sender is not bogged down by receivers at the network buffer level.

Protocol reliability is achieved by having the sender maintain a buffer of sent messages and waiting to receive ACK messages from the receivers that they have received a particular message. The senders' buffer is the limiting factor when it comes to re-transmissions to receivers who cannot pick up data from the receiver buffers fast enough and then request the sender to re-transmit the lost data. Even in this case, one can see that senders end up spending CPU cycles and memory resources tending to slow receivers and thereby bogging down the system throughput. Slow receivers are often referred to as "crybaby" receivers in network parlance.

### Slow Receivers and Cache Consistency

The ability to receive and process every piece of relevant data is critical to the functioning of a distributed cache. It is assumed that the messages coming in are relevant to the receiver and in order to maintain cache consistency, it is essential to make attempts to process the incoming data and provide some cache consistency guarantees to the consuming application.

At the same time, this desire to receive and process every message can result in a system that runs at the speed of the slowest consumer - clearly something that most distributed applications would not want to tolerate.

The solution is to define the consistency level that the cache elements within an application need and then provide a solution that deals with receiver slowdown. However, before looking at solutions, let us consider the situations that result in a slow receiver.

- Born slow receiver
  Consider a system that is comprised of 16 servers and one desktop machine, which needs to receive data from one or more servers. If the desktop is configured to be a peer, clearly its CPU will be unable to keep up with the flood of messages coming from the server machines. Eventually (defined as a couple of seconds at most in such systems), the desktop application's socket buffers fill up, bringing the publishing servers to a standstill, even though there are other consuming servers that can keep up with the publisher rate.
- Slow decline into slow receivership
  In this system, all nodes start out as equals. Activity levels on different nodes, however, tend to vary, and one of the nodes ends up having to deal with heap fragmentation issues, or garbage collection issues, or one of its threads start to run hot. In this type of a system, the performance drop is gradual and it takes a bit longer for its effects to be felt by the rest of the system - but the end result is the same nevertheless.
- Poorly written applications
  This category of slow receivers usually has two components: the component that picks up data from the socket buffers and hands it over for processing, and the component that does the actual processing. The first one may work just fine, but the second is unable to keep up. This type of slow receiver usually dies a quick death but the effects are felt later on. If the failed application is a server, the clients that it was processing quickly fail over to the other servers decreasing their throughput.
- Receiver living in a hostile neighborhood
  TCP applications are like well-mannered suburban drivers: making way for one another, going at the same speed as everyone else, and generally living a fair life from a network bandwidth perspective. When a multicast application steps into the TCP neighborhood, unless the multicast application is designed to have some group control rate, the network suddenly looks a lot like the crowded streets of big cities where fairness is no longer the norm. In such cases, a previously well behaved TCP receiver starts to look more and more like a slow receiver slowing down the whole system.

### Detecting a slow receiver

For every message that is sent from a sender to a receiver, the sender maintains some stats on the average time to completion. When the time to completion stat starts showing an upward trend and breaches a threshold, the sender flags that receiver as a slow one. This sort of detection works well in connection-oriented environments where the sender and receiver share a connection.

In connectionless environments, the sender has to maintain stats on the number of retransmission requests made by the receiver, and when that crosses a certain threshold, tag the receiver as a slow receiver.

A third class of slow receiver detection is not really detection. Instead, a slow receiver, upon failing to keep up with the rest of the system or finding excessive use of memory in its application announces itself as a slow receiver, allowing the rest of the system to activate policies that have been configured for slow receivers.

Each member of the distributed system has stats that allow the member to detect that it is entering into slow receiver mode and can be configured with policies to deal with the situation.

### Dealing with Slow Receivers

When it comes to slow receivers, there is no "one size fits all" policy that works (that works well anyway). The options that the system has once it encounters a slow receiver depend on its data consistency policy. What this implies is that a node has set certain data consistency expectations with other system members. These expectations play a major role in deciding how the member will be dealt with once it goes into slow receiver mode.

The slow receiver can choose to drop data, fire data loss notifications to the application, and catch up if the problem was temporary. This implies that not every update coming into the system has to be processed in order, and that if the application needs to fetch data from the cache, it will be fetched from other nodes on demand.

The slow receiver can send out a notification to other nodes stating that it is unable to accept any data until further notice. The remaining nodes would then ignore the member until they received a notice that the member was again open for business. Cache misses on other nodes would not be directed to this node, and data on the slow receiver would be considered suspect for the rest of the system, even though the local cache on the slow receiver would continue to serve the application and clients that it was attached to.

The system can quarantine the slow receiver thus isolating the rest of the system from the ill effects of the slow receiver. The senders could consider, store, and forward models for updates to that slow receiver. Applying interleaved updates from multiple publishers would become an issue in a system where all publishers were equal peers. In a single publisher system for a given piece of information, this would work well.

Another option is to have the notion of data ownership. This allows the slow receiver to apply updates from the owner of the data, without worrying about updates from other nodes.

A less desirable option is for the system to do nothing and run at the speed of the slow receiver. If the problem is temporary, the slow receiver comes out of that mode and the performance of the system improves.

Thus the options for dealing with slow receivers come down to the following:

- Quarantine the slow receiver until it recovers. Store and forward messages to disk-based mechanisms and let the slow receiver continue.
- The slow receiver drops messages, catches up, and fires appropriate notifications to connected applications and clients.
- Alert the system administrator about the slow receiver so that remedial action can be initiated.
- Drop messages to slow receivers and let them continue while alerting the system administrators.

### Slow Receiver Support in an Enterprise Data Fabric (EDF)

In the previous section, we discussed a problem scenario in a distributed data management system. An Enterprise Data Fabric (EDF) provides mechanisms to detect slow receivers in a distributed system by collecting stats on network activities in the system; in addition, since the EDF is an active data management platform, it can be configured to make decisions on slow receivers in real-time. These decisions can be based on the applications sharing data in the data fabric and the need for data consistency across multiple applications. It can also be based on roles played by different applications in the data fabric and the criticality of getting data to the applications in the event of slow receiver behavior in the system.

### Conclusion

A distributed data management system is a complex entity and deploying one in a production environment requires careful planning and analysis. Since we are dealing with temporal data and data consistency, it is important to have a good understanding of the network environment in which the application operates.

Every distributed system needs to have policies for dealing with slow receivers in the system. These policies have to be crafted keeping in mind the load characteristics of the system, data consistency guarantees, data loss notifications, and the system throughput requirements. Tuning the network to meet system objectives including throughput and latency has to be a part of the overall system design when you consider deploying an Enterprise Data Fabric.

Up-front capacity planning to ensure that hardware resources like network bandwidth, network partitioning, CPU, memory, and I/O characteristics of the nodes that participate in the distributed system will go a long way in avoiding unnecessary slowdowns and glitches in overall system performance. It is also important to understand the congestion characteristics of the network to ensure that the system as a whole is geared to deal with bursty traffic and temporary unavailability. Planning system redundancy, disk usage, and number of applications/instances that compete for resources on a system are factors that help prevent slow receiver problems and result in a smooth-running system.

It is also a very good idea to ask what support your distributed data management vendor has in their offering to deal with slow receivers. When it comes to dealing with slow receivers in a distributed data fabric, it is a question of "when" rather than "if."

Talkback: Discuss this article with The Answer Gang

Sudhir Menon, Director of Engineering, GemStone Systems

With over 17 years of cutting edge software experience with marquee firms like Gemstone, Intel, EDS and CenterSpan communications, Sudhir Menon is one of the key architects for the Gemfire Enterprise Data Fabric. Sudhir is the Director of Engineering for GemStone Systems, Inc. and works closely with various development teams (both onsite and offshore) working on the Gemfire Enterprise Data Fabric. His expertise in distributed data management spans multiple languages (Java, C++ and .NET) and multiple platforms and he has architected and developed network stacks for the last 10+ years. At Centerspan communications, he was one of the key architects who built the largest secure peer to peer content distribution platform over the internet.

Copyright © 2007, Sudhir Menon. Released under the Open Publication License unless otherwise noted in the body of the article. Linux Gazette is not produced, sponsored, or endorsed by its prior host, SSC, Inc.

Published in Issue 144 of Linux Gazette, November 2007

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