Anti-patterns in Cassandra
Implementation or design patterns that are ineffective and/or counterproductive in Cassandra production installations. Correct patterns are suggested in most cases.
Network attached storage
Storing SSTables on a network attached storage (NAS) device is not recommended. Using a NAS device often results in network related bottlenecks resulting from high levels of I/O wait time on both reads and writes. The causes of these bottlenecks include:
- Router latency.
- The Network Interface Card (NIC) in the node.
- The NIC in the NAS device.
If you are required to use NAS for your environment, please contact a technical resource from DataStax for assistance.
Shared network file systems
Shared network file systems (NFS) have the same limitations as NAS. The temptation with NFS implementations is to place all SSTables in a node into one NFS. Doing this deprecates one of Cassandra's strongest features: No Single Point of Failure (SPOF). When all SSTables from all nodes are stored onto a single NFS, the NFS becomes a SPOF. To best use Cassandra, avoid using NFS.
Excessive heap space size
DataStax recommends using the default heap space size for most use cases. Exceeding this size can impair the Java virtual machine's (JVM) ability to perform fluid garbage collections (GC). The following table shows a comparison of heap space performances reported by a Cassandra user:
|Heap||CPU utilization||Queries per second||Latency|
|40 GB||50%||750||1 second|
|8 GB||5%||8500 (not maxed out)||10 ms|
Cassandra's rack feature
Defining one rack for the entire cluster is the simplest and most common implementation. Multiple racks should be avoided for the following reasons:
- Most users tend to ignore or forget rack requirements that racks should be organized in an alternating order. This order allows the data to get distributed safely and appropriately.
- Many users are not using the rack information effectively. For example, setting up with as many racks as nodes (or similar non-beneficial scenarios).
- Expanding a cluster when using racks can be tedious. The procedure typically involves several node moves and must ensure that racks are distributing data correctly and evenly. When clusters need immediate expansion, racks should be the last concern.
To use racks correctly:
- Use the same number of nodes in each rack.
- Use one rack and place the nodes in different racks in an alternating pattern. This allows you to still get the benefits of Cassandra's rack feature, and allows for quick and fully functional expansions. Once the cluster is stable, you can swap nodes and make the appropriate moves to ensure that nodes are placed in the ring in an alternating fashion with respect to the racks.
Multiple-gets may cause problems. One sure way to kill a node is to buffer 300MB of data, timeout, and then try again from 50 different clients.
You should architect your application using many single requests for different rows. This method ensures that if a read fails on a node, due to a backlog of pending requests, an unmet consistency, or other error, only the failed request needs to be retried.
Ideally, use the same key reading for the entire key or slices. Be sure to keep the row sizes in mind to prevent out-of-memory (OOM) errors by reading too many entire ultra-wide rows in parallel.
Using the Byte Ordered Partitioner
The Byte Ordered Partitioner (BOP) is not recommended.
Use virtual nodes (vnodes) and use either the Murmur3Partitioner (default) or the RandomPartitioner. Vnodes allow each node to own a large number of small ranges distributed throughout the cluster. Using vnodes saves you the effort of generating tokens and assigning tokens to your nodes. If not using vnodes, these partitioners are recommended because all writes occur on the hash of the key and are therefore spread out throughout the ring amongst tokens range. These partitioners ensure that your cluster evenly distributes data by placing the key at the correct token using the key's hash value. Even if data becomes stale and needs to be deleted, this ensures that data removal also takes place while evenly distributing data around the cluster.
Reading before writingReads take time for every request, as they typically have multiple disk hits for uncached reads. In work flows requiring reads before writes, this small amount of latency can affect overall throughput. All write I/O in Cassandra is sequential so there is very little performance difference regardless of data size or key distribution.
Cassandra was designed to avoid the need for load balancers. Putting load balancers between Cassandra and Cassandra clients is harmful to performance, cost, availability, debugging, testing, and scaling. All high-level clients, such as Astyanax and pycassa, implement load balancing directly.
Be sure to test at scale and production loads. This the best way to ensure your system will function properly when your application goes live. The information you gather from testing is the best indicator of what throughput per node is needed for future expansion calculations.
To properly test, set up a small cluster with production loads. There will be a maximum throughput associated with each node count before the cluster can no longer increase performance. Take the maximum throughput at this cluster size and apply it linearly to a cluster size of a different size. Next extrapolate (graph) your results to predict the correct cluster sizes for required throughputs for your production cluster. This allows you to predict the correct cluster sizes for required throughputs in the future. The Netflix case study shows an excellent example for testing.
Lack of familiarity with Linux
Linux has a great collection of tools. Become familiar with the Linux built-in tools. It will help you greatly and ease operation and management costs in normal, routine functions. The essential list of tools and techniques to learn are:
- Parallel SSH and Cluster SSH: The pssh and cssh tools allow SSH access to multiple nodes. This is useful for inspections and cluster wide changes.
- Passwordless SSH: SSH authentication is carried out by using public and private keys. This allows SSH connections to easily hop from node to node without password access. In cases where more security is required, you can implement a password Jump Box and/or VPN.
- Useful common command-line tools include:
- top: Provides an ongoing look at processor activity in real time.
- System performance tools: Tools such as iostat, mpstat, iftop, sar, lsof, netstat, htop, vmstat, and similar can collect and report a variety of metrics about the operation of the system.
- vmstat: Reports information about processes, memory, paging, block I/O, traps, and CPU activity.
- iftop: Shows a list of network connections. Connections are ordered by bandwidth usage, with the pair of hosts responsible for the most traffic at the top of list. This tool makes it easier to identify the hosts causing network congestion.
Running without the recommended settings
Be sure to use the recommended settings in the Cassandra documentation.
Also be sure to consult the Planning a Cassandra cluster deployment documentation, which discusses hardware and other recommendations before making your final hardware purchases.