Systems

Nginx #nginx back to top

Nginx is our preferred web server software at 10up. It has proven extremely stable, performant, and reliable at high scale and offers a powerful set of tools. This is not to imply there is anything wrong with using Apache - we’ve worked on many high scale deployments that use Apache and mod_php that perform very well. In general, we’ve found Nginx to be more lightweight, use less memory, provide more flexible configuration, and perform better under heavy load than Apache. 10up maintains a public set of Nginx configuration templates that apply these best practices.

Installation

Nginx should be installed from the repos provided on Nginx.org. This will ensure timely updates. The “mainline” version is preferred over stable as mainline gets updates and improvements while stable only gets major bugfixes. “Stable” only means a stable feature set, it does not indicate that the stable version will be more reliable on a production server.

Configuration Files

The “events{ }” and “http{ }” blocks should be the only blocks in the /etc/nginx/nginx.conf file. These contain configurations that apply server-wide. Any configurations that are site specific should go in the “server{ }” blocks. As a rule of thumb, each WordPress install or website should have it’s own configuration file in /etc/nginx/conf.d/ (the /etc/nginx/sites-enabled/ and /etc/nginx/sites-available/ directory structure is also common and workable) clearly named with the domain of the site, such as “example.com.conf”. Multiple domains can be served from the same “server{ }” block and config file (for WordPress multisite for example). In general, there should be one server{ } block per configuration file, with the exception of server blocks for port 80 (HTTP) and port 443 (SSL) for the same site.

Using the include function is a good way to avoid repetitive configuration blocks by abstracting it to a separate file. Microcaching settings and security configurations are often good candidates for a separate include file.

Security

Nginx does not have a history of security vulnerabilities, but keeping it at the latest version is always a best practice. Nginx updates are extremely stable and are one of the least likely upgrades to cause problems, so it is recommended to either automatically update Nginx on a cron or proactively apply updates often.

Nginx has a number of modules that provide Web Application Firewall (WAF) style protection, but nearly all come with some significant trade-offs including the need to compile Nginx from source to install. Naxsi and modsecurity are 2 popular choices.

Even without a security module compiled in, Nginx can be used to block some common exploit requests. The basic strategy is to know what kind of traffic you are expecting and would be legitimate, and block everything else. This way, a file snuck onto the server cannot be exploited. The wordpress_security.inc file in our Nginx template provides some examples of this.

If you are certain a WordPress site is not using XML-RPC, block it in Nginx to prevent brute force amplification attacks. Our Nginx template blocks XML-RPC but allows for connections from Jetpack or whitelisted IP addresses.

Performance

There are some basic settings that can be adjusted in Nginx to improve the performance of WordPress:

Caching

Nginx has a built-in caching mechanism that can be applied to data being proxied or passed to fastCGI. Since Nginx cached data can be served without an extra hop to PHP or a dedicated caching tool like Varnish, it has the potential for being the fastest option. Solutions like Varnish, however, have a big advantage when it comes to cache management and invalidation. Varnish allows sophisticated rules to be built around cache invalidation, whereas Nginx requires extra modules be compiled in to do anything but basic cache management.

10up often uses a “microcaching” strategy with Nginx to provide a performance boost without compiling in extra modules. Cache invalidation integration with WordPress is handled at the PHP level where Batcache provides the main caching mechanism. With microcaching, a small expiration time is set so that cached pages will expire before anyone notices they were even cached in the first place. In this way, there is no need to worry about invalidating caches on new posts, or any other WordPress action that would require a page cache update. This essentially rate limits the amount of requests that are sent to PHP for any given page. A microcaching expiration time of as short as 10 seconds can be helpful on busy sites with spiky traffic patterns.

While a short microcaching time can be useful, the best practice is to set this microcaching expiration for as long a duration as is tolerable. For publishers that deal in breaking news, this may be tens of seconds. On a more static site, or a publisher where the stories are not time critical, microcaching up to 5 or 10 minutes can work and provide a big performance boost. This expiration time should be determined by collaborative discussion with the publishers and content creators.

Implementation

A handful of good blog posts cover microcaching and our Nginx templates provide the settings we commonly use with comments for context. Microcaching needs configuration in a number of places, so be sure to include configuration in the server block, the http block, and in the php location block.

PHP-FPM #php-fpm back to top

PHP-FPM is 10up’s preferred solutions for parsing PHP and serving via fastCGI through Nginx to the web. PHP-FPM has proven to be a stable and performant solution, offering a number of variables to configure in the pursuit of performance.

10up recommends keeping the PHP version updated to be within 1 release of the most recent version. For example, when PHP is on version 7.1, the version of PHP in production should be no lower than PHP 7.0.

Installation

The default repos in CentOS (and most Linux distributions) provide stable but usually well out-of-date packages for PHP. These are often more than 1 version behind the latest PHP release. Because of this, 10up relies on the Remi repos to provide up-to-date versions of PHP for CentOS. When using the Remi repos, edit /etc/yum.repos.d/remi.repo to select the PHP version to install. Alternatively, look for similarly named files that could be used for other PHP versions (such as /etc/yum.repos.d/remi-php71.repo). By enabling and disabling specific Remi repos, the desired version of PHP can be installed.

When doing a standard yum install PHP-FPM, the software will be installed in the usual places on Linux and updating to a new version of PHP will mean installing directly over the existing version. An alternative method of installation is to use Software Collections. Software Collections allow multiple versions of the same software to be installed and run at the same time. The scl command is used to specify which version of the software to use. In this scenario, multiple versions of PHP-FPM can be running simultaneously, making upgrading a website to a new version of PHP as simple as changing the upstream FastCGI process in Nginx. This allows for easy rollbacks if incompatibilities are discovered. This is also a great setup for development environments.

The following packages are recommended for WordPress installs:

php-cli
php-common
php-fpm
php-gd
php-mbstring
php-mcrypt
php-mysqlnd
php-opcache
php-pdo
php-pear
php-pecl-jsonc
php-pecl-memcache
php-pecl-memcached
php-pecl-redis
php-pecl-zip
php-process
php-redis
php-soap
php-xml

Not all of these packages are necessary for every WordPress install, but they cover the most common cases.

Configuration

There are a number of places PHP can be configured. The main configuration file is the php.ini file, normally located at /etc/php.ini. Settings in the php.ini file can often be overridden by the PHP-FPM configuration file (www.conf at /etc/php-fpm.d/www.conf) or by ini_set functions in the code. Not all settings can be changed in all locations and the scope is noted in the PHP documentation. Best practice is to set reasonable defaults in the php.ini file that apply broadly to the PHP code running on the server. If a specific subset of code or requests needs a different value, override the default settings by creating a new PHP-FPM pool with the new setting or creating a rule in the wp-config.php file that applies the override. For example, if the memory_limit in wp-admin needs to be increased for bulk post editing, create a specific PHP-FPM pool for wp-admin requests with an increased memory_limit and use Nginx to route wp-admin requests to that pool. This way, public web requests will retain the lower limit for security and stability purposes, but wp-admin requests will benefit from looser restrictions. Note that it is recommended that ini_set directives be done in the wp-config.php if possible. When ini_set is done within the code, troubleshooting errors can be challenging as not only will php.ini and www.conf settings need to be checked, but all PHP code will need to be scanned for ini_set directives. It is often difficult to determine if an ini_set statement directly impacts the code generating a vague error message. If ini_set statements can be confined to the wp-config.php file, it is much simpler for engineers to check the settings at all levels.

php.ini

The following are settings in the php.ini file that commonly are adjusted for WordPress sites.

www.conf

PHP-FPM has a useful feature where multiple pools can be defined, listening on different upstream ports or sockets, to segment traffic to be processed by a different set of rules. While these are not often necessary to employ, they can be handy in certain situations:

PHP-FPM pools are defined in brackets with the default pool being [www]. To create a new pool, add a new section to www.conf starting with [pool-name]. Then, make sure this new pool listens on a different port or socket than the main pool and configure Nginx to fastcgi_pass to this new port or socket for the desired requests.

Scaling

PHP-FPM scaling is mainly controlled by the process manager (pm). These pm settings are commonly adjusted in the PHP-FPM configuration file (www.conf) for high scale WordPress:

For further reading, “Scaling PHP Apps” by Steve Corona provides a thorough discussion of options for scaling PHP.

OpCode Cache

OpCode caching is recommended on all PHP webservers and has been observed to boost PHP performance significantly while reducing server resource usage by up to 3x. OpCode caching saves the PHP code in a post-compilation state, increasing the performance of every request beyond the first. The Zend Opcache has been the standard for PHP since version 5.5 (and is available for earlier PHP versions). There are no downsides to using the Zend Opcache and it should always be enabled.

When configuring OPcache the three most important settings are:

On CentOS these settings are typically found in /etc/php.d/10-opcache.ini. Always allocate enough memory to accommodate all code that needs to be cached. The amount required will vary depending on the number of lines of code in the WordPress theme and plugins. Simple sites may use less than 30MB of memory for Opcache while complex sites can use 100MB or more. A setting of 128MB is often appropriate.

The max_accelerated_files setting is fairly self explanatory: it should be set high enough to hold all of the files in your WordPress site. “Waste” in the cache is memory that is allocated for the cache, but is not being used. Things such as caches for code that was updated since the cache last reset are an example of waste in the cache.

For best performance OPcache should never be 100% full. When the maximum memory consumption is reached OPcache will attempt to restart (ie clear) the cache. However, if max_wasted_percentage has not been reached, OPcache will not restart and every uncached script will be compiled on the fly at request time and perform as if OPcache were not enabled at all! A number of tools are available for viewing OPcache usage. Be sure to password protect or limit access to this tool.

When you want to squeeze all the performance possible out of PHP, there is another OPcache setting to pay attention to, opcache.validate_timestamps. This default setting of 1 has OPcache frequently checking the timestamps (how frequently is configured with opcache.revalidate_freq) for each PHP file to determine if the cache needs to be updated. For maximum performance this can be set to 0, eliminating unnecessary file system calls. However, when new code is deployed to the server, OPcache will have no way of knowing that the cache should be updated and will continue to serve the outdated cached version. In this case manual purging of the OpCode cache is required. This can be accomplished by restarting php-fpm, or by using a tool such as cachetool to send commands directly to the PHP listener, over FastCGI, and instruct it to clear the OPcache:

$ php cachetool.phar opcache:reset --fcgi=127.0.0.1:9000

$ php cachetool.phar opcache:reset --fcgi=/var/run/php5-fpm.sock

PHP Sessions

PHP offers a shared space that can persist across processes to store information that needs to be accessible to all PHP processes called “sessions”. Sessions are like cookies, but stored server-side instead of on the client. By default, PHP uses files to store sessions. This presents a problem in multi web server environments where not all servers will have access to a shared filesystem. A reliable solution to this problem is to use Memcached as the storage location for PHP sessions. Memcached is recommended for WordPress object caching and is often available in a high scale WordPress environment and will make PHP sessions available across all web servers.

MySQL #mysql back to top

The impact of MySQL performance is very different site to site. A slow database might not have much noticeable impact on a well cached site with a small collection of content, but could have major impact on a busy site with many editors publishing into a very large library of content. The amount of effort to spend on MySQL tuning should be gauged by the expected impact.

Version

Both MySQL and MariaDB can serve WordPress as they are fully compatible with each other. While 10up generally has switched to MariaDB due to the more open source ethos of the project, there’s very little reason not to use MySQL if that is the easier option. In this document, “MySQL” is used as the generic term to refer to both MySQL and MariaDB.

MySQL versions have a longer lifespan than PHP versions and as long as security patches are being issued for the version in use, it is likely acceptable to use.

Hardware

The best way to ensure fast MySQL performance is fast disks. Fast hard disks in a database server can overcome poor configuration and tuning. Slow queries become more tolerable. Large datasets are no longer a challenge. If fast SSD disks can be afforded, they are the best place to invest for database speed.

If fast disks are unavailable or cost prohibitive, ample RAM is the next best resource. The general rule to follow is that all routinely queried data should be able to fit in system memory with room to spare. If, for example, a database was 2.5 GB, a server with 4GB of RAM would provide ample space to tune optimally.

Tools

MySQLTuner is a Perl script that gathers metrics on MySQL and Linux and generates a report of configuration settings with recommendations on optimisations. While these optimisations should not be used without critical evaluation, they provide an excellent basis for performance tuning. MySQLTuner should be run regularly to understand the evolving nature of the WordPress dataset and how MySQL is performing under stress. While MySQLTuner can be installed through most package managers, downloading the tool from GitHub is simple and ensures the latest version.

Sequel Pro (OS X), HeidiSQL (Windows), and MySQL Workbench (cross-platform) are among many GUI tools available for connecting to remote MySQL servers. A tool of this nature should be setup and configured on your local workstation for each database routinely worked with. Everything these tools can do can be done via the command line, but in an emergency, it is helpful to have connection information and credentials saved with a quick way to visualize tables and their contents.

Replication and HA design

Introducing database replication to any configuration is a giant leap in complexity compared to a single database setup. The maintenance burden and troubleshooting difficulty both increase substantially, so evaluate whether a project really needs this complexity before embarking.

There are a few common goals of database replication:

The three commonly encountered MySQL replication types are Master-Slave, Master-Master, and Synchronous clusters like Galera.

Master-Slave

A Master-Slave replication pair consists of the Master node, where all database writes happen, and the Slave node, where only database reads can occur. The Slave node never pushes any data back to the Master node, so any writes that are attempted on this node are refused.

Here’s how Master-Slave replication does on our replication goals:

Master-Master

Do not use MySQL Master-Master replication. It is attractive as it seems to solve all the limitations of Master-Slave replication. However, Master-Master replication can be very dangerous and can result in data collisions, lost data, or duplicated data if the replication were to break or one of the database servers were to crash. It is a fragile type of replication and, while it can be engineered to be a reliable system, there are better options available.

Galera Cluster

A Galera cluster is a synchronous multi-master database cluster for InnoDB tables where writes must happen successfully on all cluster members to finish successfully on a single member. This gives Galera a high data durability. A Galera cluster should always be setup with an odd number of nodes. This is so in the event of a replication failure of 1 node, 2 remaining nodes can remain a quorum and the source of true data, re-syncing to the lone disconnected database node when it reconnects. If the absolute lowest cost is needed, the 3rd (or odd-numbered) Galera member could be a Galera Arbitrator, which does not participate in the replication, but will maintain connections to all other Galera nodes and assist in determining a quorum.

Here’s how Galera performs on the common replication goals:

Performance Tuning

MySQL performance can be substantially improved with careful tuning of buffers, caches, and other settings. A good first step is to run MySQLTuner and carefully consider each of the recommendations. Listed near the top of the MySQLTuner output is section about MySQL memory usage. It is critical to constantly evaluate the total memory usage of MySQL and how it compares to the available memory on the server while tuning. MySQL’s memory footprint is very stable and predictable and is completely determined by the settings in the my.cnf file. There’s no reason a database server should ever run out of memory (which can lead to prolonged and difficult data recovery sessions).

When evaluating memory usage, all programs running on the database server should be considered. A typical database server will have MySQL and sometimes Memcached running, both of which have predictable memory footprints. If the full stack is running on a single server, predicting RAM usage becomes more challenging as PHP can vary substantially from one request to the next. As uncertainty increases, so should the amount of free RAM left on a server, leaving space for overruns. A good rule of thumb is to leave at least 512MB available for Linux system use (preferably more if the resources are available).

When tuning memory usage, be aware that many MyISAM buffers and caches are per thread while many InnoDB buffers are global. InnoDB is the default database engine in MySQL and is what most WordPress installs will be using at this time, so MyISAM buffers can be greatly reduced (but should be non-zero as some MySQL internal tables can use MyISAM).

This section will outline the most common variables that are tuned in the /etc/my.cnf or /etc/my.cnf.d/server.cnf files using a real example from a 10up configured site.

Once tuning moves beyond these main items, performance improvements will be minor to negligible.

MySQL tmpdir Performance Optimization

As discussed earlier, WordPress utilizes on-disk temporary tables when doing JOIN statements that reference the wp_posts table as many of the wp_posts table’s fields are of type TEXT or LONGTEXT. If a TEXT or LONGTEXT field are in the results of a JOIN, that query cannot use in-memory temporary tables and will create those temporary tables on disk. One way to optimize the performance of these on-disk temporary tables is to set MySQL’s tmpdir to a tmpfs filesystem. tmpfs appears to the operating system to be a normally mounted disk drive, but it is actually a filesystem that resides entirely in volatile memory (RAM). By mounting the tmpdir in memory, MySQL will read and write temporary tables very quickly without the input/output limitations for traditional drives. This method is even faster than most SSDs and has been shown to provide a significant performance boost for some WordPress workloads.

Most Linux servers have a few tmpfs mounts already. The /dev/shm mount is a tmpfs mount for efficiently storing temporary files for programs and we can set MySQL to use this with the tmpdir variable in the my.cnf file. tmpfs only uses space in RAM when files exist (it isn’t preallocated), but if the WordPress database is very busy, beware that MySQL could use as much RAM as is allocated for /dev/shm (which is normally 50% of total RAM). In most WordPress workloads, MySQL only uses a few megabytes in temporary tables on disk, but if /dev/shm usage were to grow, it could quickly cause an out of memory situation. Take this into account when tuning MySQL for memory usage. /dev/shm size should be monitored if used for the MySQL tmpdir.

Memcached and Redis #memcached-and-redis back to top

Memcached and Redis are in memory data stores that are used for the WordPress object cache. Implementation of the object cache in WordPress code is covered extensively in the PHP Performance section of the Best Practices and this section will focus on the hosting and setup of memcached and Redis.

Memcached and Redis are used by WordPress as simple, in memory key-value stores. By being in memory and not having the possibility for complex queries, these tools provide blazing fast retrieval of data, usually in less than 1 millisecond. Common use cases of these data stores are:

Items stored in these caching technologies are made up of 3 things: a key, a value, and an optional expiration time. After the expiration time, items are evicted from the cache. Redis can be configured with a number of eviction schemes, which control what happens when an item expires or when the cache is full, but in our use case, it rarely makes sense to use anything besides a Least Recently Used (LRU) eviction scheme. Items without an expiration time will persist in the cache until the cache is full, at which time the cache will evict the least recently used item each time a new item needs to be stored.

Sizing the Cache Pool

The LRU eviction policy means that even a small cache can be effective and the most used data will always be in the cache. However, the most effective cache pool will never evict an item before the expiration time is reached and will have enough space for a healthy collection of items with no expiration time. 10up has tested various sizes of cache pools and has found 256 MB to be appropriate for most WordPress sites. Complicated sites or large multisites may benefit from 512 MB and small sites on limited hardware can perform well with as little as 64 MB, but 256 MB should be used as a safe “rule of thumb”. Above 256 MB, the cache hit rate and eviction rate usually do not improve no matter how large the cache pool is.

A common misconception is that a full cache pool is a problem and should be avoided, or that a full cache means a bigger cache pool is needed. In reality, a full cache pool is the normal state of the cache with WordPress and means nothing. No matter how big the cache pool, it will eventually become full. This is because there will always be some cached items without an expiration time that will persist in the cache forever, until the cache is full and the LRU policy evicts it to make room for a new item.

Even a very active cache will not stress a modern CPU, so a caching server can perform very well even with a single CPU core. Similarly, since the cache activity is all in memory (unless persistence is enabled with Redis), a small and slow hard drive will work fine. As long as network bandwidth is fast, latency is low, and enough RAM is available for the cache pool, nearly any server can host the cache.

Architecture Considerations

The worst thing that can happen with an in-memory cache like memcached or Redis is the memory becoming full and swap space needing to be used. In a high availability setup, it would be prefereable if the server just crashed and went offline rather than using swap. If a server begins swapping, it is using the much slower hard disk to store some items that should be in memory, such as memcached items. A hard disk is so many times slower than memory that this will cause a dramatic worsening of the performance of the cache node and the application relying on the cache will be forced to wait for this slow server to reply. This causes a massive bottleneck for the application and in WordPress will result in pages timing out or loading very slowly. If the cache server were to just go offline, the items from the cache would be redistributed to the other cache nodes and everything would continue on, which is why it is recommended to disable swap entirely on cache nodes.

For a cache node to have stable performance, it must have a predictable amount of memory available and, preferably, devoted to the cache. For this reason, it is recommended to run memcache or redis either on a dedicated server or on a server shared with MySQL, but not on a server that is serving PHP requests. A server that is taking PHP web requests, whether with Apache and mod_php or with Nginx and PHP-FPM, will have very unpredictable memory usage. Depending on the kind of work each process is doing, PHP could use 30 MB or 300 MB and it is very difficult to predict. Therefore, webservers create the possibility that memory could unexpectedly become full. On most webservers, this isn’t a big deal as some PHP processes will fail but the rest will continue on normally. However, if a webserver were to also be hosting the cache, running out of memory could be disasterous when under load as the cache process could be killed off or worse, could start swapping. On single-server setups, careful tuning of the stack and conservative use of the available memory can make it work, but if possible, MySQL and the cache should be moved to a separate server. In constrast to PHP, MySQL is ideal software to host alongside memcached or Redis on a server. The memory usage of MySQL is predictable and can be completely controlled with the my.cnf file, leaving a stable amount of memory that can be dedicated to the cache.

Memcached

Memcached is simpler than Redis and has fewer features. While Redis can be used as a full database, memcached is only a key-value store. This simplicity is by design and makes Memcached a very low maintenance tool.

High Availability

Memcached can be installed on multiple servers that can combine into a memcached “pool”. Keeping with the theme of simplicity, each memcached server in the pool knows nothing about the other instances in the pool and operates completely independently. It is up to the client (in this case, WordPress and PHP) to distribute data across the pool of memcached servers however it sees fit. Memcached performs no replication, failover, or connection balancing itself. To distribute data evenly across multiple servers, the PHP extensions for memcached use a hashing strategy. In this way, multiple PHP web servers can read and write to multiple memcached servers and all know exactly where each key is stored.

Item Size

By default, memcached accepts items (key + value) of 1 MB or less. In most scenarios, this is fine and plenty of space. However, on larger WordPress sites or sites with a number of plugins, the “alloptions” array combining all autoloaded rows from the wp_options table can exceed 1 MB. This array will be stored in memcached if memcached is in use for the object cache, unless it exceeds the memcached item size limit. When this array is larger than the memcached item size, it can cause all sorts of odd issues and inconsistent performance. While the alloptions array is the most common way a WordPress site will exceed the 1 MB limit, many other use cases can result in this same problem, including storing HTML fragments, remote call responses, or query results. Keep the item size limit in mind whenever storing data in memcached.

Starting in memcached version 1.4.2, the item size is configurable via the -I option. Setting the item size higher than 1 MB is not recommended unless necessary as memcached becomes less efficient as the item size increases, meaning it will take more memory to store the same amount of data. If memory is available to accommodate a larger cache size, it is worth considering raising the item size to prevent this limit from ever becoming a problem.

Connecting WordPress to Memcached

WordPress connects to memcached through an object-cache.php drop-in plugin file placed in the wp-content folder. The object-cache.php file will leverage a php extension to handle communicating with memcached. There are two PHP extensions commonly used, confusingly named php-memcache and php-memcached. It is important to match the object-cache.php file with the right PHP extension, and, while there’s many object cache files that can work, 10up mostly uses one of the following:

One benefit of wordpress-pecl-memcached-object-cache is that the alloptions array discussed in the “Item Size” section has been split up and each “option” is stored as a separate key and value. This means that WordPress is much less likely to exceed the 1 MB default item size, which will allow the cache to run efficiently. The individual keys and values are retrieved with a getMulti command (only available in the php-memcached extension), which results in the only 1 additional memcached call over storing the alloptions array in a single key.

Security

Memcached offers no security. It is imperative that memcached not be exposed to the internet, both to prevent sensitive data from being exposed, but also to prevent amplification attacks. If memcached is on a dedicated server, or a server shared with MySQL, ideally it will not have a public IP address nor be accessible outside of the private network. If the entire stack is on a single server, use the -l option to bind memcached to 127.0.0.1. If a public IP must be used, restrict access using a firewall.

Tools

One of the biggest challenges with memcached is getting visibility into the items in the cache and the performance of the cache. The simplest way to interact with a memcached server is by using telnet. For example, a memcached server on localhost can be connected to with the command telnet 127.0.0.1 11211 where 11211 is the port memcached listens on. Once connected with telnet, basic statistics about the instance are available by typing stats. Most of these statistics are only useful with consistent monitoring to understand trends, but get_hits and get_misses are particularly useful for calculating the hit ratio (get_hits / (get_hits + get_misses)).

For easier to read statistics, PHPMemcachedAdmin provides an interactive, browser-based dashboard. Be diligent to keep this dashboard private by configuring it behind a password or to be accessed only by known IPs.

Beyond the stats, it is difficult to get a good idea of the content in a memcached instance. While PHPMemcachedAdmin makes some strides towards providing this visibility, there’s no good way to view all keys in memcached, or do any fuzzy matching like you would in a full database. The simplicity of memcached makes it very difficult to find anything without knowing the exact key. This kind of visibility is not often needed, but can be a challenge when debugging an issue with the cache.

Redis

While memcached strives for simplicity, Redis seeks to be a full featured, high performance database and data store. Many data types, eviction schemes, and transaction types are available, but the common use case with WordPress is very similar to memcached: a simple key-value store with an LRU eviction scheme.

High Availability

Redis offers many more options for high availability that memcached, including master-slave replication, failover architectures via Sentinel, and full multi-node clusters. Implementing any of these solutions adds complexity and hardware, which should be considered carefully. Unlike memcached, Redis cache can be made to persist to disk, so the cache values can survive a restart, making recovery from a cache failure less impactful. On high-traffic sites where the object cache uptime is mission critical, the high availability capabilities of Redis may make it the right choice.

Connecting WordPress to Redis

Similar to memcached, WordPress connects to Redis through an object-cache.php file in the wp-content folder, which in turn leverages a PHP extension. Two extensions are widely used, Predis and phpredis. The object-cache.php file also has 2 good options that are widely used:

Security

Redis supports password authentication with every request, making it suitable for use on a public endpoint. However, installing Redis within a private network is the preferred setup for security.

Tools

Redis has a robust toolset for monitoring and viewing data stored in the cache. The redis-cli tool is a must have for anyone working with a Redis site and can monitor all data in and out of a cluster, simulate replication, show real-time latency, and search for keys in the cache, among many other capabilities. For a graphical interface, many options are availble including:

For even more tools, Redis Labs has a good list.