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Memcache PHP API Overview

Python |Java |PHP |Go

High performance scalable web applications often use a distributed in-memory data cache in front of or in place of robust persistent storage for some tasks. App Engine includes a memory cache service for this purpose.

  1. When to use a memory cache
  2. Limits
  3. PHP memcache implementation
  4. How cached data expires
  5. Configuring memcache
  6. Monitoring memcache
  7. Best practices
  8. Sharing memcache between different programming languages

When to use a memory cache

One use of a memory cache is to speed up common datastore queries. If many requests make the same query with the same parameters, and changes to the results do not need to appear on the web site right away, the app can cache the results in the memcache. Subsequent requests can check the memcache, and only perform the datastore query if the results are absent or expired. Session data, user preferences, and any other queries performed on most pages of a site are good candidates for caching.

Memcache may be useful for other temporary values. However, when considering whether to store a value solely in the memcache and not backed by other persistent storage, be sure that your application behaves acceptably when the value is suddenly not available. Values can expire from the memcache at any time, and may be expired prior to the expiration deadline set for the value. For example, if the sudden absence of a user's session data would cause the session to malfunction, that data should probably be stored in the datastore in addition to the memcache.

The memcache service provides best-effort cache space by default. Apps with billing enabled may opt to use dedicated memcache, which provides a fixed cache size assigned exclusively to your app.

Limits

The following limits apply to the use of the memcache service:

  • The maximum size of a cached data value is 1 MB minus the size of the key minus an implementation-dependent overhead, which is approximately 96 bytes.
  • A key cannot be larger than 250 bytes. In the PHP runtime, if you try to set memcache with a larger key, the call will raise an exception. (Other runtimes behave differently.)
  • The "multi" batch operations can have any number of elements. The total size of the call and the total size of the data fetched must not exceed 32 megabytes.
  • A memcache key cannot contain a null byte.

PHP memcache implementation

App Engine includes implementations of the standard Memcache and Memcached APIs, which invoke the App Engine memcache service "under the hood". Some functions are callable ("stubbed") but do nothing, as they aren't needed in the context of an App Engine app. As such, calls to the following functions will be ignored:

Stubbed functions in the Memcache API

  • memcache_add_server()
  • memcache_close()
  • memcache_connect()
  • memcache_pconnect()
  • memcache_set_compress_threshold()
  • addServer()
  • close()
  • connect()
  • pconnect()
  • setCompressThreshold()

Stubbed functions in the Memcached API

  • addServer()
  • addServers()
  • getAllKeys()
  • getServerByKey()
  • getServerList()
  • getStats()
  • getVersion()
  • isPersistent()
  • isPristine()
  • quit()
  • resetServerList()
  • setSaslAuthData()

An example usage of the Memcache PHP API in App Engine:

function getData($key) {
  $memcache = new Memcache;
  $data = $memcache->get($key);
  if ($data === false) {
    $data = doSlowQuery($key);
    $memcache->set($key, $data);
  }
  return $data;
}

An example usage of the Memcached PHP API in App Engine:

$memcache = new Memcached;
$memcache->setMulti(['image' => $image, 'data' => $data], 300);
$memcache->increment('hits');

How cached data expires

By default, values stored in memcache are retained as long as possible. Values may be evicted from the cache when a new value is added to the cache if the cache is low on memory. When values are evicted due to memory pressure, the least recently used values are evicted first.

The app can provide an expiration time when a value is stored, as either a number of seconds relative to when the value is added, or as an absolute Unix epoch time in the future (a number of seconds from midnight January 1, 1970). The value will be evicted no later than this time, though it may be evicted for other reasons.

Under rare circumstances, values may also disappear from the cache prior to expiration for reasons other than memory pressure. While memcache is resilient to server failures, memcache values are not saved to disk, so a service failure may cause values to become unavailable.

In general, an application should not expect a cached value to always be available.

You can erase an application's entire cache via the API or via the Google Developers Console's memcache page.

Configuring memcache

App Engine supports two classes of the memcache service:

  • Shared memcache is the free default for App Engine applications. It provides cache capacity on a best-effort basis and is subject to the overall demand of all the App Engine applications using the shared memcache service.

  • Dedicated memcache provides a fixed cache capacity assigned exclusively to your application. It's billed by the GB-hour of cache size. Having control over cache size means your app can perform more predictably and with fewer accesses to more costly durable storage.

Both memcache service classes use the same API. Use the Google Developers Console's memcache page to select the memcache service class for an app.

Whether shared or dedicated, memcache is not durable storage. Keys may be evicted when the cache fills up, according to the cache's LRU policy. Changes in the cache configuration or datacenter maintenance events may also flush some or all of the cache.

The following table summarizes the differences between the two classes of memcache service:

Feature Dedicated Memcache Shared Memcache
Price $0.06 per GB per hour Free
Capacity 1 to 100GB No guaranteed capacity
Performance Up to 10k operations per second per GB (items < 1KB) Not guaranteed
Durable store No No
SLA None None

Dedicated memcache billing is charged in 15 minute increments. When charging in local currency, Google will convert the prices listed into applicable local currency pursuant to the conversion rates published by leading financial institutions.

If your app needs more than 100GB of cache, please contact us at cloud-accounts-team@google.com

Monitoring memcache

For information about memcache performance and the relative cost of each operation type, refer to the Google Developers Console's memcache page.

Best practices

Following are some best practices for using memcache:

  • Handle memcache API failures gracefully. Memcache operations can fail for various reasons. Applications should be designed to catch failed operations without exposing these errors to end users. This applies especially to Set operations.
  • Use the batching capability of the API when possible, especially for small items. This will increase the performance and efficiency of your app.
  • Distribute load across your memcache keyspace. Having a single or small set of memcache items represent a disproportionate amount of traffic will hinder your app from scaling. This applies to both operations/sec and bandwidth. The problem can often be alleviated by explicit sharding of your data. For example, a frequently updated counter can be split among several keys, reading them back and summing only when a total is needed. Likewise, a 500K piece of data that must be read on every HTTP request can be split across multiple keys and read back using a single batch API call. (Even better would be to cache the value in instance memory.) For dedicated memcache, the peak access rate on a single key should be 1-2 orders of magnitude less than the per-GB rating.

Sharing memcache between different programming languages

An App Engine app can be factored into one or more modules and versions. Sometimes it is convenient to write modules and versions in different programming languages. You can share the data in your memcache between any of your app's modules and versions. Because the memcache API serializes its parameters, and the API may be implemented differently in different languages, you need to code memcache keys and values carefully if you intend to share them between langauges.

Key Compatibility

To ensure language-independence, memcache keys should be bytes:

  • In Python use plain strings (not Unicode strings)
  • In Java use byte arrays (not strings)
  • In Go use byte arrays
  • In PHP use strings

Remember that memcache keys cannot be longer than 250 bytes, and they cannot contain null bytes.

Value Compatibility

For memcache values that can be written and read in all languages, follow these guidelines:

  • Byte-arrays and ASCII strings can be safely passed between languages.
  • Unicode strings are compatible, but you must encode and decode them properly in Go and PHP.
  • Be careful using integers, they are safe for increment/decrement operations, but Go does not directly support integers and PHP cannot handle 64-bit integers.
  • Avoid using floating point values and complex types like lists, maps, structs, and classes, because each language serializes them in a different way. If you need to use types like these we recommend that you implement your own language-independent serialization that uses a format such as JSON or protocol buffers.

Example

The example code below operates on two memcache items in Python, Java, Go, and PHP. It reads and writes an item with the key “who” and increments an item with the key “count”. If you create a single app with separate modules using these four code snippets, you will see that the values set or incremented in one language will be read by the other languages.

Python

self.response.headers['Content-Type'] = 'text/plain'

who = memcache.get('who')
self.response.write('Previously incremented by %s\n' % who)
memcache.set('who', 'Python')

count = memcache.incr('count', 1, initial_value=0)
self.response.write('Count incremented by Python = %s\n' % count)

Java

resp.setContentType("text/plain");

byte[] whoKey = "who".getBytes();
byte[] countKey = "count".getBytes();

byte[] who = (byte[]) memcache.get(whoKey);
String whoString = who == null ? "nobody" : new String(who);
resp.getWriter().print("Previously incremented by " + whoString + "\n");
memcache.put(whoKey, "Java".getBytes());

Long count = memcache.increment(countKey, 1L, 0L);
resp.getWriter().print("Count incremented by Java = " + count + "\n");

Go

w.Header().Set("Content-Type", "text/plain")

whoItem, err := memcache.Get(c, "who")
var who = "nobody"
if err == nil {
  who = string(whoItem.Value)
}
fmt.Fprintf(w, "Previously incremented by %s\n", who)
memcache.Set(c, &memcache.Item{
  Key:   "who",
  Value: []byte("Go"),
})

count, _ := memcache.Increment(c, "count", 1, 0)
fmt.Fprintf(w, "Count incremented by Go = %d\n", count)

PHP

header('Content-Type: text/plain');

$who = $memcache->get('who');
echo 'Previously incremented by ' . $who . "\n";
$memcache->set('who', 'PHP');

$count = $memcache->increment('count', 1, 0);
echo 'Count incremented by PHP = ' .  $count . "\n";