Cantaloupe offers a sophisticated and customizable caching subsystem that is capable of meeting a variety of needs while remaining easy to use. Several tiers of cache are available:
The Cache-Control response header, which is configurable via the
cache.client.* keys in the configuration file, can provide caching hints to clients. To enable this header, set the cache.client.enabled key to true.
The default settings look something like this:
cache.client.max_age = 2592000
cache.client.shared_max_age =
cache.client.public = true
cache.client.private = false
cache.client.no_cache = false
cache.client.no_store = false
cache.client.must_revalidate = false
cache.client.proxy_revalidate = false
cache.client.no_transform = trueThese are reasonable defaults that tell clients they can keep cached images for 2,592,000 seconds (30 days).
The source cache caches whole, pre-processed source images onto the filesystem. The only reasons to do this are:
CacheStrategy and configuring FilesystemCache.CacheStrategy, and then configuring FilesystemCache, the source cache will be utilized to automatically pre-download source images before they are accessed, as a more robust alternative to DownloadStrategy.In short, the source cache is a workaround for some kind of suboptimal situation in the image input pipeline—either a slow source or a codec limitation. Ideally, it would not have to be used.
There is only one available source cache implementation—FilesystemCache—and it is used independently of the derivative cache.
The source cache is integrated into the larger caching architecture, so all of the information about modes of operation and maintenance applies to both the source and derivative caches.
Note that when chunking is enabled on sources that support it (see the relevant documentation for HttpSource), in conjunction with a processor that supports seeking for a particular format, the source cache is bypassed even when enabled.
The derivative cache caches post-processed images in order to spare the computational expense of processing the same image over and over again. Derivative caches are pluggable in order to enable different cache stores.
In typical use, derivative caching will greatly reduce load on the server and improve response times accordingly. There are other ways of caching derivatives, such as by using a caching reverse proxy server, but the built-in derivative cache is custom-tailored for this application and easy enough to set up.
Derivative caching is disabled by default. To enable it, set cache.server.derivative.enabled to true, and set cache.server.derivative to the name of a cache, such as FilesystemCache.
The info cache caches image info objects in the Java heap independently of the derivative cache. When both are enabled, the info cache acts as a "level 1" cache in front of the "level 2" derivative cache:
The info cache can be enabled or disabled via the cache.server.info.enabled configuration key.
The info cache is cluster-safe: when multiple instances are sharing the same derivative cache, there will never (for more than a brief period) be an info in an instance's info cache that isn't also present in the derivative cache.
The maximum size of the info cache is hard-coded to a reasonable percentage of the maximum heap size, and is not configurable. As infos tend to be very small, the maximum size is unlikely to ever be reached, but if it is, the least-recently-accessed infos will be invalidated as necessary to accommodate fresher ones.
The info cache's content never expires, but it is not persisted.
The source and derivative caches can be configured to operate in one of two ways:
cache.server.resolve_first = true)cache.server.resolve_first = false)Because cached content is not automatically deleted after becoming invalid, there will likely be a certain amount of invalid content taking up space in the cache at any given time. Without periodic maintenance, the amount can only grow. If this is a problem, it can be dealt with manually using the HTTP API, or automatically using the cache worker, which periodically purges invalid items. (See the cache.server.worker.* configuration options.)
Most caches (with the exception of HeapCache) age-limit their content based on last-accessed or last-modified time. Depending on the amount of source content served, the varieties of derivatives generated, the time-to-live setting, and how often maintenance is performed, the cache may grow very large. Its size is not tracked, as this would be either expensive, or, for some cache implementations, impossible. Managing the cache size is therefore the responsibility of the administrator, and it can be accomplished by any combination of:
cache.server.source.ttl_seconds and/or cache.server.derivative.ttl_seconds configuration keys);All implementations are thread-, process-, and cluster-safe, where applicable. Multiple application instances can be pointed at the same cache store without conflicting.
FilesystemCache caches content in a filesystem tree. The tree structure looks like:
FilesystemCache.pathname/
FilesystemCache.dir.depth and FilesystemCache.dir.name_length.Cache files are created with a .tmp extension and moved into place when closed for writing.
File last-access times (atime) are used to determine validity. Filesystems used to store cached content should not be mounted with the noatime option, as this will disable recording of last-access times, forcing a fallback to last-modified times (mtime), which don't work as well for a cache.
HeapCache caches derivative images and metadata in the Java heap, which is the main area of memory available to the JVM. This is the fastest of the caches, with the main drawback being that it cannot be shared across instances.
Unlike most of the other caches, this one does not age-limit content. When the target size (HeapCache.target_size) has been exceeded, the minimum number of least-recently-accessed items are purged that will reduce it back down to this size. (The configured target size may be safely changed while the application is running.)
Because this cache is not time-limited, cache.server.derivative.ttl_seconds does not apply, and, if enabled, the cache worker will remain idle.
When using this cache, ensure that your heap is able to grow large enough to accommodate the desired target size (using the -Xmx VM option), and that you have enough RAM to accommodate this size.
This cache can persist its contents to disk using the HeapCache.persist and HeapCache.persist.filesystem.pathname configuration keys. When persistence is enabled, the contents of the cache will be written to a file at shutdown, and loaded back in at startup. If persistence is disabled, the cache contents will be lost when the application exits.
Consideration was given to storing cached data using the same on-disk format used by FilesystemCache, so that persisted data would be compatible between these caches. Unfortunately, this is not possible because of the one-way hashing used in the FilesystemCache format.
JdbcCache caches derivative images and metadata in relational database tables. To use this cache, a JDBC driver for your database must be installed on the classpath.
JdbcCache is tested with the H2 database. It is known not to work with the official PostgreSQL driver, as of version 9.4.1207. Other databases may work, but are untested.
JdbcCache can be configured with the following options:
JdbcCache.urljdbc:postgresql://localhost:5432/mydatabase.JdbcCache.userJdbcCache.passwordJdbcCache.image_tableJdbcCache.info_tableJdbcCache will not create its schema automatically—this must be done manually using the following commands, which may have to be altered slightly for your particular database:
CREATE TABLE IF NOT EXISTS {JdbcCache.derivative_image_table} (
operations VARCHAR(4096) NOT NULL,
image BLOB,
last_accessed DATETIME
);
CREATE TABLE IF NOT EXISTS {JdbcCache.info_table} (
identifier VARCHAR(4096) NOT NULL,
info VARCHAR(8192) NOT NULL,
last_accessed DATETIME
);
CREATE INDEX operations_idx ON {JdbcCache.derivative_image_table} (operations);
CREATE INDEX identifier_idx ON {JdbcCache.info_table} (identifier);S3Cache caches derivative images and metadata in a Simple Storage Service (S3) bucket. It supports both AWS and non-AWS endpoints.
Although S3 doesn't natively support the concept of a last-accessed time, S3Cache asynchronously makes copies of the objects it accesses, which enables last-modified times (which are set by S3 at object creation and immutable) to serve as last-accessed times.
S3Cache is configured (excepting credentials) using the following configuration keys:
S3Cache.endpoints3.us-east-2.amazonaws.com. (See the list of AWS S3 regions.)S3Cache.bucket.nameS3Cache.object_key_prefixSee the Credentials Sources information for S3Source. S3Cache works the same way, except that the credentials-related configuration keys, if you choose to use them, are S3Cache.access_key_id and S3Cache.secret_key.
AzureStorageCache caches derivative images and metadata into a Microsoft Azure Storage container. It can be configured with the following options:
AzureStorageCache.account_nameAzureStorageCache.account_keyAzureStorageCache.container_nameAzureStorageCache.object_key_prefixAzure Storage does not provide a last-accessed time in object metadata, so the time-to-live is on the basis of last-modified time instead.
RedisCache, available since version 3.4, caches derivative images and metadata using the Redis data structure store. It supports the following configuration options:
RedisCache.hostRedisCache.portRedisCache.sslRedisCache.passwordRedisCache.databaseUnlike with the other caches, cache policy is configured on the Redis side, and cache.server.derivative.ttl_seconds has no effect with this cache. Likewise, if enabled, the cache worker will remain idle.