NAME¶
CHI - Unified cache handling interface
VERSION¶
version 0.54
SYNOPSIS¶
use CHI;
# Choose a standard driver
#
my $cache = CHI->new( driver => 'Memory', global => 1 );
my $cache = CHI->new( driver => 'RawMemory', global => 1 );
my $cache = CHI->new( driver => 'File',
root_dir => '/path/to/root'
);
my $cache = CHI->new( driver => 'FastMmap',
root_dir => '/path/to/root',
cache_size => '1k'
);
my $cache = CHI->new( driver => 'Memcached::libmemcached',
servers => [ "10.0.0.15:11211", "10.0.0.15:11212" ],
l1_cache => { driver => 'FastMmap', root_dir => '/path/to/root' }
);
my $cache = CHI->new( driver => 'DBI',
dbh => $dbh
);
my $cache = CHI->new( driver => 'BerkeleyDB',
root_dir => '/path/to/root'
);
# Create your own driver
#
my $cache = CHI->new( driver => '+My::Special::Driver', ... );
# Cache operations
#
my $customer = $cache->get($name);
if ( !defined $customer ) {
$customer = get_customer_from_db($name);
$cache->set( $name, $customer, "10 minutes" );
}
my $customer2 = $cache->compute($name2, "10 minutes", sub {
get_customer_from_db($name2)
});
$cache->remove($name);
DESCRIPTION¶
CHI provides a unified caching API, designed to assist a developer in persisting
data for a specified period of time.
The CHI interface is implemented by driver classes that support fetching,
storing and clearing of data. Driver classes exist or will exist for the gamut
of storage backends available to Perl, such as memory, plain files, memory
mapped files, memcached, and DBI.
CHI is intended as an evolution of DeWitt Clinton's Cache::Cache package,
adhering to the basic Cache API but adding new features and addressing
limitations in the Cache::Cache implementation.
FEATURES¶
- •
- Easy to create new drivers
- •
- Uniform support for namespaces
- •
- Automatic serialization of keys and values
- •
- Multilevel caches
- •
- Probabilistic expiration and busy locks, to reduce cache
miss stampedes
- •
- Optional logging and statistics collection of cache
activity
CONSTRUCTOR¶
To create a new cache object, call "<CHI->new". It takes the
common options listed below.
driver is required; all others are
optional.
Some drivers will take additional constructor options. For example, the File
driver takes "root_dir" and "depth" options.
You can configure default options for each new cache object created - see
"SUBCLASSING AND CONFIGURING CHI".
Note that "CHI->new" returns an instance of a subclass of
CHI::Driver, not "CHI".
- compress_threshold [INT]
- A value in bytes. Automatically compress values larger than
this before storing. Requires Compress::Zlib to be installed. Defaults to
undef, meaning no automatic compression. Inspired by the parameter of the
same name in Cache::Memcached.
# Compress values larger than 1MB
compress_threshold => 1024*1024
- driver [STRING]
- Required. The name of a cache driver, for example
"Memory" or "File". CHI will prefix the string with
"CHI::Driver::", unless it begins with '+'. e.g.
driver => 'File'; # uses CHI::Driver::File
driver => '+My::CHI::Driver::File' # uses My::CHI::Driver::File
- expires_in [DURATION], expires_at [INT], expires_variance
[FLOAT]
- Provide default values for the corresponding
"set" options.
- expires_on_backend [NUM]
- If set to 0 (the default), CHI alone is aware of the
expiration time and does not pass it along to the backend driver. This
allows you to use "get_object" to retrieve expired items.
If set to 1, pass expiration times to backend driver if the driver supports
it -- for example, CHI::Driver::Memcached and CHI::Driver::CacheCache.
This may allow the driver to better manage its space and evict items. Note
that only simple expiration time will be passed along, e.g. not
"expires_variance".
If set to a number greater than 1 (e.g. 1.25), the time til expiration will
be multiplied by that number before being passed to the backend driver.
This gives you a customizable window of opportunity to retrieve expired
items.
- key_digester [STRING|HASHREF|OBJECT]
- Digest algorithm to use on keys longer than
"max_key_length" - e.g. "MD5", "SHA-1", or
"SHA-256".
Can be a Digest object, or a string or hashref which will passed to
Digest-> new(). You will need to ensure Digest is installed to
use these options.
Default is "MD5".
- key_serializer [STRING|HASHREF|OBJECT]
- An object to use for serializing keys that are references.
See "serializer" above for the different ways this can be passed
in. The default is to use JSON in canonical mode (sorted hash keys).
- label [STRING]
- A label for the cache as a whole, independent of namespace
- e.g. "web-file-cache". Used when referring to the cache in
logs, statistics, and error messages. By default, set to
"short_driver_name".
- l1_cache [HASHREF]
- Add an L1 cache as a subcache. See
"SUBCACHES".
- max_key_length [INT]
- Keys over this size will be digested. The default is
driver-specific; CHI::Driver::File, for example, defaults this to 240 due
to file system limits. For most drivers there is no maximum.
- mirror_cache [HASHREF]
- Add an mirror cache as a subcache. See
"SUBCACHES".
- namespace [STRING]
- Identifies a namespace that all cache entries for this
object will be in. This allows easy separation of multiple, distinct
caches without worrying about key collision.
Suggestions for easy namespace selection:
- •
- In a class, use the class name:
my $cache = CHI->new(namespace => __PACKAGE__, ...);
- •
- In a script, use the script's absolute path name:
use Cwd qw(realpath);
my $cache = CHI->new(namespace => realpath($0), ...);
- •
- In a web template, use the template name. For example, in
Mason, $m->cache will set the namespace to the current component
path.
Defaults to 'Default' if not specified.
- on_get_error [STRING|CODEREF]
- on_set_error [STRING|CODEREF]
- How to handle runtime errors occurring during cache gets
and cache sets, which may or may not be considered fatal in your
application. Options are:
- •
- log (the default) - log an error, or ignore if no logger is
set - see "LOGGING"
- •
- ignore - do nothing
- •
- warn - call warn() with an appropriate message
- •
- die - call die() with an appropriate message
- •
- coderef - call this code reference with three
arguments: an appropriate message, the key, and the original raw error
message
- serializer [STRING|HASHREF|OBJECT]
- An object to use for serializing data before storing it in
the cache, and deserializing data after retrieving it from the cache. Only
references will be serialized; plain scalars will be placed in the cache
as-is.
If this is a string, a Data::Serializer object will be created, with the
string passed as the 'serializer' option and raw=1. Common options include
'Storable', 'Data::Dumper', and 'YAML'. If this is a hashref,
Data::Serializer will be called with the hash. You will need to ensure
Data::Serializer is installed to use these options.
Otherwise, this must be a Data::Serializer object or another object that
implements serialize() and
deserialize() .
e.g.
# Serialize using raw Data::Dumper
my $cache = CHI->new(serializer => 'Data::Dumper');
# Serialize using Data::Dumper, compressed and (per Data::Serializer defaults) hex-encoded
my $cache = CHI->new(serializer => { serializer => 'Data::Dumper', compress => 1 });
# Serialize using custom object
my $cache = CHI->new(serializer => My::Custom::Serializer->new())
The default is to use raw Storable.
- traits [LISTREF]
- List of one or more roles to apply to the
"CHI::Driver" class that is constructed. The roles will
automatically be prefixed with "CHI::Driver::Role::" unless
preceded with a '+'. e.g.
traits => ['StoresAccessedAt', '+My::CHI::Driver::Role']
INSTANCE METHODS¶
The following methods can be called on any cache handle returned from CHI->
new(). They are implemented in the CHI::Driver package.
Getting and setting¶
- get( $key, [option => value, ...] )
- Returns the data associated with $key.
If $key does not exist or has expired, returns undef.
Expired items are not automatically removed and may be examined with
"get_object" or "get_expires_at".
$key may be followed by one or more name/value
parameters:
- expire_if [CODEREF]
- If $key exists and has not expired,
call code reference with the CHI::CacheObject as a single parameter. If
code returns a true value, "get" returns undef as if the item
were expired. For example, to treat the cache as expired if
$file has changed since the value was computed:
$cache->get('foo', expire_if => sub { $_[0]->created_at < (stat($file))[9] });
- busy_lock [DURATION]
- If the value has expired, the get will still return undef,
but the expiration time of the cache entry will be set to the current time
plus the specified duration. This is used to prevent multiple processes
from recomputing the same expensive value simultaneously. The problem with
this technique is that it doubles the number of writes performed - see
"expires_variance" for another technique.
- obj_ref [SCALARREF]
- If the item exists in cache, place the
<CHI::CacheObject|CHI::CacheObject> object in the provided
SCALARREF.
- set( $key, $data, [$expires_in | "now" |
"never" | options] )
- Associates $data with
$key in the cache, overwriting any existing entry.
Returns $data.
The third argument to "set" is optional, and may be either a
scalar or a hash reference. If it is a scalar, it may be the string
"now", the string "never", or else a duration treated
as an expires_in value described below. If it is a hash reference,
it may contain one or more of the following options. Most of these options
can be provided with defaults in the cache constructor.
- expires_in [DURATION]
- Amount of time from now until this data expires.
DURATION may be an integer number of seconds or a duration
expression.
- expires_at [INT]
- The epoch time at which the data expires.
- expires_variance [FLOAT]
- Controls the variable expiration feature, which allows
items to expire a little earlier than the stated expiration time to help
prevent cache miss stampedes.
Value is between 0.0 and 1.0, with 0.0 meaning that items expire exactly
when specified (feature is disabled), and 1.0 meaning that items might
expire anytime from now til the stated expiration time. The default is
0.0. A setting of 0.10 to 0.25 would introduce a small amount of variation
without interfering too much with intended expiration times.
The probability of expiration increases as a function of how far along we
are in the potential expiration window, with the probability being near 0
at the beginning of the window and approaching 1 at the end.
For example, in all of the following cases, an item might be considered
expired any time between 15 and 20 minutes, with about a 20% chance at 16
minutes, a 40% chance at 17 minutes, and a 100% chance at 20 minutes.
my $cache = CHI->new ( ..., expires_variance => 0.25, ... );
$cache->set($key, $value, '20 min');
$cache->set($key, $value, { expires_at => time() + 20*60 });
my $cache = CHI->new ( ... );
$cache->set($key, $value, { expires_in => '20 min', expires_variance => 0.25 });
CHI will make a new probabilistic choice every time it needs to know whether
an item has expired (i.e. it does not save the results of its
determination), so you can get situations like this:
my $value = $cache->get($key); # returns undef (indicating expired)
my $value = $cache->get($key); # returns valid value this time!
if ($cache->is_valid($key)) # returns undef (indicating expired)
if ($cache->is_valid($key)) # returns true this time!
Typical applications won't be affected by this, since the object is
recomputed as soon as it is determined to be expired. But it's something
to be aware of.
- compute( $key, $options, $code )
- Combines the "get" and "set" operations
in a single call. Attempts to get $key; if
successful, returns the value. Otherwise, calls $code
and uses the return value as the new value for $key,
which is then returned. Caller context (scalar or list) is respected.
$options can be undef, a scalar, or a hash reference.
If it is undef, it has no effect. If it is a scalar, it is treated as the
"expires_in" duration and passed as the third argument to
"set". If it is a hash reference, it may contain name/value
pairs for both "get" and "set". e.g.
# No expiration
my $value = $cache->compute($key, undef, sub {
# compute and return value for $key here
});
# Expire in 5 minutes
my $value = $cache->compute($key, '5min', sub {
# compute and return value for $key here
});
# Expire in 5 minutes or when a particular condition occurs
my $value = $cache->compute($key,
{ expires_in => '5min', expire_if => sub { ... } },
sub {
# compute and return value for $key here
});
# List context
my @value = $cache->compute($key, '5min', sub {
...
return @some_list;
});
This method will eventually support the ability to recompute a value in the
background just before it actually expires, so that users are not impacted
by recompute time.
Note: Prior to version 0.40, the last two arguments were in reverse order;
both will be accepted for backward compatibility. We think the coderef
looks better at the end.
Removing and expiring¶
- remove( $key )
- Remove the data associated with the
$key from the cache.
- expire( $key )
- If $key exists, expire it by setting
its expiration time into the past. Does not necessarily remove the data.
Since this involves essentially setting the value again,
"remove" may be more efficient for some drivers.
Inspecting keys¶
- is_valid( $key )
- Returns a boolean indicating whether
$key exists in the cache and has not expired. Note:
Expiration may be determined probabilistically if
"expires_variance" was used.
- exists_and_is_expired( $key )
- Returns a boolean indicating whether
$key exists in the cache and has expired. Note:
Expiration may be determined probabilistically if
"expires_variance" was used.
- get_expires_at( $key )
- Returns the epoch time at which $key
definitively expires. Returns undef if the key does not exist or it has no
expiration time.
- get_object( $key )
- Returns a CHI::CacheObject object containing data about the
entry associated with $key, or undef if no such key
exists. The object will be returned even if the entry has expired, as long
as it has not been removed.
Atomic operations (ALPHA)¶
These methods combine both reading and writing of a cache entry in a single
operation. The names and behaviors were adapted from memcached
<
http://memcached.org/>.
Some drivers (e.g. CHI::Driver::Memcached::libmemcached, CHI::Driver::DBI) may
implement these as truly atomic operations, and will be documented thusly. The
default implementations are not atomic: the get and set occur discretely and
another process could potentially modify the cache in between them.
These operations are labelled ALPHA because we haven't yet figured out how they
integrate with other CHI features, in particular "SUBCACHES". APIs
and behavior may change.
- add( $key, $data, [$expires_in | "now" |
"never" | options] )
- Do a set, but only if $key is not
valid in the cache.
- replace( $key, $data, [$expires_in | "now" |
"never" | options] )
- Do a set, but only if $key is valid in
the cache.
- append( $key, $new_data)
- Append $new_data to whatever value is
currently associated with $key. Does not modify
expiration or other metadata; if $key exists but is
expired, it will remain expired. Has no effect if
$key does not exist in the cache.
This is intended for simple string values only. For efficiency's sake, CHI
won't attempt to check for, or handle, the case where data is serialized
or compressed; the new data will simply be appended, and an error will
most probably occur when you try to retrieve the value.
If you use a driver with the non-atomic (default) implementation, some
appends may be lost due to race conditions.
Namespace operations¶
- clear( )
- Remove all entries from the namespace.
- get_keys( )
- Returns a list of keys in the namespace. This may or may
not include expired keys, depending on the driver.
The keys may not look the same as they did when passed into "set";
they may have been serialized, utf8 encoded, and/or digested (see
"KEY AND VALUE TRANSFORMATIONS"). However, they may still be
passed back into "get", "set", etc. to access the same
underlying objects. i.e. the following code is guaranteed to produce all
key/value pairs from the cache:
map { ($_, $c->get($_)) } $c->get_keys()
- purge( )
- Remove all entries that have expired from the namespace
associated with this cache instance. Warning: May be very inefficient,
depending on the number of keys and the driver.
- get_namespaces( )
- Returns a list of namespaces associated with the cache.
This may or may not include empty namespaces, depending on the
driver.
Multiple key/value operations¶
The methods in this section process multiple keys and/or values at once. By
default these are implemented with the obvious map operations, but some cache
drivers (e.g. Cache::Memcached) can override them with more efficient
implementations.
- get_multi_arrayref( $keys )
- Get the keys in list reference $keys,
and return a list reference of the same length with corresponding values
or undefs.
- get_multi_hashref( $keys )
- Like "get_multi_arrayref", but returns a hash
reference with each key in $keys mapping to its
corresponding value or undef. Will only work with scalar keys.
- set_multi( $key_values, $set_options )
- Set the multiple keys and values provided in hash reference
$key_values. $set_options is a
scalar or hash reference, used as the third argument to set. Will only
work with scalar keys.
- remove_multi( $keys )
- Removes the keys in list reference
$keys.
- dump_as_hash( )
- Returns a hash reference containing all the non-expired
keys and values in the cache.
Property accessors¶
- chi_root_class( )
- Returns the name of the root class under which this object
was created, e.g. "CHI" or "My::CHI". See
"SUBCLASSING AND CONFIGURING CHI".
- driver_class( )
- Returns the full name of the driver class. e.g.
CHI->new(driver=>'File')->driver_class
=> CHI::Driver::File
CHI->new(driver=>'+CHI::Driver::File')->driver_class
=> CHI::Driver::File
CHI->new(driver=>'+My::Driver::File')->driver_class
=> My::Driver::File
You should use this rather than "ref()". Due to some subclassing
tricks CHI employs, the actual class of the object is neither guaranteed
nor likely to be the driver class.
- short_driver_name( )
- Returns the name of the driver class, minus the
CHI::Driver:: prefix, if any. e.g.
CHI->new(driver=>'File')->short_driver_name
=> File
CHI->new(driver_class=>'CHI::Driver::File')->short_driver_name
=> File
CHI->new(driver_class=>'My::Driver::File')->short_driver_name
=> My::Driver::File
- Standard read-write accessors
-
expires_in
expires_at
expires_variance
label
on_get_error
on_set_error
- Standard read-only accessors
-
namespace
serializer
Deprecated methods¶
The following methods are deprecated and will be removed in a later version:
is_empty
DURATION EXPRESSIONS¶
Duration expressions, which appear in the "set" command and various
other parts of the API, are parsed by Time::Duration::Parse. A duration is
either a plain number, which is treated like a number of seconds, or a number
and a string representing time units where the string is one of:
s second seconds sec secs
m minute minutes min mins
h hr hour hours
d day days
w week weeks
M month months
y year years
e.g. the following are all valid duration expressions:
25
3s
5 seconds
1 minute and ten seconds
1 hour
CHI strives to accept arbitrary keys and values for caching regardless of the
limitations of the underlying driver.
- •
- Keys that are references are serialized - see
"key_serializer".
- •
- Keys with wide (>255) characters are utf8 encoded.
- •
- Keys exceeding the maximum length for the underlying driver
are digested - see "max_key_length" and
"key_digester".
- •
- For some drivers (e.g. CHI::Driver::File), keys containing
special characters or whitespace are escaped with URL-like escaping.
Note: All transformations above with the exception of escaping are
one-way, meaning that CHI does not attempt to undo them when returned
from "get_keys"; and
idempotent, meaning that applying them a
second time has no effect. So when you call "get_keys", the key you
get may not be exactly what you passed in, but you'll be able to pass that key
in to get the corresponding object.
- •
- Values which are references are automatically serialized
before storing, and deserialized after retrieving - see
"serializer".
- •
- Values with their utf8 flag on are utf8 encoded before
storing, and utf8 decoded after retrieving.
SUBCACHES¶
It is possible to a cache to have one or more
subcaches. There are
currently two types of subcaches:
L1 and
mirror.
L1 cache¶
An L1 (or "level one") cache sits in front of the primary cache,
usually to provide faster access for commonly accessed cache entries. For
example, this places an in-process Memory cache in front of a Memcached cache:
my $cache = CHI->new(
driver => 'Memcached',
servers => [ "10.0.0.15:11211", "10.0.0.15:11212" ],
l1_cache => { driver => 'Memory', global => 1, max_size => 1024*1024 }
);
On a "get", the L1 cache is checked first - if a valid value exists,
it is returned. Otherwise, the primary cache is checked - if a valid value
exists, it is returned, and the value is placed in the L1 cache with the same
expiration time. In this way, items fetched most frequently from the primary
cache will tend to be in the L1 cache.
"set" operations are distributed to both the primary and L1 cache.
You can access the L1 cache with the "l1_cache" method. For example,
this clears the L1 cache but leaves the primary cache intact:
$cache->l1_cache->clear();
Mirror cache¶
A mirror cache is a write-only cache that, over time, mirrors the content of the
primary cache. "set" operations are distributed to both the primary
and mirror cache, but "get" operations go only to the primary cache.
Mirror caches are useful when you want to migrate from one cache to another. You
can populate a mirror cache and switch over to it once it is sufficiently
populated. For example, here we migrate from an old to a new cache directory:
my $cache = CHI->new(
driver => 'File',
root_dir => '/old/cache/root',
mirror_cache => { driver => 'File', root_dir => '/new/cache/root' },
);
We leave this running for a few hours (or as needed), then replace it with
my $cache = CHI->new(
driver => 'File',
root_dir => '/new/cache/root'
);
You can access the mirror cache with the "mirror_cache" method. For
example, to see how many keys have made it over to the mirror cache:
my @keys = $cache->mirror_cache->get_keys();
Creating subcaches¶
As illustrated above, you create subcaches by passing the "l1_cache"
and/or "mirror_cache" option to the CHI constructor. These options,
in turn, should contain a hash of options to create the subcache with.
The cache containing the subcache is called the
parent cache.
The following options are automatically inherited by the subcache from the
parent cache, and may not be overridden:
expires_at
expires_in
expires_variance
serializer
(Reason: for efficiency, we want to create a single cache object and store it in
both caches. The cache object contains expiration information and is dependent
on the serializer. At some point we could conceivably add code that will use a
single object or separate objects as necessary, and thus allow the above to be
overridden.)
The following options are automatically inherited by the subcache from the
parent cache, but may be overridden:
namespace
on_get_error
on_set_error
All other options are initialized in the subcache as normal, irrespective of
their values in the parent.
It is not currently possible to pass an existing cache in as a subcache.
Common subcache behaviors¶
These behaviors hold regardless of the type of subcache.
The following methods are distributed to both the primary cache and subcache:
clear
expire
purge
remove
The following methods return information solely from the primary cache. However,
you are free to call them explicitly on the subcache. (Trying to merge in
subcache information automatically would require too much guessing about the
caller's intent.)
get_keys
get_namespaces
get_object
get_expires_at
exists_and_is_expired
is_valid
dump_as_hash
Multiple subcaches¶
It is valid for a cache to have one of each kind of subcache, e.g. an L1 cache
and a mirror cache.
A cache cannot have more than one of each kind of subcache, but a subcache can
have its own subcaches, and so on. e.g.
my $cache = CHI->new(
driver => 'Memcached',
servers => [ "10.0.0.15:11211", "10.0.0.15:11212" ],
l1_cache => {
driver => 'File',
root_dir => '/path/to/root',
l1_cache => { driver => 'RawMemory', global => 1 }
}
);
Methods for parent caches¶
- has_subcaches( )
- Returns a boolean indicating whether this cache has
subcaches.
- l1_cache( )
- Returns the L1 cache for this cache, if any. Can only be
called if has_subcaches is true.
- mirror_cache( )
- Returns the mirror cache for this cache, if any. Can only
be called if has_subcaches is true.
- subcaches( )
- Returns the subcaches for this cache, in arbitrary order.
Can only be called if has_subcaches is true.
Methods for subcaches¶
- is_subcache( )
- Returns a boolean indicating whether this is a
subcache.
- subcache_type( )
- Returns the type of subcache as a string, e.g. 'l1_cache'
or 'mirror_cache'. Can only be called if is_subcache is true.
- parent_cache( )
- Returns the parent cache (weakened to prevent circular
reference). Can only be called if is_subcache is true.
Developing new kinds of subcaches¶
At this time, subcache behavior is hardcoded into CHI::Driver, so there is no
easy way to modify the behavior of existing subcache types or create new ones.
We'd like to make this more flexible eventually.
SIZE AWARENESS¶
If "is_size_aware" or "max_size" are passed to the
constructor, the cache will be
size aware - that is, it will keep track
of its own size (in bytes) as items are added and removed. You can get a
cache's size with "get_size".
Size aware caches generally keep track of their size in a separate meta-key, and
have to do an extra store whenever the size changes (e.g. on each set and
remove).
Maximum size and discard policies¶
If a cache's size rises above its "max_size", items are discarded
until the cache size is sufficiently below the max size. (See
"max_size_reduction_factor" for how to fine-tune this.)
The order in which items are discarded is controlled with
"discard_policy". The default discard policy is 'arbitrary', which
discards items in an arbitrary order. The available policies and default
policy can differ with each driver, e.g. the CHI::Driver::Memory driver
provides and defaults to an 'LRU' policy.
Appropriate drivers¶
Size awareness was chiefly designed for, and works well with, the
CHI::Driver::Memory driver: one often needs to enforce a maximum size on a
memory cache, and the overhead of tracking size in memory is negligible.
However, the capability may be useful with other drivers.
Some drivers - for example, CHI::Driver::FastMmap and CHI::Driver::Memcached -
inherently keep track of their size and enforce a maximum size, and it makes
no sense to turn on CHI's size awareness for these.
Also, for drivers that cannot atomically read and update a value - for example,
CHI::Driver::File - there is a race condition in the updating of size that can
cause the size to grow inaccurate over time.
SUBCLASSING AND CONFIGURING CHI¶
You can subclass CHI for your own application and configure it in a variety of
ways, e.g. pre-defining storage types and defaults for new cache objects. Your
configuration will be independent of the main CHI class and other CHI
subclasses.
Start with a trivial subclass:
package My::CHI;
use base qw(CHI);
1;
Then, just use your subclass in place of CHI:
my $cache = My::CHI->new( ... );
print $cache->chi_root_class;
==> 'My::CHI'
This obviously doesn't change any behavior by itself. Here's an example with
actual config:
package My::CHI;
use base qw(CHI);
__PACKAGE__->config({
storage => {
local_file => { driver => 'File', root_dir => '/my/root' },
memcached => {
driver => 'Memcached::libmemcached',
servers => [ '10.0.0.15:11211', '10.0.0.15:11212' ]
},
},
namespace => {
'Foo' => { storage => 'local_file' },
'Bar' => { storage => 'local_file', depth => 3 },
'Baz' => { storage => 'memcached' },
}
defaults => { storage => 'local_file' },
memoize_cache_objects => 1,
});
1;
Each of these config keys is explained in the next section.
Configuration keys¶
- storage
- A map of names to parameter hashrefs. This provides a way
to encapsulate common sets of parameters that might be used in many
caches. e.g. if you define
storage => {
local_file => { driver => 'File', root_dir => '/my/root' },
...
}
then
my $cache = My::CHI->new
(namespace => 'Foo', storage => 'local_file');
is equivalent to
my $cache = My::CHI->new
(namespace => 'Foo', driver => 'File', root_dir => '/my/root');
- namespace
- A map of namespace names to parameter hashrefs. When you
create a cache object with the specified namespace, the hashref of
parameters will be applied as defaults. e.g. if you define
namespace => {
'Foo' => { driver => 'File', root_dir => '/my/root' },
'Bar' => { storage => 'database' },
...
}
then
my $cache1 = My::CHI->new
(namespace => 'Foo');
my $cache2 = My::CHI->new
(namespace => 'Bar');
is equivalent to
my $cache1 = My::CHI->new
(namespace => 'Foo', driver => 'File', root_dir => '/my/root');
my $cache2 = My::CHI->new
(namespace => 'Bar', storage => 'database');
- defaults
- A hash of parameters that will be used as core defaults for
all cache objects created under this root class. e.g.
defaults => {
on_get_error => 'die',
expires_variance => 0.2,
}
These can be overridden by namespace defaults, storage settings, or
"new" parameters.
- memoize_cache_objects
- True or false, indicates whether
"My::CHI->new" should memoize and return the same cache
object if given the same parameters. This can speed things up if you
create cache objects frequently. Will currently only work for 0- or 1- key
parameter hashes. e.g.
My::CHI->config({
memoize_cache_objects => 1,
});
then
# $cache1 and $cache2 will be the same object, regardless of what
# namespace and storage defaults are associated with 'Foo'
#
my $cache1 = My::CHI->new(namespace => 'Foo');
my $cache2 = My::CHI->new(namespace => 'Foo');
# $cache3 and $cache4 will be different objects
#
my $cache3 = My::CHI->new
(namespace => 'Bar', driver => 'File', root_dir => '/my/root');
my $cache4 = My::CHI->new
(namespace => 'Bar', driver => 'File', root_dir => '/my/root');
To clear the memoized cache objects, call
My::CHI->clear_memoized_cache_objects;
How defaults are combined¶
Defaults are applied in the following order, from highest to lowest precedence:
- •
- Parameters passed in "new"
- •
- Namespace defaults, if any
- •
- Storage settings, if any
- •
- Core defaults defined under 'defaults'
Inheritance of config¶
A subclass will automatically inherit the configuration of its parent if it does
not call "config" itself (ala Class::Data::Inheritable).
Reading config from a file¶
use YAML::XS qw(LoadFile);
__PACKAGE__->config(LoadFile("/path/to/cache.yml"));
AVAILABILITY OF DRIVERS¶
The following drivers are currently available as part of this distribution:
- •
- CHI::Driver::Memory - In-process memory based cache
- •
- CHI::Driver::RawMemory - In-process memory based cache that
stores references directly instead of serializing/deep-copying
- •
- CHI::Driver::File - File-based cache using one file per
entry in a multi-level directory structure
- •
- CHI::Driver::FastMmap - Shared memory interprocess cache
via mmap'ed files
- •
- CHI::Driver::Null - Dummy cache in which nothing is
stored
- •
- CHI::Driver::CacheCache - CHI wrapper for Cache::Cache
The following drivers are currently available as separate CPAN distributions:
- •
- CHI::Driver::Memcached - Distributed memory-based cache
(works with Cache::Memcached, Cache::Memcached::Fast, and
Cache::Memcached::libmemcached)
- •
- CHI::Driver::DBI - Cache in any DBI-supported database
- •
- CHI::Driver::BerkeleyDB - Cache in BerkeleyDB files
- •
- CHI::Driver::Redis - Cache in Redis
<http://redis.io/>
This list is likely incomplete. A complete set of drivers can be found on CPAN
by searching for "CHI::Driver".
See CHI::Benchmarks for a comparison of read/write times of both CHI and non-CHI
cache implementations.
"etc/bench/bench.pl" in the "CHI" distribution contains a
script to run these types of benchmarks on your own system.
DEVELOPING NEW DRIVERS¶
See CHI::Driver::Development for information on developing new drivers.
LOGGING¶
"CHI" uses Log::Any for logging events. For example, a debug log
message is sent for every cache get and set.
See Log::Any documentation for how to control where logs get sent, if anywhere.
STATS¶
CHI can record statistics, such as number of hits, misses and sets, on a
per-namespace basis and log the results to your Log::Any logger. You can then
use utilities included with this distribution to read stats back from the logs
and report a summary. See CHI::Stats for details.
RELATION TO OTHER MODULES¶
Cache::Cache¶
CHI is intended as an evolution of DeWitt Clinton's Cache::Cache package. It
starts with the same basic API (which has proven durable over time) but
addresses some implementation shortcomings that cannot be fixed in
Cache::Cache due to backward compatibility concerns. In particular:
- Performance
- Some of Cache::Cache's subclasses (e.g. Cache::FileCache)
have been justifiably criticized as inefficient. CHI has been designed
from the ground up with performance in mind, both in terms of general
overhead and in the built-in driver classes. Method calls are kept to a
minimum, data is only serialized when necessary, and metadata such as
expiration time is stored in packed binary format alongside the data.
- Ease of subclassing
- New Cache::Cache subclasses can be tedious to create, due
to a lack of code refactoring, the use of non-OO package subroutines, and
the separation of "cache" and "backend" classes. With
CHI, the goal is to make the creation of new drivers as easy as possible,
roughly the same as writing a TIE interface to your data store. Concerns
like serialization and expiration options are handled by the driver base
class so that individual drivers don't have to worry about them.
- Increased compatibility with cache implementations
- Probably because of the reasons above, Cache::Cache
subclasses were never created for some of the most popular caches
available on CPAN, e.g. Cache::FastMmap and Cache::Memcached. CHI's goal
is to be able to support these and other caches with a minimum performance
overhead and minimum of glue code required.
Cache¶
The Cache distribution is another redesign and implementation of Cache, created
by Chris Leishman in 2003. Like CHI, it improves performance and reduces the
barrier to implementing new cache drivers. It breaks with the Cache::Cache
interface in a few ways that I considered non-negotiable - for example,
get/set do not serialize data, and namespaces are an optional feature that
drivers may decide not to implement.
Cache::Memcached, Cache::FastMmap, etc.¶
CPAN sports a variety of full-featured standalone cache modules representing
particular backends. CHI does not reinvent these but simply wraps them with an
appropriate driver. For example, CHI::Driver::Memcached and
CHI::Driver::FastMmap are thin layers around Cache::Memcached and
Cache::FastMmap.
Of course, because these modules already work on their own, there will be some
overlap. Cache::FastMmap, for example, already has code to serialize data and
handle expiration times. Here's how CHI resolves these overlaps.
- Serialization
- CHI handles its own serialization, passing a flat binary
string to the underlying cache backend. The notable exception is
CHI::Driver::RawMemory which does no serialization.
- Expiration
- CHI packs expiration times (as well as other metadata)
inside the binary string passed to the underlying cache backend. The
backend is unaware of these values; from its point of view the item has no
expiration time. Among other things, this means that you can use CHI to
examine expired items (e.g. with $cache->get_object) even if this is
not supported natively by the backend.
At some point CHI will provide the option of explicitly notifying the
backend of the expiration time as well. This might allow the backend to do
better storage management, etc., but would prevent CHI from examining
expired items.
Naturally, using CHI's FastMmap or Memcached driver will never be as time or
storage efficient as simply using Cache::FastMmap or Cache::Memcached. In
terms of performance, we've attempted to make the overhead as small as
possible, on the order of 5% per get or set (benchmarks coming soon). In terms
of storage size, CHI adds about 16 bytes of metadata overhead to each item.
How much this matters obviously depends on the typical size of items in your
cache.
SUPPORT AND DOCUMENTATION¶
Questions and feedback are welcome, and should be directed to the perl-cache
mailing list:
http://groups.google.com/group/perl-cache-discuss
Bugs and feature requests will be tracked at RT:
http://rt.cpan.org/NoAuth/Bugs.html?Dist=CHI
bug-chi@rt.cpan.org
The latest source code can be browsed and fetched at:
http://github.com/jonswar/perl-chi/tree/master
git clone git://github.com/jonswar/perl-chi.git
ACKNOWLEDGMENTS¶
Thanks to Dewitt Clinton for the original Cache::Cache, to Rob Mueller for the
Perl cache benchmarks, and to Perrin Harkins for the discussions that got this
going.
CHI was originally designed and developed for the Digital Media group of the
Hearst Corporation, a diversified media company based in New York City. Many
thanks to Hearst management for agreeing to this open source release.
SEE ALSO¶
Cache::Cache
AUTHOR¶
Jonathan Swartz <swartz@pobox.com>
COPYRIGHT AND LICENSE¶
This software is copyright (c) 2011 by Jonathan Swartz.
This is free software; you can redistribute it and/or modify it under the same
terms as the Perl 5 programming language system itself.