| POE::Filter - a protocol abstraction |
POE::Filter - a protocol abstraction
$filter = POE::Filter::Something->new();
$arrayref_of_logical_chunks =
$filter->get($arrayref_of_raw_chunks_from_driver);
$arrayref_of_streamable_chunks_for_driver =
$filter->put($arrayref_of_logical_chunks);
Filters implement generic interfaces to low- and medium-level protocols. Wheels use them to communicate in basic ways without needing to know the details for doing so. For example, the Line filter does everything needed to translate incoming streams into lines and outgoing lines into streams. Sessions can get on with the business of using lines.
These methods are the generic Filter interface, and every filter must implement them. Specific filters may have additional methods.
new() creates and initializes a new filter. Specific filters may have
different constructor parameters.
get() translates raw data into records. What sort of records is
defined by the specific filter. The method accepts a reference to an
array of raw data chunks, and it returns a reference to an array of
complete records. The returned ARRAYREF will be empty if there wasn't
enough information to create a complete record. Partial records may
be buffered until subsequent get() calls complete them.
my $records = $filter->get( $driver->get( $filehandle ) );
get() processes and returns as many records as possible. This is
faster than one record per call, but it introduces race conditions
when switching filters. If you design filters and intend them to be
switchable, please see get_one_start() and get_one().
get() into two stages.
get_one_start() accepts an array reference containing unprocessed
stream chunks. It adds them to the filter's internal buffer and does
nothing else.
get_one() takes no parameters and returns an ARRAYREF of zero or more
complete records from the filter's buffer. Unlike the plain get()
method, get_one() is not greedy. It returns as few records as
possible, preferably just zero or one.
get_one_start() and get_one() reduce or eliminate race conditions when
switching filters in a wheel.
put() serializes records into a form that may be written to a file or
sent across a socket. It accepts a reference to a list of records,
and it returns a reference to a list of stream chunks.
The list reference it returns may be passed directly to a driver.
$driver->put( $filter->put( \@records ) );
get_pending() returns a filter's partial input buffer. Unlike
previous versions, the filter's input buffer is not cleared. The
ReadWrite wheel uses this for hot-swapping filters; it gives partial
input buffers to the next filter.
get_pending() returns undef if nothing is pending. This is different
from get() and get_one().
Filters don't have output buffers. They accept complete records and immediately pass the serialized information to a driver's queue.
It can be tricky keeping both ends of a socket synchronized during a filter change. It's recommended that some sort of handshake protocol be used to make sure both ends are using the same type of filter at the same time.
TCP also tries to combine small packets for efficiency's sake. In a streaming protocol, a filter change could be embedded between two data chunks.
type-1 data type-1 data change to type-2 filter type-2 data type-2 data
A driver can easily read that as a single chunk. It will be passed to a filter as a single chunk, and that filter (type-1 in the example) will break the chunk into pieces. The type-2 data will be interpreted as type-1 because the ReadWrite wheel hasn't had a chance to switch filters yet.
Adding a handshake protocol means the sender will wait until a filter change has been acknowledged before going ahead and sending data in the new format.
clone() makes a copy of the filter, and clears the copy's buffer.
3rd party modules can either implement their own clone() or inherit
from POE::Filter. If inheriting, the object MUST be an array-ref
AND the first element must be the buffer. The buffer can be either a
string or an array-ref.
The SEE ALSO section in POE contains a table of contents covering the entire POE distribution.
In theory, filters should be interchangeable. In practice, stream and block protocols tend to be incompatible.
Please see POE for more information about authors and contributors.
| POE::Filter - a protocol abstraction |