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

new() creates and initializes a new filter. Specific filters may have different constructor parameters.

get ARRAYREF

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_one_start ARRAYREF

get_one

These methods are a second interface to a filter's input translation. They split the usual 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 ARRAYREF

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

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.

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.