NAME

POE::Filter - a protocol abstraction

SYNOPSIS

  $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);

DESCRIPTION

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.

PUBLIC FILTER METHODS

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 as defined by the filter. It accepts a reference to an array of raw data chunks, and it returns a reference to an array of complete records.

Drivers' get() methods return ARRAYREFs of raw data chunks suitable for passing to filters' put() methods.

  my $records = $filter->get( $driver->get( $filehandle ) );

There needn't be a 1:1 ratio between raw data and logical records. Some filters buffer partial records until they are completed in subsequent get() calls.

get() returns a reference to an empty array if the stream doesn't include enough information to complete a record.

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, clearing it in the process. The ReadWrite wheel uses this for hot-swapping filters; it gives partial input buffers to the next filter.

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.

SEE ALSO

The SEE ALSO section in POE contains a table of contents covering the entire POE distribution.

BUGS

In theory, filters should be interchangeable. In practice, stream and block protocols tend to be incompatible.

AUTHORS & COPYRIGHTS

Please see POE for more information about authors and contributors.