# Compressed Binary CASes

## Binary CAS Compression overview

UIMA has a proprietary binary serialization format, used internally for several things, including communicating with embedded C++ annotators using UIMA-CPP. This binary format is also selectable for use with UIMA-AS. Its use requires that the source and target systems implement the identical type system (because the type system is not sent, and internal coding is used within the format that is keyed to the particular type system).

Starting with version 2.4.1, two additional forms of binary serialization are added. Both compress the data being serialized; typical size ratios can approach 50 : 1, depending on the exact contents of the CAS, when compared with normal binary serialization.

The two forms are called 4 and 6, for historical/internal reasons. The serialized forms of both of these is fixed, but not currently standardized, and the form being used is encoded in the header so that the appropriate deserializer can be chosen. Both forms include support for Delta CAS being returned from a service.

Form 6 builds on form 4, and adds: serializing only those feature structures which are reachable (that is, in some index, or referenced by other reachable feature structures), and type filtering.

Type filtering takes a source type system and a target type system, and for serializing (source to target), sends the binary representation of reachable feature structures in the target's type system. For deserializing (reading a target into a source), the filtering takes the specification being read as being encoded using the target's type system, and translates that into the source's type system. In this process, types which exist in the source but not the target are skipped (when serializing); types which exist in the target, but not the source are skipped when deserializing. Features that exist in some source type but not in the version of the same type in the target are skipped (when serializing) or set to default values (i.e., 0 or null) when being deserialized.

There are two main use cases for using compressed forms. The first one is for communicating with UIMA-AS remote services (not yet implemented).

The second use case is for saving compressed representations of CASes to other media, such as disk files, where they can be deserialized later for use in other UIMA applications.

## Using Compressed Binary CASes

The main user interface for serializing a CAS using compression is to use one of the static methods named serializeWithCompression in Serialization. If you pass a Type System argument representing a target type system, then form 6 compression is used; otherwise form 4 is used. To get the benefit of only serializing reachable Feature Structure instances, without type mapping (which is only in form 6), pass a type system argument which is null.

To deserialize into a CAS without type mapping, use one of the deserialize method in Serialization. There are multiple forms of this method, depending on the arguments. The forms which take extra arguments include a ReuseInfo may only be used with serialized forms created with form 6 compression. The plain form of deserialize works with all forms of binary serialization, compressed and non-compressed, by examining a common header which identifies the form of binary serialization used; however, for form 6, since it requires additional arguments, it will fail - and you need to use the other deserialize form.

Form 6 has an additional object, ReuseInfo, which holds information which is required for subsequent Delta CAS format serializations / deserializations. It can speed up subsequent serializations of the same CAS (before it is further updated), for instance, if an application is sending the CAS to multiple services in parallel. The serializeWithCompression method returns this object when form 6 is being used.

## Simple Delta CAS serialization

Use Form 4 for this, becuase form 6 supports delta CAS but requires that at the time of deserialization of a CAS (on the receiver side) which will later be delta serialized back to the sender, an instance of the ReuseInfo must be saved, and that same instance then used for delta serialization; furthermore, the original serialization (on the sender side) also must save an instance of the ReuseInfo and use this when deserializing the delta CAS.

Form 4 may not be as efficient as form 6 in that it does not filter the CASes either by type systems nor by only sending reachable Feature Structure instances. But, it doesn't require a ReuseInfo object when doing delta serialization or deserialization, so it may be more convenient to use when saving delta CASes to files (as opposed to the other use case of a remote service returning delta CASes to a remote client).

## Use Case cookbook

Here are some use cases, together with a suggested approach and example of how to use the APIs.

Save a CAS to an output stream, using form 4 (no type system filtering):

    // set up an output stream.  In this example, an internal byte array.
    ByteArrayOutputStream baos = new ByteArrayOutputStream(OUT_BFR_INIT_SZ);
    Serialization.serializeWithCompression(casSrc, baos);

Deserialize from a stream into an existing CAS:

    // assume the stream is a byte array input stream
    // For example, one could be created
    //   from the above ByteArrayOutputStream as follows:
    ByteArrayInputStream bais = new ByteArrayInputStream(baos.toByteArray());
    // Deserialize into a cas having the identical type system
    Serialization.deserializeCAS(cas, bais);

Note that the `deserializeCAS(cas, inputStream)` method is a general way to deserialize into a CAS from an inputStream for all forms of binary serialized data (with exceptions as noted above). The method reads a common header, and based on what it finds, selects the appropriate deserialization routine.

The `deserialization` method with just 2 arguments method doesn't support type filtering, or delta cas deserializating for form 6. To do those, see example below.

Serialize to an output stream, filtering out some types and/or features:

To do this, an additional input specifying the Type System of the target must be supplied; this Type System should be a subset of the source CAS's. The `out` parameter may be an OutputStream, a DataOutputStream, or a File.

    // set up an output stream.  In this example, an internal byte array.
    ByteArrayOutputStream baos = new ByteArrayOutputStream(OUT_BFR_INIT_SZ);
    Serialization.serializeWithCompression(cas, out, tgtTypeSystem);

Deserialize with type filtering:

The reuseInfo should be null unless deserializing a delta CAS, in which case, it must be the reuse info captured when the original CAS was serialized out. If the target type system is identical to the one in the CAS, you may pass null for it. If a delta cas is not being received, you must pass null for the reuseInfo.

    ByteArrayInputStream bais = new ByteArrayInputStream(baos.toByteArray());
    Serialization.deserializeCAS(cas, bais, tgtTypeSystem, reuseInfo);