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From "Jay Kreps" <jay.kr...@gmail.com>
Subject Re: Serialization with additional schema info
Date Wed, 03 Sep 2008 22:52:01 GMT
Hi Tom,

Your concern about cross-dependencies makes sense.  To state the
problem more generally, a serializer is a mapping of on-disk
structures to in-memory structures; one way to maintain this mapping
is to pregenerate a class for each mapping using some IDL like thrift,
but another common way is just to store the mapping as data.  Both
approaches have pluses and minuses (the usual trade-offs of
code-generation vs reflection) but I believe the current interface
only allows the code generation approach.

To remove the dependence on mapreduce from what I had below in (3) you
would change the SerializationFactory method from
SerializationFactory.getSerialization(Class<T> c)
to
SerializationFactory.getSerialization(Class<T> c, String s)

and to use it in MapTask when you create the serializer for the
MapOutputBuffer instead of
keySerializer = serializationFactory.getSerializer(job.getMapOutputKeyClass());
you would have
keySerializer =
serializationFactory.getSerializer(job.getMapOutputKeyClass(),
job.get("mapred.mapper.serialization.info"));

And similarly for reducers and deserializers. The parameter would
contain any additional schema information you needed to disambiguate a
class such as Object, List, or Map that might have different
serializations depending on its contents (e.g. it might get passed in
List.class and  "list(int32)" to instruct the serializer to read a
list of integers, or Map.class and "map(int32, list(string))" to
indicate a map of integers to lists of strings. Then implementations
could chose to use the string parameter, the class parameter, or both
when constructing the Serialization.

Thanks for the pointer to jaql, that seems very cool, but I believe
jaql would have the same problem if they tried to implement any kind
of compact structured storage.  Jaql would return a JArray or JRecord
which might have a variety of fields and you would want to store the
data about what kinds of fields separately.

Thanks,

-Jay

> From: "Tom White" <tom.e.white@gmail.com>
> To: core-dev@hadoop.apache.org
> Date: Wed, 3 Sep 2008 14:15:28 +0100
> Subject: Re: Serialization with additional schema info
> Jay,
>
> The Serialization and MapReduce APIs are very class-based - so having
> fixed types with dynamic serialization capabilities doesn't work as
> well in the current design.
>
> I like 2 better than 1, but both make the Serialization API dependent
> on MapReduce, which it currently isn't. And arguably it shouldn't be
> as you could use it simply to do serialization of data, outside a
> MapReduce context. Perhaps SerializerType is just a String, which also
> makes things more flexible (at the expense of type-safety)?
>
> What would the API changes look like for 3?
>
> Also, I believe the Jaql team has been looking at how to write JSON
> serializers, so perhaps there is an opportunity for collaboration
> here?
>
> Tom
>
> On Mon, Sep 1, 2008 at 9:52 PM, Jay Kreps <boredandroid@gmail.com> wrote:
> > Hi All,
> >
> > I am interested in hooking up a custom serialization layer I use to the new
> > pluggable Hadoop serialization framework. It appears that the framework
> > assumes there is a one-to-one mapping between java classes and
> > serializations.  This is exactly what we want to get away from--having a
> > common data format allows us to easily write generic data aggregation jobs
> > that work with any type. This is exactly how a database supports many
> > generic operations such as joins, group bys, etc--because the dataformat is
> > always a set of tuples which can be generically manipulated without
> > understanding any of the details of interpretation rather than user defined
> > complex types the db can't operate on. To do this I need to store data in a
> > standard way with supported types and have a short string schema description
> > along with each file, and pass that description to a generic
> > serializer/deserializer in order to tell it how to read the bytes in the
> > file. The problem I have is that there is no way to get the additional
> > schema information into the serializer to tell it how to serialize and
> > deserialize.
> >
> > Some Details in case the general problem is too vague:
> >
> > A very nice generic data format that maps well to programming languages is
> > JSON. For example a user could be stored like this: {"name":"Jay",
> > "date-o-birth":"05-25-1980", "age":28, "is_active": true, etc.}. But since
> > we store the same fields with each "row", this is highly inefficient. It
> > makes more sense to just store the necessary bytes for the values, and store
> > what fields we are expecting, and the expected type seperately. This let's
> > us store numbers compactly as well.
> >
> > JSON supports numbers, strings, lists, and maps, which all have natural
> > mappings in Java. The above user example would translate to a java Map
> > containing the given keys and values.
> >
> > Here is where the trouble starts. I can't do this in the existing
> > SerializationFactory because the type for the object is just Map.class, but
> > that doesn't contain enough info to properly deserialize the class. In
> > reality I need a string describing the type, such as
> >  {"name":"string", "date-o-birth":"date", "age":"int32",
> > "is_active":"boolean", ...}
> > Note that this string contains all the information needed to add in the
> > property names and to correctly interpret the bytes as Integer or Boolean,
> > or whatever.
> >
> > The obvious solution is to just add this schema into the JobConf as a
> > property such as "map.key.schema.info", and use it to construct the right
> > serializer in the Serialization implmentation. The problem with this is that
> > there is no way for the Serialization implementation to know whether it is
> > constructing the map key, map value, reduce key, or reduce value.
> >
> > Some possible solutions:
> >
> > For now I am just sticking with wrapping up map and reduce to do the
> > serialization/deserialization to solve my problem. However this seems like a
> > common case where the serialization needs information not present in the
> > class itself, and I would like to add support to do it right. Would you guys
> > accept a patch that did one of the following:
> >
> > 1. Make SerializationFactory have a getMapKeySerializer,
> > getMapValueSerializer, etc. method and allow the user to specify their own
> > SerializationFactory by setting a property with the appropriate class name.
> > This is probably the most flexible and doesn't break any user serialization
> > implementations. The getMapKeySerializer method can then check the
> > map.key.schema.info in addition to mapred.mapinput.key.class.
> > 2. Change Serialization.getSerializer(
> Class c) to
> > Serialization.getSerializer(Class c, SerializerType k) where SerializerType
> > = enum {MapKey, MapValue, ReduceKey, ReduceValue}. This allows the
> > serialization implementer to invent their own properties (map.key.schema or
> > whatever) and fetch the appropriate thing.
> > 3. Add mapred.mapinput.serializer.info, mapred.reduceinput.serializer.info,
> > etc. and pass the value of this into the constructor of the serializer if it
> > has a constructor with a single string argument.
> >
> > Or maybe there a better way to accomplish this?
> >
> > Thanks!
> >
> > -Jay
> >

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