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From "Jay Kreps" <boredandr...@gmail.com>
Subject Serialization with additional schema info
Date Mon, 01 Sep 2008 20:52:50 GMT
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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