On Oct 20, 2008, at 6:43 PM, Yih Sun Khoo wrote:
Thanks Chris and Joman for your detailed explanations.
Would this be a good example of using a shallow copy? Also I'm
trying to
wrap my head around why the shallow copy is needed. You mentioned
it is to
eliminate any state from the values the list might have formerly
contained.
Could you give me an example of when the deep copy implementation
would
have
been terrible?
MyWritable foo = new MyWritable();
// foo contains "A0", "A1", "A2" in its namelist
foo.readFields(in);
// attempt to save the collection of Strings
Collection<String> myStrings = foo.getNameList();
// clears the collection and replaces it with "B0", "B1"
foo.readFields(in);
// Expected: "A0", "A1", "A2", "B0", "B1"
// Contains: "B0", "B1", "B0", "B1"
myStrings.addAll(foo.getNameList());
@SuppressWarnings("unchecked")
*public* ArrayList<String> getNameList() {
*return* (ArrayList<String>) nameList.clone();
}
You may not need the shallow copy, depending on how you're using
the type.
If your contract with the user is that getNameList() is invalid if
readFields is called again, you can call that out in documentation.
The more
canonical pattern is to return an iterator, which will fail in this
case
(instead of silent, unexpected results).
Also is it common to have instead used a one of the existing
writables,
like
DoubleWritable, as part of my custom writable. I would imagine in
the
readFields method I would call the DoubleWritable's readFields
method.
There's usually no reason to allocate an object for a primitive
type in a
composite type. It doesn't buy you much, since you're already using
DataInput and DataOutput interfaces, which define the wire format.
What would go into (re)using a collection of Text instead of
String? Could
I perhaps use an ArrayList<Text> and in the readFields and write
methods
call each Text's readFields and write methods appropriately?
Depending on how your data looks, tracking the "valid" size of your
collection inside your type may permit you to reuse Text objects
instead of
creating new, immutable Strings. Unless you start noticing it as a
bottleneck, you're probably fine with Strings, though. -C
On Mon, Oct 20, 2008 at 11:38 AM, Chris Douglas <[EMAIL PROTECTED]
wrote:
TupleWritable is not a general-purpose type. It's used for map-side
joins,
where the arity of a tuple is fixed by construction. Its intent
is a
transient type with very, very specific applications in mind.
It sounds like you don't need a general list type, as you don't
need to
worry about encoding the type of object your list contains.
Writables are
*not* supposed to read to the end of the stream they're given;
they are
to
consume a full instance from the stream (i.e. it must consume all
"its"
bytes from a stream, even if it ultimately discards them). Given
these
constraints, Writable types of variable size almost always encode
their
length explicitly. As Joman mentioned, your constructor must
initialize
all
its elements. Further, readFields must not retain any state from
the
value
it formerly contained, so you need to clear the list before you
add more
values to it. This means your getNameList method will need to do a
shallow
copy of its elements if the caller stores a reference to the list.
This should work:
public void readFields(DataInput in) throws IOException {
nameList.clear();
score = in.readDouble();
final int len = WritableUtils.readVInt(in);
for (int i = 0; i < len; ++i) {
nameList.add(Text.readString(in));
}
}
public void write(DataOutput out) throws IOException {
out.writeDouble(score);
WritableUtils.writeVInt(out, nameList.size());
for (String name : nameList) {
Text.writeString(out, name);
}
}
You can improve your performance by (re)using a collection of Text
instead
of String (since the latter is immutable), but that requires more
work.
-C
On Oct 19, 2008, at 3:39 PM, Yih Sun Khoo wrote:
I think when it comes to the TupleWritable being part of a custm
writable,
you cannot just say tupleWritable.readFields(in) and
tupleWritable.write(out)
I might be wrong. Has anyone successfully implemented a
TupleWritable
with
,say, a DoubleWritable in a custom writable?
On Sun, Oct 19, 2008 at 3:33 AM, Joman Chu <[EMAIL PROTECTED]>
wrote:
hrm, try implementing the read(DataInput in) method, as well as a
blank constructor MyWritable() that fills dummy values into your
instance variables. For example this should be all you need for
read(DataInput in),
public static MyWritable read(DataInput in) throws IOException {
MyWritable w = new MyWritable();
w.readFields(in);
return w;
}
EDIT: I was able to sort of replicate your error. In my
constructor, i
had my instance variables assigned to null. Make sure you
assign them
to new instances of whatever Writable you are using.
Joman Chu
http://www.notatypewriter.com/
AIM: ARcanUSNUMquam
On Sun, Oct 19, 2008 at 5:10 AM, Yih Sun Khoo <[EMAIL PROTECTED]>
wrote:
Joman to add a little bit more to one of my previous mails
about the
readFields methods
Have you ever had something like this?
public class MyWritable implements Writable {
private DoubleWritable doubleWritable;
private TupleWritable tupleWritable;
public void readFields(DataInput in) throws IOException {
doubleWritable.readFields(in);
tupleWritable.readFields(in);
}
public void write(DataOutput out) throws IOException {
doubleWritable.write(out);
tupleWritable.write(out);
}
}
On Sun, Oct 19, 2008 at 1:59 AM, Joman Chu <[EMAIL PROTECTED]>
wrote:
I've never used TupleWritable, so hopefully somebody else can
help you
with that.
Joman Chu
http://www.notatypewriter.com/
AIM: ARcanUSNUMquam
On Sun, Oct 19, 2008 at 4:40 AM, Yih Sun Khoo
<[EMAIL PROTECTED]>
wrote:
Also, I've noticed TupleWritable to be quite useful.
What are good techniques for using TupleWritable in a
mapping phase
for a
"list of Text" when you do not know the size of that "list"
ahead of
time
Say I had a custom writable which implemented TupleWritable
and the
custom
writable contained a setter method
mycustomwritable.setTupleWritable( ... )
Where the ellipsis is, there lies the TupleWritable.
However I'm
wondering
since TupleWritable can be constructed using
TupleWritable(Writable[]),
how
do I dynamically resize the Writable[] and add Text elements
to it
when I
don't know the size of the Writable[] very well. Does this make
sense?
On Sun, Oct 19, 2008 at 1:32 AM, Yih Sun Khoo <[EMAIL PROTECTED]
>
wrote:
Let's say in the reduce phase your value happens to hold an
ArrayListWritable
In this example, value is of type ArrayListWritable
Maybe I've not thought about this or done this before, but
how does
one
"read data in from the DataInput stream" in the reduce phase so
that
the
ArrayListWritable which is a value already passed to the
reducer can
be
used
as ArrayListWritable
On Sun, Oct 19, 2008 at 1:25 AM, Joman Chu <[EMAIL PROTECTED]
>
wrote:
Since the ArrayListWritable extends ArrayList, you have access
to
all
the ArrayList methods as well. Once you read data in from the
DataInput stream, you should be able to use ArrayListWritable
just
like a regular ArrayList.
Joman Chu
http://www.notatypewriter.com/
AIM: ARcanUSNUMquam
On Sun, Oct 19, 2008 at 4:16 AM, Yih Sun Khoo <[EMAIL PROTECTED]
>
wrote:
Hmm, what method from ArrayListWritable allows you to access
the
different
elements of the ArrayList? Would it be readFields? for
example,
in
a
reduce phase, if I needed to know the size of the array list,
it
would
be
easy if i were dealing with an arraylist because i could
just say
arraylist.size. How would i accomplish that with the
writable
counterpart?
On Sun, Oct 19, 2008 at 1:04 AM, Joman Chu <[EMAIL PROTECTED]
>
wrote:
Hi,
For the ArrayList object, try taking a look at the
implementation
of
ArrayListWritable by Jimmy Lin at UMD here:
https://subversion.umiacs.umd.edu/umd-hadoop/core/trunk/src/edu/umd/cloud9/io/ArrayListWritable.java
But basically in the readFields methods, I prefer using each
Writable
object's readFields method to read the data in. For example,
for
your
double variable, I would use a DoubleWritable object and in the
MyWritable.readFields(DataInput in), I would use
nameofdoublewritable.readFields(in). For the
MyWritable.write(DataOutput out) method, I would use
nameofdoublewritable.write(out).
Have a good one,
Joman Chu
http://www.notatypewriter.com/
AIM: ARcanUSNUMquam
On Sun, Oct 19, 2008 at 3:30 AM, Yih Sun Khoo <[EMAIL PROTECTED]
wrote:
I don't quite know how to write the read and write
functions,
but
I
want
to
write my own writable, which should have a
DoubleWritable/double
value
followed by a list of Strings/Text. This Writable will be
used
as
a
value.
Is the code below the best way to go about writing such a
writable?
import java.io.DataInput;
import java.io.DataOutput;
import java.io.EOFException;
import java.io.IOException;
import java.util.ArrayList;
import org.apache.hadoop.io.Writable;
public class MyWritable implements Writable {
private double score;
private ArrayList<String> nameList;
public void setScore(double score) {
this.score= score;
}
public void setNameList(ArrayList<String> nameList) {
this.nameList= nameList;
}
public double getScore() {
return score;
}
public ArrayList<String> getNameList() {
return nameList;
}
public void readFields(DataInput in) throws IOException {
score= in.readDouble();
try {
do {
nameList.add(in.readUTF());
} while (true);
} catch (EOFException eofe) {
// continue; done
}
}
public void write(DataOutput out) throws IOException {
out.writeDouble(score);
for (String name: nameList) {
out.writeUTF(name);
}
}
}
