Re: [boost] Using serialization for replication

From: Robert Ramey (ramey_at_[hidden])
Date: 2006-04-03 23:02:47

Preston A. Elder wrote:
>> It would seem to me that the easiest way to do this would be
>> to make a TEE type streambuf using the stream buffer library.
>> This would duplicate each write to an additional stream.
> Do you mean with boost::iostream? or as a part of
> boost::serialization?

Remember that serialization uses streambuf for doing the actual i/o.
Hence any thing that streambuf (boost streams) implements such as
compression, duplicaiton, etc, is "inherited" by boost serialization.

>>> however the biggest barrier to this is the fact that serialization
>>> does not allow me to put the same object into the stream twice.  To
>>> be more precise, I want to be able to serialize both to a network
>>> and to disk, and I very much like the elegance of the serialization
>>> approach (plus the fact it can re-create pointer references, arrays,
>>> and such).
>>
>> I believe the above would cover this.
> TEE would handle going to both network and disk, but it would not
> obviate the 'single object only once' problem.  According to the
> serialization documentation (Reference -> Speical Considerations ->
> Object Tracking

This would be done by setting the serialization trait "tracking" to
track_never.  This would inhibit the checking for duplicates.  This
would occur before it gets to the stream buf implementation

> (http://www.boost.org/libs/serialization/doc/special.html#objecttracking)),
> an object may only be put on the stream once, I cannot put an object
> that has been changed on the stream again to be re-serialized (either
> by replacing the previously serialized entry, or adding it to be
> serialized again, but without allocating a new object).

that's what "track_never" is for.

> This is why I was asking in the first place how difficult the
> modifications would be to allow an object to be serialized twice, and
> serialization to understand this and not create a separate instance,
> but just update the existing instance.

So track_never permits the object to be written multiple times.

Using a custom streambuf would place the serialized output into multiple
streams.

Robert Ramey