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From Mark Slee <ms...@facebook.com>
Subject RE: heterogeneous collections
Date Tue, 04 May 2010 05:04:10 GMT
The protocol scheme was written the way it was because it was very simple, transparent, straightforward
to implement, safe to version changes, and reasonably defensive.

- Field identifiers are necessary for versioning
- Type identifiers are necessary so that we know how to skip fields that we don't recognize
- Therefore, the protocol sends a field identifier, then a type identifier, then the data

We could have used field-size instead of a type identifier. That simplifies the skip read-operation,
but comes at the cost of making the write operation much more complicated. It means that if
you are serializing a complex type, you have to first compact the whole thing down to determine
its total size in bytes, then write out that header. This leads to internal-buffering code
in the protocols, not fun when you're dealing with containers-of-containers. Even simple cases
are awkward, I can't know the byte-length of a list of strings without actually iterating
over all the strings first.

So, using the type-identifier system keeps the TProtocol interface incredibly flat and obvious,
just serial calls to read/write simple values, always one at a time.

The checking of types for known field ids is just basic defensiveness, protection against
someone changing the type of a field but forgetting to update its identifier. We don't generate
errors because this is considered the same class of occurrence as an invalid field identifier.

I totally agree with you about Thrift *seeming* like a partial attempt to implement dynamic
RPC. This was basically my point -- I know they look similar -- so I do happily excuse people
for thinking this. =)

Cheers,
mcslee

-----Original Message-----
From: Mayan Moudgill [mailto:mayan@bestweb.net] 
Sent: Monday, May 03, 2010 9:38 PM
To: thrift-user@incubator.apache.org; Mark Slee
Subject: Re: heterogeneous collections


If the goal of Thrift was to transport strongly typed data, then it begs 
the question: why was the protocol scheme being used currently adopted?

Clearly, if the data is typed with the types being agreed to at both 
ends, then NO type information needs to be exchanged (other than you 
container lengths - which you may not consider type information). If the 
data is typed, but the there may be disagreement over the type at the 
reciever, then you have to send the complete type information along with 
the data. TBinaryProtocol does neither.

Thrift does't have strict strict typing: the stated goal of thrift to 
support versioning, where the transmitted type and the receiver type are 
permitted to differ by the addition or deletion of fields. This means 
that the only information that needs to be transmitted are field-id and 
field-size; either the field is a known one, in which case full type 
info is known, or the field is unknown, in which case the number of 
bytes to skip is known.

Assuming that Thrift were intended to be strongly typed, the only reason 
to actually transmit as much type information as TBinaryProtocol 
actually does is that implicitly Thrift is also allowing for the type of 
fields to be changed. Was this intended to account for the case where a 
field was deleted, then reused [which does beg the question - what 
happens if the reused type is the same as the original?]; if so, there 
may very well have been different and better ways of doing this.

Other than that the only reason I can come up with is that this was some 
kind of type-checking half-measure to ensure correctness. But the 
default behavior on a type-mismatch appears to be to discard the field, 
not generate some kind of error.

So, given the implementation of TBinaryProtocol, people could be excused 
for thinking that its a partial attempt to implement a dynamically typed 
RPC.

Mark Slee wrote:
>>>If, however, you're encoding the data for demarshalling at the server, 
>>>it sounds like you want a different RPC framework.
> 
> I'm going to slightly hijack the conversation to wax philosophic for a
> minute here. I think this statement roughly captures my sentiment here.
> 
> One of Thrift's early goals was basically to do just one thing, but do
> it very simply and efficiently across lots of platforms. That thing is
> *strongly-typed* RPC and data-serialization. All of the components were
> essentially designed under the assumption that they would always be
> strongly-typed, and that they should always map to something efficient
> and obvious in a language like C++.
> 
> Now, a lot of the things Thrift does are very *similar* to other
> sorts of interesting mechanisms data-serialization, marshalling,
> containering, and whatnot. I think it can be very tempting to look at
> these similarities, analyze the distance between the two things, and
> decide since that distance looks pretty crossable, so we should just
> build a bridge to connect the two.
> 
> My fear is that in the long run this turns a small, neat, island into
> a complicated mess of bridges. If you find the right viewing angle and
> it's not a foggy day, you can sometimes still see the little island
> underneath the bridges, but this Thrift thing definitely looks like
> bridgework, not an island.
> 
> In the long term, my personal bias is that this is bad for Thrift. Most
> people interested in building these features need them to solve specific
> problems and only care about one or two target languages. If we do a lot
> of this, we end up with a patchwork set of variable feature-lists that
> are inconsistent across languages. The Thrift "brand" will invariably
> move away from "simple, lightweight, lets you do the same thing in all
> programming languages" towards "a bit complicated, does some things in
> some languages."
> 
> Part of the idea of Thrift's modular transport/protocol design
> was that it would make it easy for people to implement custom
> extensions/modifications to the system *outside of the core project.*
> Want to sub in your own weird encoding/transport/whatever? No problem,
> just write a TProtocol. Think other folks will be into it? Cool, post
> it online and send an email to the thrift-user@ list. Turns out lots of
> folks want to use it? Then maybe we should incorporate it.
> 
> For better or worse, I really think simple things like "how many source
> files appear to be in this tarball?" can matter a lot for software
> adoption. Even if a project is just 10 easy-to-read files at its core,
> when you have to locate those 10 files amongst 40 files of extensions
> and add-ons, and the default make configuration builds everything, the
> project starts feeling like a complicated, awkward thing to deal with,
> and us engineers start getting that itchy feeling of "I can't possibly
> understand this entire thing, surely it is too complicated and slow, why
> don't we just write our own from scratch."
> 
> I don't expect everyone to agree with this, and the direction of the
> project is ultimately at the behest of the developers most actively
> working on it, but when it comes to things like dynamic or heterogeneous
> containers, my opinion is that they just shouldn't be a core part of a
> strongly-typed software project with stated simplicity goals.


> Cheers,
> Mark
> 
> -----Original Message-----
> From: Mayan Moudgill [mailto:mayan@bestweb.net] 
> Sent: Monday, May 03, 2010 10:03 AM
> To: thrift-user@incubator.apache.org; alex@bizo.com
> Subject: Re: heterogeneous collections
> 
> 
> The idea of marshalling to strings seems somewhat counter-productive; 
> after all, you're marshalling the data using Thrift, which then gets 
> sent to a server, and demarshalls it. Now, on top of that you're adding 
> another layer of marshalling.
> 
> A similar thing happens in  Cassandra (except that they use binary 
> instead of strings), but at least at Cassandra the user-marshalled data 
> is uninterpreted at the server - it only handles the data as an 
> uninterpreted blob, so the marshalling/demarshalling is only confined to 
> the client [I still wonder about how version control is managed - does 
> everyone end up rolling their own?]
> 
> If, however, you're encoding the data for demarshalling at the server, 
> it sounds like you want a different RPC framework. For instance, do you 
> really need the version flexibility that is provided by Thrift? Are your 
> types fixed at source & destination? Do you need a leaner transport? In 
> fact, why did you pick Thrift in the first place?
> 
> Apropos the discussion on scalar/string compression in the 
> https://issues.apache.org/jira/browse/THRIFT-110
> I'm curious: if a particular application would tend to compress better 
> using a different algo than the one(s) provided, what happens?
> 
> 
>>On Mon, May 3, 2010 at 7:09 AM, Bryan Duxbury <bryan@rapleaf.com> wrote:
>>
>>
>>
>>>There is already a totally viable workaround, though - make a Union of the
>>>types you want in your collection, and then make the field list<YourUnion>.
>>>You get basically all the capabilities with very few drawbacks, plus the
>>>ability to include multiple logical "types" in the collection, not just
>>>physical types. Of course, if you literally need "any" possible object to
>>>go
>>>into the collection, then this won't do it for you.
>>>
>>
>>
>>Thanks for the suggestion, Bryan.
>>
>>I'm experimenting with marshalling my values to strings (I only deal with
>>basic types such as int32, int64, strings) right now.   If that doesn't
>>work, I'll go with your suggestion.
>>
>>alex
>>
> 
> 
> 
> 


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