1---
2layout: post
3title: "Promise Pipelining and Dependent Calls: Cap'n Proto vs. Thrift vs. Ice"
4author: kentonv
5---
6
7_UPDATED:  Added Thrift to the comparison._
8
9So, I totally botched the 0.4 release announcement yesterday.  I was excited about promise
10pipelining, but I wasn't sure how to describe it in headline form.  I decided to be a bit
11silly and call it "time travel", tongue-in-cheek.  My hope was that people would then be
12curious, read the docs, find out that this is actually a really cool feature, and start doing
13stuff with it.
14
15Unfortunately, [my post](2013-12-12-capnproto-0.4-time-travel.html) only contained a link to
16the full explanation and then confusingly followed the "time travel" section with a separate section
17describing the fact that I had implemented a promise API in C++.  Half the readers clicked through
18to the documentation and understood.  The other half thought I was claiming that promises alone
19constituted "time travel", and thought I was ridiculously over-hyping an already-well-known
20technique.  My HN post was subsequently flagged into oblivion.
21
22Let me be clear:
23
24**Promises alone are _not_ what I meant by "time travel"!**
25
26<img src='{{ site.baseurl }}images/capnp-vs-thrift-vs-ice.png' style='width:350px; height:275px; float: right;'>
27
28So what did I mean?  Perhaps [this benchmark](https://github.com/kentonv/capnp-vs-ice) will
29make things clearer.  Here, I've defined a server that exports a simple four-function calculator
30interface, with `add()`, `sub()`, `mult()`, and `div()` calls, each taking two integers and\
31returning a result.
32
33You are probably already thinking:  That's a ridiculously bad way to define an RPC interface!
34You want to have _one_ method `eval()` that takes an expression tree (or graph, even), otherwise
35you will have ridiculous latency.  But this is exactly the point.  **With promise pipelining, simple,
36composable methods work fine.**
37
38To prove the point, I've implemented servers in Cap'n Proto, [Apache Thrift](http://thrift.apache.org/),
39and [ZeroC Ice](http://www.zeroc.com/).  I then implemented clients against each one, where the
40client attempts to evaluate the expression:
41
42    ((5 * 2) + ((7 - 3) * 10)) / (6 - 4)
43
44All three frameworks support asynchronous calls with a promise/future-like interface, and all of my
45clients use these interfaces to parallelize calls.  However, notice that even with parallelization,
46it takes four steps to compute the result:
47
48    # Even with parallelization, this takes four steps!
49    ((5 * 2) + ((7 - 3) * 10)) / (6 - 4)
50      (10    + (   4    * 10)) /    2      # 1
51      (10    +         40)     /    2      # 2
52            50                 /    2      # 3
53                              25           # 4
54
55As such, the Thrift and Ice clients take four network round trips.  Cap'n Proto, however, takes
56only one.
57
58Cap'n Proto, you see, sends all six calls from the client to the server at one time.  For the
59latter calls, it simply tells the server to substitute the former calls' results into the new
60requests, once those dependency calls finish.  Typical RPC systems can only send three calls to
61start, then must wait for some to finish before it can continue with the remaining calls.  Over
62a high-latency connection, this means they take 4x longer than Cap'n Proto to do their work in
63this test.
64
65So, does this matter outside of a contrived example case?  Yes, it does, because it allows you to
66write cleaner interfaces with simple, composable methods, rather than monster do-everything-at-once
67methods.  The four-method calculator interface is much simpler than one involving sending an
68expression graph to the server in one batch.  Moreover, pipelining allows you to define
69object-oriented interfaces where you might otherwise be tempted to settle for singletons.  See
70[my extended argument]({{ site.baseurl }}rpc.html#introduction) (this is what I was trying to get
71people to click on yesterday :) ).
72
73Hopefully now it is clearer what I was trying to illustrate with this diagram, and what I meant
74by "time travel"!
75
76<img src='{{ site.baseurl }}images/time-travel.png' style='max-width:639px'>
77