capnproto

A simple example on Capnproto

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Introduction

Using capnproto to implement a key-value storage service. The interface between the service and client supports three different kinds of operations:

• get(key): get the value (in this demo, the value is some blog text) by key.
• store(key, blog): store the key and blog pair.
• remove(key): remove the key and corresponding blog from the storage.

Moreover, we pipeline get and store operation to implement the copy(key1, key2) operation, without changing the capnp interface file. That is, the original procedure:

value = get(key1);
store(key2, value);

is changed to something looks like:

getPromise = get(key1);
storePromise = store(key2, getPromise);
storePromise.then(...);

There is no data transfer in the intermidiate step.

Dependencies

Having been tested on Ubuntu 16.04 and Mac OS.

Linux

Install dependencies:

sudo apt update
sudo apt install make g++ pkg-config -y

Install latest capnproto:

curl -O https://capnproto.org/capnproto-c++-0.6.1.tar.gz
tar zxf capnproto-c++-0.6.1.tar.gz
cd capnproto-c++-0.6.1
./configure
make -j6 check
sudo make install
sudo ldconfig

Mac OS

brew install capnp

Run

Build it.

make all

Run with IP address and port. For exmaple:

./server *:9527
# Other host
./client <the ip of server>:9527

Or locally, using unix domain socket:

rm -f /tmp/capnp-$$
./server unix:/tmp/capnp-$$
# Other terminal
./client unix:/tmp/capnp-$$

Enjoy it!

Performance

The below table shows the average operation latency on the c3.large instance in AWS US East (Virginia), and the network condition is moderate.

Operation	Get	Store	Remove	Copy
Unix domain socket	207µs	161µs	152µs	232µs
Loopback device	246µs	163µs	152µs	267µs
Local network	446µs	372µs	301µs	381µs

The time for copy is comparable with that of get. Moreover, copy is much smaller than the sum of get and store in all cases.

Note

Some of the code is adopted from offical samples.