ddlog and OVN

My first encounter with ddlog integrated with ovn-northd

In a not distant future, OVN will be leveraging the power of ddlog to synchronize its southbound and northbound databases, thanks to the awesome work done by Ben Pfaff, Leonid Ryzhyk and others. I took some time at my Red Hat’s day of learning to see how that looks like and wrote some notes along the way.

Installation

As explained in Documentation/intro/install/general.rst, rust is needed in order to use ddlog with ovn-northd-ddlog. Here are the steps I took in order to have it working on a Centos 8 install:

$ # rust install
$ curl https://sh.rustup.rs -sSf > /tmp/rustup.sh

$ # make sure the file looks legit...
$ vi /tmp/rustup.sh

$ bash /tmp/rustup.sh

$ # check rustc version
$ source $HOME/.cargo/env
$ rustc --version
rustc 1.47.0 (18bf6b4f0 2020-10-07)

The script will give you choices for customizing the install, including the setting of the PATH variable. As for installing the ddlog portion, only a tarball is needed:

$ # Note: OVN requires DDlog v0.30.0 or newer
$ curl -L -o /tmp/ddlog.tgz  \
  https://github.com/vmware/differential-datalog/releases/download/v0.30.0/ddlog-v0.30.0-20201027231514-linux.tar.gz
$ cd && tar xzf /tmp/ddlog.tgz

At this point, I changed .bash_profile and tried the ddlog command:

$ cat <<EOT >>~/.bash_profile

DDLOG_HOME="\${HOME}/ddlog"
export DDLOG_HOME
PATH="\${HOME}/.cargo/bin:\${DDLOG_HOME}/bin:\${PATH}"
export PATH
EOT


$ . ~/.bash_profile
$ ddlog --version
DDlog v0.30.0 (56b0da1837d7ec8ea227873d0e129f5dfca179a1)
Copyright (c) 2019-2020 VMware, Inc. (MIT License)

hello world, meet ddlog

Before getting into OVN, I tried a hello world version of ddlog to appreciate what the fuss is all about. Here is a simple example I thought of doing after reading https://hexgolems.com/2020/10/getting-started-with-ddlog/. There are a few extra links I found useful at the bottom of this page.

mkdir hw ; cd hw

cat <<EOT > ./helloWorld.dl
function is_even_number(value: u32): bool { value % 2 == 0 }
input relation ANumber(name: string, value: u32)
output relation EvenNumber(name: string)
EvenNumber(name) :- ANumber(name, value), is_even_number(value).
EOT

cat <<EOT > ./helloWorld.dat
start;
insert ANumber("two", 2),
insert ANumber("two", 2),
insert ANumber("two", 2),
commit;
dump EvenNumber;
EOT

cat <<EOT > ./helloWorld.dat2
start;
insert ANumber("one", 1),
insert ANumber("ten", 10),
insert ANumber("five", 5),
insert ANumber("twelve", 12),
insert ANumber("three", 3);
commit;
dump EvenNumber;
EOT

Building helloWorld_ddlog:

$ ddlog -i helloWorld.dl && \
  (cd helloWorld_ddlog && cargo build --release)

$ ./helloWorld_ddlog/target/release/helloWorld_cli < helloWorld.dat
EvenNumber{.name = "two"}

$ ./helloWorld_ddlog/target/release/helloWorld_cli < helloWorld.dat2
EvenNumber{.name = "ten"}
EvenNumber{.name = "twelve"}

Trying out ovn-northd-ddlog

I used my fork in github to pull in Ben’s current review for ddlog. That includes changes I did to enhance meters in OVN ACL logs. More on that below.

Take a look here if you need to see the prerequisites for building OVS and OVN. After taking care of that, here is what I did in order to build OVN with northd-ddlog:

$ git clone --depth 1 https://github.com/openvswitch/ovs.git && \
  cd ovs

$ ./boot.sh && ./configure --enable-Werror && \
  time make -j$(($(nproc) + 1)) V=0 ; echo $?

$ cd .. && \
  git clone --depth 1 -b acl-meters.v5.ddlog \
  https://github.com/flavio-fernandes/ovn.git && cd ovn

$ ./boot.sh && ./configure --enable-Werror \
  --with-ovs-source=${PWD}/../ovs \
  --with-ddlog=${DDLOG_HOME}/lib --enable-ddlog-fast-build && \
  time make -j$(($(nproc) + 1)) V=0 ; echo $?

The with-ddlog param instructs OVN to build ovn-northd-ddlog. If ddlog is not installed, or you just want the non-ddlog version of ovn-northd, simply omit that. The enable-ddlog-fast-build makes compilation faster but will cause ovn-northd-ddlog to skip optimizations. Avoid using that flag outside development deployments.

The core of ddlog in ovn-northd-ddlog lives in the file ovn_northd.dl. Ben did a great job explaining the ins and outs of it when I asked for help on what would be needed to have the fair acl log meters there. You can see that conversation in the ovs mailing list.

The changes in OVN to leverage ddlog are only in northd so far (not in ovn-controller). Also, they are being done in a way that the non-ddlog version of northd is still available. In order to determine which version of northd is used, the ovn-ctl script will check for the ovn-northd-ddlog parameter. This is what it looks like to use ovn-northd-ddlog:

$ DDLG='--ovn-northd-ddlog=yes' ; \
  sudo /usr/share/ovn/scripts/ovn-ctl $DDLG start_northd

The ovn-northd tests have a loop that exercises both versions, so either one can be used until we are ready to cut the umbilical cord. Here is a sample output of how that looks like:

$ make check TESTSUITEFLAGS="--keywords=fair"
...
## ------------------------ ##
## ovn 20.09.90 test suite. ##
## ------------------------ ##

OVN northd

353: ovn-northd -- ACL fair Meters -- ovn-northd     ok
354: ovn-northd -- ACL fair Meters -- ovn-northd-ddlog ok

## ------------- ##
## Test results. ##
## ------------- ##

All 2 tests were successful.

There are still some rough edges in need of polishing, but I am certain these will get resolved soon. If you are curious about that, take a look at the logs from the upstream OVN meeting of 12/Nov/20.

Lastly, these links were super useful in getting acclimated with ddlog:

Cheers!