etcd as OpenDaylight (ODL) DataBroker database store
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README.md

This project started as a personal POC by Michael Vorburger.ch to evaluate the feasibility of using etcd as data store for YANG data in OpenDaylight (ODL), instead of its current "home made" CDS based on Akka.

See this presentation given in 2018.09 at the ODL DDF during ONS Europe in Amsterdam for some background.

The plan is contribute this to opendaylight.org, if and when successful.

How to build?

  1. To get a more recent version of jetcd-core than the currently used 0.0.2, just git clone https://github.com/coreos/jetcd.git ; cd jetcd ; mvn [-DskipTests] clean install

  2. If you're hitting an NPE in org.apache.maven.plugins:maven-javadoc-plugin:3.0.0:jar, just use mvn -Dmaven.javadoc.skip=true -Dduplicate-finder.skip -DskipTests -Dcheckstyle.skip clean install to work around it; it's possibly related to not having JAVA_HOME environment variable (what's weird is that this NPE does not seem to happen with other OpenDaylight builds).

Architecture Design

A cluster of Etcd servers appears as a single logical server to clients like this. Details about its internal clustering, Raft implementation etc. are transparent.

The EtcdDataStore implements DOMStore and internally uses a YANG DataTree, just like the sal-distributed-datastore (CDS) does.

On commit(), the put/merge/delete writes from DataTreeModification / DataTreeCandidate are sent to etcd. Each DataTreeCandidateNode is stored as an individual sub key/value - without their respective child nodes. This allows for fine-grained future updates and deletes. Changes from DataTreeCandidate are sent atomically to etcd (using TXN, not PUT).

The data is stored in a compact binary serialization format (not e.g. XML or JSON). The communication from the etcd client in ODL to the etcd server/s is similarly compact binary, not text-based over HTTP.

We watch etcd, and update our internal DataTree as and when we receive change events. Changes from watch events are applied atomically to the DataTree.

To guarantee strong consistency, we (remote) check the current revision on etcd, for a every new transaction, and await having received and processed watch events at least up to that current revision. This is what blocks reads.

If DataBroker offered an eventual consistency read API to applications, then it would be trivial to offer (optionally) blazing fast reads (directly from the local DataTree), without any remoting.

We never do any GET on etcd to read data, but always serve directly from the DataTree. There is no ser/der and tree-reconstruction overhead for reads (but there is when processing watch events).

This approach also guarantees that we have up-to-date data for validation.

Distributed Data Change Listeners also work as expected with this mechanism.

Deployment Considerations

etcd instances would typically best be localhost co-located with the ODL nodes.

Demos

Make sure you have at least 1 etcd server running:

sudo dnf install etcd
sudo systemctl start etcd
systemctl status etcd
ETCDCTL_API=3 etcdctl get --from-key ''

or even just start it directly, without systemd, in the foreground in another terminal tab:

cd /tmp
etcd

tree /tmp/default.etcd/
ETCDCTL_API=3 etcdctl get --from-key ''

TODO Document how to best easily start test cluster of 3 etcd servers locally...

If you used etcd before you may want to completely wipe it:

ETCDCTL_API=3 etcdctl del "" --from-key=true

Beware that ETCDCTL_API=3 etcdctl get --from-key '' outputs binary, so better use our own:

java -jar demo/target/*.jar read http://localhost:2379

RESTCONF

Here is how to run the asciinema POC v0.2:

Configure the etcd connection URL (TODO this is a hack which will change to be a real configuration file later):

mkdir karaf/target/assembly/../../jetcd-launcher-maven-plugin/
echo http://localhost:2379 >karaf/target/assembly/../../jetcd-launcher-maven-plugin/endpoint

Now start ODL and make sure it's happy:

./karaf/target/assembly/bin/karaf
opendaylight-user@root>feature:list -i
opendaylight-user@root>diag

Note how there is no karaf/target/assembly/etc/org.opendaylight.controller.cluster.datastore.cfg now, thus "proving" that we don't run the existing ODL datastore anymore. You can now use RESTCONF as usual - but it runs on etcd! Open apidoc/explorer (admin/admin), and watch the logs (tail -f karaf/target/assembly/data/log/karaf.log) to understand what happens when you e.g. do:

http -a admin:admin GET http://localhost:8181/restconf/config/opendaylight-etcd-test:HelloWorldContainer
echo '<HelloWorldContainer xmlns="urn:opendaylight:etcd:test"><name>hello, world</name></HelloWorldContainer>' >put.xml
http -v -a admin:admin PUT :8181/restconf/config/opendaylight-etcd-test:HelloWorldContainer @put.xml
http -a admin:admin GET :8181/restconf/config/opendaylight-etcd-test:HelloWorldContainer
java -jar demo/target/*.jar read http://localhost:2379
http -a admin:admin DELETE :8181/restconf/config/opendaylight-etcd-test:HelloWorldContainer
http -a admin:admin GET :8181/restconf/config/opendaylight-etcd-test:HelloWorldContainer

Standalone

Here is how to run the asciinema POC v0.1:

Now run this project's demo:

java -jar demo/target/*.jar write http://localhost:4001

or if you started etcd directly without systemd then:

java -jar demo/target/*.jar write http://localhost:2379

and have a closer look at the logs this will print to understand what happened, and read it via:

java -jar demo/target/*.jar read http://localhost:2379

FAQ

About this project

  • What is the status of this project? As of late July 2018, it's a Proof of Concept (POC) with the EtcdDBTest illustrating, successfully, that the ODL MD SAL DataBroker API can be implemented on top of the etcd data store.

  • What's the point of this? The main goal is to have the option in ODL to completely avoid the home grown Raft/clustering code, and completely avoid Akka. This will ease maintenance.

  • Is this project going to automagically solve all sorts of performance issues you may face in ODL today? Nope. Increasing ODL performance (compared to CDS) is not a goal of this project.

  • How can I access the data in etcd? Through ODL APIs (or RESTCONF, etc.) via the code in this project - as always. It is an explicit non-goal of this project to allow "direct" access to the YANG data in etcd. It is stored in an internal binary format, which may change. It requires the YANG model schema to really make sense. Don't read it directly. What you could do however is run a lightweight standalone "ODL" process which uses this project.

  • How can you try this out? Much work still needs to be done! ;-) This e.g. includes, roughly in order: much more unit and integration tests (notably around concurrency), some re-factorings required in ODL to remove code copy/paste here during the POC, work to make it easy to install instead of the current implementation, packaging work to make this available as a Karaf feature, then much real world testing through CSITs, etc.

  • How can you help? Please see the TODO.md and start contributing!

About some typical objections

  • But how will we upgrade the code from today's clustering solution to an etcd based datastore? The idea is that ultimately this will simply be a new alternative feature installation, and require absolutely no change to any existing application code.

  • But how will we migrate the data from today to tomorrow during customer upgrades? Replay based upgrades start with a fresh new empty datastore, so this is a non-issue. (A non replay based upgrade procedures would have to export the datastore content using DAEXIM, and re-import a dump into an instance with an etcd datastore.)

  • But how can we "shard" with this? Supporting several "shards" and/or multiple etcd stores (for sharding, not clustering) is an explicit non-goal of v1 of this project.

  • But etcd doesn't seem to have a pure in-memory mode, so what about operational vs config? So in ODL the operational data store, contrary to the configuration, does not have to survive "restarts". But perhaps it's OK if it does anyway. If not, it would certainly be easily possible to explicitly wipe the content of the operational data store sub tree in etcd on the start of the ODL cluster (not of a single ODL node, and not of the etcd cluster; which is going to have a separate lifecycle). Perhaps longer term, having an option to keep certain sub-tress only in-memory and not persisted to disk could be brought up with the etcd community as a possible feature request, purely as a performance optimization. For short and even medium term for ODL etcd adopters, this should not be a blocking issue.

  • But what about the EntityOwnershipService, EOS? It should be possible to implement it on to of etcd's Lock API, but this is still TBD. Help most welcome!

  • But what about remote RPCs? Dunno. Needs more thought and POC, discussions... TBD.

  • But I love ODL’s current datastore! This project, if successful, will be an alternative to and existing in parallel with the current implementation likely for a long time.

About alternatives

  • Why not XYZ as a KV DB? There are a number of other Key Value DBs. Some of the code from this project likely is a good basis for you to write adapters from YANG to other KV DBs. Have fun!

  • Why not just replace the Akka persistence LevelDB plugin currently used in CDS to something else like MongoDB? : Not sure that I see how an architecture where you keep Akka and would replace LevelDB (a node local single user embedded KV) to a remote multi user DB makes much sense. Using something like MongoDB as a data store like this project does for etcd perhaps makes more sense; see question above. You can, of course, just use MongoDB or whatever else you like (JDBC if you must, anything you fancy...) INSTEAD of a YANG-based data store, if that meets your requirements.

About etcd

  • What is etcd? etcd is a distributed key value store that provides a reliable way to store data across a cluster of machines. Communication between etcd machines is handled via the Raft consensus algorithm.

  • Why etcd? Among many other users, etcd is the database used in Kubernetes (and its distributions such as OpenShift). It makes sense to align ODL to this. With the Core OS acquisition, Red Hat has etcd expertise.

  • I read somewhere online that "etcd clients may have issues with operations that time out (network disruption for example) and will not send an abort respond". How do we plan on dealing with this? This will cause a timeout at the GRPC layer internally, which will lead to a failure on the MD SAL (commit) operation, which will be propagated to the ODL application client - as it should; all good.

  • I heard that "On network split (AKA split brain) read request may be served mistakenly by the minority split." How do we plan on dealing with this? According to this documentation, "there is no 'split-brain' in etcd".

  • But, but, but... Please consult with the opensource etcd community, or obtain professional support, for further doubts about and issues with etcd - just like you would say in OpenStack if you had a problem with its MariaDB (mysql) database. Relying on a well established and here-to-stay persistence engine, instead of building, debugging and maintaining a home grown one, is really the main point of this project! ;-)