Diffy

Status

This project is used in production at Twitter and is being actively developed and maintained as Opendiffy. Feel
free to contact us on gitter or @diffyproject.

What is Diffy?

Diffy finds potential bugs in your service using running instances of your new code and your old
code side by side. Diffy behaves as a proxy and multicasts whatever requests it receives to each of
the running instances. It then compares the responses, and reports any regressions that may surface
from those comparisons. The premise for Diffy is that if two implementations of the service return
“similar” responses for a sufficiently large and diverse set of requests, then the two
implementations can be treated as equivalent and the newer implementation is regression-free.
For a more detailed analysis of Diffy checkout this blogpost.

How does Diffy work?

Diffy acts as a proxy that accepts requests drawn from any source that you provide and multicasts
each of those requests to three different service instances:

1. A candidate instance running your new code
2. A primary instance running your last known-good code
3. A secondary instance running the same known-good code as the primary instance

As Diffy receives a request, it is multicast and sent to your candidate, primary, and secondary
instances. When those services send responses back, Diffy compares those responses and looks for two
things:

1. Raw differences observed between the candidate and primary instances.
2. Non-deterministic noise observed between the primary and secondary instances. Since both of these
   instances are running known-good code, you should expect responses to be in agreement. If not,
   your service may have non-deterministic behavior, which is to be expected.
   

Diffy measures how often primary and secondary disagree with each other vs. how often primary and
candidate disagree with each other. If these measurements are roughly the same, then Diffy
determines that there is nothing wrong and that the error can be ignored.

How to get started?

Running the example

The example.sh script included here builds and launches example servers as well as a diffy instance. Verify
that the following ports are available (9000, 9100, 9200, 8880, 8881, & 8888) and run ./example/run.sh start.

Once your local Diffy instance is deployed, you send it a few requests
like curl --header "Canonical-Resource: Json" localhost:8880/json?Twitter. You can then go to your browser at
http://localhost:8888 to see what the differences across our example instances look like.

Digging deeper

That was cool but now you want to compare old and new versions of your own service. Here’s how you can
start using Diffy to compare three instances of your service:

1. Deploy your old code to localhost:9990. This is your primary.

2. Deploy your old code to localhost:9991. This is your secondary.

3. Deploy your new code to localhost:9992. This is your candidate.

4. Download the latest Diffy binary from maven central or build your own from the code using ./sbt assembly.

5. Run the Diffy jar with following command line arguments:

   java -jar diffy-server.jar \
   -candidate=localhost:9992 \
   -master.primary=localhost:9990 \
   -master.secondary=localhost:9991 \
   -service.protocol=http \
   -serviceName=My-Service \
   -proxy.port=:8880 \
   -admin.port=:8881 \
   -http.port=:8888 \
   -rootUrl='localhost:8888'
   

6. Send a few test requests to your Diffy instance on its proxy port:

   curl localhost:8880/your/application/route?with=queryparams
   

7. Watch the differences show up in your browser at http://localhost:8888.

Using Diffy via Docker

Please refer to the Diffy dockerhub page for how to deploy the docker image.

FAQ's

For safety reasons POST, PUT, DELETE are ignored by default . Add -allowHttpSideEffects=true to your command line arguments to enable these verbs.

HTTPS

If you are trying to run Diffy over a HTTPS API, the config required is:

-service.protocol=https

And in case of the HTTPS port be different than 443:

-https.port=123

License

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this software except in compliance with the License.

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.