simple-loadbalancer

LPM-based load balancer with three /24 prefixes mapping to three backend ports. The example installs an initial route table as a single batched Write, reads it back to verify the device's view, modifies one route at runtime, and reads again to confirm.

Source on GitHub: examples/simple-loadbalancer/ (Java + P4 + Gradle build)

What this example demonstrates

• Pushing a pipeline + batch-inserting many entries in one RPC (sw.batch().insert(...).insert(...).execute()).
• LPM match construction with new Match.Lpm(Bytes, prefixLen) and the Match.lpm(cidr) shorthand.
• Reading entries back with sw.read("…").all() and walking the result.
• Runtime route mutation with sw.modify(...).
• IPv4 type ergonomics: Ip4.of("10.0.1.0").toBytes().

Running locally

cd examples
./gradlew :simple-loadbalancer:run

Optional: --args="my-bmv2-host:50051" to override the device address. See the example's README for the BMv2 setup line.

Expected output

[LB] connected as primary on 127.0.0.1:50051, pipeline pushed
[LB] installed 3 routes (allSucceeded=true)
[LB] backend_lookup after install: 3 entries
[LB]   10.0.1.0/24 → port 1
[LB]   10.0.2.0/24 → port 2
[LB]   10.0.3.0/24 → port 3
[LB] moved 10.0.2.0/24 to port 4
[LB] backend_lookup after modify: 3 entries
[LB]   10.0.1.0/24 → port 1
[LB]   10.0.2.0/24 → port 4
[LB]   10.0.3.0/24 → port 3
[LB] cleaned up; goodbye

The after modify block confirms the modify-then-readback round trip resolved to the new port.

Things to try

• Insert a more-specific prefix (e.g. 10.0.1.5/32 → port 9) and observe that BMv2's LPM table picks it before the /24.
• Use sw.read("…").match("hdr.ipv4.dstAddr", new Match.Lpm(...)).all() to filter the read on the server side and compare results to the unfiltered .all() (BMv2 implements server-side LPM filter strictly).
• Wrap the cleanup loop in a try/finally so a mid-run crash still tidies the device state.
• Replace one of the .insert(...) calls with an intentionally-bad one (unknown table name) and inspect WriteResult.failures() to see how per-update rejections surface without aborting the whole batch.

See also

• LPM route table batch installation — the recipe extracted from this example.
• Tables — the full Match builder surface (exact / LPM / ternary / range / optional) and the batch() API.
• Canonical bytestring — why a read-back IP address may have fewer bytes than the written form.
• P4Runtime spec mapping — how sw.batch().execute() translates to a single Write RPC with multiple Updates.