DO NOT MERGE: feat(sharding): add label-driven predicate routing

[mlwelles]

DO NOT MERGE

POC/Experiment for review / discussion purposes only.

This is a proof-of-concept implementation for label-based predicate sharding. It is intended for review and testing purposes only.

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Summary

Add @label(name) directive to predicates, allowing them to be pinned to specific alpha groups that share the same label. This creates implicit sub-clusters for data isolation.

Key Changes

• Add label field to Member, Tablet, and SchemaUpdate protos
• Parse @label(X) directive in schema (schema/parse.go)
• Route labeled predicates to matching labeled groups in Zero
• Require explicit label parameter in Tablet(), BelongsTo(), ForceTablet()
• /moveTablet validates label compatibility: blocks moves when destination group label differs from predicate label
• Prevent unlabeled predicates from being assigned to labeled groups

Label Routing Flow

1. Alpha with --label=X registers with Zero, Member.Label is set
2. Schema with @label(X) is parsed, SchemaUpdate.Label is set
3. Zero routes tablet to group where Member.Label matches
4. Rebalancer skips tablets with non-empty Label

Example Usage

# Start alphas with labels:
#   alpha1: (no label) - serves unlabeled predicates
#   alpha2: --label=secret
#   alpha3: --label=top_secret

# Schema:
name: string @index(term) .                    # -> group 1 (unlabeled)
codename: string @index(term) @label(secret) . # -> group 2 (secret)
alias: string @index(term) @label(top_secret) .# -> group 3 (top_secret)

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Testing Instructions

Prerequisites

1. Docker must be running
2. For macOS, build the Linux binary:

   GOOS=linux make install
   cp $GOPATH/bin/linux_arm64/dgraph $GOPATH/bin/dgraph

Run the Tests

# Build test runner and run label sharding tests
cd t && go build . && ./t --pkg=systest/label

# Run with verbose output
./t --pkg=systest/label -v

# Keep containers running after tests for debugging
./t --pkg=systest/label --keep

# Run a specific test
./t --pkg=systest/label --test=TestLabeledPredicateRouting

Test Cluster Configuration

The tests spin up a Docker cluster with:

Container	Group	Label	Purpose
zero1	-	-	Cluster coordinator
alpha1	1	(none)	Serves unlabeled predicates
alpha2	2	secret	Serves @label(secret) predicates
alpha3	3	top_secret	Serves @label(top_secret) predicates

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What the Tests Verify

Test	What It Verifies
TestLabeledAlphaRegistration	Alphas started with --label=X correctly register their label with Zero. Verifies Member.Label is set in cluster state.
TestLabeledPredicateRouting	Predicates with @label(X) are assigned to the group whose alpha has matching label. Verifies tablet assignments in Zero's /state endpoint.
TestLabeledPredicateDataIsolation	Data mutations and queries work correctly across predicates served by different labeled groups. Inserts data using all predicate types and queries it back.
TestLabeledPredicateCannotBeMoved	The /moveTablet API validates label compatibility. Attempting to move a labeled predicate to a group with a different (or no) label returns an error. This ensures predicates stay within their designated label boundary.
TestUnlabeledPredicateNotOnLabeledGroup	Unlabeled predicates are never assigned to groups that have a label. Prevents data leakage into labeled sub-clusters.
TestMissingLabelGroupError	Applying a schema with @label(nonexistent) returns a clear error when no alpha has that label.

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Test Plan

• TestLabeledAlphaRegistration ✓
• TestLabeledPredicateRouting ✓
• TestLabeledPredicateDataIsolation ✓
• TestLabeledPredicateCannotBeMoved ✓
• TestUnlabeledPredicateNotOnLabeledGroup ✓
• TestMissingLabelGroupError ✓

All 6 integration tests pass.