---
name: multi-tenancy
description: >
  Use when implementing tenant resolution, database or schema isolation, tenant-aware JPA,
  reactive tenant context, migrations, caching, jobs, or authorization in Spring Boot 3.
---

# Multi-Tenancy

Treat tenant identity as an authorization boundary, not a query convenience.

## Choose an isolation model

- Database per tenant: strongest isolation and highest operational cost.
- Schema per tenant: strong logical isolation with shared infrastructure.
- Shared schema with `tenant_id`: simplest operations, but every access path must enforce scope.
- Document the selected model and prohibit repositories from bypassing it.

## Resolve tenant identity

- Derive the tenant from a verified token claim, trusted host mapping, or authenticated API key.
- Reject missing, unknown, disabled, or conflicting tenant identifiers.
- Never trust a public `X-Tenant-Id` header by itself.
- Clear servlet thread-local context in `finally`; use Reactor `Context` for reactive flows.

## Enforce isolation

- Apply tenant selection before opening the persistence session or transaction.
- Include tenant identity in unique constraints, cache keys, idempotency keys, and object storage paths.
- Prevent cross-tenant joins and unrestricted administrative repositories.
- Authorize support impersonation explicitly and audit every use.

## Operations

- Run migrations per database/schema with resumable progress and version reporting.
- Propagate tenant identity into scheduled jobs, messages, and async tasks explicitly.
- Limit noisy tenants with quotas and per-tenant observability using bounded identifiers.
- Test negative cross-tenant access, not only successful tenant queries.

## Examples

- See `examples/good-tenant-filter.java` and `examples/bad-tenant-filter.java`.

## Executable shared-schema example

The [TenantOrderStore](templates/TenantOrderStore.java) and
[schema](examples/tenant-orders.sql) implement tenant-scoped reads, creates, updates and deletes.
This example uses blocking JDBC, a verified resource-server JWT and an active-tenant registry.
It is not a database/schema-routing or reactive implementation. Preserve the application's
existing ID types and isolation model rather than copying this UUID model indiscriminately.

Wire [TenantScopeFilter](templates/TenantScopeFilter.java) into the security chain before
`AuthorizationFilter` (after bearer authentication and exception translation). Supply a
`RequestMatcher` for only the tenant-protected routes and a server-side predicate checking the
tenant registry. Instantiate the filter inside the chain configuration, or disable its servlet
auto-registration if it is a bean, so it does not run twice. Missing authentication is 401;
missing, disabled or unknown tenant membership is 403. Do not read a public tenant header.

The verified `tenant_id` claim must be issued under a trusted tenant-membership policy; where
revocation must take effect before token expiry, consult current membership as well as the
active-tenant registry. Background jobs must similarly re-authorize tenant activity/membership
at execution time. A captured ID is context propagation, not perpetual authorization.

[TenantContext](templates/TenantContext.java) scopes work with try/finally cleanup, preserves
nested scopes and provides tenant-qualified cache keys:

```java
TenantId tenant = TenantAccess.resolve(authentication, tenantRegistry::isActive);
try (var scope = TenantContext.open(tenant)) {
    store.find(orderId);
    String key = TenantContext.orderCacheKey(orderId);
    executor.execute(TenantContext.capture(() -> store.find(orderId)));
}
```

Open the tenant scope before invoking a separate transactional service proxy. The example
does not propagate a transaction or SecurityContext to the worker; it starts its own unit of
work. Do not use `InheritableThreadLocal` with pooled workers. Servlet async dispatches need
explicit context propagation too. Scope every list, join and native query, and include
`tenant_id` in child foreign keys. No Java repository wrapper protects against arbitrary
unscoped SQL: restrict database credentials and consider PostgreSQL row-level security as
defense in depth.

Verify same-ID records in two tenants, forbidden cross-tenant reads/updates/deletes, distinct
cache keys, missing-context rejection, exception cleanup and sequential jobs on one worker.

## Verification scope

The repository fixture targets Boot 3.5.16 with Java 17/21 and checks
PostgreSQL 16 shared-schema access and blocking thread context; not Hibernate schema routing or Reactor. Check the
[coverage matrix](https://github.com/rrezartprebreza/spring-boot-skills/blob/main/verification/MATRIX.md)
before treating another minor version or an illustrative snippet as verified.

## Gotchas

- Agent trusts a tenant header supplied by the caller - derive tenant from authenticated context.
- Agent forgets to clear a servlet `ThreadLocal` - pooled threads can leak one tenant into another request.
- Agent scopes database queries but not cache keys - cached data can cross tenants.
- Agent starts a transaction before selecting the tenant - routing may choose the wrong database.
- Agent runs background jobs without tenant context - make tenant an explicit job parameter.