How do you design a multi-tenant Go microservice?

Multi-tenancy means one service instance serves multiple customers (tenants) with data isolation. Three common isolation models: shared database, schema per tenant, database per tenant. The choice depends on isolation requirements, scaling needs, and cost.

// Tenant context extracted from JWT or header
type TenantID string
type tenantKey struct{}

func withTenant(ctx context.Context, id TenantID) context.Context {
    return context.WithValue(ctx, tenantKey{}, id)
}

func tenantFromCtx(ctx context.Context) (TenantID, bool) {
    id, ok := ctx.Value(tenantKey{}).(TenantID)
    return id, ok
}

// Middleware: extract and validate tenant from JWT
func tenantMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        claims, err := validateJWT(r.Header.Get("Authorization"))
        if err != nil { http.Error(w, "unauthorized", 401); return }

        ctx := withTenant(r.Context(), TenantID(claims.TenantID))
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

// Shared database with tenant_id column (row-level isolation)
type TenantAwareRepo struct{ db *sql.DB }

func (r *TenantAwareRepo) FindUsers(ctx context.Context) ([]User, error) {
    tenantID, ok := tenantFromCtx(ctx)
    if !ok { return nil, errors.New("no tenant in context") }

    rows, err := r.db.QueryContext(ctx,
        // tenant_id filter on every query — NEVER omit this
        "SELECT id, name FROM users WHERE tenant_id = $1",
        tenantID)
    if err != nil { return nil, err }
    defer rows.Close()
    // ... scan rows
}

// Postgres Row Level Security (RLS) — DB enforces tenant isolation
// ALTER TABLE users ENABLE ROW LEVEL SECURITY;
// CREATE POLICY tenant_isolation ON users
//     USING (tenant_id = current_setting('app.tenant_id')::uuid);
// SET app.tenant_id = '123e4567-...' -- per connection/transaction

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