Opik Backend

Architecture

• Layered: Resource → Service → DAO (never skip layers)
• DI: Guice modules, constructor injection with @Inject
• Databases: MySQL (metadata, transactional) + ClickHouse (analytics, append-only)

Naming Conventions

Plural Names (Resources, Tests, URLs, DB Tables)

• Resource classes: TracesResource, SpansResource, DatasetsResource (not TraceResource)
• Resource test classes: TracesResourceTest, SpansResourceTest, DatasetsResourceTest (not TraceResourceTest)
• URL paths: /v1/private/traces, /v1/private/spans (not /v1/private/trace)
• DB table names: traces, spans, feedback_scores (not trace, span, feedback_score)

Singular Names (DAO, Service)

• DAO classes: TraceDAO, SpanDAO, DatasetDAO (not TracesDAO)
• Service classes: TraceService, SpanService, DatasetService (not TracesService)

// ✅ GOOD
@Path("/v1/private/traces")
public class TracesResource { }

// ✅ GOOD - DAO and Service use singular
public class TraceDAO { }
public class TraceService { }

// ✅ GOOD - test classes match plural resource name
public class TracesResourceTest { }

// ❌ BAD - singular test class
public class TraceResourceTest { }

// ❌ BAD - singular resource/URL
@Path("/v1/private/trace")
public class TraceResource { }

// ❌ BAD - plural DAO/Service
public class TracesDAO { }
public class TracesService { }

Lombok Conventions

Records and DTOs

• Always annotate records/DTOs with @Builder(toBuilder = true)
• Add @NonNull on all non-optional fields
• Use builders (not constructors) when instantiating records

// ✅ GOOD
@Builder(toBuilder = true)
record MyData(@NonNull UUID id, @NonNull String name, String description) {}

MyData data = MyData.builder()
        .id(id)
        .name(name)
        .build();

// ❌ BAD - plain constructor (positional mistakes, less readable)
new MyData(id, name, null);

// ❌ BAD - @Builder without toBuilder
@Builder
record MyData(UUID id, String name) {}

Dependency Injection

• Use @RequiredArgsConstructor(onConstructor_ = @Inject) instead of manual constructors

// ✅ GOOD
@RequiredArgsConstructor(onConstructor_ = @Inject)
public class MyService {
    private final @NonNull DependencyA depA;
    private final @NonNull DependencyB depB;
}

// ❌ BAD - boilerplate constructor
public class MyService {
    private final DependencyA depA;
    @Inject
    public MyService(DependencyA depA) {
        this.depA = depA;
    }
}

Interfaces

• Don't put validation annotations (@NonNull) on interface method parameters
• Keep interfaces free of implementation details

// ✅ GOOD
interface MyService {
    void process(String workspaceId, UUID promptId);
}

// ❌ BAD - validation on interface
interface MyService {
    void process(@NonNull String workspaceId, @NonNull UUID promptId);
}

Critical Gotchas

StringTemplate Memory Leak

// ✅ GOOD
var template = TemplateUtils.newST(QUERY);

// ❌ BAD - causes memory leak via STGroup singleton
var template = new ST(QUERY);

List Access

// ✅ GOOD
users.getFirst()
users.getLast()

// ❌ BAD
users.get(0)
users.get(users.size() - 1)

SQL Text Blocks

// ✅ GOOD - text blocks for multi-line SQL
@SqlQuery("""
        SELECT * FROM datasets
        WHERE workspace_id = :workspace_id
        <if(name)> AND name like concat('%', :name, '%') <endif>
        """)

// ❌ BAD - string concatenation
@SqlQuery("SELECT * FROM datasets " +
        "WHERE workspace_id = :workspace_id " +
        "<if(name)> AND name like concat('%', :name, '%') <endif> ")

Immutable Collections

// ✅ GOOD
Set.of("A", "B", "C")
List.of(1, 2, 3)
Map.of("key", "value")

// ❌ BAD
Arrays.asList("A", "B", "C")

API Design

• Query parameters that accept lists: Use plural names from the start (e.g., exclude_category_names not exclude_category_name). Starting with a singular name and later adding a plural variant results in two redundant query params on the same endpoint. Plural names are backward-compatible since they work for both single and multiple values.

Error Handling

Use Jakarta Exceptions

throw new BadRequestException("Invalid input");
throw new NotFoundException("User not found: '%s'".formatted(id));
throw new ConflictException("Already exists");
throw new InternalServerErrorException("System error", cause);

Error Response Classes

• Simple: io.dropwizard.jersey.errors.ErrorMessage
• Complex: com.comet.opik.api.error.ErrorMessage
• Never create new error message classes

Logging

Format Convention

// ✅ GOOD - values in single quotes
log.info("Created user: '{}'", userId);
log.error("Failed for workspace: '{}'", workspaceId, exception);

// ❌ BAD - no quotes
log.info("Created user: {}", userId);

Never Log

• Emails, passwords, tokens, API keys
• PII, personal identifiers
• Database credentials

Reference Files

• clickhouse.md - ClickHouse query patterns
• mysql.md - TransactionTemplate patterns
• testing.md - PODAM, naming, assertion patterns
• migrations.md - Liquibase format for MySQL/ClickHouse