Go Service

1. types/ — define all interfaces first so every other layer has a contract to depend on
2. database/ — implement the data access layer against the Database interface
3. client/ — implement any outbound clients against their interfaces
4. service/ — implement business logic using only the interfaces from types/
5. handler/ — implement the transport layer using only the Service interface from types/
6. main.go — wire all concrete implementations together and start the server

Core Principles

Depend on interfaces, not implementations. Every layer holds its dependencies as interfaces from types/. Nothing is imported directly from sibling packages except in main.go.

Inject all dependencies through constructors. All dependencies are passed into New* functions. Nothing is instantiated inside the service or handler.

One responsibility per layer. Transport logic does not touch the database. Business logic does not build queries. Queries do not make outbound calls.

Crash on bad config, log on recoverable errors. Missing environment variables are fatal at startup. Cache failures and best-effort side effects are logged and swallowed. Anything that prevents a correct response is returned as an error.

Shared infrastructure lives in services/common. Reusable clients, interfaces, and domain models are defined there and imported. Never copy infrastructure code into a service.

Layer Spec — types/types.go

Responsibility

Define the interfaces that every other layer in the service depends on. This file is the contract layer — it describes what each dependency must provide without specifying how.

Rules

• One interface per dependency: Service, Database, SomeClient, etc.
• The Service interface must exactly match the public methods implemented on the service struct — the handler depends on it
• No concrete types, no structs, no implementation details — interfaces only
• Shared domain models used across multiple services (e.g. common database models, queue message types) live in services/common and are imported here as needed
• Do not import from sibling packages (handler, service, database, client) — only from services/common and standard libraries

Pattern

package types

import (
    "context"

    commontypes "github.com/kabradshaw1/story/services/common"
    "github.com/kabradshaw1/story/services/common/genproto/<name>"
)

type Service interface {
    MethodOne(ctx context.Context, req *proto.Request) (*proto.Response, error)
    MethodTwo(ctx context.Context, req *proto.Request) (*proto.Response, error)
}

type Database interface {
    FindRecord(ctx context.Context, id int) (*commontypes.SomeModel, error)
    SaveRecord(ctx context.Context, record commontypes.SomeModel) error
}

type SomeClient interface {
    Notify(ctx context.Context, payload SomePayload) error
}

Checklist

• One interface defined for each dependency the service layer requires
• Service interface matches the service struct's public methods exactly
• No concrete types or structs defined here
• All shared domain types imported from services/common, not redefined

Layer Spec — database/database.go

Responsibility

Execute all data access operations. This is the only layer that communicates with the database.

Rules

• No business logic — only queries and result mapping
• A Connect function (or equivalent) accepts a connection string, establishes the connection, and returns a typed *DB struct; it calls log.Fatal on failure
• Every method accepts context.Context as its first argument and passes it through to the underlying query
• The service layer never constructs a query directly — all data access goes through this layer
• Does not call outbound clients or touch the cache

Pattern

package database

type DB struct {
    client *SomeClient
}

func Connect(connectionString string) *DB {
    // connect, fatal on error
    return &DB{client: client}
}

func (d *DB) FindRecord(ctx context.Context, id int) (*commontypes.SomeModel, error) {
    // query using ctx
}

func (d *DB) SaveRecord(ctx context.Context, record commontypes.SomeModel) error {
    // insert using ctx
}

Mutation Authorization

For any mutation scoped to a user, always verify ownership before modifying data:

1. Fetch the record by ID
2. Confirm record.User == requestingUser — return a descriptive error if not
3. Perform the update

Avoiding Extra Round-Trips

After an update query, do not re-fetch the record to return it. Instead, update the already-fetched struct in memory:

// fetch → verify → update → return modified struct
record.Field = newValue
return &record, nil

Error Handling

• Distinguish "record not found" from other errors and return a clear, specific message for each
• Wrap all errors with context before returning: fmt.Errorf("failed to find record: %w", err)

Checklist

• Connect accepts a connection string and calls log.Fatal on failure
• Every method takes context.Context as its first parameter
• No business logic, no cache access, no outbound calls
• User-scoped mutations verify ownership before updating
• Updates modify the in-memory struct and return it — no redundant re-fetch
• "Not found" errors are distinguished from other errors
• All returned errors are wrapped with context

Layer Spec — client/client.go

Responsibility

Handle all outbound calls to external services or third-party APIs. This is the only layer that makes outbound network requests.

Rules

• Wraps the underlying transport (HTTP client, gRPC connection, SDK, etc.) in a struct
• A New* constructor initializes and returns the client struct
• Every method accepts context.Context as its first argument and passes it to the underlying call
• Validate required inputs before making any call — return an error immediately for missing or invalid fields
• Return a descriptive error for any non-success response from the remote
• No business logic — only request construction, sending, and response validation

Pattern

package client

type SomeClient struct {
    // underlying transport (http.Client, grpc.ClientConn, etc.)
}

func New() *SomeClient {
    return &SomeClient{
        // initialize transport
    }
}

func (c *SomeClient) Notify(ctx context.Context, payload SomePayload) error {
    if payload.RequiredField == "" {
        return fmt.Errorf("RequiredField is required")
    }
    // construct and send request using ctx
    // check response and return error on failure
    return nil
}

Checklist

• Underlying transport is wrapped in a struct, not used directly
• New* constructor initializes the transport
• Every method accepts and passes through context.Context
• Required inputs are validated before the call is made
• Non-success responses from the remote return a descriptive error
• No business logic, no database access, no cache access

Layer Spec — service/service.go

Responsibility

Implement all business logic. Orchestrate calls to the database, cache, and outbound clients to fulfill a request.

Rules

• The struct holds all dependencies as interfaces from types/ — never concrete implementations
• The New* constructor accepts every dependency as an argument — nothing is instantiated inside
• This is the only layer that decides what to validate, what to cache, and what order to call things in
• Does not build queries directly — calls types.Database methods only
• Does not know about the transport mechanism

Pattern

package service

type Service struct {
    db     types.Database
    cache  commontypes.Cache
    client types.SomeClient
}

func NewService(db types.Database, cache commontypes.Cache, client types.SomeClient) *Service {
    return &Service{db: db, cache: cache, client: client}
}

Cache Pattern

Use a read-through cache on any method that reads data. Keys must use : as a separator between all components to prevent collisions.

cacheKey := "resource:" + userID + ":" + entityType

Order of operations:

1. Check the cache; if hit, unmarshal and return
2. On miss, query the database
3. Build the response
4. Marshal and write to cache with a TTL
5. Return the response

Cache errors (get, set, unmarshal, marshal) are always logged and swallowed — a cache failure must never prevent a valid response from being returned.

cacheData, err := s.cache.Get(ctx, cacheKey)
if err == nil && cacheData != "" {
    if err := json.Unmarshal([]byte(cacheData), &response); err != nil {
        log.Printf("<service>/service: Method: failed to unmarshal cache: %v", err)
    } else {
        return &response, nil
    }
}
// ... query db, build response ...
serializedData, err := json.Marshal(&response)
if err != nil {
    log.Printf("<service>/service: Method: failed to marshal for cache: %v", err)
} else {
    if err := s.cache.Set(ctx, cacheKey, string(serializedData), 10*time.Minute); err != nil {
        log.Printf("<service>/service: Method: failed to set cache: %v", err)
    }
}

Error Handling

• Return errors for anything required to produce a correct response
• Wrap all returned errors with context: fmt.Errorf("description: %w", err)
• Log and continue only for genuine best-effort side effects (e.g. an audit write that must not block the response); always add a comment explaining the intent

Log Message Format

All log messages must identify the service and method:

"<service>/service: MethodName: description of what failed"

Checklist

• Struct holds only types.* interfaces and commontypes.* interfaces — no concrete types
• New* constructor accepts all dependencies as arguments
• Cache keys use : separators between all variable components
• Cache errors are logged, never returned
• All returned errors are wrapped with fmt.Errorf
• Log messages follow the <service>/service: Method: description format
• Best-effort side effects have a comment explaining why failure is intentionally non-fatal