Golang Testcontainers

Golang Testcontainers

Use this skill when writing or reviewing Go integration tests that need real infrastructure dependencies in Docker.

Use it together with golang-testing for general Go test structure, t.Parallel() defaults, black-box package guidance, and assertion style.

When to Use

• Testing against a real Postgres, Redis, Kafka, NATS, or other containerised dependency
• Verifying SQL, migrations, indexes, transactions, locking, or driver behaviour
• Testing cache integration against a real Redis instance
• Replacing brittle fake environments with hermetic container-based integration tests
• Designing package-scoped test infrastructure that still allows parallel tests

Default Stance

Unless there is a clear reason not to, prefer these defaults:

• Use real containers for integration tests, not mocks
• Prefer module-specific helpers like postgres.Run and redis.Run over lower-level generic setup when a module exists
• Prefer testcontainers.Run over older GenericContainer patterns for generic services
• Register cleanup immediately with testcontainers.CleanupContainer(t, ctr) in normal tests
• For expensive services like Postgres and Redis, prefer one container per package/test binary, not one container per test
• Keep tests parallel by isolating state inside the shared container per test
• Use per-test transactions, schemas, databases, logical Redis DBs, or key prefixes instead of sharing mutable state
• Let Docker assign random host ports; do not hardcode host ports in tests
• Configure wait strategies explicitly when readiness is not guaranteed by the module
• Avoid cross-test and cross-package reuse by default; keep tests hermetic and self-contained

Terminology: Containers Are Not Mocks

For Postgres and Redis integration tests, prefer real Postgres and Redis containers.

These are not mocks.

• Mocks/fakes are best for unit tests and narrow seams
• Testcontainers are best when you want confidence in the real dependency behaviour

If the test is meant to validate SQL, migrations, transactions, Redis commands, TTL behaviour, or networked dependency behaviour, a real container is usually the right tool.

Version and API Notes

• github.com/testcontainers/testcontainers-go is the main library
• Prefer modern Run(...) APIs
• GenericContainer is the older style; prefer testcontainers.Run(...) for new generic-container examples
• Module-level RunContainer(ctx, opts...) helpers are deprecated; prefer postgres.Run(...), redis.Run(...), etc.
• WithReuseByName(...) is experimental; do not make it your default CI/test strategy

Lifecycle Scope Tradeoffs

Choose container lifetime deliberately:

Recommended Default

For Postgres and Redis, prefer:

• one container per package/test binary
• parallel tests inside that package
• per-test state isolation inside the container

Why this is the usual sweet spot:

• Starting one Postgres/Redis container per test is often too slow
• Sharing one container per package keeps startup cost acceptable
• go test runs each package in a separate process, so package-scoped fixtures already isolate one package from another
• You still need to isolate state between tests inside that package

If most tests in the package need the dependency, TestMain is a good fit. If only some tests need it, a lazy package-scoped helper can be better.

Generic Container Startup

Use generic startup when there is no higher-level module or when you need custom behaviour.

package cache_test

import (
    "context"
    "testing"
    "time"

    "github.com/stretchr/testify/require"

    "github.com/testcontainers/testcontainers-go"
    "github.com/testcontainers/testcontainers-go/wait"
)

func TestWithGenericRedis(t *testing.T) {
    t.Parallel()

    ctx := context.Background()

    ctr, err := testcontainers.Run(ctx,
        "redis:7",
        testcontainers.WithExposedPorts("6379/tcp"),
        testcontainers.WithWaitStrategy(
            wait.ForListeningPort("6379/tcp"),
            wait.ForLog("Ready to accept connections").WithStartupTimeout(30*time.Second),
        ),
    )
    testcontainers.CleanupContainer(t, ctr)
    require.NoError(t, err)

    endpoint, err := ctr.Endpoint(ctx, "")
    require.NoError(t, err)

    _ = endpoint // pass to your client under test
}

Use this pattern for services without a dedicated module, or when you need full control over files, env vars, commands, or custom wait strategies.

Postgres Module Example

Prefer the Postgres module for Postgres integration tests.

package repo_test

import (
    "context"
    "path/filepath"
    "testing"

    "github.com/stretchr/testify/require"

    "github.com/testcontainers/testcontainers-go"
    "github.com/testcontainers/testcontainers-go/modules/postgres"
)

func TestRepositoryWithPostgres(t *testing.T) {
    t.Parallel()

    ctx := context.Background()

    ctr, err := postgres.Run(ctx,
        "postgres:16-alpine",
        postgres.WithDatabase("app_test"),
        postgres.WithUsername("postgres"),
        postgres.WithPassword("postgres"),
        postgres.WithInitScripts(filepath.Join("testdata", "init.sql")),
        postgres.BasicWaitStrategies(),
    )
    testcontainers.CleanupContainer(t, ctr)
    require.NoError(t, err)

    dsn, err := ctr.ConnectionString(ctx, "sslmode=disable")
    require.NoError(t, err)

    _ = dsn // open your DB client here
}

Notes:

• postgres.BasicWaitStrategies() is the common default for Postgres readiness
• WithInitScripts(...) is useful for schema setup or seed data
• ConnectionString(...) is usually the easiest way to build a DB client

Redis Module Example

Prefer the Redis module for Redis integration tests.

package cache_test

import (
    "context"
    "testing"

    "github.com/stretchr/testify/require"

    "github.com/testcontainers/testcontainers-go"
    tcredis "github.com/testcontainers/testcontainers-go/modules/redis"
)

func TestCacheWithRedis(t *testing.T) {
    t.Parallel()

    ctx := context.Background()

    ctr, err := tcredis.Run(ctx,
        "redis:7",
    )
    testcontainers.CleanupContainer(t, ctr)
    require.NoError(t, err)

    uri, err := ctr.ConnectionString(ctx)
    require.NoError(t, err)

    _ = uri // pass to your redis client under test
}

Notes:

• Use module options like WithConfigFile(...), WithTLS(), or WithLogLevel(...) when relevant
• ConnectionString(...) returns a ready-to-use Redis URI
• WithSnapshotting(...) configures Redis persistence behaviour; it is not a per-test isolation/reset mechanism

Package-Scoped Container Pattern

When most tests in a package need the same dependency, prefer a package-scoped container.

package repo_test

import (
    "context"
    "log"
    "os"
    "testing"

    "github.com/testcontainers/testcontainers-go"
    "github.com/testcontainers/testcontainers-go/modules/postgres"
)

var (
    postgresDSN string
    postgresCtr *postgres.PostgresContainer
)

func TestMain(m *testing.M) {
    ctx := context.Background()

    var err error
    postgresCtr, err = postgres.Run(ctx,
        "postgres:16-alpine",
        postgres.WithDatabase("app_test"),
        postgres.WithUsername("postgres"),
        postgres.WithPassword("postgres"),
        postgres.BasicWaitStrategies(),
    )
    if err != nil {
        log.Fatal(err)
    }

    postgresDSN, err = postgresCtr.ConnectionString(ctx, "sslmode=disable")
    if err != nil {
        log.Fatal(err)
    }

    code := m.Run()

    if err := testcontainers.TerminateContainer(postgresCtr); err != nil {
        log.Printf("terminate postgres container: %v", err)
    }

    os.Exit(code)
}

This is a good default when:

• the container startup cost is noticeable
• most tests in the package need the dependency
• you have a clear per-test isolation strategy for data

Parallel Tests Need State Isolation, Not Just Container Isolation

Sharing one container does not make parallel tests safe by itself.

If tests run with t.Parallel(), they must not step on the same data.

Prefer one of these per-test isolation patterns.

Postgres Isolation Patterns

Preferred order:

1. Per-test transaction + rollback when your code can run inside an injected transaction
2. Per-test schema when your app can point each test at its own schema/search path
3. Per-test database inside the same Postgres server when schema isolation is not enough
4. Per-test container only when the test truly needs process-level or cluster-level isolation

Transaction pattern

Best when the application code can accept a *sql.Tx, pgx.Tx, or a narrow query interface.

• Fastest reset strategy
• Excellent for parallel tests
• Minimal container churn

Per-schema pattern

Good when each test can use a unique schema name.

• Works well with one package-level Postgres container
• Lets parallel tests run without dropping each other's tables
• Usually faster than creating a fresh container per test

Snapshot/Restore pattern

The Postgres module supports Snapshot(...) and Restore(...).

Use it when:

• migrations are expensive
• you want a quick reset to a known base state
• tests are mostly serial, or you can guarantee only one restore operation at a time

Be careful:

• Restore(...) resets shared database state
• do not call it concurrently from parallel tests that share the same Postgres container/database
• for truly parallel tests, prefer per-test transactions, schemas, or databases instead

Redis Isolation Patterns

Preferred order:

1. Per-test logical DB when using standalone Redis and your client can select DB numbers
2. Per-test key prefix/namespace when logical DB separation is not practical
3. Per-test container when you need hard isolation or destructive global operations

Per-test logical DB

Good when using a standard standalone Redis image.

client := redis.NewClient(&redis.Options{
    Addr: redisAddr,
    DB:   testDBNumber,
})

Guidance:

• Give each parallel test a distinct DB number
• Clean up with FLUSHDB for that logical DB only
• Avoid FLUSHALL in tests

Per-test key prefix

Good when DB-number isolation is unavailable or inconvenient.

• Prefix keys with a test-unique namespace such as t_<id>:
• Delete only that namespace in cleanup
• Safer for parallel tests than sharing raw keys

Be careful with global Redis operations:

• FLUSHALL will destroy every test's state
• FLUSHDB is also unsafe if multiple parallel tests share the same logical DB

Example: Package-Level Postgres + Parallel Tests

This is usually the best tradeoff for repository/service integration tests.

• Start one Postgres container in TestMain
• Run migrations once
• Let each parallel test use its own transaction, schema, or database

Prefer this over one-container-per-test when:

• container startup dominates runtime
• the main thing you need is data isolation, not full server-process isolation

Example: Package-Level Redis + Parallel Tests

This is usually the best tradeoff for cache integration tests.

• Start one Redis container for the package
• Give each test its own logical DB or unique key prefix
• Never use global destructive cleanup across all tests

Wait Strategies and Readiness

Do not assume container start means service readiness.

Prefer:

• module-provided readiness helpers when available
• wait.ForListeningPort(...) for services that only need the socket up
• wait.ForLog(...) when service logs are the most reliable readiness signal
• combined strategies for flaky services or non-Linux host setups

Postgres specifically benefits from explicit readiness checks like postgres.BasicWaitStrategies().

Ports, Addresses, and Connection Strings

Prefer runtime discovery over fixed ports:

• ctr.ConnectionString(ctx, ...) for Postgres
• ctr.ConnectionString(ctx) for Redis
• ctr.Endpoint(ctx, "") or ctr.MappedPort(ctx, ...) for generic containers

Do not hardcode localhost:5432 or localhost:6379.

Parallel tests depend on Docker assigning distinct mapped host ports.

Cleanup Guidance

In normal tests:

ctr, err := postgres.Run(ctx, "postgres:16-alpine", postgres.BasicWaitStrategies())
testcontainers.CleanupContainer(t, ctr)
require.NoError(t, err)

In TestMain:

• use testcontainers.TerminateContainer(...) explicitly after m.Run()

Register cleanup immediately after startup.

Reuse Guidance

WithReuseByName(...) exists, but it is experimental.

Avoid making reuse your default because it:

• weakens test hermeticity
• risks hidden state between runs
• can behave differently in local development vs CI

Prefer clean startup/teardown unless you have a deliberate local-only optimisation strategy.

Review Checklist

When reviewing testcontainers-go usage, check for:

• Module helpers used where available (postgres.Run, redis.Run)
• Modern Run(...) APIs instead of deprecated patterns
• Cleanup registered immediately
• No fixed host ports
• Explicit readiness/wait strategy where needed
• Package-scoped lifecycle chosen deliberately
• Clear per-test isolation strategy for parallel tests
• No global destructive cleanup that breaks parallel runs
• Real Postgres/Redis containers used for integration tests instead of mocks
• Postgres snapshot/restore used only when its shared-state tradeoff is acceptable

Anti-Patterns

Avoid these unless there is a strong reason:

• One heavy Postgres container per test when a package-scoped container plus data isolation would do
• Sharing one Postgres database across parallel tests with no transaction/schema/database isolation
• Sharing one Redis logical DB across parallel tests and calling FLUSHDB
• Calling FLUSHALL in a parallel suite
• Hardcoding localhost:5432 or localhost:6379
• Relying on container startup without readiness checks
• Treating WithSnapshotting(...) on Redis as a test reset mechanism
• Defaulting to WithReuseByName(...) in CI
• Using mocks to test SQL or Redis command behaviour that should be verified against the real service

Quick Templates

Postgres package template

var postgresDSN string

func TestMain(m *testing.M) {
    ctx := context.Background()

    ctr, err := postgres.Run(ctx,
        "postgres:16-alpine",
        postgres.WithDatabase("app_test"),
        postgres.WithUsername("postgres"),
        postgres.WithPassword("postgres"),
        postgres.BasicWaitStrategies(),
    )
    if err != nil {
        log.Fatal(err)
    }

    postgresDSN, err = ctr.ConnectionString(ctx, "sslmode=disable")
    if err != nil {
        log.Fatal(err)
    }

    code := m.Run()
    _ = testcontainers.TerminateContainer(ctr)
    os.Exit(code)
}

Then make each parallel test isolate its own transaction, schema, or database.

Redis package template

var redisURI string

func TestMain(m *testing.M) {
    ctx := context.Background()

    ctr, err := tcredis.Run(ctx, "redis:7")
    if err != nil {
        log.Fatal(err)
    }

    redisURI, err = ctr.ConnectionString(ctx)
    if err != nil {
        log.Fatal(err)
    }

    code := m.Run()
    _ = testcontainers.TerminateContainer(ctr)
    os.Exit(code)
}

Then give each parallel test its own logical DB or key prefix.

Source: brpaz/agent-skills — distributed by TomeVault.