Select default emulator on first run by anisaoshafi · Pull Request #217 · localstack/lstk

anisaoshafi · 2026-04-30T15:02:43Z

First-time users are prompted to choose their emulator (AWS or Snowflake) before lstk starts (to simulate this, rename or delete config.toml). The choice is saved to the config file immediately.

When run with --non-interactive / no TTY it silently uses the AWS by default.

Note to user they can change to the other emulator:

carole-lavillonniere · 2026-05-05T08:27:48Z

@anisaoshafi @gtsiolis it feels a bit odd to me that the --emulator flag overwrites the config. The convention for flags as far as I know is to be transient/ephemeral.

Users can still manually edit their config if they want to.

If we want to provide a way to reset the emulator in the config from command line, what about something like lstk start --reset-config or lstk config emulator that would prompt again Which emulator would you like to use?

lstk start --emulator <type> could override the config, but only temporarily.

anisaoshafi · 2026-05-05T08:33:30Z

@anisaoshafi @gtsiolis it feels a bit odd to me that the --emulator flag overwrites the config. The convention for flags as far as I know is to be transient/ephemeral.

Users can still manually edit their config if they want to.

If we want to provide a way to reset the emulator in the config from command line, what about something like lstk start --reset-config or lstk config emulator that would prompt again Which emulator would you like to use?

lstk start --emulator <type> could override the config, but only temporarily.

Good point, can be too much of a responsibility for a flag.

I like lstk config emulator option. Does the same but in a more controlled way since the user have to confirm the option, or cancel if that's not what they intended.
Will wait for @gtsiolis's opinion before making the changes 🙏🏼

carole-lavillonniere · 2026-05-05T08:44:37Z

@anisaoshafi @gtsiolis it feels a bit odd to me that the --emulator flag overwrites the config. The convention for flags as far as I know is to be transient/ephemeral.
Users can still manually edit their config if they want to.
If we want to provide a way to reset the emulator in the config from command line, what about something like lstk start --reset-config or lstk config emulator that would prompt again Which emulator would you like to use?
lstk start --emulator <type> could override the config, but only temporarily.

Good point, can be too much of a responsibility for a flag.

I like lstk config emulator option. Does the same but in a more controlled way since the user have to confirm the option, or cancel if that's not what they intended. Will wait for @gtsiolis's opinion before making the changes 🙏🏼

If we decide for another command, I think we can leave it out of the scope of this PR/create another issue for it to keep this PR small.

carole-lavillonniere · 2026-05-05T07:09:03Z

+	return loadConfig(path)
+}
+
+func switchEmulatorContent(content string, to EmulatorType) (updated string, changed bool, err error) {


nit: we can remove the err in the returned values since we never return an error

Tackled here:1684152

carole-lavillonniere · 2026-05-05T07:12:26Z

+		emName := appConfig.Containers[0].Type.DisplayName()
+		sink.Emit(output.MessageEvent{
+			Severity: output.SeverityNote,
+			Text:     fmt.Sprintf("No emulator configured; defaulting to %s. Use --emulator to change this.", emName),


question: isn't the message slightly misleading since the default emulator is written to the config?
suggestion: Configured with default emulator AWS. Pass --emulator to change.

Good point, tackled here: 1684152

carole-lavillonniere · 2026-05-05T07:22:53Z

+			return EmulatorType(strings.ToLower(m[1]))
+		}
+	}
+	return ""


nit:
If the user has such config and requests "snowflake", it would match (it would ignore the fact that line is commented out)

[[containers]] # type = "snowflake" type = "aws"

thanks for the heads up, didn't properly test it. Handled it here: 9927276

carole-lavillonniere · 2026-05-05T07:25:04Z

+	return blocks
+}
+
+var typeLineRe = regexp.MustCompile(`type\s*=\s*"(\w+)"`)


nit: this regex only matches double quotes, can we update it so it also matches single quotes? e.g.

[[containers]] type = 'aws'

and

[[containers]] type = "aws"

thoughtless of me 😬 thanks for raising. Addressed it here: 9927276

carole-lavillonniere · 2026-05-05T07:27:07Z

+	ctx := testContext(t)
+	// The process will fail at container.Start (no Docker / no real auth), but the
+	// config switch happens earlier so the file should already be updated.
+	_, _, _ = runLstk(t, ctx, "", e.With(env.AuthToken, "test-token"), "--emulator", "snowflake", "--non-interactive")


nit: can we still check returned error/add assertions for the expected failures, to make sure we pass for the right reason?

Yes, addressed here: 3706dc8

carole-lavillonniere · 2026-05-05T07:30:09Z

+		return "", fmt.Errorf("unsupported emulator %q: must be 'aws' or 'snowflake'", s)
+	}
+}
+


nit: can we move this function next to EmulatorType in internal/config/containers.go to keep it in its domain?

Moved it here, good point: b85896c

carole-lavillonniere · 2026-05-05T07:37:11Z

 	}

+	if requestedEmulator != "" {
+		emType, err := parseEmulatorType(requestedEmulator)


nit: can we parse the emulator type earlier, right after reading the flag in cmd/root.go line 40, to fail early and strong-type it when passing it down. I would also make it a pointer *config.EmulatorType so that an unset value is clear (nil), rather than relying on empty string.

Attempted here: aa2efc9

Looks good! Are we now able to inline ParseEmulatorType in ParseOptionalEmulatorType (if not used anywhere else)?

Good idea, tried here: fe5fa04

carole-lavillonniere · 2026-05-05T07:51:25Z

+tag  = "latest"
+port = "4566"
+# volume = ""    # Host directory for persistent state (default: OS cache dir)
+# env = []       # Named environment profiles to apply (see [env.*] sections below)`


question:
At the moment those blocks are duplicated and there is a chance of drift.
How hard it is to parse those blocks from default_config.toml?
Another solution could be to use go templates in default_config.toml so the container blocks could come from another file.

Addressed here 🙏🏼 0aeca91

carole-lavillonniere · 2026-05-05T08:29:37Z

+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+


nit: these tests seem to be a good fit for table-driven tests, WDYT?

great observation, done: abc89ac

carole-lavillonniere · 2026-05-05T08:41:17Z

+			EmulatorLabel:          config.CachedPlanLabel(),
+			LabelCh:                labelCh,
+			NeedsEmulatorSelection: needsEmulatorSelection,
+			OnEmulatorSelected: func(emType config.EmulatorType) ([]config.ContainerConfig, error) {


issue: such callback is not very "go-like". It's not clear to me why we need it, don't we have all dependencies in ui/run.go to execute what we must execute after selection?

Attempted to refactor here, let me know if I'm doing sth terrible 🙏🏼5e7723a

gtsiolis · 2026-05-05T12:22:33Z

Looking at this now! 👀

gtsiolis · 2026-05-05T12:57:20Z

+type = "snowflake"
+tag  = "latest"
+port = "4566"


issue: Similar to what @carole-lavillonniere mentioned above, this leads to content duplication.

# [[containers]] # type = "aws" # Emulator type. Currently supported: "aws", "snowflake" # tag = "latest" # Docker image tag, e.g. "latest", "2026.03" # port = "4566" # Host port the emulator will be accessible on # # volume = "" # Host directory for persistent state (default: OS cache dir) # # env = [] # Named environment profiles to apply (see [env.*] sections below) [...] [[containers]] type = "snowflake" tag = "latest" port = "4566" # volume = "" # Host directory for persistent state (default: OS cache dir) # env = [] # Named environment profiles to apply (see [env.*] sections below)

I'd expect if I select SNO on start, to simply use snowflake in the first [[containers]] section.

[...] [[containers]] type = "snowflake" # Emulator type. Currently supported: "aws", "snowflake" tag = "latest" # Docker image tag, e.g. "latest", "2026.03" port = "4566" # Host port the emulator will be accessible on volume = "" # Host directory for persistent state (default: OS cache dir) env = [] # Named environment profiles to apply (see [env.*] sections below) [...]

I'm going to tackle this 💯

Addressed here 🙏🏼 0aeca91

gtsiolis · 2026-05-05T14:52:40Z

+		Prompt: "Which emulator would you like to use?",
+		Options: []output.InputOption{
+			{Key: "a", Label: "AWS [A]"},
+			{Key: "s", Label: "Snowflake [S]"},


issue(non-blocking): Here's what most users with no access to SNO emulation will see. I guess this comes from the emulator, right? Could we mask now or later this error?

SNO users in trial see no error currently.

Have to check what SNO-only users in production see.

yes, comes from emulator side. I created a follow up task: DRG-818 for handling this error in a nicer way since it's out of the scope for this PR 🙏🏼

gtsiolis · 2026-05-05T14:55:48Z

+
+	msg := selected.DisplayName() + " emulator selected."
+	if configPath != "" {
+		msg += fmt.Sprintf(" You can change this anytime in %s.", configPath)


issue: The message here wraps early on line 2, can we make the message wrap normally in two lines, I think we do so for other messages, right?

@gtsiolis Does this look a bit better? Also changed the message as you proposed in the other comment

What do you think of using the same success message as the AWS profile creation? This will avoid ending up with multiple NOTE messages when the emulator is running (although edge case) and make the output more functional upon selection.

1/2 2/2

same success message as the AWS profile creation

What does this mean when the user selects snowflake on first try?

@gtsiolis sorry, I'm not following. Can you rephrase your suggestion?
Did you mean Change configuration in ~/.config/lstk/config.toml should have a green tick in front?

gtsiolis · 2026-05-05T14:57:39Z

+	labelCh <- label
+}
+
+func selectEmulatorInTUI(


praise: Thanks for reusing this UX!

…ckType

…ation message is secondary

anisaoshafi · 2026-05-06T12:00:07Z

Apologies for the back and forth and the huge PR. I removed the --emulator flag from the scope of this PR (discussing it in linear), should have removed that before tackling the review comments which were mostly around the switching between emulators, but a bit too late now.
This simplifies the PR. Thanks for your patience @carole-lavillonniere @gtsiolis 🙏🏼

anisaoshafi added the semver: patch label Apr 30, 2026

anisaoshafi force-pushed the drg-381-add-option-to-select-default-emulator branch 17 times, most recently from e1db26a to 4204284 Compare May 4, 2026 15:34

anisaoshafi marked this pull request as ready for review May 4, 2026 15:39

anisaoshafi requested review from carole-lavillonniere and silv-io as code owners May 4, 2026 15:39

anisaoshafi requested a review from gtsiolis May 4, 2026 15:39

carole-lavillonniere reviewed May 5, 2026

View reviewed changes

anisaoshafi marked this pull request as draft May 5, 2026 11:24

anisaoshafi force-pushed the drg-381-add-option-to-select-default-emulator branch from ada0134 to 5e7723a Compare May 5, 2026 12:34

gtsiolis reviewed May 5, 2026

View reviewed changes

anisaoshafi force-pushed the drg-381-add-option-to-select-default-emulator branch from b4e27ea to e1d9e9c Compare May 5, 2026 16:47

anisaoshafi marked this pull request as ready for review May 5, 2026 17:08

anisaoshafi requested review from carole-lavillonniere and gtsiolis May 5, 2026 17:08

anisaoshafi marked this pull request as draft May 6, 2026 08:27

anisaoshafi force-pushed the drg-381-add-option-to-select-default-emulator branch 3 times, most recently from 66f5972 to 33e0742 Compare May 6, 2026 11:54

anisaoshafi added 16 commits May 6, 2026 13:55

Select default emulator on first run

af4f7f3

Parallelize new tests

5ce5a73

Remove error & rename message for default emulator

fba3335

Match single quotes in config & handle commented section in detectBlo…

cb621fc

…ckType

Assert error from runLstk

7018421

Move ParseEmulatorType to its domain package

cfe2273

Make switch_test.go tests table-driven

7620257

Strong-type --emulator, validate early

9739902

Remove callback in startEmulator: handle in run.go

b7d7801

Remove ParseEmulatorType in favor of ParseOptionalEmulatorType

0f681da

Gentler message on changing configuration

1d627a9

Avoid duplicating config content

2aa3fff

Skip keyboard shortcuts for emulator selection

bbd7f69

Nits

96aa343

Split emulator-selected note into two message events: change configur…

ef8a3a5

…ation message is secondary

Simplify: Remove --emulator flag on start

9b1d670

anisaoshafi force-pushed the drg-381-add-option-to-select-default-emulator branch from 33e0742 to 9b1d670 Compare May 6, 2026 11:55

anisaoshafi marked this pull request as ready for review May 6, 2026 12:00

anisaoshafi mentioned this pull request May 6, 2026

Improve error for users with no snowflake license #230

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gtsiolis commented May 5, 2026

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anisaoshafi commented Apr 30, 2026 •

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