# Logs

Logs are what your live app prints: requests, warnings and errors. Use them to find out why a live app misbehaves. They are off until you turn them on.

For why a <Term id="deploy">deploy</Term> itself failed, use **See why** on the deploy (see [Deploy an app](/docs/deploy/deploy-an-app)). For an <Term id="agent">agent</Term>'s runs, use **Log and files** on the run (see [Runs and approvals](/docs/agents/runs)).

## Before you start [#before-you-start]

<Callout type="info" title="Logs cost by the gigabyte kept">
  They are off by default. Your spend shows in <AppLink to="settings.usage" /> (see [Credits](/docs/workspace/credits)).
</Callout>

- A deployed <Term id="project">project</Term>.
- To turn logs on or off, you need to be able to change the project.

## Turn logs on [#turn-logs-on]

<ForEveryone>

<Steps>
<Step>
### Open the project's settings [#open-the-projects-settings]
Open the project from <AppLink to="projects" />, go to **Settings** (see [Project settings](/docs/projects/settings)) and choose the **Deploy** section ("How your app runs once it is live, where it deploys from, and what it prints").
</Step>
<Step>
### Turn on Keep logs [#turn-on-keep-logs]
Under **Logs**, turn on **Keep logs**. It takes effect within a minute, no new deploy needed.
</Step>
<Step>
### Choose how long to keep them [#choose-how-long-to-keep-them]
Under **Keep for**, choose 1, 3, 5, 7, 14 or 30 days. Older lines are removed on their own.
</Step>
</Steps>

<Screenshot src="/docs/_media/deploy-page-overview.png" alt="The Logs section in a project's Deploy settings" />

</ForEveryone>

<ForDevelopers>

```python
from klarety import Nexus

nx = Nexus.connect()
print(nx.projects.logs("PROJECT_ID"))
nx.projects.set_logs("PROJECT_ID", enabled=True, days=7)
```

`days` is one of 1, 3, 5, 7, 14 or 30. `set_logs(..., enabled=False)` turns them off.

</ForDevelopers>

## Read the last hour [#read-the-last-hour]

<ForEveryone>

**Last hour** shows the newest lines your app printed; select **Refresh** for more. When nothing was printed, it says "Nothing printed in the last hour."

</ForEveryone>

<ForDevelopers>

```python
for line in nx.projects.log_lines("PROJECT_ID", minutes=60):
    print(line)

api_lines = nx.projects.log_lines("PROJECT_ID", minutes=15, service="api")
```

Lines come newest last. Pass `service` to read one part of the app. The answer is empty while logs are off.

</ForDevelopers>

## What you see when it worked [#what-you-see-when-it-worked]

Within a minute, new lines appear under **Last hour**. If the app is quiet, open it with **Visit** and select **Refresh**.

## Choose how long to keep logs [#choose-how-long-to-keep-logs]

A longer period costs more for a busy app.

- **1 or 3 days** suit day-to-day checks.
- **7 or 14 days** help when a problem shows up only now and then.
- **30 days** suits an app where you need to look back over a month.

Change the period or turn logs off at any time.

## If something goes wrong [#if-something-goes-wrong]

- **Nothing shows under Last hour**: check that **Keep logs** is on, then give it a minute. An app that **Sleeps when idle** prints nothing while asleep; visit it to wake it.
- **The app prints too much**: lower the period, or have the app log less (for example, only warnings and errors).
- **A new version failed many requests and visitors went back to the one before**: read the logs for those errors, fix them and deploy again.

## Where to go next [#where-to-go-next]

- [Deploy an app](/docs/deploy/deploy-an-app)
- [Run settings and app keys](/docs/deploy/run-settings)
- [Project settings](/docs/projects/settings)