---
title: "Processing Logs | Grafana Agent documentation"
description: "Learn how to process logs"
---

# Processing Logs

This tutorial assumes you are familiar with setting up and connecting components. It covers using `loki.source.api` to receive logs over HTTP, processing and filtering them, and sending them to Loki.

## Receive logs over HTTP and Process

**Recommended reading**

- Optional: [loki.source.api](/docs/agent/v0.43/flow/reference/components/loki.source.api/)

The `loki.source.api` component can receive logs over HTTP. It can be useful for receiving logs from other Grafana Agents or collectors, or directly from applications that can send logs over HTTP, and then processing them centrally.

Your pipeline is going to look like this:

Let’s start by setting up the `loki.source.api` component:

Alloy ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```alloy
loki.source.api "listener" {
    http {
        listen_address = "127.0.0.1"
        listen_port    = 9999
    }

    labels = { "source": "api" }

    forward_to = [loki.process.process_logs.receiver]
}
```

This is a simple configuration. You are configuring the `loki.source.api` component to listen on `127.0.0.1:9999` and attach a `source="api"` label to the received log entries, which are then forwarded to the `loki.process.process_logs` component’s exported receiver. Next, you can configure the `loki.process` and `loki.write` components.

## Process and Write Logs

**Recommended reading**

- [loki.process#stage.drop](/docs/agent/v0.43/flow/reference/components/loki.process/#stagedrop-block)
- [loki.process#stage.json](/docs/agent/v0.43/flow/reference/components/loki.process/#stagejson-block)
- [loki.process#stage.labels](/docs/agent/v0.43/flow/reference/components/loki.process/#stagelabels-block)

Alloy ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```alloy
// Let's send and process more logs!

loki.source.api "listener" {
    http {
        listen_address = "127.0.0.1"
        listen_port    = 9999
    }

    labels = { "source" = "api" }

    forward_to = [loki.process.process_logs.receiver]
}

loki.process "process_logs" {

    // Stage 1
    stage.json {
        expressions = {
            log = "",
            ts  = "timestamp",
        }
    }

    // Stage 2
    stage.timestamp {
        source = "ts"
        format = "RFC3339"
    }

    // Stage 3
    stage.json {
        source = "log"

        expressions = {
            is_secret = "",
            level     = "",
            log_line  = "message",
        }
    }

    // Stage 4
    stage.drop {
        source = "is_secret"
        value  = "true"
    }

    // Stage 5
    stage.labels {
        values = {
            level = "",
        }
    }

    // Stage 6
    stage.output {
        source = "log_line"
    }

    // This stage adds static values to the labels on the log line
    stage.static_labels {
        values = {
            source = "demo-api",
        }
    }

    forward_to = [loki.write.local_loki.receiver]
}

loki.write "local_loki" {
    endpoint {
        url = "http://localhost:3100/loki/api/v1/push"
    }
}
```

You can skip to the next section if you successfully completed the previous section’s exercises. If not, or if you were unsure how things worked, let’s break down what is happening in the `loki.process` component.

Many of the `stage.*` blocks in `loki.process` act on reading or writing a shared map of values extracted from the logs. You can think of this extracted map as a hashmap or table that each stage has access to, and it is referred to as the “extracted map” from here on. In subsequent stages, you can use the extracted map to filter logs, add or remove labels, or even modify the log line.

> Note
> 
> `stage.*` blocks are executed in the order they appear in the component, top down.

Let’s use an example log line to illustrate this, then go stage by stage, showing the contents of the extracted map. Here is our example log line:

JSON ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```json
{
    "log": {
        "is_secret": "true",
        "level": "info",
        "message": "This is a secret message!",
    },
    "timestamp": "2023-11-16T06:01:50Z",
}
```

### Stage 1

Alloy ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```alloy
stage.json {
    expressions = {
        log = "",
        ts  = "timestamp",
    }
}
```

This stage parses the log line as JSON, extracts two values from it, `log` and `timestamp`, and puts them into the extracted map with keys `log` and `ts`, respectively.

> Note
> 
> Supplying an empty string is shorthand for using the same key as in the input log line (so `log = ""` is the same as `log = "log"`). The *keys* of the `expressions` object end up as the keys in the extracted map, and the *values* are used as keys to look up in the parsed log line.

If this were Python, it would be roughly equivalent to:

Python ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```python
extracted_map = {}
log_line      = {"log": {"is_secret": "true", "level": "info", "message": "This is a secret message!"}, "timestamp": "2023-11-16T06:01:50Z"}

extracted_map["log"] = log_line["log"]
extracted_map["ts"]  = log_line["timestamp"]
```

Extracted map *before* performing this stage:

JSON ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```json
{}
```

Extracted map *after* performing this stage:

JSON ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```json
{
    "log": {
        "is_secret": "true",
        "level": "info",
        "message": "This is a secret message!",
    },
    "ts": "2023-11-16T06:01:50Z",
}
```

### Stage 2

Alloy ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```alloy
stage.timestamp {
    source = "ts"
    format = "RFC3339"
}
```

This stage acts on the `ts` value in the map you extracted in the previous stage. The value of `ts` is parsed in the format of `RFC3339` and added as the timestamp to be ingested by Loki. This is useful if you want to use the timestamp present in the log itself, rather than the time the log is ingested. This stage does not modify the extracted map.

### Stage 3

Alloy ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```alloy
stage.json {
    source = "log"

    expressions = {
        is_secret = "",
        level     = "",
        log_line  = "message",
    }
}
```

This stage acts on the `log` value in the extracted map, which is a value that you extracted in the previous stage. This value is also a JSON object, so you can extract values from it as well. This stage extracts three values from the `log` value, `is_secret`, `level`, and `log_line`, and puts them into the extracted map with keys `is_secret`, `level`, and `log_line`.

If this were Python, it would be roughly equivalent to:

Python ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```python
extracted_map = {
    "log": {
        "is_secret": "true",
        "level": "info",
        "message": "This is a secret message!",
    },
    "ts": "2023-11-16T06:01:50Z",
}

source = extracted_map["log"]

extracted_map["is_secret"] = source["is_secret"]
extracted_map["level"]     = source["level"]
extracted_map["log_line"]  = source["message"]
```

Extracted map *before* performing this stage:

JSON ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```json
{
    "log": {
        "is_secret": "true",
        "level": "info",
        "message": "This is a secret message!",
    },
    "ts": "2023-11-16T06:01:50Z",
}
```

Extracted map *after* performing this stage:

JSON ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```json
{
    "log": {
        "is_secret": "true",
        "level": "info",
        "message": "This is a secret message!",
    },
    "ts": "2023-11-16T06:01:50Z",
    "is_secret": "true",
    "level": "info",
    "log_line": "This is a secret message!",
}
```

### Stage 4

Alloy ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```alloy
stage.drop {
    source = "is_secret"
    value  = "true"
}
```

This stage acts on the `is_secret` value in the extracted map, which is a value that you extracted in the previous stage. This stage drops the log line if the value of `is_secret` is `"true"` and does not modify the extracted map. There are many other ways to filter logs, but this is a simple example. Refer to the [loki.process#stage.drop](/docs/agent/v0.43/flow/reference/components/loki.process/#stagedrop-block) documentation for more information.

### Stage 5

Alloy ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```alloy
stage.labels {
    values = {
        level = "",
    }
}
```

This stage adds a label to the log using the same shorthand as above (so this is equivalent to using `values = { level = "level" }`). This stage adds a label with key `level` and the value of `level` in the extracted map to the log (`"info"` from our example log line). This stage does not modify the extracted map.

### Stage 6

Alloy ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```alloy
stage.output {
    source = "log_line"
}
```

This stage uses the `log_line` value in the extracted map to set the actual log line that is forwarded to Loki. Rather than sending the entire JSON blob to Loki, you are only sending `original_log_line["log"]["message"]`, along with some labels that you attached.

This stage does not modify the extracted map.

## Putting it all together

Now that you have all of the pieces, let’s run the Grafana Agent and send some logs to it. Modify `config.river` with the config from the previous example and start the Grafana Agent with:

Bash ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```bash
/path/to/agent run config.river
```

To get the current time in `RFC3339` format, you can run:

Bash ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```bash
date -u +"%Y-%m-%dT%H:%M:%SZ"
```

Try executing the following, replacing the `"timestamp"` value:

Bash ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```bash
curl localhost:9999/loki/api/v1/raw -XPOST -H "Content-Type: application/json" -d '{"log": {"is_secret": "false", "level": "debug", "message": "This is a debug message!"}, "timestamp": <YOUR TIMESTAMP HERE>}'
```

Now that you have sent some logs, let’s see how they look in Grafana. Navigate to [localhost:3000/explore](http://localhost:3000/explore) and switch the Datasource to `Loki`. Try querying for `{source="demo-api"}` and see if you can find the logs you sent.

Try playing around with the values of `"level"`, `"message"`, `"timestamp"`, and `"is_secret"` and see how the logs change. You can also try adding more stages to the `loki.process` component to extract more values from the logs, or add more labels.

## Exercise

Since you are already using Docker and Docker exports logs, let’s get those logs into Loki. You can refer to the [discovery.docker](/docs/agent/v0.43/flow/reference/components/discovery.docker/) and [loki.source.docker](/docs/agent/v0.43/flow/reference/components/loki.source.docker/) documentation for more information.

To ensure proper timestamps and other labels, make sure you use a `loki.process` component to process the logs before sending them to Loki.

Although you have not used it before, let’s use a `discovery.relabel` component to attach the container name as a label to the logs. You can refer to the [discovery.relabel](/docs/agent/v0.43/flow/reference/components/discovery.relabel/) documentation for more information. The `discovery.relabel` component is very similar to the `prometheus.relabel` component, but is used to relabel discovered targets rather than metrics.

Solution

Alloy ![Copy code to clipboard](/media/images/icons/icon-copy-small-2.svg) Copy

```alloy
// Discover docker containers to collect logs from
discovery.docker "docker_containers" {
    // Note that if you are using Docker Desktop Engine this may need to be changed to
    // something like "unix:///${HOME}/.docker/desktop/docker.sock"
    host = "unix:///var/run/docker.sock"
}

// Extract container name from __meta_docker_container_name label and add as label
discovery.relabel "docker_containers" {
    targets = discovery.docker.docker_containers.targets

    rule {
        source_labels = ["__meta_docker_container_name"]
        target_label  = "container"
    }
}

// Scrape logs from docker containers and send to be processed
loki.source.docker "docker_logs" {
    host    = "unix:///var/run/docker.sock"
    targets = discovery.relabel.docker_containers.output
    forward_to = [loki.process.process_logs.receiver]
}

// Process logs and send to Loki
loki.process "process_logs" {
    stage.docker { }

    forward_to = [loki.write.local_loki.receiver]
}

loki.write "local_loki" {
    endpoint {
        url = "http://localhost:3100/loki/api/v1/push"
    }
}
```