---
title: Merge
description: Learn about merging rules
keywords: compose, compose specification, merge, compose file reference
aliases: 
 - /compose/compose-file/13-merge/
weight: 100
---

{{% include "compose/merge.md" %}}

These rules are outlined below. 

## Mapping

A YAML `mapping` gets merged by adding missing entries and merging the conflicting ones.

Merging the following example YAML trees:

```yaml
services:
  foo:
    key1: value1
    key2: value2
```

```yaml
services:
  foo:
    key2: VALUE
    key3: value3
```

Results in a Compose application model equivalent to the YAML tree:

```yaml
services:
  foo:
    key1: value1
    key2: VALUE
    key3: value3
```

## Sequence

A YAML `sequence` is merged by appending values from the overriding Compose file to the previous one.

Merging the following example YAML trees:

```yaml
services:
  foo:
    DNS:
      - 1.1.1.1
```

```yaml
services:
  foo:
    DNS: 
      - 8.8.8.8
```

Results in a Compose application model equivalent to the YAML tree:

```yaml
services:
  foo:
    DNS:
      - 1.1.1.1
      - 8.8.8.8
```

## Exceptions

### Shell commands

When merging Compose files that use the services attributes [command](services.md#command), [entrypoint](services.md#entrypoint) and [healthcheck: `test`](services.md#healthcheck), the value is overridden by the latest Compose file, and not appended.

Merging the following example YAML trees:

```yaml
services:
  foo:
    command: ["echo", "foo"]
```

```yaml
services:
  foo:
    command: ["echo", "bar"]
```

Results in a Compose application model equivalent to the YAML tree:

```yaml
services:
  foo:
    command: ["echo", "bar"]
```

### Unique resources

Applies to the [ports](services.md#ports), [volumes](services.md#volumes), [secrets](services.md#secrets) and [configs](services.md#configs) services attributes.
While these types are modeled in a Compose file as a sequence, they have special uniqueness requirements:

| Attribute   | Unique key               |
|-------------|--------------------------|
| volumes     |  target                  |
| secrets     |  target                  |
| configs     |  target                  |
| ports       |  {ip, target, published, protocol}   |

When merging Compose files, Compose appends new entries that do not violate a uniqueness constraint and merge entries that share a unique key.

Merging the following example YAML trees: