---
title: "The read-only guarantee"
description: "monedula-sim-link connect never writes to your cluster: the read requests it sends, the one that is opt-in, what it never does, and where to verify it."
url: https://monedula.dev/flock/docs/sim-link/concepts/read-only-guarantee/
---
`connect` takes its snapshot with eight kinds of Kafka request, all of them reads, and a ninth, also a read, only when you ask for it with `--producers`.

## The requests

| Request | What `connect` uses it for |
| --- | --- |
| `ApiVersions` | The handshake of the Kafka client library, `franz-go`, to agree on protocol versions. |
| `Metadata` | The brokers, the topics, and the leader, replicas and in-sync replicas of every partition. |
| `ListOffsets` | The earliest and latest offset of every selected partition, which gives the relative fill of each. |
| `ListGroups` | The list of consumer groups. Only groups with the protocol type `consumer` are used. |
| `FindCoordinator` | The Kafka client library's lookup of the broker that coordinates a group, which `DescribeGroups` and `OffsetFetch` go through. |
| `DescribeGroups` | The state and the members of those groups. |
| `OffsetFetch` | The offsets those groups have committed, for the selected topics. |
| `DescribeConfigs` | The configs of the selected topics: `cleanup.policy`, `min.insync.replicas`, `retention.ms` and `retention.bytes`. |
| `DescribeProducers` | Only with `--producers`. The idempotent and transactional producers that wrote to the selected partitions, each with the timestamp of its last write. |

The `Metadata` request is sent with `AllowAutoTopicCreation=false`, so looking up a topic can never create it. That matters on clusters that have `auto.create.topics.enable` turned on: asking for a topic that does not exist fails with a message and leaves the cluster as it was.

### Why `DescribeProducers` is opt-in

`DescribeProducers` (KIP-664, Kafka 2.8 or newer) needs `READ` on each topic it asks about, where the other requests need only `DESCRIBE` (and `DESCRIBE_CONFIGS` for the topic configs). So the tool sends it only when you pass [`--producers`](/flock/docs/sim-link/reference/cli/connect/), or set `producers` on the MCP tool `cluster_to_url`. Without the flag the request is never sent. `READ` is also the permission that lets a client fetch records. `connect` never sends a fetch, and everything on this page holds with the flag too, but that is a reason to grant `READ` to the principal only when you want producers ([Connect to a secured cluster](/flock/docs/sim-link/how-to/connect-to-a-secured-cluster/#4-allow-reading-the-topics-for-producers-optional) shows how).

## What it never does

- It produces no records, and creates or alters no topics and no configs.
- It joins no consumer group: there is no `JoinGroup`, `SyncGroup` or `Heartbeat`.
- It commits no offset: there is no `OffsetCommit`.
- It never reads record contents. None of these requests fetches records, so the tool never sees the keys and values of your data. `DescribeProducers` reports a producer id and the timestamp of its last write, not what it wrote.

The MCP tool `cluster_to_url` uses the same code path, so it has the same guarantee. The other MCP tools never connect to a cluster.

## Where to check it yourself

The whole client surface lives in one file, [`snapshot/kafka.go`](https://github.com/monedula-dev/monedula-sim-link/blob/main/snapshot/kafka.go), so the guarantee can be audited in one place. Tests back it up:

- `TestFetchMissingTopicDoesNotAutoCreate` proves that a failed topic lookup does not create the topic, and `TestFetchGroups` proves that reading the committed offsets of a group twice never changes them. Both are in `snapshot/kafka_test.go`.
- `TestFetchWithoutProducersSendsNoDescribeProducers`, in `snapshot/producers_test.go`, proves that a snapshot without `--producers` sends no `DescribeProducers` request.
- An end-to-end suite runs the pipeline against a real three-node KRaft cluster, with Apache Kafka and with Confluent Platform images. It checks the snapshot, the link, the `connect` binary and the MCP server, and that nothing on the cluster changed: no topic was created, no offset committed and no member joined. Its read-only test takes a snapshot with the producers read as well.

## Defense in depth

The guarantee comes from the tool, and your cluster can enforce it as well. Give `connect` a principal that can only describe the cluster, its topics, its groups and its topic configs, and its permissions match what it does; add `READ` on the topics only for `--producers`. [Connect to a secured cluster](/flock/docs/sim-link/how-to/connect-to-a-secured-cluster/) shows how to pass the credentials.

## Related

- [What the link shows](/flock/docs/sim-link/concepts/what-the-link-shows/) says what the snapshot becomes.
- The [`connect` command reference](/flock/docs/sim-link/reference/cli/connect/) lists its flags.