--- title: "Preview a link before you share it" description: "Use --dry-run to read the size parameter, the real-to-simulator name tables and the decoded action list that a monedula-sim-link link is made of." url: https://monedula.dev/flock/docs/sim-link/how-to/preview-with-dry-run/ --- ## Before you start - `monedula-sim-link` installed. No cluster is needed: `demo` runs a built-in fixture through the same pipeline, and `--dry-run` works on it too. ## Steps ### 1. Run with `--dry-run` ```bash monedula-sim-link demo --dry-run ``` For your own cluster, add `--dry-run` to the `connect` command you already use. The output below is from `demo`, whose fixture is a four-broker cluster with an `orders` topic and a `payments` topic: ```text size: 4x3x3xpayments~1~2~1~d~~~~ brokers: 1 -> broker-1 2 -> broker-2 3 -> broker-3 4 -> broker-4 topics: orders -> orders payments -> payments actions (19 entries): RP:p1@500 RG:c1@750 RA:orders:2:broker-3.broker-1.broker-2@1000 K:broker-4@1250 SR:broker-2:1@1500 AP:kp1:orders@1750 P:kp1:orders:b@2000 P:kp1:orders:e@2250 P:kp1:orders:k@2500 P:kp1:orders:n@2750 P:kp1:orders:o@3000 P:kp1:orders:f@3250 P:kp1:orders:g@3500 RP:kp1@5500 AP:kp2:payments@5750 P:kp2:payments:a@6000 P:kp2:payments:b@6250 P:kp2:payments:c@6500 RP:kp2@8500 https://monedula.dev/kafka-simulator/playground?scenario=free&actions=RP%3Ap1%40500%2C...&size=4x3x3xpayments~1~2~1~d~~~~ ``` Warnings go to standard error, so they do not mix with this output. In this run there is one, about broker 2 being out of the in-sync replicas of `orders` partition 2. ### 2. Read the size line `size` is the cluster shape, in the form `xx`, followed by one segment for each extra topic. Here that is 4 brokers and the `orders` topic with 3 partitions and replication factor 3, then `payments` with 1 partition, replication factor 2, `min.insync.replicas` 1 and the `delete` cleanup policy. The [mapping reference](/flock/docs/sim-link/reference/mapping/#the-size-parameter) describes the segments. ### 3. Check the name tables Each table maps a real name to the name the simulator shows: - **`brokers`:** the simulator numbers brokers `broker-1` to `broker-N` in the order of the real ids, so this table tells you which real broker each simulator broker stands for. - **`topics`:** a name the simulator cannot carry, such as `billing.events`, is renamed ([naming rules](/flock/docs/sim-link/reference/mapping/#topic-and-broker-naming)), and each rename also prints a warning. - **`groups`:** the consumer groups that made it into the link. A group is absent when it has no committed offset on a shown topic, or when it was not among the 4 groups kept ([Handle large clusters](/flock/docs/sim-link/how-to/handle-large-clusters/)). The table appears only when there is at least one group. ### 4. Read the action list The `actions` list is what the playground replays on its simulated clock. Each entry is `@`. The tool starts at t=500 ms and spaces the actions 250 ms apart, except that a synthetic producer is removed 2 seconds after its last record. These are the actions in the output above: | Entry | What it does | Why it is there | | --- | --- | --- | | `RP:p1@500` | Removes the free-play template producer `p1`. | So the playground does not add traffic of its own on top of the data in the link. | | `RG:c1@750` | Removes the free-play template consumer group `c1`, with its member. | So a fake consumer does not sit beside the real groups. It is there even when the cluster has no group. | | `RA:orders:2:broker-3.broker-1.broker-2@1000` | Reassigns the replicas of `orders` partition 2 to those brokers. | The real placement of that partition differs from the simulator's default. A partition on the default placement emits nothing. | | `K:broker-4@1250` | Kills `broker-4`. | In the fixture, broker 4 is offline. | | `SR:broker-2:1@1500` | Slows the follower replication of `broker-2`. | `broker-2` is in the replica list of a partition but out of its in-sync replicas. | | `AP:kp1:orders@1750` | Adds a short-lived producer `kp1` for the topic `orders`. | It carries the synthetic records. | | `P:kp1:orders:b@2000` | Produces one record with the key `b` to `orders`. | One record of the data-volume sample. The key was chosen so that the record lands on the intended partition. | | `RP:kp1@5500` | Removes the producer `kp1`, two seconds after its last record. | At most one synthetic producer is alive at a time. | A consumer group adds more entries: `AG:analytics:orders@7250` adds a group on a topic, `AC:` lines join its members, and `C:group.analytics.consumer.autoOffsetReset:earliest@7500` makes a lagging group start from the earliest offset. With `--producers`, one `AP:pid-:` entry per producer comes last: `monedula-sim-link demo --producers --dry-run` ends with `AP:pid-1001:orders@8750` and `AP:pid-1002:payments@9000`, and prints a warning for the second, which wrote from inside an open transaction. The [mapping reference](/flock/docs/sim-link/reference/mapping/) explains each family of actions. A long action list makes a long link; [Handle large clusters](/flock/docs/sim-link/how-to/handle-large-clusters/) covers the action and length limits. ### 5. Print the link When the tables look right, run the same command without `--dry-run`. It prints only the link. ## Related - [What the link shows](/flock/docs/sim-link/concepts/what-the-link-shows/) explains the mapping in plain terms. - The [`demo` command reference](/flock/docs/sim-link/reference/cli/demo/) lists its flags.