Challenge #6a: Single-Node, Totally-Available Transactions

In this challenge, you’ll need to implement a key/value store which implements transactions. These transactions contain micro-operations (read & write) and the results of those operations depends on the consistency guarantees of the challenge. Your goal is to support weak consistency while also being totally available. We begin with a single-node service and then write a multi-node version.


Your node will support the txn-rw-register workload by implementing a key/value store that accepts only one message. How easy, right?? This message is the txn message which passes in a list of operations to perform.

Writes in this workload are unique per-key so key 100 would only ever see a write of 1 once, a write of 2 once, etc. This helps Maelstrom to verify correctness.

RPC: txn

This message passes in an array operations in the "txn" key. Each operation is represented by a 3-element array containing the operation name, the integer key to operate on, and a possibly-null integer value.

For example, your node will receive a request message body that looks like this:

  "type": "txn",
  "msg_id": 3,
  "txn": [
    ["r", 1, null],
    ["w", 1, 6],
    ["w", 2, 9]

This represents three operations:

  • Read from key 1
  • Write the value of 6 to key 1
  • Write the value of 9 to key 2

In response, it should send a txn_ok message that contains the same operation list, however, read ("r") operations should have their value filled in with the current value. For example, if the value of key 1 was 3 before this transaction, it should be returned in the read operation. Non-existent keys should be returned as null.

  "type": "txn_ok",
  "msg_id": 1,
  "in_reply_to": 3,
  "txn": [
    ["r", 1, 3],
    ["w", 1, 6],
    ["w", 2, 9]


Build your Go binary as maelstrom-txn and run it against Maelstrom with the following command:

./maelstrom test -w txn-rw-register --bin ~/go/bin/maelstrom-txn --node-count 1 --time-limit 20 --rate 1000 --concurrency 2n --consistency-models read-uncommitted --availability total

This will verify your single-node system works before we move on to distributing our writes across nodes.

If you’re successful, continue on to the Totally-Available, Read Uncommitted Transactions challenge. If you’re having trouble, jump over to the Community forum for help.

  1. Read More About Echo
  2. Read More About Unique ID Generation
  3. Read More About Broadcast
  4. Read More About Grow-Only Counter
  5. Read More About Kafka-Style Log
  6. Read More About Totally-Available Transactions