# Consistency Level Calculator

    <style>
        .calculator {
            width: fit-content;
        }
        .form-inline {
            display: flex;
            flex-flow: row wrap;
        }
        .form-inline label {
            margin: 5px 10px 5px 0;
        }
        .form-inline input {
            padding: 10px;
            margin: 5px 10px 5px 0;
        }
    </style>
    <div class="calculator">
        <form class="form-inline" action="">
            <div class="input">
                <label for="nodes">Nodes</label><input style="width: 8ch;" type="number" min="1" value="3" name="nodes" id="nodes">
            </div>
            <div class="input">
                <label for="replication-factor">Replication factor</label><input style="width: 8ch;" min="1" value="3" type="number" name="replication-factor" id="replication-factor">
            </div>
            <div class="input">
                <label for="read-consistency">Read consistency level</label><select name="read-consistency" id="read-consistency">
                    <option value="ONE">ONE</option>
                    <option value="TWO">TWO</option>
                    <option value="THREE">THREE</option>
                    <option value="QUORUM">QUORUM</option>
                    <option value="ALL">ALL</option>
                </select>
            </div>
            <div class="input">
                <label for="write-consistency">Write consistency level</label><select name="write-consistency" id="write-consistency">
                    <option value="ONE">ONE</option>
                    <option value="TWO">TWO</option>
                    <option value="THREE">THREE</option>
                    <option value="QUORUM">QUORUM</option>
                    <option value="ALL">ALL</option>
                </select>
            </div>
        </form>
        <div class="results"></div>
    </div>
    <br/>
    <br/>

    <script>
        function consistency(c, rf) {
            switch(c) {
                case "ONE":
                    return 1;
                case "TWO":
                    return 2;
                case "THREE":
                    return 3;
                case "QUORUM":
                    return Math.floor(rf / 2) + 1;
                case "ALL":
                    return rf;
            }
        }

        function calculate() {
            const nodes = Number.parseInt(document.querySelector("#nodes").value);
            const readConsistency = document.querySelector("#read-consistency").value;
            const writeConsistency = document.querySelector("#write-consistency").value;
            const replicationFactor = Number.parseInt(document.querySelector("#replication-factor").value);
            const writeConsistencyN = consistency(writeConsistency, replicationFactor);
            const readConsistencyN = consistency(readConsistency, replicationFactor);

            
            const resultsDiv = document.querySelector(".results");
            
            if (nodes < replicationFactor) {
                resultsDiv.textContent = "Replication factor must be smaller or equal to the number of nodes";
                resultsDiv.classList.add("warning", "admonition");
            } else if (writeConsistencyN > replicationFactor || readConsistencyN > replicationFactor) {
                resultsDiv.textContent = "Consistency level must be lower than replication factor";
                resultsDiv.classList.add("warning", "admonition");
            } else {
                const msg = [];
                msg.push("<ul>")
                if (writeConsistencyN + readConsistencyN > replicationFactor) {
                    msg.push("<li>Your reads are <b>consistent</b></li>");
                } else {
                    msg.push("<li>Your reads are <b>eventually consistent</b></li>");
                }
                msg.push(\`<li>You can lose ${replicationFactor - readConsistencyN} nodes without failing reads</li>\`);
                msg.push(\`<li>You can lose ${writeConsistencyN - 1} nodes without data loss</li>\`);
                msg.push(\`<li>Each node holds ~ ${Math.round(replicationFactor / nodes \* 100)}% of your data</li>\`);
                msg.push("</ul>")
                
                resultsDiv.classList.remove("warning", "admonition");
                resultsDiv.innerHTML = msg.join("<br/>"); 
            }
        }

        document.querySelectorAll(".input input").forEach((element) => {
            element.addEventListener("input", calculate);
        })

        document.querySelectorAll(".input select").forEach((element) => {
            element.addEventListener("input", calculate);
        })

        calculate();
    </script>

## Additional Information

* [Consistency Levels](https://docs.scylladb.com/manual/master/cql/consistency.md)
* [Fault Tolerance](https://docs.scylladb.com/manual/master/architecture/architecture-fault-tolerance.md)
* [Consistency Level Compatibility](https://docs.scylladb.com/manual/master/using-scylla/cassandra-compatibility.md#consistency-level-read-and-write)
* [Consistency Quiz](https://docs.scylladb.com/manual/master/kb/quiz-administrators.md)
* Take a course on [Consistency Levels at ScyllaDB University](https://university.scylladb.com/courses/scylla-essentials-overview/lessons/high-availability/topic/consistency-level/)