Tair (Redis® OSS-Compatible):Implement high-performance optimistic locking by using TairString

How version-number optimistic locking works

Optimistic locking works by attaching a version number to each key. When you read a key, you also read its version. When you write, you supply the version you read. If another client has written to the key in the meantime, the server version has incremented and no longer matches yours — the write is rejected rather than silently applied. You then retry with the updated value and version.

This is the problem that TairString solves natively: native Redis strings consist of only keys and values, with no mechanism to detect whether a key changed between a read and a write.

The race condition: why last-write-wins causes data loss

The following diagram illustrates a typical race condition.

Consider a counter that two clients increment concurrently. Both read the current value (10), independently compute 11, and write back. The result is 11 instead of the expected 12 — one increment is silently lost. The same pattern applies to any shared key:

1. key_1 starts with the value hello.

2. Application 1 reads hello.

3. Application 2 reads hello.

4. Application 1 writes world to key_1.

5. Application 2 writes universe to key_1.

Application 1 expects key_1 to be world, but the actual value is universe. To prevent this, the read-modify-write sequence must be atomic — which is exactly what TairString's version-number mechanism provides.

TairString commands for optimistic locking

TairStrings consist of keys, values, and version numbers. TairString integrates all Redis string features except bit operations.

For the full command reference, see exString.

Implement optimistic locking with EXGET and EXSET

The basic optimistic locking pattern uses a retry loop: read the current value and version with EXGET, compute the new value, then write with EXSET, passing the version you read. If a competing write occurred between your read and write, the version check fails and the loop retries.

while(true){
    {value, version} = EXGET(key);      // Retrieve the value and version number of the key.
    value2 = update(...);               // Save the new value as value 2.
    ret = EXSET(key, value2, version);  // Update the key and assign the return value to the ret variable.
    if(ret == OK)
       break;                           // If the return value is OK, the update is successful and the while loop exits.
    else if (ret.contains("version is stale"))
       continue;                        // If the return value contains the "version is stale" error message, the update fails and the while loop is repeated.
}

This guarantees that if any other client updated the key after your EXGET, your EXSET fails rather than silently overwriting their change. Each iteration of this loop requires two round trips to Tair: one EXGET to read the current value and version, and one EXSET to attempt the write.

Reduce I/O with EXCAS

The EXGET-then-EXSET loop requires two I/O operations per retry cycle. The EXCAS (Compare-And-Swap) command reduces this to one I/O per cycle — a significant improvement under high concurrency.

EXCAS atomically checks the version and applies the write in a single round trip. On failure, it returns all three of the following in one response:

• "ERR update version is stale"

• The current value

• The current version number

Because the current state is included in the failure response, the next retry cycle can start immediately — no extra EXGET call is needed.

while(true){
    {ret, value, version} = excas(key, new_value, old_version)    // Use the CAS command to replace the original value with a new value.
    if(ret == OK)
       break;                                                     // If the return value is OK, the update is successful and the while loop exits.
    else (if ret.contains("update version is stale"))            // If the return value contains the "update version is stale" error message, the update fails. The values of the value and old_version variables are updated.
       update(value);
       old_version = version;
 }

Related topics

• exString command reference