Conditional execution

Understand how NRedisStack uses conditional execution

Most Redis client libraries use transactions with the WATCH command as the main way to prevent two clients writing to the same key at once (see Transactions for more information). Unfortunately, this approach is difficult to use explicitly in NRedisStack. Its multiplexing system is highly efficient and convenient but can also cause bad interactions when different connections use watched transactions at the same time.

Instead, NRedisStack relies more heavily on conditional execution. This comes in two basic forms, When conditions and transaction conditions, both of which are explained in the sections below.

When conditions

Several commands have variants that only execute if the key they change already exists (or alternatively, if it doesn't already exist). For example, the SET command has the variants SETEX (set when the key exists), and SETNX (set when the key doesn't exist).

Instead of providing the different variants of these commands, NRedisStack lets you add a When condition to the basic command to access its variants. The following example demonstrates this for the HashSet() command.

bool resp7 = db.HashSet("Details", "SerialNumber", "12345");
Console.WriteLine(resp7); // >>> true

db.HashSet("Details", "SerialNumber", "12345A", When.NotExists);
string resp8 = db.HashGet("Details", "SerialNumber");
Console.WriteLine(resp8); // >>> 12345

db.HashSet("Details", "SerialNumber", "12345A");
string resp9 = db.HashGet("Details", "SerialNumber");
Console.WriteLine(resp9); // >>> 12345A

The available conditions are When.Exists, When.NotExists, and the default When.Always.

Transaction conditions

NRedisStack also supports a more extensive set of conditions that you can add to transactions. They are implemented internally using WATCH commands in a way that is guaranteed to be safe, without interactions between different clients. Although conditions don't provide exactly the same behavior as explicit WATCH commands, they are convenient to use and execute efficiently.

The example below shows how to use the AddCondition() method on a transaction to let it run only if a specified hash key does not already exist. See Pipelines and transactions for more information about transactions.

var watchedTrans = new Transaction(db);

watchedTrans.AddCondition(Condition.KeyNotExists("customer:39182"));

watchedTrans.Db.HashSetAsync(
    "customer:39182",
    new HashEntry[]{
        new HashEntry("name", "David"),
        new HashEntry("age", "27")
    }
);

bool succeeded = watchedTrans.Execute();
Console.WriteLine(succeeded); // >>> true

The table below describes the full set of conditions you can add to a transaction. Note that you can add more than one condition to the same transaction if necessary.