# Conditional execution ```json metadata { "schema_version": 2, "title": "Conditional execution", "description": "Understand how `StackExchange.Redis` uses conditional execution.", "categories": ["docs","develop","stack","oss","rs","rc","oss","kubernetes","clients"], "tableOfContents": {"sections":[{"id":"when-conditions","title":"When conditions"},{"id":"transaction-conditions","title":"Transaction conditions"}]} , "codeExamples": [] } ``` Most Redis client libraries use transactions with the [`WATCH`](https://redis.io/docs/latest/commands/watch) command as the main way to prevent two clients writing to the same key at once (see [Transactions](https://redis.io/docs/latest/develop/using-commands/transactions) for more information). Unfortunately, this approach is difficult to use explicitly in `StackExchange.Redis`. Its [multiplexing](https://redis.io/docs/latest/develop/clients/pools-and-muxing) system is highly efficient and convenient but can also cause bad interactions when different connections use watched transactions at the same time. Instead, `StackExchange.Redis` 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`](https://redis.io/docs/latest/commands/set) command has the variants [`SETEX`](https://redis.io/docs/latest/commands/setex) (set when the key exists), and [`SETNX`](https://redis.io/docs/latest/commands/setnx) (set when the key doesn't exist). Instead of providing the different variants of these commands, `StackExchange.Redis` lets you add a `When` condition to the basic command to access its variants. The following example demonstrates this for the [`HashSet()`](https://redis.io/docs/latest/commands/hset) command. ```csharp 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 `StackExchange.Redis` also supports a more extensive set of conditions that you can add to transactions. They are implemented internally using [`WATCH`](https://redis.io/docs/latest/commands/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](https://redis.io/docs/latest/develop/clients/dotnet/transpipe) for more information about transactions. ```csharp var watchedTrans = db.CreateTransaction(); watchedTrans.AddCondition(Condition.KeyNotExists("customer:39182")); var updateTask = watchedTrans.HashSetAsync( "customer:39182", new HashEntry[]{ new HashEntry("name", "David"), new HashEntry("age", "27") } ); bool succeeded = db.Wait(watchedTrans.ExecuteAsync()); db.Wait(updateTask); 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. | Condition | Description | | :-- | :-- | | `HashEqual` | Enforces that the given hash-field must have the specified value. | | `HashExists` | Enforces that the given hash-field must exist. | | `HashNotEqual` | Enforces that the given hash-field must not have the specified value. | | `HashNotExists` | Enforces that the given hash-field must not exist. | | `KeyExists` | Enforces that the given key must exist. | | `KeyNotExists` | Enforces that the given key must not exist. | | `ListIndexEqual` | Enforces that the given list index must have the specified value. | | `ListIndexExists` | Enforces that the given list index must exist. | | `ListIndexNotEqual` | Enforces that the given list index must not have the specified value. | | `ListIndexNotExists` | Enforces that the given list index must not exist. | | `StringEqual` | Enforces that the given key must have the specified value. | | `StringNotEqual` | Enforces that the given key must not have the specified value. | | `HashLengthEqual` | Enforces that the given hash length is a certain value. | | `HashLengthLessThan` | Enforces that the given hash length is less than a certain value. | | `HashLengthGreaterThan` | Enforces that the given hash length is greater than a certain value. | | `StringLengthEqual` | Enforces that the given string length is a certain value. | | `StringLengthLessThan` | Enforces that the given string length is less than a certain value. | | `StringLengthGreaterThan` | Enforces that the given string length is greater than a certain value. | | `ListLengthEqual` | Enforces that the given list length is a certain value. | | `ListLengthLessThan` | Enforces that the given list length is less than a certain value. | | `ListLengthGreaterThan` | Enforces that the given list length is greater than a certain value. | | `SetLengthEqual` | Enforces that the given set cardinality is a certain value. | | `SetLengthLessThan` | Enforces that the given set cardinality is less than a certain value. | | `SetLengthGreaterThan` | Enforces that the given set cardinality is greater than a certain value. | | `SetContains` | Enforces that the given set contains a certain member. | | `SetNotContains` | Enforces that the given set does not contain a certain member. | | `SortedSetLengthEqual` | Enforces that the given sorted set cardinality is a certain value. | | `SortedSetLengthEqual` | Enforces that the given sorted set contains a certain number of members with scores in the given range. | | `SortedSetLengthLessThan` | Enforces that the given sorted set cardinality is less than a certain value. | | `SortedSetLengthLessThan` | Enforces that the given sorted set contains less than a certain number of members with scores in the given range. | | `SortedSetLengthGreaterThan` | Enforces that the given sorted set cardinality is greater than a certain value. | | `SortedSetLengthGreaterThan` | Enforces that the given sorted set contains more than a certain number of members with scores in the given range. | | `SortedSetContains` | Enforces that the given sorted set contains a certain member. | | `SortedSetNotContains` | Enforces that the given sorted set does not contain a certain member. | | `SortedSetEqual` | Enforces that the given sorted set member must have the specified score. | | `SortedSetNotEqual` | Enforces that the given sorted set member must not have the specified score. | | `SortedSetScoreExists` | Enforces that the given sorted set must have the given score. | | `SortedSetScoreNotExists` | Enforces that the given sorted set must not have the given score. | | `SortedSetScoreExists` | Enforces that the given sorted set must have the specified count of the given score. | | `SortedSetScoreNotExists` | Enforces that the given sorted set must not have the specified count of the given score. | | `StreamLengthEqual` | Enforces that the given stream length is a certain value. | | `StreamLengthLessThan` | Enforces that the given stream length is less than a certain value. | | `StreamLengthGreaterThan` | Enforces that the given stream length is greater than a certain value. |