Add support for handling failures within transactions in course-definition.yml.

Author: rohitpaulk

course-definition.yml

@@ -3629,7 +3629,34 @@ stages:
     name: "Failures within transactions"
     difficulty: medium
     description_md: |
-      **🚧 We're still working on instructions for this stage**. You can find notes on how the tester works below.
+      In this stage, you'll add support for handling failures within a transaction.
+
+      ### Failures within transactions
+
+      When executing a transaction, if a command fails, the error is captured and returned within the response for `EXEC`. All other commands in
+      the transaction are still executed.
+
+      You can read more about this behaviour [in the official Redis docs](https://redis.io/docs/latest/develop/interact/transactions/#errors-inside-a-transaction).
+
+      Example:
+
+      ```bash
+      $ redis-cli
+      > MULTI
+      OK
+      > SET foo xyz
+      QUEUED
+      > INCR foo
+      QUEUED
+      > SET bar 7
+      > EXEC
+      1) OK
+      2) (error) ERR value is not an integer or out of range
+      3) OK
+      ```
+
+      In the example above, note that the error for the `INCR` command was returned as the second array element. The third command (`SET bar 7`) was
+      still executed successfully.
 
       ### Tests
 
@@ -3642,21 +3669,41 @@ stages:
       The tester will then connect to your server as a Redis client, and send multiple commands using the same connection:
 
       ```bash
-      $ redis-cli SET foo abc
-      > SET bar 7
+      $ redis-cli
+      > SET foo abc
+      OK
+      > SET bar 41
+      OK
       > MULTI
+      OK
       > INCR foo
+      QUEUED
       > INCR bar
+      QUEUED
       > EXEC
+      1) (error) ERR value is not an integer or out of range
+      2) (integer) 42
       ```
 
-      The tester will then create another connection to your server as a Redis client, and send a single command:
+      The expected response for `EXEC` is a [RESP array](https://redis.io/docs/latest/develop/reference/protocol-spec/#arrays) of
+      the responses of the queued commands. The exact bytes will be:
 
       ```bash
-      $ redis-cli GET foo
-      > GET bar
+      *2\r\n-ERR value is not an integer or out of range\r\n:42\r\n
+      ```
+
+      The tester will also verify that the last command was successfully executed and that the key `bar` exists:
+
+      ```bash
+      $ redis-cli
+      > GET foo (expecting "$3\r\nabc\r\n" as the response)
+      > GET bar (expecting "$2\r\n42\r\n" as the response)
       ```
 
+      ### Notes
+
+      - There are a subset of command failures (like syntax errors) that will cause a transaction to be aborted entirely. We won't
+        cover those in this challenge.
     marketing_md: |
       In this stage, you'll implement handling failures while executing a transaction.