DOC-5881 first draft of error handling page

Author: andy-stark-redis

content/develop/clients/error-handling.md

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+---
+title: Error handling
+description: Learn how to handle errors when using Redis client libraries
+linkTitle: Error handling
+weight: 50
+---
+
+When working with Redis, errors can occur for various reasons—network issues, invalid commands, or resource constraints. This guide explains the types of errors you might encounter and how to handle them effectively.
+
+## Categories of errors
+
+### Connection errors
+
+Connection errors occur when your application cannot communicate with Redis. These are typically temporary and often recoverable.
+
+**Common causes:**
+- Network connectivity issues
+- Redis server is down or unreachable
+- Authentication failure
+- Connection timeout
+- Connection pool exhaustion
+
+**Examples:**
+- `ConnectionError`: Network failure or server unreachable
+- `TimeoutError`: Operation exceeded the configured timeout
+- `AuthenticationError`: Invalid credentials
+
+**When to handle:** Almost always. Connection errors are usually temporary, so implementing retry logic or fallback strategies is recommended.
+
+**Example strategy:**
+
+```mermaid {width="100%"}
+graph TB
+    A["Try to connect<br/>to Redis"]
+    A -->|Success| B(["Use the result"])
+    A -->|Failure| C{Error type?}
+    C -->|Timeout| D(["Retry with<br/>exponential backoff"])
+    C -->|Auth failure| E(["Check credentials<br/>and fail"])
+    C -->|Network error| F(["Fallback to<br/>alternative data source"])
+```
+
+### Command errors
+
+Command errors occur when Redis receives an invalid or malformed command. These typically indicate a bug in your code.
+
+**Common causes:**
+- Typo in command name
+- Wrong number of arguments
+- Invalid argument types
+- Using a command that doesn't exist in your Redis version
+
+**Examples:**
+- `ResponseError`: Invalid command or syntax error
+- `WRONGTYPE Operation against a key holding the wrong kind of value`
+- `ERR unknown command`
+
+**When to handle:** Rarely. These usually indicate a programming error and should be fixed in your code, not handled at runtime. However, some cases (like invalid user input) may warrant handling.
+
+**Example:**
+
+```mermaid {width="60%"}
+graph TB
+    A["User provides<br/>JSONPath expression"]
+    A --> B["Try to execute it"]
+    direction TB
+    B -->|ResponseError| C["Log the error"]
+    C --> D(["Return default value<br/>or error message to user"])
+    B -->|Success| E(["Use the result"])
+```
+
+### Data errors
+
+Data errors occur when there's a problem with the data itself—serialization failures, corrupted data, or type mismatches.
+
+**Common causes:**
+- Object cannot be serialized to JSON
+- Cached data is corrupted
+- Attempting to deserialize invalid data
+- Type mismatch (e.g., trying to use string operations on a list)
+
+**Examples:**
+- `JSONDecodeError`: Cannot deserialize JSON data
+- `SerializationError`: Cannot serialize object
+- `WRONGTYPE`: Operating on wrong data type
+
+**When to handle:** Sometimes. If the error is due to user input or external data, handle it gracefully. If it's due to your code, fix the code.
+
+**Example:**
+
+```mermaid {width="60%"}
+graph TB
+    A["Read cached data"]
+    A --> B["Try to deserialize"]
+    B -->|Success| C(["Use the data"])
+    B -->|Deserialization fails| D["Log the error"]
+    D --> E["Delete corrupted<br/>cache entry"]
+    E --> F(["Fetch fresh data<br/>from source"])
+```
+
+### Resource errors
+
+Resource errors occur when Redis runs out of resources or hits limits.
+
+**Common causes:**
+- Memory limit reached
+- Connection pool exhausted
+- Too many connections
+- Key eviction due to memory pressure
+
+**Examples:**
+- `OOM command not allowed when used memory > 'maxmemory'`
+- Connection pool timeout
+- `LOADING Redis is loading the dataset in memory`
+
+**When to handle:** Sometimes. Some resource errors are temporary (Redis loading), while others indicate a configuration problem.
+
+**Example:**
+
+```mermaid {width="80%"}
+graph TB
+    A{Resource error<br/>occurred?}
+    A -->|Redis loading| B(["Retry after<br/>a delay"])
+    A -->|Memory full| C(["Check Redis<br/>configuration and data"])
+    A -->|Pool exhausted| D(["Increase pool size<br/>or reduce concurrency"])
+```
+
+## Error handling patterns
+
+### Pattern 1: Fail fast
+
+Use this when the error is unrecoverable or indicates a bug in your code.
+
+**When to use:**
+- Command errors (invalid syntax)
+- Authentication errors
+- Programming errors
+
+**Example:**
+```python
+try:
+    result = r.get(key)
+except redis.ResponseError as e:
+    # This indicates a bug in our code
+    raise  # Re-raise the exception
+```
+
+### Pattern 2: Graceful degradation
+
+Use this when you have an alternative way to get the data.
+
+**When to use:**
+- Cache reads (fallback to database)
+- Session reads (fallback to default values)
+- Optional data (skip if unavailable)
+
+**Example:**
+```python
+try:
+    cached_value = r.get(key)
+    if cached_value:
+        return cached_value
+except redis.ConnectionError:
+    logger.warning("Cache unavailable, using database")
+
+# Fallback to database
+return database.get(key)
+```
+
+### Pattern 3: Retry with backoff
+
+Use this when the error is temporary and the operation is idempotent.
+
+**When to use:**
+- Connection timeouts
+- Temporary network issues
+- Redis loading data
+
+**Example:**
+```python
+import time
+
+max_retries = 3
+retry_delay = 0.1
+
+for attempt in range(max_retries):
+    try:
+        return r.get(key)
+    except redis.TimeoutError:
+        if attempt < max_retries - 1:
+            time.sleep(retry_delay)
+            retry_delay *= 2  # Exponential backoff
+        else:
+            raise
+```
+
+### Pattern 4: Log and continue
+
+Use this when the operation is not critical to your application.
+
+**When to use:**
+- Cache writes (data loss is acceptable)
+- Non-critical updates
+- Metrics collection
+
+**Example:**
+```python
+try:
+    r.setex(key, 3600, value)
+except redis.ConnectionError:
+    logger.warning(f"Failed to cache {key}, continuing without cache")
+    # Application continues normally
+```
+
+## Decision tree: How to handle errors
+
+```mermaid
+graph LR
+    Start{Error occurred?}
+
+    Start -->|Connection error| C1{Operation type?}
+    C1 -->|Read| C2["Graceful degradation<br/>fallback"]
+    C1 -->|Write| C3["Log and continue<br/>or retry"]
+    C1 -->|Critical| C4["Retry with backoff"]
+
+    Start -->|Command error| Cmd1{Error source?}
+    Cmd1 -->|User input| Cmd2["Log and return<br/>error to user"]
+    Cmd1 -->|Your code| Cmd3["Fail fast<br/>fix the bug"]
+
+    Start -->|Data error| D1{Operation type?}
+    D1 -->|Read| D2["Log, invalidate,<br/>fallback"]
+    D1 -->|Write| D3["Log and fail<br/>data is invalid"]
+
+    Start -->|Resource error| R1{Error type?}
+    R1 -->|Redis loading| R2["Retry with backoff"]
+    R1 -->|Pool exhausted| R3["Increase pool size"]
+    R1 -->|Memory full| R4["Check configuration"]
+```
+
+## Logging and monitoring
+
+### What to log
+
+- **Error type and message:** What went wrong?
+- **Context:** Which key? Which operation?
+- **Timestamp:** When did it happen?
+- **Retry information:** Is this a retry? How many attempts?
+
+**Example:**
+```python
+logger.error(
+    "Redis operation failed",
+    extra={
+        "error_type": type(e).__name__,
+        "operation": "get",
+        "key": key,
+        "attempt": attempt,
+        "timestamp": datetime.now().isoformat(),
+    }
+)
+```
+
+### What to monitor
+
+- **Error rate:** How many errors per minute?
+- **Error types:** Which errors are most common?
+- **Recovery success:** How many retries succeed?
+- **Fallback usage:** How often do we use fallback strategies?
+
+These metrics help you identify patterns and potential issues.
+
+## Common mistakes
+
+### ❌ Catching all exceptions
+
+```python
+try:
+    result = r.get(key)
+except Exception:  # Too broad!
+    pass
+```
+
+**Problem:** You might catch unexpected errors and hide bugs.
+
+**✅ Better:**
+```python
+try:
+    result = r.get(key)
+except redis.ConnectionError:
+    # Handle connection error
+    pass
+```
+
+### ❌ Not distinguishing error types
+
+```python
+try:
+    result = r.get(key)
+except redis.ResponseError:
+    # Retry? This won't help if it's a syntax error!
+    retry()
+```
+
+**Problem:** Different errors need different handling.
+
+**✅ Better:**
+```python
+try:
+    result = r.get(key)
+except redis.TimeoutError:
+    retry()  # Retry on timeout
+except redis.ResponseError:
+    raise   # Fail on syntax error
+```
+
+### ❌ Retrying non-idempotent operations
+
+```python
+# This increments the counter each retry!
+for attempt in range(3):
+    try:
+        r.incr(counter)
+        break
+    except redis.TimeoutError:
+        pass  # Retry
+```
+
+**Problem:** Retrying non-idempotent operations can cause data corruption.
+
+**✅ Better:** Only retry idempotent operations (GET, SET with same value) or use transactions.
+
+### ❌ Ignoring connection pool errors
+
+```python
+# Pool is exhausted, but we don't handle it
+result = r.get(key)  # Might timeout waiting for connection
+```
+
+**Problem:** Connection pool errors indicate a configuration or concurrency issue.
+
+**✅ Better:** Monitor pool usage and increase size if needed.
+
+## Client-specific error handling
+
+For detailed information about exceptions in your client library, see:
+
+- [redis-py error handling]({{< relref "/develop/clients/redis-py/error-handling" >}})
+- [Node.js error handling]({{< relref "/develop/clients/nodejs/error-handling" >}})
+- [Java (Jedis) error handling]({{< relref "/develop/clients/jedis/error-handling" >}})
+- [Go (go-redis) error handling]({{< relref "/develop/clients/go/error-handling" >}})
+- [.NET (NRedisStack) error handling]({{< relref "/develop/clients/dotnet/error-handling" >}})
+
+## Next steps
+
+- Learn about [connection pooling]({{< relref "/develop/clients/pools-and-muxing" >}}) to avoid resource errors
+- Explore [client-side caching]({{< relref "/develop/clients/client-side-caching" >}}) for resilient applications
+- See [use-case guides]({{< relref "/develop/use-cases" >}}) for pattern-specific error handling examples
+