stemedb/ai-lookup/patterns/error-handling.md

Error Handling Pattern

Last Updated: 2026-01-31 Confidence: High

Summary

StemeDB uses thiserror for library errors with context chains. No panics in production code. All fallible operations return Result<T, E>.

Key Facts:

• Library code: thiserror for custom error types — ALL error enums MUST use #[derive(thiserror::Error)]
• Binary code: anyhow for error chaining
• Never use unwrap(), expect(), panic!() in production
• Add context with .context("what we were doing")?
• NEVER use manual impl Display + impl Error for error types — use thiserror derives instead

Error types in workspace (all use thiserror):

• stemedb-core/src/serde.rs — SerdeError
• stemedb-wal/src/error.rs — QuarantineError
• stemedb-storage/src/error.rs — StorageError
• stemedb-ingest/src/error.rs — IngestError
• stemedb-query/src/error.rs — QueryError
• stemedb-api/src/error.rs — ApiError

The Pattern

use thiserror::Error;

#[derive(Debug, Error)]
pub enum StemeError {
    #[error("assertion not found: {0:?}")]
    NotFound(Hash),

    #[error("invalid signature for agent {agent:?}")]
    InvalidSignature { agent: AgentId },

    #[error("storage error: {0}")]
    Storage(#[from] sled::Error),

    #[error("serialization error: {0}")]
    Serialization(String),
}

// Usage with context
fn load_assertion(&self, hash: &Hash) -> Result<Assertion, StemeError> {
    let bytes = self.store
        .get(hash.as_bytes())
        .context("failed to read assertion from store")?
        .ok_or(StemeError::NotFound(*hash))?;

    rkyv::from_bytes(&bytes)
        .map_err(|e| StemeError::Serialization(e.to_string()))
}

Error Categories

Type	Description	Example
NotFound	Data doesn't exist	Missing assertion
InvalidSignature	Crypto verification failed	Tampered assertion
Storage	Underlying KV error	Disk full
Serialization	Encode/decode failed	Corrupt data

Related Topics

• Rust Guidelines
• CODING_GUIDELINES.md