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When building Event-Sourced microservices on sandboxed WebAssembly runtimes (like Wasmtime or Fermyon Spin), you are running database operations across strict host boundaries. This can sometimes lead to unexpected serialization or type-mapping behaviors from low-level database drivers. This document lists known driver-specific quirks, common errors, and how to resolve them.

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🐘 PostgreSQL Driver Pitfalls

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Issue: “Failed to parse JSON: expected value at line 1 column 1”

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Symptom

When querying aggregated events or JSONB payloads from a PostgreSQL database under a WebAssembly runtime (such as Fermyon Spin), you encounter a parsing/deserialization error like: 
Constraint Validation Error
error running server function: Failed to parse latest_events JSON: expected value at line 1 column 1

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Root Cause

This error occurs when using PostgreSQL-native aggregation functions (e.g., json_agg or json_build_object) inside custom raw SQL queries. Because the low-level Fermyon Spin PostgreSQL WASI driver doesn’t have native, high-level rust-postgres type-mappings for aggregated JSON results, it returns them under the low-level DbValue::Unsupported(Vec<u8>) (or SpinPgDbVal::Unsupported) variant rather than standard strings or structured types. If this unsupported byte payload is unmatched or printed raw via its debug representation, it generates a non-JSON debug string like "DbValue::Unsupported([91, 123, ...])". Feeding this raw string into serde_json::from_str throws the expected value at line 1 column 1 error (because the string begins with "D" instead of valid JSON brackets/braces [ or {).

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Solution

Our database adapter (adapters.rs) has been updated to explicitly pattern-match SpinPgDbVal::Unsupported(b) and SpinPgDbVal::Jsonb(j) variants. Instead of falling back to debug string formatting, the adapter attempts to parse the raw byte slices directly using serde_json::from_slice: 
SpinPgDbVal::Unsupported(b) => {
    if let Ok(jv) = serde_json::from_slice::<serde_json::Value>(b) {
        jv
    } else {
        // Fallback to lossy UTF-8 if it is not valid JSON
        serde_json::Value::String(String::from_utf8_lossy(b).into_owned())
    }
}
If you write custom repository methods or custom raw database query handlers in your application:
  • Avoid treating JSON-aggregated query columns as simple text strings (DbValue::Str).
  • Always check if your driver returns them as DbValue::Unsupported or DbValue::Jsonb byte arrays.
  • Safely parse using serde_json::from_slice to prevent decoding crashes.

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🗄️ SQLite Driver Pitfalls

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Issue: Dynamic Type Conversions

In SQLite, columns are dynamically typed (affinity-based). If a query column contains json_object or json_group_array, the driver may return the result as a raw Value::Blob(Vec<u8>) or Value::Text(String).
  • Pitfall: Attempting to extract a text string using .as_str() directly on a value returned as a Blob will fail.
  • Best Practice: Use our core adapter’s helper methods, which safely handle both text strings and binary bytes dynamically: 
    let text = match value {
    Value::Text(s) => s.clone(),
    Value::Blob(b) => String::from_utf8_lossy(b).into_owned(),
    _ => return Err("Unexpected sqlite type".to_string()),
};

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🔄 Concurrency Collision vs. Database Locking

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Symptom: RepositoryError::Concurrency vs Gateway Timeout

  • Concurrency Collision: Occurs when two request threads try to commit events with the same revision sequence under Optimistic Concurrency Control (OCC). This is a healthy, expected system transition. The transaction rolls back cleanly, and the client should load the latest event stream and retry.
  • Database Deadlock / Timeout: Occurs when your database connection pool is exhausted or a connection block is held open indefinitely by long-running transactions.
    • Tip: In Spin or Wasmtime serverless environments, avoid holding long-running synchronous locks or blocks. Projections should write and checkpoint asynchronously to prevent blocking the write transaction paths.