recommit

Signed-off-by: TennyZhuang <[email protected]>

rfcs/0068-error-record-table.md

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+---
+feature: error_record_table
+authors:
+  - "TennyZhuang"
+start_date: "2023/07/31"
+---
+
+# Error Record Table
+
+## Summary
+
+Our current streaming engine does not help users to discover, debug, and handle errors well. When user met an data record error, they can only find a log record like ``ExprError: Parse error: expected `,` or `]` at line 1 column 10 (ProjectExecutor: fragment_id=19007)``.
+
+User can't view the eror record, and can't replay with the error record.
+
+We want to introduce the Error Record Table (ERT) to resolve the problem.
+
+## Motivation
+
+There are several benefits to maintain the error records ourselves:
+
+1. We can ensure that our storage engine can handle the volume of erroneous data, as it is of the same magnitude as the source.
+2. Users can view the error records directly over psql.
+3. Users can reproduce the error easily by the similar SQL.
+
+## Design
+
+### Creating
+
+The ERTs are automatically created as internal tables when an operator is created. In most cases, an operator will have n ERTs, where n corresponds to the number of inputs it has.
+
+### Naming
+
+Same as other internal tables while suffixed by `error_{seq}`.
+
+### Schema
+
+The schema of ERT should have the same fields as their input, with several extra columns:
+
+1. `id bigint`: The ID can be generated by the similar method like `row_id` (vnode + local monotical ID).
+2. `error_reason varchar`: A human-readable error message.
+
+### Modification
+
+To keep things simple, we do not permit any DML operations over the ERT. Only the `TRUNCATE TABLE` operation is permitted.
+
+### The relationship between ERT and the log system
+
+We should keep the warning entry in our log, and we can give the error record ID in the log entry.
+
+We can even give a SQL to query the error record in the log entry if it's helpful to user.
+
+## Unresolved questions
+
+Should we allow creating sink over ERT?
+
+## Alternatives
+
+One alternative solution is to output the complete error record directly to the log system. There are some concerns:
+
+1. The data record may be too large to record, e.g. several tens of KB.
+2. Errors may occur continuously, causing the log system to fill up quickly.
+
+## Future possibilities
+
+### Data correction
+
+ERT could potentially be used to correct data, for example, users could clean up the data within ERT and then reimport it into the source.
+
+```sql
+SELECT v1, v2, error_reason FROM __rw_internal_1023_source_1134_error_1;
+# 10000, 0, "division by zero"
+CREATE TEMP TABLE fixing_1234 (v1 int, v2 int);
+INSERT INTO fixing_1234 (
+  SELECT v1, v2 FROM __rw_internal_1023_source_1134_error_1);
+UPDATE fixing_1234 SET (v2 = 1) WHERE v2 = 0;
+INSERT INTO source_table (
+  SELECT * FROM fixing_1234
+);
+TRUNCATE TABLE __rw_internal_1023_source_1134_error_1;
+```
+
+### Sink
+
+For advanced users, we can still allow them sink the error records to their own system.