Guard production Databricks data pipelines — Delta Lake, Liquid Clustering,
Structured Streaming, Auto Loader, and DLT — against the twelve foot-guns that
fire at scale: OPTIMIZE/auto-compaction conflicts, Liquid-Clustering merge
conflicts, VACUUM breaking a streaming checkpoint, RocksDB OOM, Auto Loader
schema-evolution stops, and DLT refresh data loss. Includes a PreToolUse hook
that blocks DROP/CREATE-OR-REPLACE/VACUUM on a table with active streaming
consumers. Use when a Delta MERGE/OPTIMIZE fails with a concurrency exception, a
stream breaks after VACUUM or a table replace, an Auto Loader stream stops on a
new column, a DLT refresh drops data, or before running a destructive op on a
streamed-from table. Trigger with "ConcurrentAppendException",
"ConcurrentDeleteDeleteException", "DELTA_FILE_NOT_FOUND", "streaming checkpoint
broke", "vacuum broke my stream", "autoloader UnknownFieldException", "dlt full
refresh".
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Guard production Databricks data pipelines — Delta Lake, Liquid Clustering,
Structured Streaming, Auto Loader, and DLT — against the twelve foot-guns that
fire at scale: OPTIMIZE/auto-compaction conflicts, Liquid-Clustering merge
conflicts, VACUUM breaking a streaming checkpoint, RocksDB OOM, Auto Loader
schema-evolution stops, and DLT refresh data loss. Includes a PreToolUse hook
that blocks DROP/CREATE-OR-REPLACE/VACUUM on a table with active streaming
consumers. Use when a Delta MERGE/OPTIMIZE fails with a concurrency exception, a
stream breaks after VACUUM or a table replace, an Auto Loader stream stops on a
new column, a DLT refresh drops data, or before running a destructive op on a
streamed-from table. Trigger with "ConcurrentAppendException",
"ConcurrentDeleteDeleteException", "DELTA_FILE_NOT_FOUND", "streaming checkpoint
broke", "vacuum broke my stream", "autoloader UnknownFieldException", "dlt full
refresh".
The data-ops spine of the pack. Delta Lake, Liquid Clustering, Structured
Streaming, and DLT each ship a different set of foot-guns that fire most visibly
when production data flows through them at scale — and most of them are documented
platform decisions that surprise engineers, not bugs. This skill's job is
friction at trigger time (a hook that blocks the genuinely-irreversible op) plus
deterministic recovery when something already broke.
Overview
Twelve foot-guns, grouped by the surface that triggers them. Eleven are owned
outright (D01–D10, D12); the twelfth — D11, DLT rebuild cost — is shared with
databricks-cost-leak-hunter: this skill checks the rebuild cost as part of
pre-refresh safety, that skill owns ongoing cost optimization.
Delta write conflicts. D01 ConcurrentDeleteDeleteException — a manual
OPTIMIZE colliding with auto-compaction, which is silently enabled on any table
touched by MERGE/UPDATE/DELETE. D02 ConcurrentAppendException after moving
to Liquid Clustering — LC keeps file-set-level writer conflicts; a fan-out MERGE
breaks unless its predicate is narrowed to the clustering keys.
Streaming + checkpoint. D03 DELTA_FILE_NOT_FOUND_DETAILED — VACUUM deletes
files the checkpoint pins to. D04 silent checkpoint corruption / reset to batch 0.
D05 RocksDB state-store off-heap OOM (the heap looks fine while off-heap state pins
multi-GB). D12 DIFFERENT_DELTA_TABLE_READ_BY_STREAMING_SOURCE — CREATE OR REPLACE mints a new UUID and kills every active consumer.
Migration + evolution. D06 Liquid-Clustering migration's hidden full-rewrite
cost + downstream partition-predicate breakage. D07 time travel breaking silently
when VACUUM crosses the retention boundary. D10 Auto Loader
UnknownFieldException stopping the stream on every new column.
DLT. D08 the @dlt.table thread race (out-of-order registration). D09 full
refresh silently dropping data from a non-replayable source. D11 the rebuild cost
multiplier (checked before a full refresh; ongoing DLT cost is
databricks-cost-leak-hunter's job).
The hook (AP02/AP06 — this pack's only blocking hook). A PreToolUse hook
(hooks/streaming-guard-hook.py) intercepts a Bash command that runs DROP TABLE,
CREATE OR REPLACE TABLE, or against a table and — only when it confirms
via that an active stream reads that table —
with a message naming the consumers and the pain. It is precise by
design: it matches only real SQL-execution surfaces (never a mentioning
"drop table"), and it — if it cannot verify consumers, it allows
rather than false-block. Blocking is reserved for the genuinely irreversible.
VACUUM
system.streaming.query_progress
blocks it
git commit
fails open
Deterministic work lives in scripts/; deep knowledge in references/; the
Liquid-Clustering predicate rewrite in the merge-rewriter subagent. Two data
planes: the databricks-workspace-mcp control plane (cluster/pipeline events) and
the CLI Statement Execution API for system.* reads. Either absent → advisory mode
on pasted input.
Prerequisites
databricks-workspace-mcp registered — for clusters_events (RocksDB OOM
correlation) and pipelines_get (DLT event log). Absent → advisory mode.
Databricks CLI authenticated + jq, and DATABRICKS_WAREHOUSE_ID set —
for the system.streaming.query_progress reads the hook and recovery flows use.
The hook fails open (allows) if these are absent, so it never false-blocks.
The hook is a plugin-level PreToolUse hook — it runs on Bash commands once
the pack is installed. It is silent on everything except a confirmed-unsafe
destructive op.
Instructions
Pick the flow by symptom. Always name the exact, searchable Databricks error
string — ConcurrentAppendException, ConcurrentDeleteDeleteException,
DELTA_FILE_NOT_FOUND_DETAILED, DIFFERENT_DELTA_TABLE_READ_BY_STREAMING_SOURCE,
UnknownFieldException — even when the user paraphrases it or gives a short form;
the full code is what an operator greps logs and docs for.
Step 1: Before a destructive op (the hook does this automatically)
Running DROP TABLE / CREATE OR REPLACE TABLE / VACUUM on a table? The hook
checks for active streaming consumers first and blocks if any exist. To check
manually, query system.streaming.query_progress for a stream whose
source_description names the table. If consumers exist: do NOT CREATE OR REPLACE (use ALTER/in-place — D12) and do NOT VACUUM below the consumers'
checkpoint lag (D03/D07). See
${CLAUDE_SKILL_DIR}/references/checkpoint-recovery.md.
Step 2: A Delta write conflict (D01, D02)
ConcurrentDeleteDeleteException (D01) — before a manual OPTIMIZE, probe
the table for auto-compaction:
ConcurrentAppendException on a Liquid-Clustering table (D02) — hand the
failing MERGE to the merge-rewriter subagent; it fetches the target's
clustering keys via DESCRIBE DETAIL and narrows the ON predicate so writers
touch disjoint file sets.
Step 3: A broken streaming source (D03, D04, D12)
Map the symptom to the failure class and its exact error code, then get the
recovery tier from the decision tree:
It echoes the canonical error code and recommends SAFE_RESTART /
REPROCESS_FROM_OFFSET / RESTORE_FROM_TIME_TRAVEL / FULL_RESET_BACKFILL with the
data-loss tradeoff stated up front. Name that full code in your answer — not just
the short class. The full
three-tier reasoning is in
${CLAUDE_SKILL_DIR}/references/checkpoint-recovery.md.
Step 4: RocksDB state-store OOM (D05)
A driver/executor OOM while the JVM heap looks healthy points at off-heap RocksDB
state. Correlate the OOM to state size with clusters_events, then bound the
memory and enable changelog checkpointing per
${CLAUDE_SKILL_DIR}/references/rocksdb-state-store-tuning.md.
Step 5: Auto Loader schema evolution (D10)
A stream stopping with UnknownFieldException on a new column is the default
addNewColumns mode. Choose the mode deliberately (evolve-and-restart vs
rescue's silent widening) and pin types with schemaHints per
${CLAUDE_SKILL_DIR}/references/autoloader-schema-evolution.md.
Step 6: DLT rebuild safety (D08, D09, D11)
Before a DLT full refresh, confirm every source is replayable (a Kafka topic past
retention or a truncate-and-load source loses data on refresh — D09) and that
@dlt.table registration is deterministic (the thread race — D08). Read the DLT
event log with pipelines_get; the checklist is in
${CLAUDE_SKILL_DIR}/references/dlt-rebuild-safety.md.
Output
A hook decision — a destructive op on a streamed-from table is blocked with
the active consumers named and the pain (D12 / D03-D07) explained; everything
else passes silently.
A pre-OPTIMIZE verdict — SAFE or COLLISION RISK (auto-compaction on) with the
disable-or-serialize fix.
A rewritten MERGE — the LC clustering-key-scoped predicate (from
merge-rewriter) that stops ConcurrentAppendException.
A recovery recommendation — the recovery tier + steps + the data-loss risk,
for the specific failure class.
A tuning / mode / refresh-safety recommendation — RocksDB bounds, Auto Loader
mode, or the DLT full-refresh checklist, from the matching reference.
Error Handling
Error
Cause
Solution
ConcurrentDeleteDeleteException
Manual OPTIMIZE races auto-compaction (D01)
Run pre-optimize-check.sh; don't manually OPTIMIZE an auto-compacted table, or disable auto-compact first.
ConcurrentAppendException on an LC table
MERGE predicate not scoped to clustering keys (D02)
Route the MERGE to merge-rewriter; narrow the ON predicate to the clustering keys.
DELTA_FILE_NOT_FOUND_DETAILED
VACUUM deleted checkpoint-pinned files (D03)
recover-streaming-source.py --failure file-not-found; restore via time travel if in retention, else reprocess/reset.
DIFFERENT_DELTA_TABLE_READ_BY_STREAMING_SOURCE
CREATE OR REPLACE minted a new UUID (D12)
The old checkpoint is dead — new checkpoint + backfill; the hook prevents this going forward.
Driver OOM, heap looks fine
Off-heap RocksDB state (D05)
Bound state-store memory + changelog checkpointing; size state with a watermark.
UnknownFieldException, stream stopped
Auto Loader addNewColumns default (D10)
Restart to evolve (idempotent sink), or choose rescue/schemaHints deliberately.
Hook allowed a destructive op with a warning
Could not verify consumers (no CLI/warehouse)
Advisory — the hook fails open; verify system.streaming.query_progress manually before running it.
Examples
Example 1: "About to CREATE OR REPLACE a table other jobs stream from."
The PreToolUse hook fires, confirms 2 active consumers via
system.streaming.query_progress, and blocks with: "CREATE OR REPLACE mints a
new UUID → both consumers die with DIFFERENT_DELTA_TABLE_READ_BY_STREAMING_SOURCE;
use ALTER / in-place."
Example 2: "My MERGE into a Liquid-Clustering table fails with ConcurrentAppendException."
The merge-rewriter subagent reads the target's clustering keys via DESCRIBE DETAIL and rewrites the ON predicate to include them, so concurrent writers
touch disjoint file sets — the exception stops without serializing the jobs.
Example 3: "My stream died with DELTA_FILE_NOT_FOUND after a VACUUM."
recover-streaming-source.py --failure file-not-found --time-travel yes →
RESTORE_FROM_TIME_TRAVEL (no data loss): restore the source to a pre-VACUUM version,
restart on the existing checkpoint, then align VACUUM retention with the checkpoint lag.
Example 4: "Before I OPTIMIZE this table."
pre-optimize-check.sh --table main.sales.orders reports COLLISION RISK because
delta.autoOptimize.autoCompact is on — so the skill recommends letting
auto-compaction do it, or disabling it for the maintenance window first.
