ClickHouse Writer programmer's reference

UploadPolicy

String

No

eventcount:10000, interval:30s

Controls how often ClickHouse Writer flushes a batch. eventcount:<N> flushes after N events; interval:<duration> flushes after the given duration (for example, 5s, 30s, 2m). Either condition triggers a flush.

Target Table Definition

String (JSON)

No

""

Per-table DDL clauses applied when Striim creates a target table: Order By (Sorting Key), Primary Key, Partition By, Sample By, TTL, and Settings. Ignored if the target table already exists. Has no effect for Replicated engines, Distributed tables, or AggregatingMergeTree, because Striim does not manage table creation for these engines.

Tables

String

Yes

""

Maps source tables to target tables. Supports explicit mapping (src.EMP, target_db.employee), wildcard mapping (SCH.%, target_db.% — wildcards are supported for tables, not databases), and single-table mapping. Events that do not match any mapping are discarded and counted in the discard metric. Supports the KeyColumn and ColumnMap sub-clauses (see Target table definition).

Table Engine

String

No

MergeTree

The ClickHouse table engine used for all tables written by this target. See Choosing a table engine for supported values and guidance.

ParallelThreads

Integer

No

1

Creates multiple writer instances to increase throughput. Events are distributed by target table, and events for the same table are always routed to the same instance to preserve ordering. If a writer targets only one table, increasing ParallelThreads beyond 1 has no effect; see Scaling writes to a single table for an alternative.

OptimizedMerge

Boolean

No

false

Applies only when Table Engine is MergeTree and Mode is MERGE. Enable when the source emits partial-row updates (for example, Oracle supplemental logging set to capture changed columns only). Enabling OptimizedMerge with any other table engine halts the application at startup.

NullMarker

String

No

NULL

The token ClickHouse Writer writes to represent a NULL value in the batched CSV data. Change this only if your data legitimately contains the literal string "NULL" as a non-null value.

Mode

String

No

APPENDONLY

Applies only when Table Engine is MergeTree. APPENDONLY appends every event as a new row and does not apply UPDATE, DELETE, or PKUPDATE semantics; use it for initial loads, audit tables, or non-CDC sources. MERGE reconciles INSERT, UPDATE, DELETE, and PKUPDATE operations from the source.

IgnorableExceptionCode

String

No

""

The only accepted value is the literal string TABLE_NOT_FOUND (case-insensitive); any other value is rejected at compile time. When set, a TABLE_NOT_FOUND error for a target table is logged and the triggering event is discarded and acknowledged instead of halting the application; the missing table is remembered so later events for that table are also discarded without re-querying ClickHouse. Use with care: ignoring this error means events for a missing table are silently dropped rather than retried or surfaced.

Excluded Tables

String

No

""

A semicolon-separated list of source tables to exclude from replication. Events for a listed table are discarded rather than written to ClickHouse. Supports the same case-sensitive, multi-schema, and wildcard matching as the Tables property.

DeleteStrategy

String

No

LightweightDelete

Applies when Mode is MERGE (MergeTree) or when Table Engine is CoalescingMergeTree. LightweightDelete issues DELETE FROM ... WHERE ...; ClickHouse marks the rows deleted immediately and reclaims them physically during a later background merge. MutationDelete issues ALTER TABLE ... DELETE WHERE ...; ClickHouse rewrites the affected data parts immediately, which is useful for compliance-driven erasure requirements but causes higher write amplification.

ConnectionRetryPolicy

String

No

initialRetryDelay=10s, retryDelayMultiplier=2, maxRetryDelay=1m, maxAttempts=10, totalTimeout=10m

Controls how ClickHouse Writer retries a failed connection before halting the application.

connectionProfileName

String

Yes

""

The name of the ClickHouse Connection Profile attached to this writer. Credentials are not configurable inline; they come from the Connection Profile.

CDDLAction

String

No

Process

Controls how ClickHouse Writer responds to DDL events from the source. Process applies supported CREATE, ALTER, DROP, and TRUNCATE TABLE changes to ClickHouse automatically. Ignore skips the DDL event and continues processing DML. Halt flushes pending data and stops the application; use this when the target schema is managed outside Striim, for example by dbt or Terraform. See Schema evolution (CDDLAction) for supported changes and known limitations.