deltalite

Streaming, partition-level upsert for Delta Lake tables that replaces delta-rs's SQL MERGE with a bounded-memory merge engine. Memory is bounded by the size of the incoming batch and a few concurrency knobs — never by the size of the target table.

delta-rs stays the storage and protocol layer (transaction log, checkpoints, Parquet encoding, S3 conditional-put commits, conflict resolution). deltalite replaces only the merge execution, and writes output files through a streaming writer that produces the same files and Add-action statistics as delta-rs's RecordBatchWriter without holding a second copy of each file.

import deltalite

table = deltalite.DeltaLiteTable.open("s3://bucket/my_table")
stats = table.upsert(record_batch, primary_keys=["id"], partition_key="day")
print(f"v{stats.version}: +{stats.rows_inserted} / ~{stats.rows_updated}")

Why

delta-rs MERGE executes a DataFusion hash join whose memory scales with the scanned target, and it can deadlock under a bounded memory pool (delta-io/delta-rs#4614). For a large, slowly-changing table merged against a comparatively small batch — the typical incremental-sync shape — that means either OOM risk or a hang.

deltalite takes a different route:

1. Build a primary-key hash set over the (small) source batch.
2. Stream the (large) target one Parquet row group at a time, dropping rows whose key is in the source set.
3. Write survivors plus the source rows into new files.
4. Commit every touched partition in one atomic Delta commit.

Peak memory is bounded by the source batch and the concurrency knobs, not by the table. In validation, resident memory stayed flat from 62k- to 1M-row partitions (~4× below MERGE) at matching write volume, via exact content-based file selection.

Installation

pip install deltalite
# or
uv add deltalite

Prebuilt cp312-abi3 wheels are published for manylinux (2_28) and musllinux on x86_64/aarch64, and macOS on arm64/x86_64. A single wheel works on any CPython 3.12 or newer. No Rust toolchain is needed to install.

Usage

upsert accepts anything with the pyarrow C-stream interface — a pyarrow.Table, a RecordBatch, or a RecordBatchReader:

import pyarrow as pa
import deltalite

table = deltalite.DeltaLiteTable.open(
    "s3://bucket/events",
    storage_options={
        "AWS_REGION": "us-east-1",
        "AWS_ACCESS_KEY_ID": "...",
        "AWS_SECRET_ACCESS_KEY": "...",
    },
)

batch = pa.table({
    "id":  [1, 2, 3],
    "day": ["2026-01-01", "2026-01-01", "2026-01-02"],
    "val": ["a", "b", "c"],
})

stats = table.upsert(
    batch,
    primary_keys=["id"],
    partition_key="day",          # omit for an unpartitioned table
    commit_metadata={"source": "my-sync"},
)

print(stats)
# UpsertStats(version=42, partitions_touched=2, files_added=2, files_removed=1, ...)

Rows in the batch whose primary key already exists are replaced; new keys are inserted. Deletes are not expressed through upsert — it is an insert-or-replace by key. Duplicate primary keys within a single batch are rejected (raising DeltaLiteError) rather than silently double-inserted.

API

DeltaLiteTable

version, table_id, configuration, schema_*, partition_columns, num_files, files and file_uris read the loaded snapshot in memory and do no I/O; call reload() first when the table may have moved. history reads the log.

UpsertStats

Returned by upsert. Counts: version, partitions_touched, files_added, files_removed, files_carried_over, files_probed, rows_updated, rows_inserted, rows_copied, source_rows, null_pk_rows. Per-phase wall-clock timings (milliseconds): ingest_ms (importing the pyarrow source), initial_open_ms (the full snapshot load in DeltaLiteTable.open, reported on the first upsert through a handle and 0 afterwards, so a sum over a handle's upserts counts it once), open_ms (snapshot refreshes around the upsert), relax_ms (nullability-relax check), plan_ms (listing + pruning files), rewrite_ms (reading + rewriting the touched partitions), commit_ms (committing to the Delta log), maintenance_ms (checkpoint/log cleanup, non-zero only on checkpoint-boundary commits). columns_relaxed counts non-nullable columns flipped to nullable before the write.

Compaction

compact plans bins exactly as delta-rs's optimize.compact does, from the loaded log with no storage reads, and commits them the same way (ZSTD level 4, one OPTIMIZE commit, dataChange=false). Each bin is streamed one file and one byte-bounded batch at a time through the upsert's fetch and decode budgets, so memory follows the knobs, not the bin. Output files and row groups are also capped by decoded size, so a bin that decodes past a cap writes more than one file where delta-rs writes one.

Conflicts: when another writer commits first, compaction keeps each bin whose input files are all still live and drops the others (deleting their output), so it never resurrects rows a merge replaced. A concurrent schema, partitioning or protocol change raises DeltaLiteCommitConflictError. Tables the upsert refuses are refused here too (DeltaLiteUnsupportedTableError). The result dict carries deltalite's counters plus delta-rs's numFilesAdded, numFilesRemoved, partitionsOptimized, numBatches, totalConsideredFiles and totalFilesSkipped.

Exceptions

All inherit from DeltaLiteError, so you can catch the base or branch on kind:

Operational knobs

Per-call — keyword arguments to upsert (defaults in parentheses):

Process-global — environment variables, enforced on top of the per-call knobs so that many concurrent upsert threads in one process cannot multiply the budgets:

DELTALITE_PROCESS_MAX_PARALLEL_PARTITIONS (8), DELTALITE_PROCESS_MAX_PARALLEL_FILES (16), DELTALITE_PROCESS_MAX_BUFFERED_BYTES (256 MiB), DELTALITE_PROCESS_MAX_FETCH_BYTES (256 MiB), DELTALITE_MAX_SOURCE_BYTES, DELTALITE_MULTIPART_THRESHOLD_BYTES, DELTALITE_MULTIPART_PART_SIZE_BYTES.

Checkpoint prefetch. Opening or reloading a table reads its latest checkpoint Parquet file whole in one GET and serves the Parquet reader's many small range reads from memory, instead of one round trip per footer, metadata block and column chunk. DELTALITE_CHECKPOINT_PREFETCH_MAX_BYTES (128 MiB) bounds the total checkpoint bytes cached by one table load; larger files or multipart checkpoints that exhaust the budget fall back to range reads, and 0 disables the prefetch. Cached bytes also reserve space from DELTALITE_PROCESS_MAX_BUFFERED_BYTES, so concurrent table opens share the process-wide budget. The bytes are released as soon as the load finishes.

Commit snapshot adoption. After a successful upsert the handle keeps the table state delta-rs derived for the commit it just wrote (the same state every delta-rs operation returns) instead of listing the log and reading that commit back. The next upsert still refreshes before it plans, so commits from other writers are observed exactly as before, and a checkpoint the upsert's own maintenance wrote is adopted on that refresh. DELTALITE_ADOPT_COMMIT_SNAPSHOT=0 restores the post-commit refresh.

Data-file reads. Every Parquet data file is opened with a 64 KiB footer size hint, so a footer that fits arrives in one GET instead of two, and a file the probe found a match in hands its parsed footer to the rewrite, so the file is not opened a second time.

Metrics

deltalite emits via the Rust metrics facade (static labels only): deltalite_upserts_total (outcome, prune_strategy, error_kind), deltalite_upsert_duration_seconds, deltalite_files_{added,removed,carried_over,probed}_total, deltalite_rows_{updated,inserted,copied}_total, and deltalite_checkpoint_prefetch_total (outcome).

Compatibility & status

• Built against deltalake (delta-rs) 0.32.x as the storage/protocol layer. Correctness is guaranteed by a differential parity suite that runs the same batch sequences through real delta-rs MERGE and through deltalite.upsert and asserts identical logical content — not by version equality.
• Not supported: tables with deletion vectors or column mapping (detected and raised as DeltaLiteUnsupportedTableError), and SCD2 merges.
• deltalite rejects duplicate source primary keys that MERGE silently double-inserts — check pre-existing data if you migrate an existing pipeline.

This package is developed in the PostHog monorepo under rust/deltalite/. Issues and source live there.