make zstandard optional (#2453)

Resolves #2452

Author: angus-langchain

python/langsmith/_internal/_background_thread.py

@@ -15,7 +15,7 @@
 
 from langsmith import schemas as ls_schemas
 from langsmith import utils as ls_utils
-from langsmith._internal._compressed_traces import CompressedTraces
+from langsmith._internal._compressed_traces import ZSTD_AVAILABLE, CompressedTraces
 from langsmith._internal._constants import (
     _AUTO_SCALE_DOWN_NEMPTY_TRIGGER,
     _AUTO_SCALE_UP_NTHREADS_LIMIT,
@@ -612,10 +612,16 @@ def tracing_control_thread_func(client_ref: weakref.ref[Client]) -> None:
     # 1 for this func, 1 for getrefcount, 1 for _get_data_type_cached
     num_known_refs = 3
 
-    # Disable compression if explicitly set or if using OpenTelemetry
+    # Disable compression if explicitly set, using OpenTelemetry, or zstd unavailable
+    if not ZSTD_AVAILABLE:
+        logger.debug(
+            "zstandard package is not installed. "
+            "Falling back to uncompressed multipart ingestion."
+        )
     disable_compression = (
         ls_utils.is_env_var_truish("DISABLE_RUN_COMPRESSION")
         or client.otel_exporter is not None
+        or not ZSTD_AVAILABLE
     )
     if not disable_compression and use_multipart:
         if not (client.info.instance_flags or {}).get(

python/langsmith/_internal/_compressed_traces.py

@@ -2,10 +2,15 @@
 import threading
 from typing import Optional
 
-from zstandard import ZstdCompressor  # type: ignore[import]
-
 from langsmith import utils as ls_utils
 
+try:
+    from zstandard import ZstdCompressor  # type: ignore[import]
+
+    ZSTD_AVAILABLE = True
+except ImportError:
+    ZSTD_AVAILABLE = False
+
 compression_level = int(ls_utils.get_env_var("RUN_COMPRESSION_LEVEL") or 1)
 compression_threads = int(ls_utils.get_env_var("RUN_COMPRESSION_THREADS") or -1)
 
@@ -14,6 +19,13 @@
 
 class CompressedTraces:
     def __init__(self, max_uncompressed_size_bytes: Optional[int] = None) -> None:
+        if not ZSTD_AVAILABLE:
+            raise ImportError(
+                "zstandard is required for compressed trace ingestion. "
+                "Install it with `pip install zstandard` or set the environment "
+                "variable LANGSMITH_DISABLE_RUN_COMPRESSION=true to disable "
+                "compression."
+            )
         # Configure the maximum total uncompressed size for the in-memory queue.
         if max_uncompressed_size_bytes is None:
             max_bytes_str = ls_utils.get_env_var("MAX_INGEST_MEMORY_BYTES")