Async INSERTs is a ClickHouse feature tha enables batching data automatically and transparently on the server-side. Although async inserts work, they still have issues, but have been improved in latest versions. We recommend to batch at app/ingestor level because you will have more control and you decouple this responsibility from ClickHouse. Being said that here some insights about Async inserts you should now:

  • Async inserts give acknowledgment immediately after the data got inserted into the buffer (wait_for_async_insert = 0) or by default, after the data got written to a part after flushing from buffer (wait_for_async_insert = 1).
  • INSERT .. SELECT is NOT async insert. (You can use matView + Null table OR ephemeral columns instead of INPUT function, then ASYNC insert work)
  • Async inserts will do (idempotent) retries.
  • Async inserts can collect data for some offline remote clusters: Yandex self-driving cars were collecting the metrics data during the ride into ClickHouse installed on the car computer to a distributed table with Async inserts enabled, which were flushed to the cluster once the car was plugged to the network.
  • Async inserts can do batching, so multiple inserts can be squashed as a single insert (but in that case, retries are not idempotent anymore).
  • Async inserts can loose your data in case of sudden restart (no fsyncs by default).
  • Async inserted data becomes available for selects not immediately after acknowledgment.
  • Async inserts generally have more moving parts there are some background threads monitoring new data to be sent and pushing it out.
  • Async inserts require extra monitoring from different system.tables (see system.part_log, system.query_log and system.asynchronous_inserts for 22.8). Previously such queries didn’t appear in the query log. Check: #33239.
  • Important to use wait_for_async_insert = 1 because with any error you will loose data without knowing it. For example your table is read only -> losing data, out of disk space -> losing data, too many parts -> losing data.

22.10+ bugfixes/features

  • Fixed bug which could lead to deadlock while using asynchronous inserts. See #43233.
  • Async insert dedup: Support block deduplication for asynchronous inserts. Before this change, async inserts did not support deduplication, because multiple small inserts coexisted in one inserted batch. See #38075 and #43304.
  • Added system table asynchronous_insert_log. It contains information about asynchronous inserts (including results of queries in fire-and-forget mode. (with wait_for_async_insert=0)) for better introspection. See #42040.
  • Support async inserts in clickhouse-client for queries with inlined data (Native protocol).
    • Check:
    • Also new feature opened by @alexey-milovidov to use async inserts with prepared blocks like a normal insert #54381
  • Async insert backpressure:

To improve observability / introspection

In 22.x versions, it is not possible to relate part_log/query_id column with asynchronous_insert_log/query_id column. We need to use query_log/query_id:

asynchronous_insert_log shows up the query_id and flush_query_id of each async insert. The query_id from asynchronous_insert_log shows up in the system.query_log as type = 'QueryStart' but the same query_id does not show up in the query_id column of the system.part_log. Because the query_id column in the part_log is the identifier of the INSERT query that created a data part, and it seems it is for sync INSERTS but not for async inserts.

So in asynchronous_inserts table you can check the current batch that still has not been flushed. In the asynchronous_insert_log you can find a log of all the async inserts executed.

But in ClickHouse 23.7 Flush queries for async inserts (the queries that do the final push of data) are now logged in the system.query_log where they appear as query_kind = 'AsyncInsertFlush'.


FROM system.columns
WHERE (table = 'metric_log') AND (name ILIKE '%Async%')

Query id: 3d0b7cbc-7990-4498-9c18-1c988796c487

 CurrentMetric_AsynchronousInsertThreadsActive       part

FROM system.metrics
WHERE metric ILIKE '%async%'

 AsynchronousInsertThreads                0  Number of threads in the AsynchronousInsert thread pool.                         
 AsynchronousInsertThreadsActive          0  Number of threads in the AsynchronousInsert thread pool running a task.          
 AsynchronousReadWait                     0  Number of threads waiting for asynchronous read.                                 
 PendingAsyncInsert                       0  Number of asynchronous inserts that are waiting for flush.                       
 AsyncInsertCacheSize                     0  Number of async insert hash id in cache                                          
Last modified 2023.12.07: Update (#63) (16f792a)