[[indices-flush]] === Flush Flushing an index is the process of making sure that any data that is currently only stored in the <> is also permanently stored in the Lucene index. When restarting, {es} replays any unflushed operations from the transaction log into the Lucene index to bring it back into the state that it was in before the restart. {es} automatically triggers flushes as needed, using heuristics that trade off the size of the unflushed transaction log against the cost of performing each flush. Once each operation has been flushed it is permanently stored in the Lucene index. This may mean that there is no need to maintain an additional copy of it in the transaction log, unless <>. The transaction log is made up of multiple files, called _generations_, and {es} will delete any generation files once they are no longer needed, freeing up disk space. It is also possible to trigger a flush on one or more indices using the flush API, although it is rare for users to need to call this API directly. If you call the flush API after indexing some documents then a successful response indicates that {es} has flushed all the documents that were indexed before the flush API was called. [source,console] -------------------------------------------------- POST twitter/_flush -------------------------------------------------- // TEST[setup:twitter] [float] [[flush-parameters]] ==== Request Parameters The flush API accepts the following request parameters: [horizontal] `wait_if_ongoing`:: If set to `true` the flush operation will block until the flush can be executed if another flush operation is already executing. If set to `false` then an exception will be thrown on the shard level if another flush operation is already running. Defaults to `true`. `force`:: Whether a flush should be forced even if it is not necessarily needed i.e. if no changes will be committed to the index. This can be used to force the generation number of the transaction log to be incremented even if no uncommitted changes are present. This parameter should be considered internal. [float] [[flush-multi-index]] ==== Multi Index The flush API can be applied to more than one index with a single call, or even on `_all` the indices. [source,console] -------------------------------------------------- POST kimchy,elasticsearch/_flush POST _flush -------------------------------------------------- // TEST[s/^/PUT kimchy\nPUT elasticsearch\n/] [float] [[synced-flush-api]] ==== Synced Flush See <>.